UNIVERSIT ` A DEGLI STUDI DI TRENTO Facolt` a di Scienze Matematiche Fisiche e Naturali Corso di Laurea in Fisica Applicata Elaborato Finale Calibration system for the muon veto scintillators employed in the CRESST-II dark matter search experiment. Sistema di calibrazione per gli scintillatori impiegati come “muon veto” nell’esperimento CRESST-II per la ricerca di dark matter. Relatori: Prof. Giuseppina Orlandini Prof. Dr. Peter Grabmayr Laureando: Daniele Nicolodi 22 Febbraio 2006 Anno Accademico 2004 - 2005
Transcript
UNIVERSITA DEGLI STUDI DI TRENTOFacolta di Scienze Matematiche Fisiche e Naturali
Corso di Laurea in Fisica Applicata
Elaborato Finale
Calibration system for the muon vetoscintillators employed in the CRESST-II
dark matter search experiment.
Sistema di calibrazione per gli scintillatori impiegati
Nonostante le persuasive evidenze indirette dell’esistenza di dark matter nell’universola sua diretta individuazione rimane una delle piu importanti sfide sperimentali dellamoderna fisica e cosmologia. Da considerazioni teoriche un plausibile candidato ad essereil principale costituente della dark matter e la Weakly Interacting Massive Particle oWIMP. Questa particella e prevista interagire con la materia ordinaria solo attraversoscattering elastico con i nuclei. Ad oggi tutti i metodi di rivelamento diretto si sonoconcentrati su questa possibilita.
Il Cryogenic Rare Event Search using Superconducting Thermometers o semplice-mente esperimento CRESST cerca di rilevare lo scattering WIMP – nuclei utilizzandorivelatori criogenici. Esso costituisce attualmente la piu avanzata installazione criogenicaa basso background esistente.
La cross section per lo scattering WIMP – nuclei e molto piccola. Nel principalerange di energie investigato non piu di qualche centinaio di eventi per kg di materiale delrivelatore per anno di esposizione sono attesi. Una efficiente soppressione del backgrounde dunque obbligatoria nelle ricerche sperimentali di dark matter. L’esperimento CRESSTsoddisfa questo requisito con la sua collocazione all’installazione a basso background deiLaboratori Nazionali del Gran Sasso, per lo schermaggio della radiazione cosmica, e l’usodi schermi di piombo e rame, per lo schermaggio dell’area sperimentale dal backgroundambientale. Ovviamente, per evitare contaminazioni, esclusivamente materiali privi dicontaminanti radioattivi sono utilizzati all’interno dello schermo.
Nonostante l’impiego di tecniche passive per la riduzione del background una noncompleta schermatura dall’ambiente esterno e una inevitabile lieve contaminazione ra-dioattiva dei materiali prossimi al rivelatore e del rivelatore stesso sono causa di radi-azione di background. Tale radiazione contribuisce alla generazione di eccitazione termicaall’interno del calorimetro e dunque limita la sensibilita dell’esperimento. Nell’esperimentoCRESST si cerca di ottenere migliore mediante sopressione attiva del background.
All’interno del calorimetro particelle α e β danno origine ad eccitazioni elettronichementre neutroni e WIMP producono eccitazioni nucleari. In un materiale scintillatore en-trambe generano luce di scintillazione e fononi ma le eccitazioni nucleari producono moltimeno fotoni delle eccitazioni elettroniche della stessa energia. L’uso di un cristallo scin-tillatore come calorimetro e la misura simultanea di fotoni e fononi permettono dunquela distinzione delle eccitazioni elettroniche causate da radiazione di background dalleeccitazioni nucleari causate da WIMP e neutroni.
Le eccitazioni nucleari indotte da neutroni non possono essere discriminate da quelleindotte da WIMP. I neutroni rimangono la principale fonte di background e limitano la
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4 INTRODUZIONE
sensibilita dell’esperimento. La maggior parte di essi e soppressa utilizzando moderatoridi polyethylene ma i neutroni ad alta energia prodotti da reazioni nucleari indotte damuoni nel materiale di schermo non possono essere eliminati. Per identificare i neutroniindotti dai muoni e prevista l’installazione di un sistema di veto per i muoni. Esso erealizzato con estesi pannelli a scintillazione e l’identificazione delle particelle incidenti eeffettuata in base all’energia che esse cedono nel rivelatore.
Per evitare di registrare eventi legati ad innocui raggi γ e dunque necessario impostareun’energia di soglia adeguata. A questo scopo una accurata calibrazione degli scintillatorie necessaria. In scintillatori molto ampi la risposta in energia dipendente dalla posizionedi impatto della particella incidente. Per questo e necessario misurare la relazione tral’ampiezza dell’impulso prodotto e l’energia della particella incidente in molti punti dellasuperficie dei pannelli. Come riferimenti energetici per la calibrazione degli scintillatorisono usati il gradino Compton prodotto per scattering Compton di raggi γ provenientida due sorgenti radioattive naturali. Questo e un processo che richiede molto tempo edeve essere automatizzato per completare la calibrazione in un tempo ragionevole.
Il mio lavoro e consistito nella progettazione e realizzazione di un dispositivo peril posizionamento della sorgente radioattiva sulla superficie dei rivelatori e lo sviluppodel pacchetto software necessario a controllare tale dispositivo e ad acquisire i dati dalsistema elettronico a cui gli scintillatori sono interfacciati. In questo scritto riporto unadescrizione del sistema realizzato e i dati ottenuti dall’analisi dei dati acquisiti.
Va notato anche che le conoscenze ed i materiali acquisiti durante questo lavorosaranno riutilizzati per la realizzazione di un sistema di posizionamento di campioni perun altro esperimento. Tale esperimento verra realizzato presso la sorgente di elettronidel laboratorio MAX-Lab dell’Universita di Lund. L’obiettivo di questo esperimento ela misura della cross section totale di assorbimento di fotoni da parte di campioni diLitio isotopicamente puri al di sotto della soglia energetica dei pioni. Per raggiungeretale scopo e necessario l’accurato posizionamento di un campione nella forma di cilindrodella lunghezza di due metri nel fascio di fotoni.
Introduction
Despite persuasive indirect evidence for the existence of dark matter in the universe, thedirect detection remains one of the outstanding experimental challenges of present dayphysics and cosmology. From theoretical considerations a plausible candidate for beingthe main dark matter constituent is the Weakly Interacting Massive Particle or WIMP.This particle is expected to interact with ordinary matter only by elastic scattering onnuclei. Up to now all direct detection schemes have focused on this possibility.
The Cryogenic Rare Event Search using Superconducting Thermometers or simplyCRESST experiment attempts to detect WIMPs – nucleus scattering using cryogenicdetectors and is presently the most advanced, deep underground, low background, cryo-genic facility.
The expected cross section for WIMPs – nucleus scattering is very small. In the mainenergy range that will be investigated no more than some hundreds of events per kg ofdetector material per year of exposure are expected. Therefore efficient background sup-pression is mandatory in experimental search for dark matter. The CRESST experimentmeets this requirement with its location at the low background facility of LaboratoriNazionali del Gran Sasso for shielding against cosmic radiation and the use of lead andcopper for shielding of the experimental area against environmental background. Ofcourse only radiopure materials are used inside the shielding to avoid contamination.
In spite of the employement of passive background reduction techniques a non perfectshielding from the external environment and a unavoidable radioactive contabinants inthe material close to the detector and in the detector itself are cause of backgroundradiation. This radiation contributes to the generation of thermal excitation in thecalorimeter and then limits the experiment sensivity. In the CRESST experiment abetter sensibility achievement is attempted with active backgroud suppression.
In the calorimeter α and β particles lead to electron recoils while neutrons and WIMPsproduce nuclear recoils. In a scintillating material both nuclear and electron recoilsgenerate scintillation light and phonons. However nuclear recoils produce much lessphotons than electron recoils of the same energy. The use of a scintillating crystall asabsorber and the simultaneous measurement of photons and phonons permit then todistinguish electron recoils from nuclear recoils and the rejection of the formers.
Nuclear recoils induced by neutrons can not be discriminated from nuclear recoils in-duced by WIMPs. Therefore neutrons remain the main source of background and limitthe sensitivity of the experiment. While the low energetic neutrons can be suppressedusing polyethylene moderators, the high energy neutrons induced by muons in the sur-ronding material cannot be eliminated by passive shielding. Muons are charged particles
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6 INTRODUCTION
and then they are more easily detected than neutrons. To cope with muon inducedneutrons background the installation of a muon veto system was then planned to detectprimary muons instead of the produced neutrons.
Large scintillator panels are used to identify muons on the base of the energy lossin the detector. To avoid triggering on signals from harmless γ-rays it is necessary toset a sensible low threshold on the scintillator detectors. For this purpose an accuratecalibration of the scintillators is necessary. As energy reference for the calibration of thescintillators the Compton edge obtained from Compton scattering of γ-rays produced bytwo natural sources is used. The energy response of large scintillators is dependent onthe impact position. That makes necessary the measurement of the pulse height – energyrelation in many points over the surface of the panels. This is a time consuming processand must be automated in order to complete the calibration in a reasonable amount oftime.
My work was the design and the construction of a device for the positioning of theradioative source over the surface of the detector panels. This required the realizationof the hardware and the development of the software package necessary to control thedevice and to acquire data from the electronics to which the scintillators are interfaced.This paper is a description of the realised system and of the data obtained from theanalysis of the acquired data along with the analysis method used.
It should be pointed out also that the knowledge and the materials acquired duringthis work will be reused for the realisation of a target positioning system for anotherexperiment. That experiment will be performed at the MAX-Lab electron beam facilityof the University of Lund. The goal of this experiment is the measurement of the totalphotoabsorption cross section of isotopic pure lithium targets below pion threshold. Forachieving that goal the positioning of a two meter long cylinder of target material in thephoton beam with high accuracy is necessary.
Chapter 1
Motivation
The first part of this chapter starts with an introduction to dark matter and an overview of theimportance of his detection and continues with the description of the CRESST experiment andwhy muon veto detectors are needed. The second part explains the choice of the scintillatordetectors and why the work described in the following chapters is needed.
1.1 Dark Matter
First hints of the existence of dark matter were observed by the Swiss astronomer Zwickyalready in the 1930s. He observed that in galaxy clusters the galaxies move too fastaround each other to be bound gravitationally if there is not a large amount of invisi-ble matter holding the system together. Nevertheless only in recent years it has beensupposed that the universe is dominated by particles whose nature is almost unknownand which have never been directly observed. The existence of such particles is not onlypostulated by astronomy but also by cosmology and theoretical particle physics. At thepresent time there are significant efforts to detect them in laboratory experiments anddetermine their physical properties. However the extremely low interaction between darkmatter and ordinary matter requires extremely sensitive detectors.
1.1.1 Indirect Evidence
The kinematics of the universe is determined by its energy density. If the average energydensity in the universe is larger than a critical density then the attractive gravitationalforces can be large enough to eventually turn the expansion of the universe into a con-traction. Otherwise if the density is less than critical the universe will expand forever.Just few years ago it was learned that this simple picture is not complete and there isalso gravitational repulsion. Anyway the critical density remains a widely used massunit for the universe. Apart from its influence on the kinematics of the universe theoverall energy density also determines the geometry of the universe. Only in a universewhere the overall matter density is equal to the critical density the space has no overallcurvature and is flat.
From astronomical observations it is known that optically visible matter is condensed
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into stars and that the mass of a star determines his luminosity. A good estimation ofthe density of the visible matter in the universe could then be obtained counting starsand galaxies and measuring their luminosity. Summing up all the visible mass in theuniverse gives about only 1% of the critical mass.
Much of the evidence for the presence of not luminous matter comes from someinconsistency in the study of the kinematics of spiral galaxies. Matter in the disk portionof a spiral galaxy should orbit the centre of the galaxy in a way where the centrifugalforce is balanced by gravitational attraction determined by the total mass inside theorbit. Based on this it is expected that the average orbital speed of an object at aspecific distance away from the majority of the mass distribution would decrease inverselywith the square root of the radius of the orbit. However from astronomical observationsserious deviations from this model were revealed. The speed does not decrease in theexpected inverse square root relationship moving away from the centre of the galaxy asexpected but it remains constant. There are many evidences of the correctness of thelaws of gravity and dynamics on a galactic scale thus an additional gravitational force isneeded to balance the centrifugal force. As the mass of the visible galaxy is not enough toexplain the stability of the motion there must be invisible matter as source of additionalgravity. High orbital speeds are observed even at very large distances from the galaxycentre then such matter seems to be distributed in an ample halo around the galaxy.
The presence of an additional fraction of matter is moreover motivated by cosmologyand particle physics. The nature of this matter is however not clarified and is generallycalled dark matter because it neither emits or absorbs light. A wide variety of indepen-dent observations point toward the existence of dark matter in much larger quantitiesthan the visible mass. To explain all observational evidence dark matter must sum upto about 27% of the critical density compared to only 1% of visible matter.
1.1.2 The Nature of Dark Matter
As the main component of matter in the universe, dark matter should have significantinfluence on the evolution of the universe and the formation of the structures. With thecurrent interpretation of early stages of universe dark matter is necessary to explain thelarge scale structure of the universe. Clarifying the nature of dark matter is then centralfor cosmology.
Ordinary matter becomes optically luminous if it is condensed into stars that areheavy enough to start nuclear fusion processes in their centres. Fusion processes cannotbe ignited in gas, dust or small planets. Also black holes do not emit light for completelydifferent reasons. Such forms of matter might then contribute to the matter densitywithout contributing to the luminosity. Ordinary matter consists of similar numbers ofprotons, neutrons and electrons where proton and neutrons together are identified asbaryons.
Dark matter however seems to be different. It cannot consist of a single type ofcharged particle, otherwise the Coulomb repulsion between the charged particles wouldovercompensate the gravitational attraction such that the particles could not be boundto galaxies. Also baryons are excluded. Free neutrons decay into protons, electrons andanti neutrinos. Due to high energies in the early universe also the inverse reactions,
1.1. DARK MATTER 9
where a proton and an electron form a neutron and a neutrino, are possible. Thereforethere were equal densities of protons and neutrons. As the energy density dropped, thebalance between the reactions shifted towards the decay of neutrons, rising the density ofprotons over the density of neutrons. When the first composed nuclei formed the decayof neutrons stopped, because they were bound into nuclei, freezing the ratio betweenproton and neutron density. The abundance of light elements in the universe dependson the baryon density at the time of nucleosyntesis. Abundance measurements allowthe derivation of the baryonic contribution to the matter density in the universe. Thosecalculations point out that the absolute amount of baryonic matter corresponds to onlyabout 5% of the critical density, about five times the amount of visible matter. Thismeans that there is a large amount of non luminous baryonic matter in form of dust,gas, planets and black holes usually referred as baryonic dark matter but it cannotaccount for all the missing mass.
Another possible dark matter candidate are neutrinos. Neutrinos are very abundant,non baryonic particles that participate only in weak interactions and gravity. As neu-trinos do not participate in electromagnetic interactions they are not part of the visiblematter and weakly interact with ordinary matter. The mass of neutrinos is not yet wellestimated but recently was determined that neutrinos are at least 250 000 times lessheavy than electrons but not massless. The product of the experimental mass limitsand the number density of neutrinos, as calculated to be produced in the early stages ofuniverse, restrict the contribution of neutrinos to the matter density of the universe tosomething between 0.2% and 20% of the critical mass.
In the early universe the formation of structures was started by density fluctuationsand driven by gravitational attraction. The overdense regions grew by attracting ad-ditional matter that built up stars, galaxies and galaxy clusters. The duration of thecondensation processes is long compared to the time highly relativistic particles, suchas neutrinos, need for traversing and leave these structures. Then, if the missing massis accounted for mainly by relativistic particles, the formation of small scale structurescould not be possible. From this constraint the limits of neutrinos contribution to mat-ter density in the universe can be restricted to something between 0.2% and 4% of thecritical mass. That rules out neutrinos as the main explanation for dark matter. Fortheir relativistic velocities neutrinos are usually referred as hot dark matter.
It looks as if only new particles can explain the nature of dark matter. Those mustinteract with ordinary matter as rarely as neutrinos, otherwise they would have beendetected already, and must be sufficiently massive and slow to allow structure formation.Until the real existence and the nature of such particles are clarified them are calledWeakly Interacting Massive Particles or WIMPs on the base of the properties they areexpected to have.
The standard model of particle physics is the most powerful and widely accepted toolfor describing subatomic particles and their interaction, in agreement with experimentalobservations. According to this model, matter is made of a set of elementary particlesordered in three families: electrons, muons and tauons. For every particle there is ananti particle with identical mass but opposite charge. The spin quantum number of allthe elementary particles is semi integer so them are fermions. Those particles interact by
10 CHAPTER 1. MOTIVATION
four fundamental forces: gravitational, weak, electromagnetic and strong interactions.The forces between the particles are mediated by exchange particles whose spin quantumnumbers are integers so them are bosons.
The standard model is still not able to provide a convincing explanation for theparticle masses or a common description of the fundamental forces. Therefore othermodel were developed. In a supersymmetric extension of the standard model there arenew bosonic partner particles for each fermion and new fermionic partner particle for eachboson. As these supersymmetric particles are heavier than the common particles theyare expected to be unstable. A new preserved quantum number is also introduced and iscalled R parity. This quantum number is different for particles and their supersymmetricpartners and makes the lightest supersymmetric particle – LSP – stable. The mass ofthis particle is calculated to be in the range between a few tens and some thousandsof GeV and if it is electrically neutral it participates only in weak interactions and ingravity. Such a particle is called neutralino and is a very promising candidate for darkmatter even if it was invented for completely other reasons.
1.1.3 Dark Matter Detection
In agreement with the hypothesis that dark matter is distributed in an halo around thegalaxy, earth is moving through a gas of dark matter particles gravitationally bounded toour galaxy. However WIMPs interact mainly via weak interaction and their interactionrate with ordinary matter is extremely small.
The cross section is expected smaller than one attobarn which is more than 18 ordersof magnitude smaller than the scattering cross section of neutrons. Moreover, even ifWIMPs are the main constituents of matter, their number density is very low. If thedensity of matter necessary to explain the dynamics in our galaxy is about 3000 GeVper m3, then a plausible WIMP mass of 300 GeV corresponds to about one thousandWIMPs per cubic meter. To be gravitationally bound to our galaxy WIMPs must havevelocity lower than about 220 km/s. It is then possible to determine a flux of aboutone hundred million WIMPs per square meter and per second while only about fiveinteraction per day and per a cubic meter are expected.
The transfer of recoil energy due to WIMPs – nucleus scattering offers a chance fordirect dark matter detection. As WIMPs masses are supposed to be in the mass rangeof atomic nuclei, the maximum energy transfer in elastic scattering is the kinetic energyof the incident particle. The energy transfer is then not expected to be more than fewtens of keV which is the limit of detectability.
In order to increase the probability of detecting a WIMP interaction, detectors haveto be both as large and a sensitive possible. However the sensitivity of large monolithicdetectors usually is limited by its high energy threshold. Most dark matter detectorstherefore consist of many small detectors modules each having a low energy threshold.Anyhow the extreme low counting rate expected is the most severe problem. Backgroundevents caused by radioactivity in the surrounding of the detector and in the detector itselfmust be strictly avoided, shielded against or identified as such.
To cope with the background, dark matter research experiments use both passiveand active background suppression techniques: deep underground location for shielding
1.1. DARK MATTER 11
of cosmic radiation, shielding with lead and copper against photons and charge particles,shielding with hydrogen rich materials against neutrons, veto detectors to reject eventsdue to background particles that cannot be easily shielded, of course use of radiopurematerials. In most advanced arrangements also special detection schemes are used wherethe detector itself is able to discriminate between nuclear recoil due to WIMPs, but alsoneutrons, and electron recoils due to background.
Despite all these efforts it is impossible to definitely suppress background. Further-more the spectrum of recoil energy due to WIMPs scattering rises towards low energymaking it very reminiscent of the shape of typical background spectra. Without a spe-cial signature to identify it as being caused by WIMPs it can be easily misidentified asbackground and vice versa. Fortunately some prediction of the theoretical model can beused to gain some more confidence on the acquired data.
Pulse height and time distribution of the recorded signals can be used for furtherbackground suppression. Recorded events that do not match the characteristic predictedby kinematics for the WIMPs scattering can be rejected. Also the dependence of theWIMPs recoil spectrum on the detector material can be used as a signature. A change inthe mass of the target nucleus affects the scattering kinematics and therefore the shapeof the recoil spectrum.
Furthermore if the accepted events are not caused by an unknown source of back-ground a small seasonal variation is expected to occur in the recoil spectrum. Differentfrom the galaxy, the WIMP halo that surround our galaxy is not expected to be inrotational motion. Therefore the motion of the solar system around the galactic centreintroduces a preferential direction of the earth relative to the WIMP halo. Nowadays therelative velocity of earth referred to the halo is supposed to be about 220 km/s. Season-ally this speed varies by about 7% due to the earth motion around the sun. Accordinglythe WIMPs recoil spectrum should be seasonally modulated. During the summer in thenorthern hemisphere the speed is somewhat higher causing on average slightly higherrecoil energy.
As reported above constraints from cosmology as well as predictions of the super-symmetric theory exclude cross sections for WIMPs elastic scattering on atomic nucleilarger than one attobarn but still leave a possible range of several orders of magnitude.If background cannot be further reduced and no unambiguously positive WIMP signalis detected only upper limits on the WIMPs scattering cross section can be derived. Theexperiment can then only exclude cross sections which would cause more events than theobserved background.
1.1.4 Achieved Results
Several WIMP searches based on conventional charge or scintillator detectors have beenstarted already. As the background events rate was successfully reduced a sufficientsensitivity was reached to investigate the upper end of the parameters predicted by thesupersymmetric model.
A upper limit of few attobarn for the scattering cross section was pointed out butup to now positive evidence in dark matter searches were achieved only by DAMAcollaboration. The DAMA experiment consists of thirteen sodium iodide crystals with
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a total absorber mass of about 100 kg operated as a scintillation detector. Each crystalis viewed by two photomultiplier tubes that detect scintillation light being producedwhen radiation is produced in one of the crystals. After seven years of data taking theexposure of the DAMA experiment amount to about 100 000 kg · day necessary to gaina sufficiently high statistic to cope with the low sensitivity of the detector.
The scattering cross section derived by this experiment is just the upper end of pos-sible value predicted by the supersymmetric models. In confirmation that the observedsignal is due to dark matter scattering also a seasonal modulation in the recorded eventrate had been observed with phase and frequency in agreement with the expected mod-ulation explained in section 1.1.3. A lot of possible side effects that can produce suchmodulation have been analysed leading to the conclusion that only WIMPs remain asan acceptable explanation. Anyway independent confirmation by another experiment isnecessary.
The goal of new dark matter searches is then to improve the present sensitivity bytwo or if possible three orders of magnitude such that DAMA results can be confirmedor rejected unambiguously. An increased sensitivity requires an improved signal to back-ground ratio and then background has to be further reduced. Ambitious projects tryto gain this improvement with event better materials but this is everything but easy.Another approach used by other projects is to recognise remaining background events assuch.
In this perspective cryogenic detectors are a very promising technology. Since theenergy of nuclear recoils is primarily converted into phonons and only partially intoelectronic excitations, calorimeters are well suited for direct dark matter detection. Ad-ditionally with cryogenic detectors is possible to distinguish between nuclear recoils andelectrons recoils making possible to reject background events from α and β particles.This is done using semiconducting or scintillating material as absorbers such that thedetector can be run simultaneously in calorimetric and charge sensitive or scintillationmode.
The CMDS experiment exploits this possibility reading both the charge and thetemperature signal from germanium and silicon crystals. This experiment was the firstusing this kind of technology to challenge the results of the DAMA experiment even withbetter sensitivity. Very recently published results from the CDMS collaboration, thatmake it presently the most sensitive experiment in the search for dark matter, revealsthat still no WIMP signal has been detected. It must be observed that the improvedsensitivity was achieved with an extremely low detector exposure compared to the oneof the DAMA experiment. That confirms cryogenic detectors as a well suited tool fordark matter research.
This discrepancy between the results of the two experiments increases the interest inimproving experiments performance to clarify the DAMA evidence and furthermore toinvestigate the properties of such particles and identify their role in particle physics.
1.2. CRESST EXPERIMENT 13
1.2 CRESST Experiment
The goal of the Cryogenic Rare Event Search using Superconducting Thermometersproject is the direct detection of elementary particle dark matter and the elucidationof its nature using cryogenic detectors. As stated in section 1.1.2 a plausible candidatefor the dark matter search is the Weakly Interacting Massive Particle or WIMP. Thoseparticles are expected to interact with ordinary matter by elastic scattering on nucleiand all direct detection schemes have focused on this possibility.
Conventional methods for direct detection rely on the ionisation or scintillation causedby the recoiling nucleus. This leads to certain limitations connected with the relativelyhigh energy involved in producing ionisation and with the sharply decreasing ionisationefficiency of slow nuclei. The principal physical effect of a WIMP nuclear recoil is thegeneration of phonons. Cryogenic calorimeters are then well suited for detection of thoseparticles and, while conventional methods are probably close to their limits, cryogenictechnology can still make great improvements. Furthermore when this technology iscombined with charge or light detection the resulting background suppression leads to apowerful technique to search for the rare nuclear recoils due to WIMPs scattering.
The CRESST experiment is reaching its second phase [1]. The first phase endedin year 2001 and the update to the second phase setup is ongoing. In the first stagesapphire crystals where used as absorbers and only a temperature rise measurement wasperformed. The achieved sensitivity was quite good and the obtained results [2] con-firmed the high reliability and good stability of the system. In the second stage CaWO4
scintillating crystals with a mass of 300 g each for a total mass of about 10 kg are used.Both temperature rise and scintillation light are measured introducing the possibilityto discriminate between nuclear recoils, from WIMPs and neutrons, and electron re-coils, from background α and β particles. Preliminary results [3] [4] obtained with thistechnology show a greatly improved sensitivity. It will be further increased with the in-stallation of neutron moderators and muon veto panels to cope with remaining neutronsbackground.
1.2.1 Experimental Setup
The central part of the CRESST low background facility at the LNGS is the cryostat.The design of this cryostat had to combine the requirements of low temperatures withthose of low background. Due to cryogenic requirements some non radiopure materials,for example stainless steel, cannot be completely avoided. Thus the design shown infigure 1.1 is chosen in which a well separated cold box houses the experimental volumeat some distance from the cryostat. The experimental volume can house up to 100 kg oftarget mass. The cold box is made of low background copper, with high purity lead usedfor the vacuum seals. It is surrounded by shielding consisting of 14 cm of low backgroundcopper and 20 cm of lead.
Special consideration was given to the space between the dilution refrigerator and thecold box. The separation was chosen large enough so that the upper part of the externalshielding, together with the internal shields, eliminates any direct line of sight from theoutside world into the cold box. The low temperature of the dilution refrigerator is
14 CHAPTER 1. MOTIVATION
Figure 1.1: Layout of dilution refrigerator and cold box
transferred to the cold box by a 1.5 m long cold finger protected by thermal radiationshields, all of low background copper. A 20 cm thick lead shield inside a copper canis placed between the mixing chamber and the cold finger, with the low temperaturetransmitted here by the copper can. This internal shield, combined with another onesurrounding the cold finger, serves to block any line of sight for radiation coming fromthe dilution refrigerator into the experimental volume. The entire shielding is enclosedin a gas tight box that is flushed with nitrogen and maintained at a small overpressure.A Faraday cage surrounds all the experiment.
1.2.2 Cryogenic Detector
A calorimeter detector uses an absorber of known thermal capacity C to determine theamount of absorbed energy by measuring the temperature rise in the absorber. Thetemperature rise ∆T depends on the absolute amount of transfered energy E but not on
1.2. CRESST EXPERIMENT 15
how this energy is transfered to the absorber:
∆T =E
C
The temperature rise then does not depends on whether the energy is deposited by anelectron or by nuclear recoil. The temperature rise is measured by a dedicated temper-ature sensor in good thermal contact with the target. After the detector has relaxedback to the temperature of the thermal bath via a defined thermal link the detectoris ready for the detection of the next particle. The duration of the relaxation processis determined by the thermal capacity of the detector and its thermal coupling to thetemperature bath.
Whereas at room temperature the temperature rise due to the deposition of few keVis too tiny to be measured, the situation is different in the milli Kelvin temperaturerange. At those temperatures the heat capacity is sufficiently small so that the deposi-tion of a few keV results is a measurable temperature rise also in macroscopic absorbers.To operate such detectors at very low temperature in low background conditions re-quires experimental effort but cryogenic calorimeters have important advantages overconventional detectors:
1. High energy resolution. The energy resolution of a detectors is ultimately limited bystatistical fluctuations in the number of excitations causing the signal. Cryogenicdetectors are based on excitations such phonons whose energy is in the meV range,tiny compared to the several eV of excitation required in scintillation or chargedetectors. Many more excitations are involved for each event and signal statisticand energy resolution can be considerably improved.
2. Low energy threshold. Reducing the statistical fluctuations in the number of ex-citations also reduces the energy threshold of the detector. In cryogenic detectoradditionally the detection principle does not rely on secondary effects such as chargeproduction.
3. Wide choice of target materials. Almost any material is suitable for calorimeterswhile only a small range of semiconductor or scintillating material can be used asabsorbers in ionisation detectors or scintillators.
The calorimeter developed by the CRESST collaboration consists of a dielectric crystal,in which the particle interaction takes place, and a small superconducting film evapo-rated onto the surface, serving as a thermometer. The device is operated within thesuperconducting to normal transition of the thermometer, where a small temperaturerise ∆T of the thermometer leads to a relatively large rise ∆R of its resistance. Thetemperature rise induced by a particle is usually much smaller than the width of thetransition, which leads to an approximately linear relation between ∆T and ∆R. Theresistance of the film is measured by passing a constant current through a circuit inwhich the strip is in parallel with a small resistor and the pickup coil of a SQUID. A risein the film resistance is then measured via the current rise through the input coil of theSQUID amplifier.
16 CHAPTER 1. MOTIVATION
To a good approximation the high frequency phonons created by an event do notthermalize in the crystal before being directly absorbed in the superconducting film. Thusthe energy resolution is only moderately dependent on the size of the crystal extendingthe application of this technique to absorbers up to masses of several hundreds grams.
Passive techniques of background reduction are of course imperative in WIMP darkmatter searches. Deep underground site, efficient shielding against radioactivity of sur-rounding rocks and use of radiopure materials inside the shielding are mandatory. How-ever, there is a remaining background dominated by β and γ emissions from radioactivecontaminants inside the detectors and its surrounding. These produce exclusively elec-tron recoils in the detector. In contrast WIMPs and neutrons lead to nuclear recoils.Therefore dramatic improvements in sensitivity can be achieved if the detector itself iscapable to distinguish electron recoils from nuclear recoils and reject them.
A particle interaction in a scintillating crystal produces mainly phonons and scin-tillation light. Low energy nuclear recoils create much less scintillation light than elec-tron recoils of the same energy. Thus a measurement of the energy branching betweenscintillation light and phonons has the potential to clearly distinguish between nuclearand electron recoils. The new cryogenic detector employed in the second phase of theCRESST experiment exploit this possibility using a scintillating crystal as absorber.
The standard technique to detect single photons is the employment of photomultipliertubes. However they contain too many radioactive impurities and are not well suited forvery low temperatures. The high sensitivity of the cryogenic detector allows the use ofa small separate detector of the same type as light detector.
Among different scintillating crystals, CaWO4 was selected for the absorber, becauseof its light yield at low temperatures and the absence of noticeable degradation of lightemission for events near the crystal surface. In addition the large atomic mass of tungstenmakes it a very favourable target for WIMPs scattering. The detector modules consistof a 300 g cylindrical crystal with 40 mm diameter. The second phase of the CRESSTwill employ 33 of such modules up to a total mass of about 10 kg.
At the operating temperature about 1% of the energy deposited in the CaWO4 crystalis detected as scintillation light then a detector with a great efficiency is required. Thelight detector consist of a 20× 20× 0.4 mm3 sapphire substrate. The scintillation lightescaping from the crystal must be concentrated onto the light detector by means ofa reflector. Phonon and photon detectors are then mounted together inside a highlyreflective housing made of specularly reflecting plastic foil and of diffusive teflon endcapsas shown in figure 1.2.
Both phonon and photon detectors are equipped with superconducting phase tran-sition thermometers. Those thermometers consist of a thin film of tungsten evaporatedon one side of the crystals. The opposite side of the light detector is covered by sputterdeposited Si to improve the light absorption. Additionally each detector is provided witha heater for the control of the temperature. The heater is constituted by a gold pad inthe middle of the thermometer and two aluminium pads on the sides connected with theheating circuit. The thermal coupling of the detectors with the heat bath is obtainedby additional gold pads connected to the mixing chamber of the dilution refrigeratorwhich is stabilised at 6 mK. All the electrical and thermal connections are realized with
1.2. CRESST EXPERIMENT 17
Figure 1.2: Prototype detector module. On the left side is the dismounted end cap of the detectormodule with the light detector inside consisting sapphire crystal with phonon sensor in the centre.The inside of the end cap is made of teflon and polymeric reflector foil. On the right side is thecylindrical CaWO4 crystal with its tungsten thermometer on the top. It is completely surrounded bypolymeric reflector foil.
superconducting wires of negligible resistance and thermal capacity. All the connectionsare bonded or screwed to avoid the radioactivity of soldered joins.
The detectors run at the temperature of about 9 mK which is kept constant to withinfew µK to obtain a highly stable response. The data acquisition system, in addition topulses from particle interactions, also records the response to periodic test pulses whichare applied to each thermometer in order to monitor the behaviour of the detector andthe stability of the system.
Figure 1.3 shows a scatter plot of the pulse height in the phonon detector versus thepulse height in the light detector recorded with a test detector when it was irradiatedwith photons, electrons and neutrons [3]. The plot shows two well separated bands. Thelower band is caused by neutron induced nuclear recoils while the upper band is causedby electron recoils induced by γ and β. It is to be observed that electron and nuclearrecoils can be clearly distinguished down to a threshold of about 10 keV . This allows arejection of photon and electron events with an efficiency of better than 99.7% for nuclearrecoil energies above 15 keV .
Another methodology to discriminate between nuclear and electron recoils is thesimultaneous measurement of charge and phonons. However light and phonon measure-ment offers several advantages. In the measurement of charge and phonons, electricalcontacts always produce an unfortunate dead layer on the surface, which causes sur-face events, especially electrons from outside, to leak into the nuclear recoil band. Lightcollection does not suffer neither from problems such as space charge build up, field inho-mogeneities or phonons produced by drifting the charges. Many of the effects known fromcharge-phonon measurement to cause leakage of electron recoils into the nuclear recoilband are absent. As a result the background suppression efficiency of the light-phonon
18 CHAPTER 1. MOTIVATION
Figure 1.3: Pulse height in the light detector versus pulse height in the phonon detector measuredwhile the detector was irradiated with photons, electrons and neutrons. The lower band is caused byneutron induced nuclear recoils, the upper band by electron recoils from photons and electrons [1].
detection is excellent and it works equally well for photons and electron backgrounds,thus avoiding particle dependent systematic uncertainties in discrimination.
As stated before in this section, the CREEST detector technology offers the oppor-tunity to employ different absorbers with different target nuclei. This gives a powerfulhandle for understanding and then reducing the background. More important is the pos-sibility to further investigate the properties of dark matter elementary particles changingthe characteristics of the scattering nucleus and checking the variations in the detectedsignal against the predictions of the theoretical models. The use of different materialspermits also the use of the system for different rare events searches. With the cur-rent setup in the CRESST experiment was already observed the natural alpha decay oftungsten [4].
1.2.3 Background
As described is section 1.2.1 the dark matter detector is protected against backgroundradiation with the location at the low background facility of LNGS, that provides ashielding of about 3500 m of water equivalent, against cosmic radiation and with 20 cmof lead and 14 cm of low background copper, against photons and electrons backgroundfrom surrounding materials. Additionally the discrimination of electron recoils fromnuclear recoils, by the measurement of both scintillation light and phonons, permits thenearby complete rejection of events induced by α and β active contaminants inside theshielding. However neutrons induced nuclear recoils cannot be discriminated from WIMPinduced nuclear recoils, as already pointed out in section 1.2.2.
Neutrons remain as main background source and will limit the sensitivity of theexperiment. Previous publications have studied such background [5] [6]. The neutrons
1.3. MUON VETO DETECTOR 19
flux present in the CRESST experimental area at LNGS comes from four different origins:
1. Low energy neutrons induced by fission and (α, n) reactions due to uranium andthorium activities in the surrounding rock and concrete.
2. Low energy neutrons induced by fission in the shielding material and in the otherexperimental setup parts.
3. High energy neutrons induced by muons in the rock. These neutrons could inducespallation reactions in the experimental shield and produce additional neutrons.
4. High energy neutrons induced by muons in the shielding material.
Low energy neutrons from the surrounding rock and concrete gives the bulk of the totalflux in the laboratory. To suppress those sources of neutrons the installation of a neutronmoderator is used. Neutron moderators consists of hydrogen rich materials such as wateror plastic. As neutrons are about as heavy as hydrogen nuclei, neutrons scattering onhydrogen nuclei efficiently lose their kinetic energy and thus their ability to producenuclear recoils in the dark matter detector. In the CRESST experiment the installationof a 50 cm polyethylene neutrons moderator is planned as shown in figure 1.4. This willreduce the background count rate in the detector by more than three orders of magnitude.
Low energy neutrons induced by fission in the shielding cannot be avoided if not withthe use of low background materials. With the usual radioactive contaminations in thematerial employed in this kind of experiments this source of background is negligible.High energy neutrons induced by muons in the rock cannot be vetoed easily and cannot besuppressed with an affordable amount of shielding material. They remain as unavoidablebackground source.
High energy neutrons induced by muons in the shielding also cannot be suppressed.However neutron induced nuclear recoils in the detector can be identified as such detect-ing entering muons which produce such neutrons. Muons are charged particles and assuch can be easily detected by scintillators. In the CRESST experiment the installationof muon veto scintillator detectors is planned inside the polyethylene shield in the con-figuration shown in figure 1.4. The muon veto is expected to have an efficiency of morethan 90%. This leads to the rejection of background events reducing the backgroundcount rate in the detector by only a factor of three, unless high energy neutrons from therock can be overcome. The muon veto scintillator detectors are the object of this work.
To further reduce the neutrons background is possible to detect multiple scatteringin the detectors. Since WIMPs are expected not to scatter multiply those events can berejected as neutron induced.
1.3 Muon Veto Detector
As explained in section 1.2.2 the new detection concept applied to the second phaseof the CRESST experiment enables a clear discrimination between electron recoils andnuclear recoils. That leaves neutrons as the main background. Therefore, to achievethe desired sensitivity it is necessary to suppress the neutron background. As detailed
20 CHAPTER 1. MOTIVATION
polyetyleneneutron moderator
muon vetoplastic scintillatorsPPPPPPPPPPPPPPPPq
������1
�������)
PPPPPPPPPPPPPPPi
Figure 1.4: The cryostat surrounded by the shielding material with the additional muon veto scintillatorpanels and the polyethylene neutron moderator.
in section 1.2.3 the neutron flux in the experimental area comes from different origins.Some of those can be suppressed with a passive neutron moderator but it is impossibleto shield against neutrons that come from high energy neutrons induced by muons inthe shielding material. The only way to suppress that source of background is to use amuon veto system. The muon veto system consists of a batch of detectors able to detectincoming muons in the experimental area. Detecting the incoming muons permits toreject events registered by the cryogenic dark matter detectors right after a muon entersin the experimental area.
A large area must be covered by these detectors and strict particle identification isnot necessary. Scintillator detectors where chosen because they can be scaled to largedimensions at low cost and small reduction in energy resolution.
1.3.1 Detector Characteristics
The scintillator panels employed in the building of the muon veto are drawn in figure 1.5.The detectors are made of Bicron BC-408 plastic scintillator. This material is charac-terised by a wavelength of maximal emission at 425 nm and matches the request of a lowattenuation factor having an attenuation length of 210 cm. The scintillator is wrapped
1.3. MUON VETO DETECTOR 21
� � � �� � � �
� � �� � �
16x SIDE PANELS
�����
�����
� �� �� �� �� �
�����
2x BOTTOM PANELS2x TOP PANELS
1m
Figure 1.5: The plastic scintillators employed in the building of the muon veto. The dashed gridrepresents the points where the calibration measurements are performed.
in aluminium foil for achieving optimal light reflection and in black opaque plastic foilfor shielding against external light. The aluminium foil is applied to the scintillatorin a way that some air stays in the between. This minimises the reflection index ofthe medium that stays is contact with the scintillator obtaining a maximisation of theinternal reflection due to the total reflection.
This scintillator is coupled to an Electron Tubes 9900B photomultiplier powered byan Electron Tubes C637A voltage divider. This is an end-window photomultiplier withsidewall sensitivity for wide-angle light detection and is well suited for this applicationdue to the geometry of the system. This photomultiplier is equipped with an enhancedgreen sensitive bialkali photocathode that achieve the requirement of optimal couplingto the scintillator with its maximum quantum efficiency of 26% at about 400 nm. Theamplification is obtained with 11 dynodes of SbSc with box and grid design. This designimplies that the photomultiplier is characterised by a slow time response but this is nota problem for its application in this experimental setup.
It must be pointed out that the environment where those detectors will operate isalmost free from electromagnetic noise so no electromagnetic shielding is placed aroundthe photomultiplier. The laboratory where the calibration is performed, on the otherhand is rich of electromagnetic noise. Some care must be taken on that side.
1.3.2 Detector Calibration
The purpose of the scintillator detectors is the detection of the incoming muons in orderto measure background free signals in the cryogenic detector. Signals from the cryogenicdetector are rejected if they come right after a muon is detected by the muon vetoscintillators. Muons are identified only on the base of the energy loss in the scintillator.To avoid the triggering of the rejection on signals from harmless γ-rays it is necessary toset a sensible low energy threshold on the scintillator detector.
In large scintillator detectors there is a non-uniformity in energy response over theirvolume because of the differences in the path photons have to travel between the pointof impact and the photocathode. For a good calibration is then necessary to measurethe energy response of the scintillator panels over their complete volume.
Muons are background radiation that is very difficult to suppress. A localised muonflux can be obtained by taking coincidence between two small pads detectors which thendefine the muons path through the scintillators. However this procedure reduces the rate
22 CHAPTER 1. MOTIVATION
dramatically. The response of the scintillator to muon and electron interactions is almostthe same and electrons are easily produced. For that reason electrons can be used forthe calibration. The easiest way to obtain electrons that can serve as energy reference isto look at the Compton edge obtained from Compton scattering of γ-rays produced bynatural radiative sources.
Light Loss
The loss of light inside a scintillator can occur in two basic ways: escaping through theboundaries and absorption by the scintillator material. Absorption is usually negligiblebut takes a prominent role in large detectors, where the dimensions are such that the pathtravelled by photons is comparable to the attenuation length of the scintillator material.The attenuation length of the used material is 210 cm but the scintillator panels size upto about 160×80 cm2 and that implies an attenuation factor definitely not unimportant.
Escape of light from the detector boundaries happens because the surfaces of thescintillator are not completely reflective. Light emitted at any point of the scintillatortravels in all directions and only a fraction reaches the photomultiplier directly. Theremainder travels toward the scintillator boundaries where only if the angle of incidenceis greater than the total reflection angle all the light is turned back into the scintillator.Otherwise partial reflection occurs and the transmitted light fraction escapes from thescintillator. The major fraction of this light is then reflected back by the aluminium foil.
For the present geometry the light fraction that is emitted in the direction of themain surfaces of the scintillator panel needs many reflection and thus a long path toreach the photocathode. That causes a great attenuation. The important fraction ofthe light that reaches the photocathode is then direct light or light that is reflected fortotal reflection on the sides of the panel as confirmed by the numerical simulation of thedetector.
Compton Scattering
Compton scattering is the scattering of photons on free electrons. In matter the electronsare bound, however if the energy of the photon is hight with respect to the binding energythe electrons can be considered quasi-free. Applying energy and momentum conservationit is possible for an incoming photon of energy hν to calculate the energy hν ′ of thescattered photon:
hν ′ =hν
1 + γ (1− cos α)
where α is the angle between the incoming and the outgoing photon and γ = hν/mec2.
The kinetic energy T of the recoil electrons is:
T = hν − hν ′ = hν
{γ (1− cos α)
1 + γ (1− cos α)
}The maximum recoil energy is limited by the kinematics and is obtained in case ofbackward scattering when α = π:
Tmax = hν
(2γ
1 + 2γ
)
1.3. MUON VETO DETECTOR 23
this energy is known as Compton edge. Because of the low atomic number of elementsthat build up a plastic scintillators there is a low probability for the photoelectric effect orpair production. Therefore light pulses arising from incident γ-rays are mainly producedby recoil electrons in Compton scattering.
Sources
Two sources are used. The less energetic source is a 60Co decaying to 60Ni via β−. Theuseful decays in 60Ni are the one from the 4+ state to the 2+ state emitting a gamma of1.173 MeV and the subsequent decay from the 2+ state to the ground state emitting agamma of 1.332 MeV . The energy resolution of our detector is too low to discriminatebetween those peaks so we assume to have a source of gamma rays with an energy thatis the average of the two. The described decays have not exactly the same probabilitybut the difference is less than 1% so we can simply make the mean of the two energiesobtaining an energy of 1.253 MeV . The Compton edge for photons of such energy iscalculated to 1.040 MeV .
The more energetic source is a 228Th source but the gamma emitter is an element inits decay chain. The useful gamma is emitted with an energy of 2.615 MeV during thetransition in 208Pb from the exited 3+ state to the ground state. The Compton edge forphotons of such energy is calculated to 2.382 MeV .
Data Acquisition
For electron energies above 100 keV the relation of light pulses height to electron energyis considered linear. Therefore it is necessary to measure the detector response only attwo defined energies and fit a line to the obtained points in the pulse height – electronsenergy graph. Thus two γ-rays sources are sufficient.
It is supposed that the variation of pulse height over the scintillator volume does notdepend on the energy of the incident particles. Therefore it is not necessary to map thepanel with both sources. The Thorium source was chosen because the related Comptonedge is higher and thus more distant from the noise peak and then the data analysis ismore comfortable. In order to determine the pulse height – particle energy relation isanyway necessary to acquire data using both sources. To make them easily reproducible,it is chosen to do those measurements at the middle-point of the surface of the panel.
For the mapping is supposed that a measure every ten centimetres is enough toestimate with sufficiently good precision the position dependency of the scintillatorsresponse. The measurements are then performed on the crossing points of an imaginarygrid of square cells with sides of ten centimetres. This grid is placed in a way that makesthe photomultiplier lie in the centre of one of such cells – see figure 1.5. That impliesthat about one hundred measurements are needed to map each detector.
It is expected that panels of the same shape have similar energy responses. Wherethen planned to map only one panel for each kind with a long acquisition time of twentyminutes for each point. The other panels will be measured with a acquisition time of onlytwo minutes for each point. This choice permits to finish the calibration in a reasonableamount of time while still guaranteeing a very good statistic for the calibration of each
24 CHAPTER 1. MOTIVATION
Figure 1.6: An example of acquired spectra. The continuous line is a 20 minutes measure with theThorium source. The dashed line is normalised background obtained with the sum of two 40 minutesmeasurements.
kind of panels and a reasonable one for the others.As stated before two known sources are used for the calibration. It is however clear
that it is impossible to measure only the radiation coming from a known source. En-vironmental background is always present and its suppression is not always affordable.For that reason it is necessary to measure the background and then subtract it from thereal measurements. For each panel three background spectra are measured: one beforeand one after the mapping and one after the measurement with the cobalt source. Thethree histograms are then added to get a sort of average of the measurements. Makingbackground measurements at different times during the calibration permits to analysethe time-stability of the system.
The scintillator panels are labelled by the manufacturer with a product identifierranging from 108 to 127. To identify the scintillators during the calibration and in thefinal setup it is chosen to use a two digits code obtained subtracting 107 from all theidentifiers. The data acquisition software uses that code to name the files produced foreach point of the mapping. The data reported are about the panel identified with thecode 08, one of the scintillators measured with a long acquisition time. This scintillator isone of the detectors used for the bottom part of the muon veto. Its size if 1430×720 mmand the photomultiplier is placed on the middle of the longer side.
Data Analysis Principle
For electron energies above 100 keV the relation of light pulses height to electrons energyis considered linear. Therefore the detector response is measured only at two definedenergy using two γ-rays sources that define two Compton edge energies.
In the bare acquired spectra the Compton edge is not clearly visible as can be seen infigure 1.6. The first peak is due to environmental background and electromagnetic noise.
1.3. MUON VETO DETECTOR 25
Figure 1.7: The previous spectra after background subtraction. The fitted slope is also shown withthe supposed energy position of the Compton edge. The strange shape of the low energy part of thespectra is due to electromagnetic noise.
The second, enlarged in the detailed view, is due to cosmic radiation. The differenceinduced by the source between the measure and the background is slightly visible inthe high energy part of the first peak. To locate the energy position of the Comptonedge the measurement spectra and the background are normalised and the backgroundis subtracted. An exponential slope is then fitted to the Compton edge peak generatedby the radiative source. What is shown in figure 1.7 is obtained. The strange shape ofthe low energy part of the spectra is due to electromagnetic noise. It is assumed that theCompton edge is located at the position corresponding to the half height of the slope.The energy calibration is then realized fitting a line to the obtained points in the pulseheight – Compton edge energy graph.
As pointed out in section 1.3.2 it is supposed that the energy attenuation is energyindependent. Thus the mapping of the panel is done with only a source. The normalisa-tion and the background subtraction are done also for those spectra and the exponentialslope is fitted in order to locate the Compton edge. The variation of the energy positionof the Compton edge for different positions of the source over the scintillator surface isidentified with the light attenuation in the detector volume. The different response ofthe scintillator to particles that impact on different points of his surface is expressed asratio of the Compton edge energy position between the considered point and the averageresponse of detector. The overall response is obtained calculating the average of theposition of the Compton edge in all the points considered during the mapping.
26 CHAPTER 1. MOTIVATION
Chapter 2
Hardware
This chapter describes the hardware system used during this work. A positioning systemfor placing the radioative source is designed and an electronic system for the readout of theoutput of the scintillator panels is build from standard modular NIM and VME devices.
2.1 Positioning System
2.1.1 Requirements
The main goal of this work is the automation of the mapping of the scintillator panels.For that purpose a movement system able to move the radioative source over the panelsis needed. Not only the source but also the shielding necessary to confine the particlesemitted by the source in the smallest area possible over the panel must be handled bythe positioning system. With the source used in the mapping – see section 1.3.2 – asufficient shielding is obtained with 5.0 cm of lead. A top and a side view of the sourceand of the shielding is sketched in figure 2.1.
Considering the dimensions of the source, the dimensions of the shielding and thesize of the biggest scintillator panel, the system should be able to handle a weight ofabout 30 kg over a surface of about 160× 80 cm2. To ensure that all the particles thatreach the scintillator have the same energy the distance between the source and the panelmust be constant during the measurement. To minimize the spread of the particles onthe scintillator surface the distance between the shielding and the panel must be as shortas possible.
2.1.2 Realization
Parts of an old system were available and it was chosen to reuse them. The availablecomponents include stepper motors with the required control system and worm gearunits. The old device is a target system with two axes of about 40 cm and about 80 cmin length. Those axes are too short for cover the complete surface of one scintillatingpanel. The longer axis is used to scan the complete length of the shortest side of thepanel. The shorter axis is used to scan a part of the longest side. The idea is to use
Figure 2.1: Sketch of horizontal and vertical sections of source and required shielding.
the units to scan the biggest possible surface of the panel and then require user actionto change the position of the system over the panel. The panel is no more completelyautomatic but becomes a semi-automatic procedure.
Considering the size of the panels and the length of the shorter worm gear unit theintervention of the user to relocate the system is required three times for each mapping.With the data acquisition times chosen in section 1.3.2 the intervention of the user isrequired once in a hour for the low statistic measurements and once every ten hours forthe high statistic ones. Those times make the system quite comfortable to operate.
In the old system the movement is obtained using two worm gears operated by twostepper motors. The combination of the shaft with the nut and the number of steps forrevolution of the stepper motor produces a linear movement of 0.0125 mm for step. Theobtained positioning is very accurate but those actuator are not suited to handle therequested weight. The solution is to place the moving parts on wagons that move onrails that carry the full weight of the system. A sketch of the realized system is reportedin figure 2.2.
In the figure 2.2 wagon A is responsible of handling the source and is driven by thelonger actuator over rails connected also to wagon B. Wagon B moves in the orthogonaldirection of wagon A and is driven by the shorter actuator placed on wagon C. The shorteractuator is placed on a wagon allowing the relocation of the movement system makingpossible the complete mapping of a panel with the procedure described in section 2.1.3.
The stepper motors are driven by a control unit that generates the required signalsthrough a serial RS232 interface. The interface accepts smart movement commands andstores the status of the system making possible movement commands relative to thecurrent position. The actuators are provided with an hardware reference that definesthe zero position. There is no feedback of the real position the actuator but the currentplacement is obtained counting the motor steps starting from the zero reference. Thiscould be a problem because stepper motors under heavy stress can lose the step and thatcan not be noticed. To prevent a loss, the movement speed must not exceed a well tested
Figure 2.2: Sketch of a top view of the positioning system.
value that guarantees correct operation of the device.In the figure 2.2 is shown the cartesian coordinate system chosen to identify the points
in the device and which all measurements refers to. The mounting of the panels in theright position and alignment with the system is achieved with an hardware reference.For the shape of his enclosure and for positioning the motor’s power cable in a suitableposition the longer actuator must be placed with the hardware zero position in theopposite side of the one showed for the source of the coordinates. That is adjustedreverting the y coordinate in the controlling software – see section 3.3.4.
All the structure of the movement system is realized with modular profiles. Thatmakes the building procedure simpler and the parts reusable as exercised with the onesof the previous experimental setup.
2.1.3 Coordinate Translation
As described in previous section, for limitations in the available hardware, it is necessaryto move the system to realize a complete scan of one panel. This in principle requires todefine more references position on the x axis and to align the system to those positionsmultiple times in a single mapping. This obviously introduces more possibilities of errorsand complexity in the system use.
The problem is solved in the following way. Both wagons B and C are equipped witha locking device that fix them to the rest of the system. Wagon C is locked and theactuator that resides on it can move wagon B until its length limit. Wagon B is then
30 CHAPTER 2. HARDWARE
locked and wagon C unlocked. The actuator is now driven to his zero position movingwagon C forward from the locking position of wagon B. The procedure ends with theunlock of the wagon B and the lock of wagon C. This produces a shift of the coordinatesystem of the full length of the actuator.
2.1.4 Checks
The system is tested for accuracy and reproducibility of the placement connecting amarker to the source handler and tracing lines on a sheet placed over a scintillator panel.The drawings are then checked against the commands supplied to the device through thecontrol software. With those tests the estimated uncertainty and reproducibility error isless than 1 mm without coordinate translation and less that 2 mm over the full size ofthe system.
The weight of the scintillator panel is to big to permit a very careful positioning.The error in the placement over the full size of the panel is not due to the coordinatetranslation procedure but mainly to the misalignment of the scintillator panel with thesystem. It must be also argued that the tests are done while the system is not loadedby the weight of the shielding lead. With the shielding in place a worsening of theperformance is not expected but could be that the system presents oscillations duringthe movement for the increased mass of the moving parts.
2.1.5 Problems
While the system proves to be mechanically stable an underestimation of the mechanicalstress of those parts during the design of the rails where the wagons are placed causedthe choice of too thin metal profiles. That produces a bending of the rails under theweight of the source and other moving parts of system.
This bending is not dangerous for the stability of the system but causes the distancefrom the source to the scintillator to vary from the sides to the inner part of the panelchanging the conditions in which the measurements is done in each point. The variation ofthe distance produced by the bending is in the worse case quantifiable to 5 mm. For thiswork that difference is not important so no action to correct this problem is performed.A simple solution to avoid the bending could be coupling the existing profiles with otherones of the same type, increasing the strength of the system.
2.1.6 Possible Improvements
Some improvement to the system other than the solution to the problem analyzed inthe previous sections are possible. As stated in section 2.1.2 the mapping procedure issemi-automatic. One big improvement to the system is making an entire mapping beable to run without user action. An expensive solution could be to buy a new worm gearunit long enough to scan the longest side of the panel. A less expensive solution is tomake the locking system electronically controlled. Achieving that for example is possibleby replacing the current mechanical mechanism with an electromagnetic one.
Other things that are not yet automated are the measurements for the energy cali-bration and background measurements. For those the radiative source must be replaced
2.2. READOUT ELECTRONIC 31
with another source in the first case and away from the scintillator in the second one. Forthe background measurement probably it is enough to simply move the source outsidethe surface of the scintillator. For making measurements with two different sources thesolution is to completely shield the source and build an electro-mechanic device thatopens and closes a gap in the shielding. That of course increases a lot the complexityof the device and the advantages for the measure of so few detectors are not so big tojustify the work and the expense.
2.2 Readout Electronic
2.2.1 Requirements
The electronic should provide the high voltage to the photomultiplier inserted into thescintillator panels, discriminate signals related to physical events against the noise in thephotomultiplier output and convert those signals in a digital value.
The electronic readout system used during the calibration is not the one that willbe used during the operation of the panels in their final location. That means that theused system must be able to elaborate only the signal of a single panel and there are notstrict requirements about the timing.
2.2.2 Realization
A logical diagram of the electronic system is shown in figure 2.3. The system could besplit into three logical parts: the charge to digital converter, the discriminator circuitand the hight voltage supply. To avoid noise and spikes on the power lines the electronicequipment is physically divided in three different crates as evidenced in figure 2.3. Adescription of each component follows.
Charge to Digital Converter
The main part of the electronic system is the charge to digital converter – QDC. Thisdevice translates the output of the photomultiplier into a digital value that is thenacquired by the data acquisition software – see section 3.3.1.
The QDC takes two inputs: the analog signal and a gate signal that defines the timefor the integration of the voltage amplitude of the input signal to obtain the chargeto digital conversion. In that application the analog signal is directly taken from thephotomultiplier and the gate is generated by the discriminator circuit.
The QDC used is a module that houses 32 channels and that offers an event buffermemory that can store the result of up to 32 successive conversions. In this work onlyone channel is used and only 20 values will be stored in the buffer. The module uses theVME Bus as a communication channel with the data acquisition system. This is one ofthe de-facto standards in particle physics experiments.
32 CHAPTER 2. HARDWARE
Figure 2.3: Logical diagram of the electronic system.
Discriminator Circuit
The output signal of the photomultiplier is branched into two equal signals by a linearfan-in fan-out. One of the obtained signals is routed through a delay of 150 ns to theQDC and the other to the discriminator circuit. The delay is introduced to compensatethe time that the discriminator circuit takes to generate the gate signal.
The discriminator circuit is built up by an adjustable voltage threshold that discrim-inates between signals produced by light emitted by recoils in the scintillator and noise.The output signal of the threshold is used as one of the inputs for a coincidence unit.The output signal of the coincidence is used to drive a 700 ns gate generator – gate A –that generates the gate signal that defines the time in which the signal is integrated toachieve the charge to digital conversion.
In the QDC module event buffer 20 events are stored so the output of the gate isalso routed to a rate divider which reduces the rate of the input signal by 20. The signalgiven by the rate divider is then used to drive a latch that is connected to an IO modulethat, like the QDC, uses the VME Bus to signal to the acquisition system to read theconverted values. When the input line of the IO module becomes high the acquisitionsystem should read the event buffer of the QDC, empty the buffer and, using the outputline of the IO module, reset the latch.
The latch output is used also as input for the coincidence. That ensures that notrigger can take place during the reading of the event buffer of the QDC module. That isnecessary because during that operation the module is unable to perform any conversion.The reading of the events is done in groups of twenty to reduce the dead time of thesystem. The output signal of the coincidence is also routed through a 20 µs gate generator– gate B – and then negated and used as input for the coincidence itself. This is the
2.2. READOUT ELECTRONIC 33
time the QDC requires to do the conversion. That ensures that no more events can takeplace during one conversion.
High Voltage Supply
The high voltage supply provides the voltage needed by the photomultiplier to work.Because the gain curve of the new photomultiplier is not known and should be determi-nated during this work is necessary that the high voltage is adjustable between 700 Vand 1000 V . The gain of the photomultiplier is strongly dependant on the high voltage.The provided voltage should be then very stable in time to guarantee stability to thesystem.
The current required by the photomultiplier is simply the one that goes through theresistors network that builds the voltage divider because the current of the output signalis negligible in common operative condition. If the current absorbed is greater than thisvalue the photomultiplier is not working properly and could be damaged by a too highcurrent. For that reason it is good to use a power supply that drops the output if thecurrent limit is exceeded.
2.2.3 Parameters
The electronic system has at least four parameters that influence its behaviour. Twoparameters are never changed during measurements done in this works. Those are thegates times used for triggering the charge to digital conversion – gate A – and for inhibitthe incoming signals during the conversion – gate B. While the second one is not criticalto the operation of the system the first one modifies strongly the acquired data.
The length of the gate B must be greater than the time that the QDC takes to doits work, otherwise the system simply stops to work. An overestimation of it causes onlyan increase of the dead time of the system that is not critical for this work as long as theprobability of a second event occurring during a dead time period is small. The detectorremains sensitive to other events during dead time so if successive events pile up on thefirst resulting in a distortion of the signal and loss of information on both events.
The length of the gate A must be exactly the length of the longest possible signalfrom the photomultiplier. If it is shorter the full signal produced by the photomultiplieris not taken in account and thus wrong data are acquired. If it is longer also some noise isintegrated by the QDC producing values greater than the real ones introducing randomvariations in the pulse height.
Two parameters that will be changed during this work are the high voltage valueand the threshold level. The high voltage value influences the photomultiplier gain.The relation between the gain and the voltage was measured by another group. Itshould be considered in first approximation linear in a small interval around the nominalvalue supplied by the manufacturer of the photomultiplier. The threshold level definewhat is the minimum signal that an event should have to be considered by the system.This influences the noise rejection of the system. The effects of the changing of thoseparameters are analyzed in chapter 4.
34 CHAPTER 2. HARDWARE
Chapter 3
Software
This chapter describes the requirements of the software package, the framework used for hisimplementation and the technical solutions adopted during the development for achievingthe requirements.
3.1 Requirements
The main goal of this work is the automation of the mapping of the scintillator panels.Therefore the minimum requirement is a system able to acquire data and to put theradiative source in the right position without human supervision. In particular thesoftware system should be able to: calculate the grid of points where the radiative sourcemust be positioned for the measurements given the size of the panel and the position ofthe photomultiplier; position the radiative source on each point; for each point perform adata acquisition run for a fixed amount of time. The software should communicate withthe hardware devices through the VME Bus with the data acquisition electronic systemand through the RS232 serial port with the positioning device.
Given that some software for controlling the same hardware devices used in this workwas already developed using the Midas framework, an obvious choice is to continue touse this system. That gives us a complete data acquisition framework with networkingcapabilities that will be described later. For the proven stability of the system and forthe availability of good, well tested and, last but not least, free developing tools theGNU/Linux platform is chose for this work.
3.2 Midas
The Maximum Integrated Data Acquisition System – MIDAS – is a general purposesystem for event based data acquisition in small and medium scale physics experiments.It is developed at the Paul Scherrer Institute – Switzerland – and at TRIUMF – Canada– since 1993. While the system is already in use in several laboratories, the developmentcontinues with addition of new features and tools.
Midas is based on a modular design with networking capability and on a centraldatabase system. It is implemented through a library and several applications that
35
36 CHAPTER 3. SOFTWARE
can run on many different operating systems and hardware architectures. The modularscheme that allows scalability and flexibility.
The following description should familiarize the reader with the basic services andsystem components to make the description of the implementation of the software writtenfor this work more understandable. It does not cover all the features of the system. Formore exhaustive and detailed information refer to the Midas documentation [7].
3.2.1 Basic Services
The Midas library implements some basic services that are the basis framework for allthe components that constitute the full software data acquisition system. Those basicservices are the buffer manager, the online database and the message system.
Buffer Manager
The buffer manager consists of a set of library functions for event collection and distri-bution. A buffer is a shared memory region which can be accessed by several processes,called clients. Processes sending events to a buffer are called producers, processes readingevents are called consumers. Consumers can specify which type of events they want toreceive from a buffer. For this purpose each event contains a Midas header with an eventID and other pertinent information. Buffers can be accessed locally or remotely via theMidas server.
Message System
Any client can produce status and error messages that are forwarded to any other clientswho maybe susceptible to receive these messages and react accordingly as well as to acentral log file system. The message system is based on the buffer manager scheme. Adedicated buffer is used to receive and distribute messages.
Online Database
In a distributed data acquisition environment configuration data is usually stored inseveral files on different computers. Midas uses a different approach. All relevant datafor a particular experiment are stored in a central database called online database orODB. This database contains run parameters, logging channel information, conditionparameters for frontends and analyzers as well as status and performance data.
In the ODB data is stored as pairs of key and value hierarchically structured in atree. Any ODB client can register a hot link between a local variable and a element ofthe ODB. Whenever a client changes a value in this subtree the variable automaticallyreceives an update of the changed data. Additionally, a client can register a callbackfunction which will be executed as soon as the hot link’s update has been received.
Midas Server
For remote access to a Midas experiment a remote procedure call – RPC – server isavailable. Is possible to run every component of the data acquisition software system
3.2. MIDAS 37
on different nodes of a network and then make them inter-operate through the Midasserver. In that way an experiment could be controlled and the data can be analyzed andarchived on different hosts.
3.2.2 System Components
A complete data acquisition system is formed by several components that can run ondifferent nodes of the Midas network communicating through the Midas server. Thebasic software components needed to run an useful data acquisition system are: one ormore frontends, an analyzer, a data logger and a run control program.
Frontend
The frontend program refers to a task running on a particular computer which hasaccess to hardware equipment for collecting physical data. Several frontend can beattached simultaneously to a given experiment. Each frontend can be composed ofmultiple equipments. An equipment is a single or a collection of tasks meant to collectand regroup logically or physically data under a single and uniquely identified event.
The frontend program registers its equipment list to the Midas system, provides themean of collecting the data from the hardware source defined in each equipment, gathersthese data in a known format for each equipment, sends these data to the buffer manager,collects periodically statistical informations of the acquisition task and sends it to theonline database.
The data collection in the frontend framework can be triggered by several mechanismsthat can be grouped into: periodic events, scheduled event based on a fixed time interval,polled events, hardware trigger information read continuously which in turns if the signalis asserted it will trigger the equipment readout, interrupt events, generated by particularhardware device supporting interrupt mode.
Analyzer
An analyzer program refer to a task running on a particular computer connected to theMidas network receiving events from the buffer manager. The purpose of that programis to do some basic calculations on the data contained in each event and store the resultin the event itself. The received and the calculated values can then be booked in a formatsuitable for easy successive analysis such HBOOK or ROOT from CERN.
The analyzer then takes care of receiving events, initializes the booking system andautomatically books all events. The analyzer is structured into stages, where each stageanalyzes a subset of the event data. Low level stages can perform calibration and scalingof read values, high level stages can calculate physics results. The same analyzer exe-cutable can be used to run online, receiving events from the buffer manager, and off line,reading events from a file.
38 CHAPTER 3. SOFTWARE
Data Logger
The data logger is a client, usually running on the backend computer, receiving eventsfrom the buffer manager and saving them onto disk, or other storage, for successiveanalysis. The data logger provided by default by the framework supports several parallelslogging channels with individual event selection criteria and different file formats forrecording events. This program fulfil all the requirements and then will be used in oursetup.
Run Control Program
As mentioned earlier, the online database contains all the pertinent information regardingan experiment. For that reason a run control program requires only to access thatdatabase.
Three run states defines the state of Midas: stopped, paused, running. In orderto change from one state to another four basic transitions are provided: start, pause,resume, stop. During these transition any Midas client register to receive notificationof such message will be able to perform its task within the overall run control of theexperiment.
3.3 Dedicated Software
Following the Midas recommendations the software is split into several different programseach one dedicated to a specific task. The following programs build up the completesystem: frontend is responsible for data collection, analyzer has the only purpose ofbooking data into a format suitable for successive analysis, timer is responsible for thetiming of the measure, arms has the role of moving the positioning system and controlis the main control program and the user interface to the system. A diagram of thesystem is reported in figure 3.1. A detailed description of the implementation details foreach component follows.
3.3.1 Frontend
The frontend program registers only an equipment named Scintillator that generatesone event for every successful read from the scintillator panel. This program talks to theelectronics through the VME Bus using the library of functions provided by the driverof the VME adapter. This library provides a platform-independent interface so this codecould be ported easily on all the platform for which a driver for the VME adapter exists.
In the initialization function the IO and the QDC modules are configured for theongoing measurement according to the description in the chapter 2 and then the state ofthe system is reseted. The equipment uses the polling operation model and in the event-polling function the state of the input line is checked. If the state of the line is high thenthe function return a true value, false otherwise. On a true value returned by the pollingfunction the event read function is called. That function reads the events buffer of theQDC module. According to the hardware configuration that buffer contains 20 readouts
3.3. DEDICATED SOFTWARE 39
Figure 3.1: Block diagram of the data acquisition system.
of the scintillator output. The first of them is immediately returned as Midas event to thesystem and the remaining are kept in a local buffer. Until there are unreported readoutsin the local buffer the pooling function returns true without reading the state of the inputline and the event read function iterates over buffer entries generating a Midas event foreach of the readouts. When the local buffer no more contains unreported readouts theQDC event buffer is cleared and a high signal on the output line is given to make theelectronic record another group of 20 readouts.
3.3.2 Analyzer
Given that no calibration, scaling or further analysis of the data read from the hardwaresystem is needed the analyzer program is very simple. It registers itself for receivingall events produced by the Scintillator equipment and registers a single analyzer mod-ule that simply build up an histogram of the received data without any manipulation.Two alternative analyzer program are provided. The one contained in the hanalyzersubdirectory of the source distribution uses the HBOOK format for histogram booking andthe one contained in the ranalyzer subdirectory uses the ROOT framework for data stor-age. Only one of the two alternative programs must be used at the same time for onlineanalysis.
3.3.3 Timer
The timer program is a very simple frontend consisting of a single equipment calledTimer that generates an event after a fixed amount of time. This time is configurablein the online database through the /Equipment/Timer/Settings/Time entry.
For achieving a sufficient precision of the measured time the status of the timer ischecked periodically with a period of 100 ms. An internal timer is used to store the timesince the start of the measure and is compared to the wanted measuring time at each
40 CHAPTER 3. SOFTWARE
recursion. Only when the time is over the equipment generates an event. Every time anevent is raised the Midas framework checks that the events limit is not exceeded. If thenumber of the event raised is equal to the maximum one the measurement is stopped.The stop of the measurement is then obtained by setting the maximum number of eventsfor the Timer equipment to one.
3.3.4 Arms
The arms program is basically a client of the online database. It monitors the contentof the /Control/Arms/Position/Demand subtree and enforce the demanded positioningcommunicating with the stepper motors control unit. That subtree contains the entriesX and Y that represent the wanted position of the device in millimeters from the zeropoint.
At each modification of the structure, if no other movements are taking place and ifthe data acquisition is in the stopped status, the /Control/Arms/Status/Moving entryis set to a true value and a new process is forked for achieving the requested positioning,otherwise an error is raised. The process is then monitored in the main loop and if itends successfully the /Control/Arms/Status/Moving entry is restored to zero and the/Control/Arms/Position/Current entry is updated to the new value. As described insection 3.2.1 the client could not stay inactive for a long time waiting for the movementto take place because otherwise the Midas system recognizes it as in a dead state andremoves it from the notification system so forking a new process is required.
Also some working parameters for the positioning system are stored in the onlinedatabase. The /Control/Arms/Settings/Device contains the name of the serial deviceused for the communication with the hardware and the /Control/Arms/Settings/Speedspecify the speed used in all the movements in millimeters per second.
The communication with the hardware is done in a three layer system granting mod-ularization and easier code reuse beside the fact that changing the hardware requires thereplacement of only one of the layers. The upper layer, implemented in the driver.csource file, stores the device status in an opaque structure and implements some genericmovements function that takes parameters in an human intelligible form. The implemen-tation of those function translates the received commands is hardware specific commandsexported by the middle layer, implemented in the commands.c source file. The functionexposed by the middle layer are a simple mapping between the commands understoodby the hardware device and C functions. The implementation of those functions sendscommands to the hardware in form of strings through the lower layer, implemented inthe serial.c source file, that exports communication functions over the serial line.
This program prevent the possibility that the user start a data acquisition run whilethe positioning system is moving hooking to the transition notification system – seethe the Control Program paragraph in section 3.2.2. If the program recognizes that amovement is going on when the user requests a start transition the transition is abortedand an error is reported to the user.
As described in section 2.1.2 for obtaining a cartesian system of coordinates startingfrom the chosen zero point the direction of the y axis must be reverted. That is done inthe driver layer.
3.3. DEDICATED SOFTWARE 41
3.3.5 Control
The control program represent the run control program previously described. It isresponsible for the automation of the data taking procedure and represents the user in-terface to the system. It takes a file with the sequence of actions to perform and executesthem. Beside the states defined by Midas that refer to the single data acquisition runtwo new states of the experiment are defined: started and stopped. While the experimentis in the started status the control program takes the next command from the file andexecutes it. This program offers to the user a simple command line interface where thefollowing commands are available: load tells the program which file is the one with thecommands to execute; panel sets the right identifier of the panel that will be measuredin the database; start starts the data tacking procedure; stop stops the data tackingprocedure before the end of the commands file; quit exits from the control program.
The file with the sequence of the actions to perform can be manually edited orgenerated with the points program. That program takes the size of the scintillatorpanel and the position of the photomultiplier tube as command line arguments andoutputs the procedure needed to map the scintillator. Possible commands that can beput in the control file are: move tells the system to move the radiative source at thegiven position on the panel; run starts the data acquisition; set sets some variables inthe online database; end tells to the system that the procedure is terminated; pausemake the system wait for user input before going on with the procedure.
3.3.6 Convenience Programs
Some of the convenience programs written during the development of the system couldbe of interest to the user. The setposition, setspeed and settime programs are basiconline database clients that set the corresponding parameters in the database. Thereset program uses functions exported by driver.c for drive the positioning deviceto the zero position. This operation is needed when the system is powered on to setthe right starting point to which refer the movement commands to the motors driver.The graph program is a python program that takes as input the file that describes thesequence of the action to perform to do a measure run and uses the gnomecanvas libraryto give a graphical representation of the points where the measurements will take place.It is useful for debugging the functionality of the points program.
42 CHAPTER 3. SOFTWARE
Chapter 4
Data Analysis
In this chapter the procedure used to analyse the data collected during the measurement isdescribed. The results obtained from the data analysis are also pointed out.
4.1 Tools
For the data acquisition the analyzer that saves the histograms of the collected events inHBOOK format is selected – see section 3.3.2. This format is chosen because it is the oneused by the PAW program suite and some routines for the analysis of the spectra werealready developed for this environment. PAW stands for Physics Analysis Workstationand is a powerful data analysis package developed by CERN to assist physicists in theanalysis and presentation of their data. It provides interactive graphical presentationand statistical or mathematical analysis and it is well suited to handle most types ofphysics data. Additionally the ROOT framework is used for the analysis of the dataobtained from the mapping and for the comparison of such data with the data obtainedfrom the numerical simulation of the scintillator panels. ROOT is an Object OrientedData Analysis Framework and is another powerful data analysis package developed byCERN as modern replacement of PAW.
4.2 Normalisation
To analyse the acquired pulse height spectra the background must be subtracted. Thispermits to obtain an histogram with the spectra due to only the well know source.For obtaining sensible data from the background subtraction it is necessary that thenumber of events that compose the background in the background measurement and inthe real one are the same. The background is given due to the cosmic radiation and tothe environment radioactivity. In principle those sources of radiation are constant overthe time and all the measurements are done over a fixed amount of time, as explainedin section 1.3.2. A simple normalisation of the histograms that takes care only of thedifferent duration of the measure is then possible. The problem is the dead time of thesystem. As described in section 2.2.2 the system is built in a way that permits to collect
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44 CHAPTER 4. DATA ANALYSIS
only an event in a fixed time frame. If multiple particles come in the same time frameall the signals not generated by the first one are discarded. Is then clear that if the rateof the incoming particles change also the measured background changes.
What could be done is to measure the live time of the system instead of the realtime and then do the normalisation using this time. That can be achieved, for example,counting the impulses generated from a clock gated through the acquisition system signalgate. This obviously introduces more complexity in the electronic system and is notnecessary for this work. Another way to proceed is to normalise the area of a peak inthe histogram that is known to be not influenced by the physic of the experiment. Theselected peak could be due to a physic event or could be an electronically generated signalof known amplitude and shape. The option to insert electronically generated signals inthe system causes an increase of the complexity of the electronic system and does notgive any advantage in this application.
In this work the normalisation is done referring to the area of the muon peak, becausethe muon peak is always present and is easily identifiable. The problem is that in somemeasurement there is an overlapping of the muon peak with the peak due to the particleemitted by the used source. This overlapping is limited to the less energetic side of thepeak so the problem is solved referring the normalisation to only the high energy half ofthe peak. The position of the muon peak in the spectra is fixed for every scintillator oncethat the high voltage for the photomultiplier is fixed. The range of bins in the histogramto sum the area of the muon peak is then the same all the measurements done with thesame panel.
4.3 Fitting
The use of the Compton edge as reference for the calibration rises the problem of de-termining where the edge lies. The finite resolution of the detector smears out thetheoretically rough edge into a sloppy edge so a sharp structure is no more available. Forthe calibration this slope is fitted with an exponential decreasing function in the form:
f(x) =c
1 + exp {a (x− b)}
and the Compton edge is assumed to be at the position corresponding to the half heightof the fitted slope. In the literature there is no real agreement on where the edge lies [8] [9]and is not possible, without additional measurements, to say if this is the correct energyposition of the Compton edge. However that assumption influences only the value ofthe energy offset value must be anyway corrected to fit the different electronic readoutsystem that will be used in the final setup.
From the fitting three parameters are gained. The c parameter is the height of theCompton edge and is related only to the number of counts then to the measure time.The b parameter is the energy position of middle height point of the sloope and is usedas energy position of the Compton edge. The a parameter is related to the smoothnessof the curve and can be used as an estimation of the energy resolution of the system andthen as estimation of the uncertainty of the energy position.
4.4. PARAMETERS 45
4.4 Parameters
As pointed out in section 2.2.3 there are two parameters that influences the operation ofthe electronic system used for the data acquisition and then the acquired data that mustbe set for the measurements: the threshold level used to discriminate between signalsand noise and the high voltage used to power the photomultiplier. A brief analysis ofthe choice and of the effect of their variations follow.
Figure 4.1: Difference between two background measurements done with a threshold level of 20 mV.As reference the two background are also plotted with dashed and dotted lines.
4.4.1 Threshold Level
The threshold level is determined with some test measurements and is set to a commonvalue for all the panels. The threshold must be selected in a way that permits the systemto acquire all the events generated by the energy reference source while rejecting as muchnoise as possible. The selected value is 10 mV . However after some data analysis it wasobserved that the selected threshold is too low and the noise rejection is not optimal.
Many factors influence the electromagnetic field in the surrounding of the photomul-tiplier. Also the radiative source with the lead shielding and the aluminium profiles of thepositioning device act roughly as electromagnetic shielding. That induces a dependencyof the noise to the position of the source producing a loss of the random behaviour ofthe electromagnetic noise in the mid term. This effect can be observed comparing thespectra from different measurements. For example the spectra of figure 1.7 presents astrange shape in the low energy part. If the background and the measurement wheredone with the same noise conditions after the background subtraction the graph shouldshow only the peak relative to the Compton edge.
Another cause of the bad shape of the spectra could be identified. As described insection 4.2 the number of events recorded by the system is limited by the dead time.
46 CHAPTER 4. DATA ANALYSIS
Adding the γ source the number of the events that must be recorded increases andthen increases the dead time. That can lead to the rejection of some events. As thenormalisation is done referring to the area of the cosmic peak we see that difference inthe number of events recorded in the environmental background peak. Anyway thatpeak is not interesting for the purpose of the measurement.
To see the real effect of the threshold level some test measurements are done withdifferent thresholds. In figure 4.1 a graph of the subtraction of two background spectraacquired with a threshold of 20 mV is plotted. To further decrease the effect of theelectromagnetic noise also an aluminium foils was applied on the external surface of thescintillator trying to shield as much noise as possible. Is observed that the fluctuations ofthe spectra at low energies are still present but with a more regular shape. All the spectralooks noisier but that is only due to a shorter acquisition time. Excluding electromagneticnoise, for the increased threshold and for the added shielding, it is possible to identify anadditional cause of that problem in fluctuations of the threshold level. Small fluctuationscan indeed produce recording of a different number of low pulse height signals.
However, the small fluctuations in the lowest energy part of the spectra does not in-fluence the accuracy of our measurements and any effort is put in correcting the problemsidentified.
Figure 4.2: Background spectra acquired with different high voltage settings: 800 V, 850 V and 900 Vrespectively for the spectra drawn with the continuous, the dashed and the dotted line.
4.4.2 Hight Voltage Value
The high voltage value determines the gain and the efficiency of the photomultiplier. Therelation between the voltage and the behaviour of the photomultiplier was measured byanother group. The relation between the voltage and the gain could be considered infirst approximation linear in a small interval around the nominal value of 850 V suppliedby the manufacturer of the photomultiplier.
4.5. TIME STABILITY 47
In the measurements for each panel the high voltage is set to a value that makes themiddle of the cosmic peak to place always in the same channel of the histogram. Thatis to make the data acquired from different panel more easy to compare. Obviously forpanels of the same shape the high voltage used is the same. Voltage values used rangesbetween 825 V and 875 V to place the middle of the cosmic peak at about channel 2750of the histogram.
In the graph of figure 4.2 three background spectra obtained with three differentvalues of high voltage are plotted. Each spectra is normalised to the same area of thecosmic peak. Accordingly with the expectations, it is possible to see a shift of the positionof the cosmic peak toward higher energies, going from the lower to the higher voltage.With the increase of the voltage an increase of the area of the environmental backgroundpeak is also observed. That is due to the increased gain that amplify low height pulses,before discarded, to amplitudes higher than the threshold. Those low height pulses arenot only related to physical events but also to noise. Increasing the high voltage also thethreshold should be increased to avoid to record non physical events.
Figure 4.3: Difference between two background measurements. As reference the two background arealso plotted with dashed and dotted lines.
4.5 Time Stability
As pointed out in section 1.3.2 the acquisition for each panel of more than one backgroundspectra at different times permit to do a basic time-stability analysis of the system.What can be checked is the stability of the electronic data acquisition system and of thephotomultiplier.
In the electronic system the threshold level, the QDC calibration and the high voltagevalue can be subject to time drifts and cause noticeable differences in the acquired data.A modification of the threshold level is noticed with a change in the events count at thevery low energy part of the spectra. A change in the QDC calibration induces an error
48 CHAPTER 4. DATA ANALYSIS
in the measurement of the pulse height of the signal that reflects in the spectra witha shift of the peaks proportional to the energy. The high voltage value influences thegain of the photomultiplier, as previously described. A drift of the voltage induces inthe acquired spectra a shift of the peak proportional to the energy. The photomultiplieris a sensible device. Its response is temperature dependent and the gain can change ifthe photomultiplier is turned off and on again. When the high voltage is applied to thephotomultiplier there is also a settling time that it needs to stabilise. For that reason ispreferable to not switch the high voltage on and off during the data acquisition and towait some minutes before start to acquire data after the photomultiplier is turned on.
An evaluation of the difference between two spectra can be obtained plotting thesubtraction of the two histograms. An example of the general shape of the spectraobtained from the subtraction of two background spectra is plotted in figure 4.3. Itis possible to see that the difference between the two spectra is always null but in thelower energy part. The causes of such difference are analysed in section 4.4.1. Importantfor the work is the gain stability and the analysis proved it stable in the limits of theuncertainty on the acquired data.
4.6 Results
For the installation of the scintillator panels as muon veto is necessary to perform anaccurate energy calibration of the detectors. The response of the scintillators to muonsimpacting in different position of their volume should be measured. The goal of thiswork was not the calibration of the scintillator panels but the development of the systemto automate the data acquisition process necessary to perform the calibration. This goalis reached with the realization of the system described in the previous chapters and withthe tests performed to prove its functionality and stability. Anyway is interesting topresent an example of calibration.
Normalisation, background subtraction and fitting of the Compton edge are per-formed for each of the acquired spectra accordingly to section 1.3.2 to extract informa-tions from the acquired data. This fitting procedure extract from each spectra threeparameters representing the Compton edge as described in section 4.3. Two of themare interesting for the calibration. The b parameter that represent the position of theCompton edge and the a parameter that can be used as estimation of the uncertainty ofthe measurements.
As anticipated in section 1.3.2 the pulse height – energy response of the scintillators issupposed linear for energies above 100 keV . The energy calibration can then be obtainedfrom the two spectra acquired positioning the two radioactive sources emitting γ-rays ofdifferent energies in the middle of the scintillator panels. The energy of the Comptonenergy as calculated in section 1.3.2 is plotted as function of channel position of theCompton edge in the measured spectra. A line is fitted thought the two points obtainedin the graph. The intersection of this line with the x axis gives the energy offset whilethe angular coefficient of the line gives the pulse height – energy ratio.
With those data for each of the spectra acquired for every panel the channel positionof the Compton edge is translated into an energy. The differences in the pulse height –
4.6. RESULTS 49
energy response of the scintillator for different impact positions are then determined asthe ratio between the response of the scintillator in a specific point and the overall re-sponse. The overall response is determined computing the average of the energy positionof the Compton edge for the spectra acquired with the radioactive source positioned inall the different points over the surface of the detector.
Figure 4.4: Differences in pulse height – energy response measured for scintillator panel 08 expressedas ratio between the punctual response and the average response of the detector.
Figure 4.5: Differences in pulse height – energy response obtained from the data produced by thesimulation expressed as ratio between the punctual response and the average response of the detector.
The graph of figure 4.4 shows the pulse height – energy response of the scintillatorpanel 08. As anticipated a computer simulation of the scintillator detectors where alsoperformed. In the graph of figure 4.5 the results from the simulation are reported. Thecomparison of the two graphs shows that the general trend expected from the simulationis confirmed by the measurements. While the simulation and the measurements are inagreement within an error of about the 10% in the external part of the scintillator panels,
50 CHAPTER 4. DATA ANALYSIS
on the central part of the detector, near the photomultiplier, the discrepancy is muchbigger. The point of the maximum efficiency is shifted away from the photocathode andthe ratio between the maximum efficiency and the average response is much lower in thereal measurements.
Close to the photocathode the relative importance of the direct light compared tothe reflected one changes. The simulation software package used uses only a basic bi-dimensional model of the detector. Only light reflection on the borders of the scintillatoris taken in account while the reflection on the main surfaces of the scintillators in ne-glected. This model works well at high distances from the photocathode. Here the paththe photons have to travel becomes very long compared to the attenuation length incase of reflection on the surfaces. However the reflected light play a bigger role close tothe photomultiplier. That can explain the differences between the simulation and themeasurements. The collected data can then be trusted with sufficient confidency.
For further development of the muon veto and for the selection of the right thresholdvalue more simulation of the full system are performed. Those simulation are based onthe mapping data collected during the measurements.
Introduction of a model that takes in account the three dimensional geometry of thedetectors and the the reflection on the surfaces of the scintillators along with furtheranalysis of the obtained data and comparison with the simulation can driven to thedevelopment of a better software package capable to effectively reproduce the behaviourof such big scintillators panels with shape similar to the ones used in this application.Anyway that is out of the goals of this work.
Conclusions
The main goal of this work was achieved with the realization of the device for achievingthe mapping of all the scintillator panels employed in the second stage of the CRESSTexperiment. The software package necessary to control this device and to acquire datafrom the electronics to which the scintillators are interfaced was also developed. Duringthe measurements the positioning device proved to be accurate and stable enough to fulfilthe requirement of this job. The software package proved to be reliable and adequate forthe data acquisition and successive analysis.
Because of time restrictions, enforced by the deadline for the installation of the scin-tillator detectors in the experimental setup, not all the panels were measured as planned.One panel for each shape was measured with a high acquisition time of ten minutes perpoint. For the major part of the other panels only the energy calibration and a smallnumber of additional measures were performed. Those measures guarantee with suffi-cient confidence that there are no significant variations in the energy response of panelsof the same kind. With that assumption all the panel are modelled accordingly to thedata acquired with the high statistic measures.
The data analysis was not completed during my stay at the Universitat Tubingenbut was finished by Marcel Kimmerle. The data acquired during this work are nowused to perform new numerical simulations of the full muon veto system to calculate anappropriate energy threshold level and to check for muons leakage.
It should be pointed out also that the apparatus developed during this work was alsoused by another group to test detectors for COSY-TOF which investigates the reactionpp → ppππ. The reuse of the system for another work confirmed the quality of thedesign and of the realization that, while being focused on a specific experiment, wasgeneral enough to be used for similar purposes.
The apparatus, in a different configuration, will be also used at the MAX-Lab electronbeam facility of the University of Lund where it will work as a target positioning systemfor another experiment. The goal of this experiment is the measurement of the totalphotoabsorption cross section of isotopic pure lithium targets below pion threshold. Forachieving that goal the positioning of a two meter long cylinder of target material in thephoton beam with high accuracy is necessary. The built system proved to be able tofulfil this requirement.
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52 CONCLUSIONS
Conclusioni
L’obiettivo principale del mio lavoro e stato raggiunto con la realizzazione del disposi-tivo utilizzato per effettuare la mappatura di tutti gli scintillatori impiegati nella secondafase dell’esperimento CRESST e con lo sviluppo del pacchetto software necessario al con-trollo del dispositivo e all’acquisizione dei dati dall’elettronica a cui gli scintillatori sonointerfacciati. Durante le misure il sistema di posizionamento si e dimostrato accurato estabile a sufficienza per soddisfare le richieste di questo lavoro. Il pacchetto software sie dimostrato affidabile ed adeguato per l’acquisizione dei dati e per la successiva analisi.
A causa di restrizioni temporali imposte dalla scadenza per l’installazione degli scin-tillatori nel setup sperimentale non tutti i pannelli sono stati misurati come previsto.Un pannello per ogni tipo e stato misurato con un elevato tempo di acquisizione di dieciminuti per ogni punto. Per la maggior parte dei pannelli restanti soltanto la calibrazionein energia e un piccolo numero di misure addizionali e stato realizzato. Queste mis-ure garantiscono con sufficiente confidenza che non esistono significative variazioni nellarisposta in energia dei detector dello stesso tipo. In accordo con questa assunzione tuttii pannelli sono modellati secondo i dati acquisiti con le misure ad alta statistica.
L’analisi dei dati raccolti non e stata completata durante la mia permanenza all’Uni-versitat Tubingen ma e stata conclusa da Marcel Kimmerle. I dati acquisti durantequesto lavoro sono adesso utilizzati per effettuare nuove simulazioni dell’intero sistemadi muon veto per calcolare un adeguata soglia in energia e per escludere la possibilitache muoni entrino nell’area sperimentale senza essere individuati dal muon veto.
L’apparato sviluppato durante questo lavoro e stato utilizzato da un altro gruppoper il test di detector impiegati nell’esperimento COSY-TOF che mira ad investigare lareazione pp → ppππ. La possibilita di riutilizzare il sistema per un altro lavoro confermala qualita della progettazione e della realizzazione che nonostante fosse focalizzata su unesperimento specifico si e rivelata sufficientemente generale per essere utilizzata per altriscopi simili.
L’apparato verra utilizzato in una differente configurazione anche presso il MAX-Labdell’University of Lund dove funzionera come sistema di posizionamento del target perun altro esperimento. Lo scopo di tale esperimento e la misura della sezione d’urto totaledi foto assorbimento di campioni di litio isotopicamente puri al di sotto della soglia diemissione di pioni. Per raggiungere tale scopo e necessario l’accurato posizionamento nelfascio di fotoni di un target composto da un cilindro di litio della lunghezza di due metri.Il successo nello sviluppo di questo sistema ha permesso di verificare che le tecnologieimpiegate nella progettazione e realizzazione offrono caratteristiche in grado di soddisfaretale richiesta di accurato posizionamento.
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54 CONCLUSIONI
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