IEEE Transactions on Nuclear Science, Vol. NS-3 1, No. 1, February 1984
A TIME-OF-FLIGHT HODOSCOPE SAVING THE NUMBER OF PHOTOMULTIPLIERS
Tullio BRESSANI, Mario CARIA and Sergio SERCIDipartimento di Scienze Fisiche, Universita di Cagliari, 09100 Cagliari, Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
and
Felice IAZZI and Bruno MINETTIDipartimento di Fisica, Politecnico di Torino, 10129 Torino, Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
and
Emilio CHIAVASSA, Sergio COSTA, Giuseppe DELLACASA, Nora DE MARCO, Mauro GALLIO and Alfredo MUSSO
Istituto di Fisica Superiore dell'Universita di Torino, 10125 Torino, Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
and
Patrick FASSNACHTCentre de Recherches Nucl6aires and Universite Louis Pasteur, Strasbourg, France
AbstractResults of measurements of the performances of a time-of-flight hodoscope consisting of 16 slabs of plasticscintillator, 90x4xl cm3 each, viewed by only 8 (in-stead of 32) photomultipliers are described. The slabsare coupled to the photomultipliers in such a way that
they are considered as the elements of a square matrix
of rank 4. Thus only 2x4 photomultipliers are needed toidentify unambiguously each of the 16 slabs, if only a
particle at a time has to be detected. By means of a
very simple electronics we obtained a local time-of-flight resolution of 570 ps fwhm and a spatial resolu-tion along each slab of 5 cm fwhm. The cross-talk due
to unrecognized events was less than 5%.
The use of scintillator hodoscopes of largedimensions for the precise measurement of the time-of-
flight (TOF) with a simultaneous determination of the
impact position of the particles is rather frequent in
experiments on nuclear and particle physics. The usual
technique adopted to achieve the best performances is
that of coupling each element of the hodoscope at the
two sides to two fast photomultipliers (PM). The infor-
mation on the position is given by the time difference
At1 measured by the two PMs, whereas the arithmetic
mean between the time differences, At2 and At3, measu
red by the two PMs relative to another counter placedupstream, gives the TOF, independent from the impact
position (see e.g. [1],[2] and references quoted there-in). For fine-grained hodoscopes consisting of many ele
ments for a localization in both directions, a seriouslimitation can sometimes arise from the high cost ofthe good quality PMs and associated fast electronics.
We were faced with this problem in design-ing the final counter of a spectrometer used to measureat the CERN Synchro-cyclotron the pion spectra producedby the interaction of ions (3He of 303 MeV/nucl. and12C of 86 MeV/nucl.) with nuclear targets. We needed a
fine-grained scintillator hodoscope in order to discri-
minate by TOF the pions from other lighter or heavierproducts and possibly to solve by an additional independent measurement some ambiguities that could be givenby the drift chambers used for the reconstruction ofthe trajectories of the particles.
The hodoscope (T2) consists of 16 slabs ofNE 110 plastic scintillator, 90x4xl cm3 each (total de-tecting area = 5760 cm2). Since we had to trigger onlyon a particle at a time over the whole surface of thehodoscope, we coupled the scintillator elements to the
PMs at the two sides following a coding system which
was already used for multiwire proportional counters
[3],[4]. The coupling was done through Lucite light guiaes, 60 cm long each. Fig. 1 shows the scheme of the
counter. It appears that each scintillator slab is con-
sidered as the element of a square matrix of rank 4,and then only 2x4 = 8 PMs and electronic channels are
needed to identify unambiguously the hit scintillator
and process its signal, instead of 32 as with the usualmethod of coupling. The first four PMs determine therow number and the last four the column number of thisideal matrix.
The scintillators and the light guides are
wrapped by aluminized- mylar. The PMs are Philips XP2020, with supply voltages adjusted in order to give signals of equal amplitude for minimum ionizing particlespassing near the center of each slab. Light guidesglued at the same position were also used to this purpose. The start signal for TOF measurements was given byanother counter (T1) consisting in 6 slabs of NE 110scintillator, 30x5x0.5 cm3 each, all together coupledthrough Lucite light guides to an XP 2020 PM at eachside.
Before mounting the hodoscope in the spec-trometer, we tested it and measured its performances ina beam of secondary particles (e-, t-, g-) of variableenergy from an internal target of the CERN Synchro-cyclotron. The magnetic elements of the beam were adju-sted in order to give a simultaneous irradiation of thewhole hodoscope, with a total flux exceeding 5x105 par-ticles/sec. Fig. 2 shows the layout of the counters
Fig. 2 - Apparatus used for the study of the hodoscope.Notation: Tl = start counter, T2 = hodoscope,Cg = gas Cerenkov counter, Cl, C2 = scintilla-tion counters.
used for the test. Cg was a gas (Co2 at 10 atm) Ceren-
kov counter placed immediately behind Tl and allowingthe selection of the electrons with an efficiency bet-
ter than 99.9%. Cl and C2 were small scintillationcounters placed on a remotely controlled beam scanner
and crossed in order to define by means of their coinci
dence a surface of less than 1 cm2 on T2. With this simple apparatus we could study all the features of ou-rcounter.
The block diagram of the electronics usedto process the signals is shown in Fig. 3. Commercial
Fig. 3 - Simplified block diagram of the electronics.Notation: CFD = constant fraction discrimina-tor, MT = mean timer circuit, FI - logic fanin/out, C = coincidence, SC = strobed coinci-dence, TAC = time-to-amplitude converter,G = gate. Passive delay units as well as out-put circuits are omitted for simplicity.
NIM modules (Le Croy) have been used throughout. Theconstant fraction discriminators (CFD) were ORTEC, Mod.934. From this scheme one can notice the compactnessand the considerable saving in electronic modules offe-red by our solution compared to the conventional one[1].
The measured total efficiency of T2 overits whole surface was 99.8 + 0.1%. The coincidence be-tween the two opposite groups of PMs (1 + 4 and 5 + 8),required in order to define a good event from T2, wassimply given by the mean timer (MT) circuit. Fig. 4shows the TOF spectrum measured for particles of 200MeV/c (e-, t-, jr-) passing near the center of one slab(nr. 8). The total TOF resolution, as measured by thewidth of the electron's peak (B = 1) is 570 ps fwhm.The pion's peak (0 = 0.82) is broader due to the momen-tum spread of the beam (i+ 5%). Similar spectra were ob-tained from all the other slabs and at different posi-tions, in particular near the edges. The worse spectrumexhibited a total TOF resolution of 630 ps fwhm.
Fig. 5 shows the results of the study of
148
the localization properties of T2. We have plotted thetime difference Att1 measured by the two oppositegroups of PMs in coincidence with the two defining scintillators (Cl, C2) displaced successively in steps of10 cm along one slab (nr. 8). From the width of thepeaks we could evaluate a spatial resolution of 5 cmfwhm. The same figure shows also the relation between
Fig. 4 - TOF spectrum relative to the slab nr. 8.
the centroid of the peaks and the displacement X. Itis clearly linear. We could then evaluate a constantof proportionality X/At1 - 7.85 + 0.1 cm/ns, in agree-ment with previous measurements on similar detectors
[1], [5]. It is the half of the velocity of propagationof the light in the scintillator, averaged over all thepossible trajectories.
The major drawback that we could expectfrom the adopted scheme of coding was that of the cross-talk involving PMs not directly coupled to the detec-ting element. It is due to some photons that can bebackward reflected from the various separation surfacesat different places along the optical path. For exemple,a scintillation produced in the element 7 (see Fig. 1)could fire not only the PMs 2 and 7, but also the 6
and/or 8 in the case of backward reflections near thePM2. When more than two opposite PMs are giving a si-gnal it is impossible to obtain an unambiguous identification of the scintillating slab since two or more solutions are possible. The obvious way of eliminating or,at least, reducing at the minimum the cross-talk is a
proper adjustement of the threshold settings on theCFDs. It is not straightforward since the attenuationof the light pulse along the whole length of the scin-tillator's slab (about a factor 3) has to be taken intoaccount.
We did a careful study of this effect bymeans of a source of Sr90 and a beam of minimum ioni-zing particles, for different positions on all theslabs and for different values of the threshold. Thedetermination of the best operating conditions was doneby means of the strobed coincidence (SC, see Fig. 3).The cryterion was that of measuring no losses of effi-ciency on the pair of PMs viewing the activated slabwith a minimum of counts on the other PMs. With the
Fig. 5 - Results of the study of the localization properties. See the text for details.
best setting we measured less than 5% of cross-talk e-
vents.Since the results of these preliminary
tests were completely satisfactory, we mounted the hodoscope in the spectrometer and we used it for all the
data-taking runs. With a more sophisticated off-line a-
nalysis we succeeded in reducing the amount of cross-
talk events to 1.3%.If we compare the performances of our "eco-
nomic" hodoscope with those of similar conventional in-
struments (see e.g. [1]), we found no substantial differences. In particular we stress that our quoted TOF re-
solution includes the intrinsic time resolution of Tl,the intrinsic time resolution of T2 and finally thetime resolution of the MT circuit. If we assume that Tl
contributes with an intrinsic time resolution of 250
ps and the MT circuit with 500 ps (according to the ope
rating performances given by Le Croy), we can quite rea
sonably assume an intrinsic time resolution in the or-
der of 300 ps fwhm for our hodoscope.In conclusion we have shown that economic
large-area scintillator hodoscopes having the best per-
formances can be built with the adoption of a simple co
ding system for the connection of the scintillator'sslabs to the PMs. The only limitation is the require-ment of a particle at a time within the gate width (40ns). We are now trying to improve the system to a greatter number of grouped elements (matrices of rank 6 or
7) and to the detection of neutrons, which give a continuous pulse height spectrum. We plan to use such a neu-
tron hodoscope, with about 100 elements, in an experi-ment on the measurement of the antineutron mass at theLEAR facility at CERN [a].
We wish to thank Dr. E. Lazzaroli for thecollaboration in the early stages of the work and Mr.P.P. Trapani and Mr. K. Ratz for the precious technicalassistance.
tillation counter for precise measurement of timeof flight and impact position", Nucl.Instr.Meth.,vol. 125, pp. 357-363, March/April 1975.
[6] T. Bressani and B. Minetti, "On the measurement ofthe antineutron mass", presented at the Internatio-nal Workshop on Physics with Low Energy Cooled Antiproton Beams, Erice, Italy, May 10-15, 1982.
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