Preliminary Results on Compton Electrons in SiliconDrift Detector

T. Çonka-Nurdan, K. Nurdan, K. Laihem, A. H. Walenta, C. Fiorini, B. Freisleben, N. Hörnel, N. A. Pavel, andL. Strüder

Abstract—Silicon drift detectors (SDD) with on-chip electronicshave found many applications in different fields. A detector systemhas recently been designed and built to study the electrons fromCompton scatter events in such a detector. The reconstruction ofthe Compton electrons is a crucial issue for Compton imaging. Theequipment consists of a monolithic array of 19 channel SDDs andan Anger camera. Photons emitted from a finely collimated sourceundergo Compton scattering within the SDD where the recoil elec-tron is absorbed. The scattered photon is subsequently observed byphotoelectric absorption in the second detector. The coincidenceevents are used to get the energy, position, and direction of theCompton electrons. Because the on-chip transistors provide thefirst stage amplification, the SDDs provide outstanding noise per-formance and fast shaping, so that very good energy resolutioncan be obtained even at room temperature. The drift detectors re-quire a relatively low number of readout channels for large de-tector areas. Custom-designed analog and digital electronics pro-vide fast readout of the SDDs. The equipment is designed suchthat the measurements can be done in all detector orientations andkinematical conditions. The first results obtained with this detectorsystem will be presented in this paper.

Index Terms—Compton camera, compton electron, silicon driftdetector (SDD).

I. INTRODUCTION

S ILICON DRIFT DETECTORS (SDDs) [1] with integratedelectronics are used in various applications such as mate-

rial analysis [2], scintillator readout [3], and holography [4].This paper presents a new application which is focused on thereconstruction of the electrons from Compton events to get in-formation about the initial gamma rays. The reconstruction ofthe Compton events is an interesting task in many fields such asCompton camera imaging [5], astrophysics, radiation therapy,and of course, the very interesting application for imaging ofnuclear tracers in small animals.

The main idea is to have a detector system where the trackof the recoil electron is determined as precisely as possible.This is necessary if the events are used for image reconstructionwhere incomplete events lower the contrast and the modulationtransfer function. The collimated photons undergo a Comptonscattering in the SDD where the Compton electron is to be de-tected. Another detector is used in coincidence with the SDD toabsorb the scattered photon and measure its energy so that onlythe true Compton events are processed. To track the electron,the first detector should have good energy resolution, good po-sition resolution and fast readout capability. SDD was chosenas the scattering detector due to its superior properties. The firsttransistor is implemented directly on the chip which reduces thedetector capacitance considerably. This results in a good energyresolution with fast shaping even at room tempera-ture. It has a large detection area with relatively low number ofreadout channels. A monolithic array of 19 channel SDD is usedfor detection of the Compton electrons. The Compton events inthis detector are selected for reconstruction if they are in coinci-dence with the absorption of the scattered photon in the seconddetector. The absorption detector is an Anger camera without acollimator. The large area of the camera provides a wide rangeof angular acceptance for the scattered photons. The analog anddigital readout electronics of the SDD have been custom de-signed with fast readout capability.

A. System Overview

The detector system for reconstructing the Compton electronis shown in Fig. 1. The radioactive source is placed inside thecollimator and for the first measurements is used. Thecollimation of the beam down to a sub-mm-scale allows us toscan the SDD cells. The SDD and the readout electronics aremounted on a motor system which moves in horizontal, verticaland rotational directions. In this way, Compton electrons in alldirections and kinematics can be studied. The Anger camera isplaced on a moving stage to cover a larger region where thescattered photons enter. It has also an embedded motor systemwhich enables a vertical translation and two-axes rotational mo-tion of the camera head.

B. Silicon Drift Detector

The cross section of the single cell SDD and possibleCompton events are shown in Figs. 2 and 3, respectively. Whena photon of 662 keV Compton scatters in silicon, recoil elec-trons with a maximum energy of about 470 keV are created.If only the forward scattering region is considered, then theenergy is about 370 keV. Considering the detection efficiency

Fig. 1. The detector system for Compton electron detection.

Fig. 2. The section of a single SDD cell.

of 300 thick silicon detector, some of these electrons leavethe detector without being absorbed completely [case II inFig. 2(b)], whereas the lower energy electrons may be fullycaptured (case I). A stack of these detectors would increase thedetection efficiency and it is planned for the future system [6].The optimum thickness is about 1.5 cm if only single Comptonevents are considered [5] and this value reduces down to 2 mmif multiple events are utilized [7]. The optimum area of thedetector depends on the practical application.

The SDD used for the Compton camera test setup consists ofan array of 19 hexagonal cells with a total detection area of 95

. The chip was produced at Max Planck Institute semicon-ductor laboratory, the bonding and the ceramic layout were de-signed at the detector physics group of Siegen Universityand thecip was mounted and bonded at Ketek GmbH. The detector ismounted on an aluminum nitride (AlN) ceramic and the layoutis done by using a thick film technology. The detector can becooled by using a two-stage Peltier element which is thermallycoupled to the ceramic. AlN was chosen as the ceramic materialfor its excellent heat conduction properties.

Fig. 3. Sample event view at the SDD where an incoming photon Comptonscatters and the recoil electron either deposits its full energy in SDD or itescapes.

The detector chip is shown in Fig. 4. This is the front side ofthe chip where the field strips cover the surface. Field strips ofeach cell are connected to outer rings, therefore the bias volt-ages are common to all channels. The radiation entrance side isthe nonstructured -junction on the back side. Each cell of the

Fig. 4. The 19-cell SDD mounted on an AlN ceramic.

SDD has a junction field-effect transistor (JFET) integrated di-rectly on the detector chip. This is particularly suitable for multi-channel detectors because it avoids high stray capacitance whichmay occur due to the connection of each cell to an external tran-sistor.

C. Front-End Electronics

The on-chip JFET works in a source follower configurationand it is driven by a constant current source. Emitter followerswere successfully used in the past as the first stage preamplifica-tion for proportional counters [8]. Similarly, an emitter followerstage [9] is used before the preamplifier to reduce the rise time ofthe output signals which allows us to use a shorter shaping timefor the readout. The rise-time of the preamplier output decreasedby a factor of 5 with the addition of this transistor stage. Usuallythe rise-time of the SDD signals vary between 200 and 300 nsdepending on the detector and the preamplifier setup. With theemitter follower this was reduced to 55–60 ns. The system be-came more immune to stray capacitances and the preamplifiercould be placed further from the detector.

A low-noise preamplifier developed particularly for this de-tector was used. A 10-channel hybrid board was designed andproduced. The preamplifier contributes about five electrons rmsof equivalent noise charge to the whole detector and readoutelectronics system. This was experimentally measured and inparallel confirmed by SPICE simulations. Preamplified signalsare further amplified and shaped by a CR-RC shaper which isalso a hybrid design. Fig. 5 shows the preamplifier and shapercarrier used for the readout of 10 channels.

Fig. 5. 10 channel preamplifier and five of two channel shaper hybrid boardson a carrier printed circuit board for analog readout of 10 cells of SDD.

Fig. 6. Data acquisition system implemented on a 19 crate with the eventbuilder and channel processor modules connected to a common bus.

D. Data Acquisition System

The differential shaper output is then transfered to a custom-designed field programmable gate array (FPGA) based data ac-quisition (DAQ) system [10], [11]. The DAQ system consists ofchannel processor modules, an event builder module and a bus.The channel processor modules receive the shaped analog sig-nals, digitize them with 12-bit 65 MHz ADCs. The digital signalis then transfered to the FPGA where the operations like timestamping, integration, peak finding are performed. Each channelprocessor module has four channels and a single XILINX FPGAfor all of them. With the help of the flexible and programmablefeature of FPGAs, same channel processor design has been usedboth for the SDD and the Anger Camera. Therefore, 19 chan-nels for SDD and four channels for , , and corrected energysignals of an Anger Camera need 6 such modules. The data aretransfered via the bus to the master event builder card with a rateof 100 MBps. The event builder card has a static RAM whichstores up to 250 kevents. When the buffer is filled up to a pro-grammable capacity, the data are transfered to a PC via parallelport. Fig. 6 shows the DAQ system for the whole setup.

Fig. 7. Fe and Cd spectra obtained with one of the 19 channels of SDD at 10 C with 100 ns shaping time.

E. Spectroscopic Measurements

The spectroscopic measurements were performed with thedetector irradiated with a source. The average equivalentnoise charge of the channels is about 40 electrons rms with afluctuation of few electrons rms from channel to channel. Thisresult is obtained at room temperature and with a shaping timeof 100 ns. Fig. 7 shows the spectrum obtained at 10 with

and sources. The resolution improves even furtherwith shaping time of the order of 250 ns and - shapingbut for this application a shorter shaping time is more desiredthan a better energy resolution. The resolution is good enoughto discriminate Mn and lines even at short shaping timesand at room temperature. The energy resolution was measuredto be better with a single cell SDD [9] at room temperature andwith the same readout electronics. This detector had a leakagecurrent of 100 pA and the 19 cell SDD seems to have a higherleakage current which results in a degraded energy resolution.

A proportional, integral, derivative (PID) controller has beendesigned and produced to control the Peltiers [12]. The warmside of the Peltiers is cooled by water circulating within thedetector housing. In addition to water flux, a constant nitrogenflux is provided which prevents any condensation that mayoccur below dew temperatures. It is important to perform themeasurements at constant temperature because not only thedetector chip but also the transistors are sensitive to tempera-ture changes. Stable temperatures are maintained by the PIDsystem. The spectroscopic measurements were performed atvarious temperatures and the FWHM energy resolution of Mn

line as a function of temperature is shown in Fig. 8. Theresolution does not improve considerably below 10 due tothe electronics noise limits.

F. Coincidence Measurements

The Compton electrons captured in silicon are taken as trueevents only when there is a coincidence with the Anger camera.The Anger camera (AC) is composed of a 5/8-in–thick NaIwith a radius of 10 in that is coupled to 37 photomultipliertubes. The large area of the camera provides large field of

Fig. 8. Energy resolution of one SDD channels as a function of temperature.

view for the scattered photons. The time stamps of two de-tectors were collected in continuous mode without enablingthe coincidence logic. Fig. 9 shows the time coincidencecurve obtained by subtracting these time stamps. The timestamp has a resolution of 15 ns and events within of

- have been scanned.The coincidence events are concentrated in a 300-ns wideregion which may result from the drift time of electrons anddecay time of NaI.

The signals were also examined by direct observation on theoscilloscope. The shaper output when the SDD is irradiated with

source is shown in Fig. 10. The signal looks fine in termsof pole-zero cancellation, etc. However, when the detector isirradiated with source, various signal shapes (Fig. 11)are observed. The signal form shown in Fig. 11(a) occurs oftenwhen the energy deposited in the SDD is high. The signal witha double peak in Fig. 11(b) seems to be produced by doubleCompton events. Only few out of 2000 signals were observedto have such a shape which agrees with a small probability ofhaving double Compton events in 300- -thick silicon. The

Fig. 9. Time coincidence curve obtained by taking the difference of time stamps from SDD and AC.

Fig. 10. Shaper output when SDD is irradiated with an Fe source.

bipolar signal [Fig. 11(c)] probably occurs when some of theelectrons move toward the drain of the on chip JFET instead ofmoving toward the anode. The electron cloud diffuses duringits travel to the anode and it diffuses more when the interactionoccurs at the edge of the cell where the electric field is weaker.When they reach the anode region, some electrons could go tothe transistor channel which is at more positive potential thanthe anode. In this case, they move away from the anode and thisproduces the negative tail of the signal. This type of signals wasobserved to occur more frequently if the Compton scatteringhappens in a region where the electric field is weaker and alsowhen the energy of the incoming beam is higher. When the en-ergy is higher, more charge is created, the first part of the cloudreaches the anode and this makes the anode more negative com-pared to the drain which causes the remaining part of the cloud

to tend to be attracted by the transistor channel. These resultsshould be compared with simulated signals to explain such sig-nals more precisely. These signals may give some hints aboutthe direction of the Compton electrons in silicon.

II. CONCLUSION

A detector system to track the Compton electrons in a sil-icon drift detector array has been designed and constructed.The determination of the direction and the position of electronsis crucial in many applications including Compton imaging inmedicine and astrophysics. Good energy resolution, high ratecapacity, and fast readout are promising features for this detectorsystem. Initial measurements suggest the system is promisingfor tracking Compton electrons. The comparisons with signalsimulations are necessary for more accurate conclusions.

Fig. 11. Various shaper outputs obtained with Cs source. In each figure: middle signal: SDD signal, upper signal: Anger camera signal, lower signal:coincidence gate.

ACKNOWLEDGMENT

The authors would like to thank to Dr. H. J. Besch and Dr.A. Rudge for their support on various technical issues. We aregrateful to Dr. D. Gunter for proofreading the manuscript.

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