SILICON DRIFT DETECTORS (SDDs) [1] with integrated. Preliminary Results on Compton Electrons in Silicon Drift Detector

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1 Preliminary Results on Compton Electrons in Silicon Drift Detector T. Çonka-Nurdan, K. Nurdan, K. Laihem, A. H. Walenta, C. Fiorini, B. Freisleben, N. Hörnel, N. A. Pavel, and L. Strüder Abstract Silicon drift detectors (SDD) with on-chip electronics have found many applications in different fields. A detector system has recently been designed and built to study the electrons from Compton scatter events in such a detector. The reconstruction of the Compton electrons is a crucial issue for Compton imaging. The equipment consists of a monolithic array of 19 channel SDDs and an Anger camera. Photons emitted from a finely collimated source undergo Compton scattering within the SDD where the recoil electron is absorbed. The scattered photon is subsequently observed by photoelectric absorption in the second detector. The coincidence events are used to get the energy, position, and direction of the Compton electrons. Because the on-chip transistors provide the first stage amplification, the SDDs provide outstanding noise performance and fast shaping, so that very good energy resolution can be obtained even at room temperature. The drift detectors require a relatively low number of readout channels for large detector areas. Custom-designed analog and digital electronics provide fast readout of the SDDs. The equipment is designed such that the measurements can be done in all detector orientations and kinematical conditions. The first results obtained with this detector system will be presented in this paper. Index Terms Compton camera, compton electron, silicon drift detector (SDD). I. INTRODUCTION SILICON DRIFT DETECTORS (SDDs) [1] with integrated electronics are used in various applications such as material analysis [2], scintillator readout [3], and holography [4]. This paper presents a new application which is focused on the reconstruction of the electrons from Compton events to get information about the initial gamma rays. The reconstruction of the Compton events is an interesting task in many fields such as Compton camera imaging [5], astrophysics, radiation therapy, and of course, the very interesting application for imaging of nuclear tracers in small animals. The main idea is to have a detector system where the track of the recoil electron is determined as precisely as possible. This is necessary if the events are used for image reconstruction where incomplete events lower the contrast and the modulation transfer function. The collimated photons undergo a Compton scattering in the SDD where the Compton electron is to be detected. Another detector is used in coincidence with the SDD to absorb the scattered photon and measure its energy so that only the true Compton events are processed. To track the electron, the first detector should have good energy resolution, good position resolution and fast readout capability. SDD was chosen as the scattering detector due to its superior properties. The first transistor is implemented directly on the chip which reduces the detector capacitance considerably. This results in a good energy resolution with fast shaping even at room temperature. It has a large detection area with relatively low number of readout channels. A monolithic array of 19 channel SDD is used for detection of the Compton electrons. The Compton events in this detector are selected for reconstruction if they are in coincidence with the absorption of the scattered photon in the second detector. The absorption detector is an Anger camera without a collimator. The large area of the camera provides a wide range of angular acceptance for the scattered photons. The analog and digital readout electronics of the SDD have been custom designed with fast readout capability. A. System Overview The detector system for reconstructing the Compton electron is shown in Fig. 1. The radioactive source is placed inside the collimator and for the first measurements is used. The collimation of the beam down to a sub-mm-scale allows us to scan the SDD cells. The SDD and the readout electronics are mounted on a motor system which moves in horizontal, vertical and rotational directions. In this way, Compton electrons in all directions and kinematics can be studied. The Anger camera is placed on a moving stage to cover a larger region where the scattered photons enter. It has also an embedded motor system which enables a vertical translation and two-axes rotational motion of the camera head. B. Silicon Drift Detector The cross section of the single cell SDD and possible Compton events are shown in Figs. 2 and 3, respectively. When a photon of 662 kev Compton scatters in silicon, recoil electrons with a maximum energy of about 470 kev are created. If only the forward scattering region is considered, then the energy 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.

A stack of these detectors would increase the detection efficiency and it is planned for the future system [6]. The optimum thickness is about 1.

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

2 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 leave the detector without being absorbed completely [case II in Fig. 2(b)], whereas the lower energy electrons may be fully captured (case I). A stack of these detectors would increase the detection efficiency and it is planned for the future system [6]. The optimum thickness is about 1.5 cm if only single Compton events are considered [5] and this value reduces down to 2 mm if multiple events are utilized [7]. The optimum area of the detector depends on the practical application. The SDD used for the Compton camera test setup consists of an array of 19 hexagonal cells with a total detection area of 95. The chip was produced at Max Planck Institute semiconductor laboratory, the bonding and the ceramic layout were designed at the detector physics group of Siegen Universityand the cip was mounted and bonded at Ketek GmbH. The detector is mounted on an aluminum nitride (AlN) ceramic and the layout is done by using a thick film technology. The detector can be cooled by using a two-stage Peltier element which is thermally coupled to the ceramic. AlN was chosen as the ceramic material for its excellent heat conduction properties. Fig. 3. Sample event view at the SDD where an incoming photon Compton scatters and the recoil electron either deposits its full energy in SDD or it escapes. The detector chip is shown in Fig. 4. This is the front side of the chip where the field strips cover the surface. Field strips of each cell are connected to outer rings, therefore the bias voltages are common to all channels. The radiation entrance side is the nonstructured -junction on the back side. Each cell of the

Fig. 5. 10 channel preamplifier and five of two channel shaper hybrid boards on a carrier printed circuit board for analog readout of 10 cells of SDD. Fig. 4.

This is particularly suitable for multichannel detectors because it avoids high stray capacitance which may occur due to the connection of each cell to an external transistor. C.

Emitter followers were successfully used in the past as the first stage preamplification for proportional counters [8].

3 Fig channel preamplifier and five of two channel shaper hybrid boards on a carrier printed circuit board for analog readout of 10 cells of SDD. Fig. 4. The 19-cell SDD mounted on an AlN ceramic. SDD has a junction field-effect transistor (JFET) integrated directly on the detector chip. This is particularly suitable for multichannel detectors because it avoids high stray capacitance which may occur due to the connection of each cell to an external transistor. C. Front-End Electronics The on-chip JFET works in a source follower configuration and it is driven by a constant current source. Emitter followers were successfully used in the past as the first stage preamplification for proportional counters [8]. Similarly, an emitter follower stage [9] is used before the preamplifier to reduce the rise time of the output signals which allows us to use a shorter shaping time for the readout. The rise-time of the preamplier output decreased by a factor of 5 with the addition of this transistor stage. Usually the rise-time of the SDD signals vary between 200 and 300 ns depending on the detector and the preamplifier setup. With the emitter follower this was reduced to ns. The system became more immune to stray capacitances and the preamplifier could be placed further from the detector. A low-noise preamplifier developed particularly for this detector was used. A 10-channel hybrid board was designed and produced. The preamplifier contributes about five electrons rms of equivalent noise charge to the whole detector and readout electronics system. This was experimentally measured and in parallel confirmed by SPICE simulations. Preamplified signals are further amplified and shaped by a CR-RC shaper which is also a hybrid design. Fig. 5 shows the preamplifier and shaper carrier used for the readout of 10 channels. Fig. 6. Data acquisition system implemented on a 19 crate with the event builder and channel processor modules connected to a common bus. D. Data Acquisition System The differential shaper output is then transfered to a customdesigned field programmable gate array (FPGA) based data acquisition (DAQ) system [10], [11]. The DAQ system consists of channel processor modules, an event builder module and a bus. The channel processor modules receive the shaped analog signals, digitize them with 12-bit 65 MHz ADCs. The digital signal is then transfered to the FPGA where the operations like time stamping, integration, peak finding are performed. Each channel processor module has four channels and a single XILINX FPGA for all of them. With the help of the flexible and programmable feature of FPGAs, same channel processor design has been used both for the SDD and the Anger Camera. Therefore, 19 channels for SDD and four channels for,, and corrected energy signals of an Anger Camera need 6 such modules. The data are transfered via the bus to the master event builder card with a rate of 100 MBps. The event builder card has a static RAM which stores up to 250 kevents. When the buffer is filled up to a programmable capacity, the data are transfered to a PC via parallel port. Fig. 6 shows the DAQ system for the whole setup.

4 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 the detector irradiated with a source. The average equivalent noise charge of the channels is about 40 electrons rms with a fluctuation of few electrons rms from channel to channel. This result is obtained at room temperature and with a shaping time of 100 ns. Fig. 7 shows the spectrum obtained at 10 with and sources. The resolution improves even further with shaping time of the order of 250 ns and - shaping but for this application a shorter shaping time is more desired than a better energy resolution. The resolution is good enough to discriminate Mn and lines even at short shaping times and at room temperature. The energy resolution was measured to be better with a single cell SDD [9] at room temperature and with the same readout electronics. This detector had a leakage current of 100 pa and the 19 cell SDD seems to have a higher leakage current which results in a degraded energy resolution. A proportional, integral, derivative (PID) controller has been designed and produced to control the Peltiers [12]. The warm side of the Peltiers is cooled by water circulating within the detector housing. In addition to water flux, a constant nitrogen flux is provided which prevents any condensation that may occur below dew temperatures. It is important to perform the measurements at constant temperature because not only the detector chip but also the transistors are sensitive to temperature changes. Stable temperatures are maintained by the PID system. The spectroscopic measurements were performed at various temperatures and the FWHM energy resolution of Mn line as a function of temperature is shown in Fig. 8. The resolution does not improve considerably below 10 due to the electronics noise limits. F. Coincidence Measurements The Compton electrons captured in silicon are taken as true events only when there is a coincidence with the Anger camera. The Anger camera (AC) is composed of a 5/8-in thick NaI with a radius of 10 in that is coupled to 37 photomultiplier tubes. 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 detectors were collected in continuous mode without enabling the coincidence logic. Fig. 9 shows the time coincidence curve obtained by subtracting these time stamps. The time stamp has a resolution of 15 ns and events within of - have been scanned. The coincidence events are concentrated in a 300-ns wide region which may result from the drift time of electrons and decay time of NaI. The signals were also examined by direct observation on the oscilloscope. The shaper output when the SDD is irradiated with source is shown in Fig. 10. The signal looks fine in terms of pole-zero cancellation, etc. However, when the detector is irradiated with source, various signal shapes (Fig. 11) are observed. The signal form shown in Fig. 11(a) occurs often when the energy deposited in the SDD is high. The signal with a double peak in Fig. 11(b) seems to be produced by double Compton events. Only few out of 2000 signals were observed to have such a shape which agrees with a small probability of having double Compton events in 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.

The electron cloud diffuses during its travel to the anode and it diffuses more when the interaction occurs at the edge of the cell where the electric field is weaker.

5 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 the electrons move toward the drain of the on chip JFET instead of moving toward the anode. The electron cloud diffuses during its travel to the anode and it diffuses more when the interaction occurs at the edge of the cell where the electric field is weaker. When they reach the anode region, some electrons could go to the transistor channel which is at more positive potential than the anode. In this case, they move away from the anode and this produces the negative tail of the signal. This type of signals was observed to occur more frequently if the Compton scattering happens in a region where the electric field is weaker and also when the energy of the incoming beam is higher. When the energy is higher, more charge is created, the first part of the cloud reaches the anode and this makes the anode more negative compared to the drain which causes the remaining part of the cloud to tend to be attracted by the transistor channel. These results should be compared with simulated signals to explain such signals more precisely. These signals may give some hints about the direction of the Compton electrons in silicon. II. CONCLUSION A detector system to track the Compton electrons in a silicon drift detector array has been designed and constructed. The determination of the direction and the position of electrons is crucial in many applications including Compton imaging in medicine and astrophysics. Good energy resolution, high rate capacity, and fast readout are promising features for this detector system. Initial measurements suggest the system is promising for tracking Compton electrons. The comparisons with signal simulations are necessary for more accurate conclusions.

6 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.

7 ACKNOWLEDGMENT The authors would like to thank to Dr. H. J. Besch and Dr. A. Rudge for their support on various technical issues. We are grateful to Dr. D. Gunter for proofreading the manuscript. REFERENCES [1]. [Online] [2] C. Fiorini and A. Longoni, In-situ, nondestructive identification of chemical elements by means of portable EDXRF spectrometer, IEEE Trans. Nucl. Sci., vol. 46, pp , Dec [3] C. Fiorini, A. Longoni, F. Perotti, C. Labanti, E. Rossi, P. Lechner, H. Soltau, and L. Strüder, A monolithic array of silicon, drift detectors for high-resolution gamma-ray imaging, IEEE Trans. Nucl. Sci., vol. 49, pp , June [4] K. Hansen and L. Tröger, A novel multicell silicon drift detector module for X-ray spectroscopy and imaging applications, IEEE Trans. Nucl. Sci., vol. 47, pp , Dec [5] T. Çonka-Nurdan, K. Nurdan, F. Constantinescu, B. Freisleben, N. A. Pavel, and A. H. Walenta, Impact of the detector parameters on a Compton camera, IEEE Trans. Nucl. Sci., pt. 1, vol. 49, pp , June [6] A. H. Walenta, A. B. Brill, A. Castoldi, T. Çonka-Nurdan, C. Guazzoni, K. Hartmann, A. Longoni, K. Nurdan, and L. Strüder, Vertex detection in a stack of Si-drift detectors for high resolution gamma-ray imaging, in IEEE Medical Imaging Conf. Proc., Portland, OR, Oct. 2003, submitted for publication. [7] W. L. Rogers, N. H. Clinthorne, and A. Bolozdynia, Compton Cameras for Nuclear Medical Imaging, 2001, ch. V3. [8] W. D. Farr and G. C. Smith, Emitter followers as low noise pre-amplifiers for gas proportional detectors, Nucl. Instrum. Methods, vol. 206, pp , [9] T. Çonka-Nurdan, K. Nurdan, A. H. Walenta, H. J. Besch, C. Fiorini, and N. A. Pavel, Silicon drift detector readout electronics for a Compton camera, Nucl. Instrum. Methods A 523, pp , [10] K. Nurdan, T. Çonka-Nurdan, H. J. Besch, B. Freisleben, N. A. Pavel, and A. H. Walenta, FPGA based data acquisition system for a Compton camera, in Proceedings of SAMBA (Symposium on Applications of Particle Detectors in Medicine, Biology and Astrophysics) II, Nucl. Instrum. Methods A 510, 2003, pp [11] K. Nurdan et al., Development of a Compton camera data acquisition system using FPGAs, in Proc. ISPC2003 (International Signal Processing Conference), vol. 406, Dallas, TX, Mar. Apr. 31 3, [12] K. Laihem, Effect of temperature on the perfprmance of Si-drift detector, analysis and technical implementation, Master Thesis, University of Siegen, 2004.

Silicon Drift Detector Readout Electronics for a Compton Camera

Silicon Drift Detector Readout Electronics for a Compton Camera arxiv:physics/0311019v1 [physics.ins-det] 5 Nov 2003 T. Çonka Nurdan a,, K. Nurdan c, A.H. Walenta a, H.J. Besch a, C. Fiorini b, B. Freisleben