- Email: [email protected]
Development of CdZnTe X-ray detectors at DSRI
Nuclear Instruments and Methods in Physics Research A 439 (2000) 625}633
Development of CdZnTe X-ray detectors at DSRI M.A.J. van Pamelen!, C. Budtz-J+rgensen",*, I. Kuvvetli" !VOF de G~s, Kalymnosdreef 228, 3562 XP Utrecht, The Netherlands "Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen East, Denmark
Abstract An overview of the development of CdZnTe X-ray detectors at the Danish Space Research Institute is presented. Initiated in the beginning of 1996, the main motivation at that time was to develop focal plane detectors for the novel type of hard X-ray telescopes, which are currently under study at DSRI. With the advent of the Danish Micro Satellite program it was, however, recognised that this type of detector is very well suited for two proposed missions (eXCALIBur, AXO). The research at DSRI has so far been concentrated on the spectroscopic properties of the CZT detector. At DSRI we have developed a technique, which, with the use of microstrip electrodes, is able to compensate for the signal loss caused by trapping of positive charge carriers. This technique leads to a dramatic improvement of the achievable energy resolution, even for crystals of poor quality. With the technique, hole trapping has now little in#uence and the spectrum displays a pronounced Gaussian peak at 661 keV with a width (FWHM) of 6.9 keV. Also a small peak produced by CdTe escape events can now be observed. At the same time, no events have to be rejected. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: X-ray detectors; Semiconductor detectors; Unipolar sensing; CdZnTe; Detector: modi"cation and application
1. Introduction The potential of X-ray and gamma-ray detectors based on heavy element semiconductor materials is now recognised in many "elds. One of the most promising materials is Cd(Zn)Te (CZT). The principal advantages of CZT material are high quantum e$ciency and no requirement for cooling to cryogenic temperatures. CZT detector systems will be very compact and coupled with modern electronics which will require a minimum of power consumption. These properties make the CZT
detector very interesting in relation to space applications and ideally suited for a small-sized satellite. DSRI initiated a CZT development program in the beginning of 1996. The main motivation at that time was to develop focal plane detectors for the novel type of hard X-ray telescope [1] which is currently being studied at DSRI. With the advent of the Danish Small Satellite program it was however recognised that this type of detector is very well suited for two proposed missions: eXCALIBur and AXO:
* Corresponding author: Tel.: #45-3532-5726; fax: #45-35362475. E-mail address: [email protected] (C. Budtz-J+rgensen)
(a) eXCALIBur: This is a dedicated calibration mission with the aim of establishing accurate standard sources on the X-ray sky. It consists of a simple pinhole camera with a 100 cm2 CZT
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 8 3 9 - 6
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detector array and X-rays in the 1}100 keV band are observed through a 50 cm2 aperture. It will be the main purpose of the mission to determine the Crab Nebula X-ray spectrum with an accuracy of a few percent. (b) AXO: This mission is dedicated to the observation of X-rays generated in the earth's atmosphere. Of special interest will be simultaneous optical and X-ray observations of sprites which are massive but weak luminous #ashes that appear directly above an active thunderstorm system and are coincident with lightning strokes. They have only recently been documented using low lightlevel television technology [2]. AXO will contain an X-ray imager using a two-dimensional CZT detector array (21]21 pixels and total area of 900 cm2) in combination with a two-dimensional coded mask. It is planned to observe for sprite correlated X-ray events in the band from 3 to 300 keV. The research at DSRI has so far been concentrated on the spectroscopic properties of the CZT detector. Although the quality of CZT semiconductor material has improved substantially in recent years it is still a drawback that most CZT semiconductors contain defects, which can trap the charge carriers generated by the ionising radiation. This will perturb the signal generation and as consequence reduce the energy resolution of the detector. The problem is most severe for holes, which have trapping probabilities that are more than an order of magnitude higher than the one of the electrons. Various methods already exist to diminish the e!ects caused by hole trapping. The methods can be divided in two groups: The events with a large contribution of holes are distinguished and can be rejected or the contribution of the holes to the signal is reduced. Examples of the "rst are rise-time discrimination [3], and dual shaping time, Ref.[4]. With these techniques, a large number of events are rejected to obtain a good spectral resolution. An example of reducing the sensitivity to the contribution of holes is the coplanar technique [5]. This is achieved by subtracting the signals on two sets of coplanar strips yielding a signal that is nearly independent of holes.
At DSRI, a technique was developed, which, with the use of microstrip electrodes, has two characteristics: (1) the sensitivity to the holes is reduced; and (2) compensation for the loss of holes is possible. This technique leads to a dramatic improvement of the achievable energy resolution, even for crystals of poor quality. Especially for space applications, it is important that the technique does not require any event rejection. The sensitivity to the holes is reduced by placing microstrips between the anode strips that screen the anodes form the holes. In the extreme case, only one anode strip is used, and all these cathode strips on both sides act as screens for the in#uence of the holes. In the extreme case the screening strips are biased in such a way that the electrons move toward the anode strip. To compensate for the loss of holes, the signal on the planar electrode is also measured. This signal is still in#uenced strongly by the holes. With the combination of both, signal on the planar electrode and the signal on the anode strips, it is possible to correct for the contribution of the holes that is still present in the signal on the anode strip. Fig. 1 shows a cross-section of a detector using the principle of cathode strips.
2. Theoretical principles 2.1. Induced charge To determine the e!ect of cathode strips between the anode strips, the induced charge on the anode strips is calculated. In general, the total induced charge on an electrode q is the sum of the */$,505 contributions of the electrons q and the holes */$,/ q : */$,1 q (E; x)"q (E; x)#q (E; x) (1) */$,505 */$,/ */$,1 where x is the position of the absorbed photon in the crystal. The photon energy, E, determines the size of the primary charge cloud Q(E; x). The induced charge on an electrode can be calculated by the theorem of Ramo that uses the weighing potential [6], in which the weighing potential < W is de"ned as the potential that would exist in the detector with the selected electrode at `unita
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627
Fig. 1. Schematic view of strip detector with cathode strip.
potential, while holding all other electrodes at zero potential. The weighing electric "eld, E , is the W gradient of the weighing potential. The induced charge q at an electrode is the charge of the */$ charge cloud Q(E; x, s) times the weighing electric "eld integrated over the path of the charge cloud:
P
q (E; x)" Q(E; x, s)E (s) ds */$ W
(2)
where s is the centre of charge position of the charge cloud in the crystal. When a uniform weighing "eld and a uniform distribution of charge traps are assumed, this equation reduces to the so-called Hecht relations [7]. To illustrate the contribution of di!erent charge carriers on the detector electrodes, charge trapping is neglected. Then the induced charge becomes independent of the actual path of the charge carrier: q (E; x)"Q(E)*< (x). (3) */$ W The potential di!erence *< is the di!erence in W weighing potential between the position where the photon was absorbed and the electrode where the charge carriers are collected. Since the induced charge is measured on the electrode where the electrons are collected, this electrode has a weighing potential of one, while it is zero on all other electrodes. Thus, the fraction induced by the electrons f , and by the holes /
f depends only on the < (x): 1 W f (x)"1!< (x) / W f (x)"< (x). (4) 1 W As an example, the contribution of each charge carrier is calculated for a CdZnTe strip detector. The 15 anode strips were 15 lm wide with a pitch of 300 lm. The cathode strips were varied from 15 to 150 lm. All strips were 3 mm long. The CdZnTe crystal had a thickness of 1.5 mm and an area of 7]15 mm2. The weighing potential was calculated with the simulation program ELFI [8]. Across the strips, the weighing potential varies less than 1% when the distances to the strip electrodes become larger than 200 lm. For simplicity the x is therefore reduced to a one-dimensional position x on the line that connects the centre of a read-out strip with the planar electrode with x"0 at the planar electrode. Fig. 2a and b show the contribution from the holes and the electrons as a function of x. Compared to a planar electrode, the cathode strips always reduce the in#uence of the holes on the signal. Throughout the crystal, the induced charge depends mostly on the contributions of the electrons. Only when the photon is absorbed in the last 15% of the crystal, does the contribution of the electrons drop drastically. An increase of the width from 15 to 150 lm decreases the contribution of the holes
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Fig. 2. The total induced charge q as a function of the position x where the photon is absorbed. (a) Contribution of the electrons. (b) */$ Contribution of the holes.
from 40 to 20%. If only a single anode strip is used, the screening is even better, since the area of all other strips screens the anode strip. 2.2. Charge trapping Since the strip signal, q , is less sensitive to the 453*1 holes, it is also less sensitive to the hole trapping than q . Moreover, combining q and q , 1-!/!3 453*1 1-!/!3 it is possible to determine the hole trapping in the strip detector and accurately correct q for it. 453*1 Since the induced signal on the cathode strip can be bipolar, only q and q are used. 453*1 1-!/!3 In order to quantify the hole trapping, the ratio R,q /q is formed. This quantity is indepe1-!/!3 453*1 ndent of the photon energy. The correlation between R and q , was calculated assuming that the 453*1 charge trapping can be described by the following equation: Q(E; x, s)"Q (E) e~jd(x,s)
(5)
where the trapping per unit length j is uniform. The trapping of holes per unit length is j while j is 1 % trapping of electrons per unit length. The path length between x and s is d(x, s). Applying Eqs. (1) and (2) it is possible to calculate both, the
strip signal, q , and the signal, q , of the 453*1 1-!/!3 planar electrode. When no trapping is present (j "j "0), the maximum value, q , is reached p e .!9 for both q and q . 1-!/!3 453*1 The e!ect of trapping for the strip patterns discussed in Section 2.1, is shown in Fig. 3. As an example, the result for the strip with 150 lm wide cathode strips is given. In Fig. 3a, the correlation between q /q and R calculated for di!erent 453*1 .!9 values of j is shown. Electron trapping is neglect1 ed, j "0. It is worth noticing that correlated % values of R and j fall on a single curve that is 1 independent of j but determined only by the strip 1 pattern. However, for large j values, R is distrib1 uted between one and zero while this distribution becomes narrower for small j values. But, inde1 pendently of the actual values of j , the curve 1 allows determination of the in#uence of hole trapping on the strip signal accurately and thereby also the correction for it. This is not the case if electron trapping is not uniform. The correlation of R and q /q is 453*1 .!9 shown in Fig. 3b for di!erent values of j while the % contribution of holes is neglected, j "R. The 1 q /q and R relation is obviously not indepen453*1 .!9 dent of j . Thus, q cannot be corrected for % 453*1
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Fig. 3. The relation between ratio R and the induced charge on the strips q /q , with q as the induced charge when neither hole 453*14 .!9 .!9 nor electron trapping is present. (a) Dependence on the hole trapping j ; (b) dependence on electron trapping j . 1 %
electron trapping and, unless j is constant, the % correction for j will be less accurate. 1 3. Experimental procedure Two patterns of microstrips were deposited on a CdZnTe crystal to demonstrate the e!ect of cathode strips in between the anode strips. The "rst
pattern used cathode strips between 15 anode strips, where the cathode strips were already interconnected on the crystal (see Fig. 1). The anode strips were 15 lm wide and had a pitch of 300 lm. The cathode strips were 150 lm wide. The average resistance between the anode strips and the other strips was 17 G)/strip. For the single anode strip measurements, a second pattern of 20 anode strips with a width of 40 lm and a pitch of 100 lm, was
Fig. 4. Schematic view of strip detector in a single anode strip con"guration.
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Fig. 5. Spectra of an 241Am source. (a) Planar electrode (multiple anode measurement); (b) planar electrode (single anode measurement); (c) multiple anode strips; (d) single anode strip.
deposited on another area of the CdZnTe crystal. The average resistance between two strips was 7.38 G). The length of the strips was 3 mm for both patterns. A gold layer was deposited as a planar electrode on the backside of the CdZnTe crystal. The CdZnTe crystal was 1.5 mm thick and had an area of 7]15 mm2. The crystal was glued on an aluminium oxide substrate with a fan-out electrode. The strips were wire bonded to the fan-out electrode connected to a PCB. A #at cable connector was used to connect the PCB to an eV Product
9049 preampli"er. An Ortec 423c preampli"er was connected to the planar electrode. The signals were Gaussian shaped with a shaping time of 1 ls and the FAST multiparameter system was used to obtain the pulse-height spectra. All the anode strips of the "rst pattern were interconnected to measure directly the summed signal of all anode strips. However, one anode strip was not connected because it made short circuit with the intermediate strips due to a lithographic error. The voltage on the planar electrode, < , B*!4
M.A.J. van Pamelen et al. / Nuclear Instruments and Methods in Physics Research A 439 (2000) 625}633
was !200 V, and the voltage on the cathode strips was !30 V. The anode strips were held at ground potential. Fig. 1 shows a schematic diagram of this set-up. In this con"guration, almost all electrons were collected on the anode strips, except for events occurring near the edge of the strip pattern. The single anode strip detector was operated as a drift detector to ensure the collection of the electrons on the anode strip. Creating a voltage di!erence of !20 V between adjacent strips achieved this. The bias of the planar electrode, < , was B*!4 !200 V, while the anode strip was held at ground potential. Fig. 4 shows a schematic diagram of the set-up. The voltage divider uses nine resistors of 2 M), which is much smaller than the inter strip resistance of 7.38 G). Calculations of the electric "elds inside the detector con"rmed that electrons created in the active volume de"ned by the 15 strips would drift to the central anode strip. Measurements done with the 241Am source used a beam collimated to 4 mm2 in order to ensure that only active the volume was illuminated. This was not possible for the measurements done with the 137Cs source. However, events for which the "eld lines do not end on the central strip will not generate a signal here and will therefore not be analysed.
631
The smaller area of the single anode detector explains the lower electronic noise on the planar electrode. The small area and the low leakage current for the single anode strip explain its low electronic noise. Fig. 5 compares the spectra obtained with the 241Am source. Fig. 5a and b, which were measured at the planar electrodes, shows clearly that the crystal su!ers from poor hole collection. The poor hole collection results in the low-energy tailing observed for the 59.6 keV line. For the multiple anode measurement, the resolution improved signi"cantly as can be seen in Fig. 5c. However, the resolution becomes completely electronic noise limited for the Table 1 Energy resolution for di!erent con"gurations Measurements
FWHM at 59.6 keV (%)
Planar electrode (multiple anode measurement) Planar electrode (single anode measurement) Multiple anode Single anode
24.1 18.5 15.1 5.4
4. Measurements The two detectors were illuminated with photons from an 241Am source in order to investigate the behaviour in the X-ray energy band. In both cases the planar electrodes faced the source. A valid event required a signal on the strip electrode and a coincident signal on the planar electrode. For the multiple anode strip measurements (pattern 1), the electronic noise for the interconnected anode strips was 9.0 keV FWHM. The leakage current contributed 5.7 keV FWHM. The rest of the electronic noise was caused by stray capacity and pick-up noise in the set-up. The electronic noise for the planar signal corresponded to 6.7 keV FWHM. For the single anode strip detector (pattern 2), the electronic noise on the planar electrode was 4.68 and 3.1 keV FWHM on the anode strip.
Fig. 6. Two-dimensional spectrum of a 137Cs source as a function of ratio R and the photon energy measured on the single anode strip.
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Fig. 7. Spectra of an 137Cs source showing the di!erent improvements that can be achieved. (a) Planar electrode; (b) single anode strip; (c) single anode strip with correction for hole trapping.
single anode measurement shown in Fig. 5d. It should be noted that single anode situation has lower electronic noise and also has lesser sensitivity to the contribution of holes. The measured energy resolutions are given in Table 1. The behaviour of the strip detector at higher photon energies was investigated with the help of a 137Cs source. The planar electrode spectra were completely washed out due to the large variation of the photon interaction depth, x. The cathode strip spectra were dramatically improved and could be corrected for hole trapping as described in the following. The measured correlation between R and q for the single anode con"guration is shown in 453*1 Fig. 6. The logarithmic grey scale indicates the
number of counts. Channel 205 corresponds to an R-value of 1.0. The slope of the R and q curve 453*1 agrees well with the calculations shown in Fig. 3a. Since R is distributed from small values up to 1.0, j must be large. The one-to-one relation between 1 R and q can be used to correct for the hole 453*1 trapping. In Fig. 7, the di!erent stages of improvements that are achieved for the 137Cs peak at 661 keV are shown. No peak could be observed with the planar electrode (Fig. 7a), while the peak has a width of 17.9 keV for the single anode strip detector (Fig. 7b). In Fig. 7c, the resolution is improved further to 6.9 keV after correction for hole trapping. The peak-to-valley ratio is 40:1 and a peak-toCompton ratio is 4:1.
M.A.J. van Pamelen et al. / Nuclear Instruments and Methods in Physics Research A 439 (2000) 625}633
5. Conclusion It has been demonstrated that strip technology can improve the spectral resolution of the CdZnTe detector at energies above 40 keV drastically. It was also demonstrated that the spectral resolution was improved further by correcting for hole trapping. These corrections were made using both the planar electrode signals and the strip signals. But although the resolution increased considerably, there is still place for further improvement. Since the resolution was mainly limited by the electronic noise and the poor quality of the CdZnTe material, strips will be deposited on spectroscopic grade CdZnTe materials in the near future and more care will be taken to achieve low electronic noise. This will in our opinion lead to very good spectral resolutions over a large dynamic range that is Fano factor limited at high X-ray energies. At DSRI, the next phase of CdZnTe detector development will be the application of ASICs (VA32C from IDE A/S) for the read-out of the
633
CdZnTe strip detectors. Such read-out is needed for the eXCALIBur mission and the AXO mission, two candidates in the Danish Micro Satellite Program.
References [1] F.E. Christensen, Multilayer mirrors for future X-ray missions, Leicester Special Report XRA97/02, 1997, p. 133. [2] S.B. Mende, D.D. Sentman, E.M. Wescott, Lightning between earth and space, Sci. American, August 1997, p. 36. [3] V.I. Ivanov et al., IEEE Trans. Nucl. Sci. NS-42 (4) (1995) 258. [4] J.C. Lund et al., IEEE Trans. Nucl. Sci.-NS 43 (3) (Part II of III) (1996) 1411. [5] P.N. Luke, IEEE Trans. Nucl. Sci.-Ns 42 (4) (1995) 207. [6] S. Ramo, Current induced by electron motion, Proceedings of the IRE, Vol. 27, Sept. 1939, pp. 584-585. [7] See for example, T.E. Schlesinger, R.B. James, Semiconductors and Semimetals Vol. 43, Academic Press, New York, 1995. [8] SiR- Simulated Reality, Benutzerhandbuch ELFI, Ein Programm zur numerischen Berechnung zweidimensionaler elektrischer Felder, Siegen, 1993.
VII. OTHER DETECTORS
Development of CdZnTe X-ray detectors at DSRI M.A.J. van Pamelen!, C. Budtz-J+rgensen",*, I. Kuvvetli" !VOF de G~s, Kalymnosdreef 228, 3562 XP Utrecht, The Netherlands "Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen East, Denmark
Abstract An overview of the development of CdZnTe X-ray detectors at the Danish Space Research Institute is presented. Initiated in the beginning of 1996, the main motivation at that time was to develop focal plane detectors for the novel type of hard X-ray telescopes, which are currently under study at DSRI. With the advent of the Danish Micro Satellite program it was, however, recognised that this type of detector is very well suited for two proposed missions (eXCALIBur, AXO). The research at DSRI has so far been concentrated on the spectroscopic properties of the CZT detector. At DSRI we have developed a technique, which, with the use of microstrip electrodes, is able to compensate for the signal loss caused by trapping of positive charge carriers. This technique leads to a dramatic improvement of the achievable energy resolution, even for crystals of poor quality. With the technique, hole trapping has now little in#uence and the spectrum displays a pronounced Gaussian peak at 661 keV with a width (FWHM) of 6.9 keV. Also a small peak produced by CdTe escape events can now be observed. At the same time, no events have to be rejected. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: X-ray detectors; Semiconductor detectors; Unipolar sensing; CdZnTe; Detector: modi"cation and application
1. Introduction The potential of X-ray and gamma-ray detectors based on heavy element semiconductor materials is now recognised in many "elds. One of the most promising materials is Cd(Zn)Te (CZT). The principal advantages of CZT material are high quantum e$ciency and no requirement for cooling to cryogenic temperatures. CZT detector systems will be very compact and coupled with modern electronics which will require a minimum of power consumption. These properties make the CZT
detector very interesting in relation to space applications and ideally suited for a small-sized satellite. DSRI initiated a CZT development program in the beginning of 1996. The main motivation at that time was to develop focal plane detectors for the novel type of hard X-ray telescope [1] which is currently being studied at DSRI. With the advent of the Danish Small Satellite program it was however recognised that this type of detector is very well suited for two proposed missions: eXCALIBur and AXO:
* Corresponding author: Tel.: #45-3532-5726; fax: #45-35362475. E-mail address: [email protected] (C. Budtz-J+rgensen)
(a) eXCALIBur: This is a dedicated calibration mission with the aim of establishing accurate standard sources on the X-ray sky. It consists of a simple pinhole camera with a 100 cm2 CZT
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 8 3 9 - 6
VII. OTHER DETECTORS
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detector array and X-rays in the 1}100 keV band are observed through a 50 cm2 aperture. It will be the main purpose of the mission to determine the Crab Nebula X-ray spectrum with an accuracy of a few percent. (b) AXO: This mission is dedicated to the observation of X-rays generated in the earth's atmosphere. Of special interest will be simultaneous optical and X-ray observations of sprites which are massive but weak luminous #ashes that appear directly above an active thunderstorm system and are coincident with lightning strokes. They have only recently been documented using low lightlevel television technology [2]. AXO will contain an X-ray imager using a two-dimensional CZT detector array (21]21 pixels and total area of 900 cm2) in combination with a two-dimensional coded mask. It is planned to observe for sprite correlated X-ray events in the band from 3 to 300 keV. The research at DSRI has so far been concentrated on the spectroscopic properties of the CZT detector. Although the quality of CZT semiconductor material has improved substantially in recent years it is still a drawback that most CZT semiconductors contain defects, which can trap the charge carriers generated by the ionising radiation. This will perturb the signal generation and as consequence reduce the energy resolution of the detector. The problem is most severe for holes, which have trapping probabilities that are more than an order of magnitude higher than the one of the electrons. Various methods already exist to diminish the e!ects caused by hole trapping. The methods can be divided in two groups: The events with a large contribution of holes are distinguished and can be rejected or the contribution of the holes to the signal is reduced. Examples of the "rst are rise-time discrimination [3], and dual shaping time, Ref.[4]. With these techniques, a large number of events are rejected to obtain a good spectral resolution. An example of reducing the sensitivity to the contribution of holes is the coplanar technique [5]. This is achieved by subtracting the signals on two sets of coplanar strips yielding a signal that is nearly independent of holes.
At DSRI, a technique was developed, which, with the use of microstrip electrodes, has two characteristics: (1) the sensitivity to the holes is reduced; and (2) compensation for the loss of holes is possible. This technique leads to a dramatic improvement of the achievable energy resolution, even for crystals of poor quality. Especially for space applications, it is important that the technique does not require any event rejection. The sensitivity to the holes is reduced by placing microstrips between the anode strips that screen the anodes form the holes. In the extreme case, only one anode strip is used, and all these cathode strips on both sides act as screens for the in#uence of the holes. In the extreme case the screening strips are biased in such a way that the electrons move toward the anode strip. To compensate for the loss of holes, the signal on the planar electrode is also measured. This signal is still in#uenced strongly by the holes. With the combination of both, signal on the planar electrode and the signal on the anode strips, it is possible to correct for the contribution of the holes that is still present in the signal on the anode strip. Fig. 1 shows a cross-section of a detector using the principle of cathode strips.
2. Theoretical principles 2.1. Induced charge To determine the e!ect of cathode strips between the anode strips, the induced charge on the anode strips is calculated. In general, the total induced charge on an electrode q is the sum of the */$,505 contributions of the electrons q and the holes */$,/ q : */$,1 q (E; x)"q (E; x)#q (E; x) (1) */$,505 */$,/ */$,1 where x is the position of the absorbed photon in the crystal. The photon energy, E, determines the size of the primary charge cloud Q(E; x). The induced charge on an electrode can be calculated by the theorem of Ramo that uses the weighing potential [6], in which the weighing potential < W is de"ned as the potential that would exist in the detector with the selected electrode at `unita
M.A.J. van Pamelen et al. / Nuclear Instruments and Methods in Physics Research A 439 (2000) 625}633
627
Fig. 1. Schematic view of strip detector with cathode strip.
potential, while holding all other electrodes at zero potential. The weighing electric "eld, E , is the W gradient of the weighing potential. The induced charge q at an electrode is the charge of the */$ charge cloud Q(E; x, s) times the weighing electric "eld integrated over the path of the charge cloud:
P
q (E; x)" Q(E; x, s)E (s) ds */$ W
(2)
where s is the centre of charge position of the charge cloud in the crystal. When a uniform weighing "eld and a uniform distribution of charge traps are assumed, this equation reduces to the so-called Hecht relations [7]. To illustrate the contribution of di!erent charge carriers on the detector electrodes, charge trapping is neglected. Then the induced charge becomes independent of the actual path of the charge carrier: q (E; x)"Q(E)*< (x). (3) */$ W The potential di!erence *< is the di!erence in W weighing potential between the position where the photon was absorbed and the electrode where the charge carriers are collected. Since the induced charge is measured on the electrode where the electrons are collected, this electrode has a weighing potential of one, while it is zero on all other electrodes. Thus, the fraction induced by the electrons f , and by the holes /
f depends only on the < (x): 1 W f (x)"1!< (x) / W f (x)"< (x). (4) 1 W As an example, the contribution of each charge carrier is calculated for a CdZnTe strip detector. The 15 anode strips were 15 lm wide with a pitch of 300 lm. The cathode strips were varied from 15 to 150 lm. All strips were 3 mm long. The CdZnTe crystal had a thickness of 1.5 mm and an area of 7]15 mm2. The weighing potential was calculated with the simulation program ELFI [8]. Across the strips, the weighing potential varies less than 1% when the distances to the strip electrodes become larger than 200 lm. For simplicity the x is therefore reduced to a one-dimensional position x on the line that connects the centre of a read-out strip with the planar electrode with x"0 at the planar electrode. Fig. 2a and b show the contribution from the holes and the electrons as a function of x. Compared to a planar electrode, the cathode strips always reduce the in#uence of the holes on the signal. Throughout the crystal, the induced charge depends mostly on the contributions of the electrons. Only when the photon is absorbed in the last 15% of the crystal, does the contribution of the electrons drop drastically. An increase of the width from 15 to 150 lm decreases the contribution of the holes
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Fig. 2. The total induced charge q as a function of the position x where the photon is absorbed. (a) Contribution of the electrons. (b) */$ Contribution of the holes.
from 40 to 20%. If only a single anode strip is used, the screening is even better, since the area of all other strips screens the anode strip. 2.2. Charge trapping Since the strip signal, q , is less sensitive to the 453*1 holes, it is also less sensitive to the hole trapping than q . Moreover, combining q and q , 1-!/!3 453*1 1-!/!3 it is possible to determine the hole trapping in the strip detector and accurately correct q for it. 453*1 Since the induced signal on the cathode strip can be bipolar, only q and q are used. 453*1 1-!/!3 In order to quantify the hole trapping, the ratio R,q /q is formed. This quantity is indepe1-!/!3 453*1 ndent of the photon energy. The correlation between R and q , was calculated assuming that the 453*1 charge trapping can be described by the following equation: Q(E; x, s)"Q (E) e~jd(x,s)
(5)
where the trapping per unit length j is uniform. The trapping of holes per unit length is j while j is 1 % trapping of electrons per unit length. The path length between x and s is d(x, s). Applying Eqs. (1) and (2) it is possible to calculate both, the
strip signal, q , and the signal, q , of the 453*1 1-!/!3 planar electrode. When no trapping is present (j "j "0), the maximum value, q , is reached p e .!9 for both q and q . 1-!/!3 453*1 The e!ect of trapping for the strip patterns discussed in Section 2.1, is shown in Fig. 3. As an example, the result for the strip with 150 lm wide cathode strips is given. In Fig. 3a, the correlation between q /q and R calculated for di!erent 453*1 .!9 values of j is shown. Electron trapping is neglect1 ed, j "0. It is worth noticing that correlated % values of R and j fall on a single curve that is 1 independent of j but determined only by the strip 1 pattern. However, for large j values, R is distrib1 uted between one and zero while this distribution becomes narrower for small j values. But, inde1 pendently of the actual values of j , the curve 1 allows determination of the in#uence of hole trapping on the strip signal accurately and thereby also the correction for it. This is not the case if electron trapping is not uniform. The correlation of R and q /q is 453*1 .!9 shown in Fig. 3b for di!erent values of j while the % contribution of holes is neglected, j "R. The 1 q /q and R relation is obviously not indepen453*1 .!9 dent of j . Thus, q cannot be corrected for % 453*1
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Fig. 3. The relation between ratio R and the induced charge on the strips q /q , with q as the induced charge when neither hole 453*14 .!9 .!9 nor electron trapping is present. (a) Dependence on the hole trapping j ; (b) dependence on electron trapping j . 1 %
electron trapping and, unless j is constant, the % correction for j will be less accurate. 1 3. Experimental procedure Two patterns of microstrips were deposited on a CdZnTe crystal to demonstrate the e!ect of cathode strips in between the anode strips. The "rst
pattern used cathode strips between 15 anode strips, where the cathode strips were already interconnected on the crystal (see Fig. 1). The anode strips were 15 lm wide and had a pitch of 300 lm. The cathode strips were 150 lm wide. The average resistance between the anode strips and the other strips was 17 G)/strip. For the single anode strip measurements, a second pattern of 20 anode strips with a width of 40 lm and a pitch of 100 lm, was
Fig. 4. Schematic view of strip detector in a single anode strip con"guration.
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Fig. 5. Spectra of an 241Am source. (a) Planar electrode (multiple anode measurement); (b) planar electrode (single anode measurement); (c) multiple anode strips; (d) single anode strip.
deposited on another area of the CdZnTe crystal. The average resistance between two strips was 7.38 G). The length of the strips was 3 mm for both patterns. A gold layer was deposited as a planar electrode on the backside of the CdZnTe crystal. The CdZnTe crystal was 1.5 mm thick and had an area of 7]15 mm2. The crystal was glued on an aluminium oxide substrate with a fan-out electrode. The strips were wire bonded to the fan-out electrode connected to a PCB. A #at cable connector was used to connect the PCB to an eV Product
9049 preampli"er. An Ortec 423c preampli"er was connected to the planar electrode. The signals were Gaussian shaped with a shaping time of 1 ls and the FAST multiparameter system was used to obtain the pulse-height spectra. All the anode strips of the "rst pattern were interconnected to measure directly the summed signal of all anode strips. However, one anode strip was not connected because it made short circuit with the intermediate strips due to a lithographic error. The voltage on the planar electrode, < , B*!4
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was !200 V, and the voltage on the cathode strips was !30 V. The anode strips were held at ground potential. Fig. 1 shows a schematic diagram of this set-up. In this con"guration, almost all electrons were collected on the anode strips, except for events occurring near the edge of the strip pattern. The single anode strip detector was operated as a drift detector to ensure the collection of the electrons on the anode strip. Creating a voltage di!erence of !20 V between adjacent strips achieved this. The bias of the planar electrode, < , was B*!4 !200 V, while the anode strip was held at ground potential. Fig. 4 shows a schematic diagram of the set-up. The voltage divider uses nine resistors of 2 M), which is much smaller than the inter strip resistance of 7.38 G). Calculations of the electric "elds inside the detector con"rmed that electrons created in the active volume de"ned by the 15 strips would drift to the central anode strip. Measurements done with the 241Am source used a beam collimated to 4 mm2 in order to ensure that only active the volume was illuminated. This was not possible for the measurements done with the 137Cs source. However, events for which the "eld lines do not end on the central strip will not generate a signal here and will therefore not be analysed.
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The smaller area of the single anode detector explains the lower electronic noise on the planar electrode. The small area and the low leakage current for the single anode strip explain its low electronic noise. Fig. 5 compares the spectra obtained with the 241Am source. Fig. 5a and b, which were measured at the planar electrodes, shows clearly that the crystal su!ers from poor hole collection. The poor hole collection results in the low-energy tailing observed for the 59.6 keV line. For the multiple anode measurement, the resolution improved signi"cantly as can be seen in Fig. 5c. However, the resolution becomes completely electronic noise limited for the Table 1 Energy resolution for di!erent con"gurations Measurements
FWHM at 59.6 keV (%)
Planar electrode (multiple anode measurement) Planar electrode (single anode measurement) Multiple anode Single anode
24.1 18.5 15.1 5.4
4. Measurements The two detectors were illuminated with photons from an 241Am source in order to investigate the behaviour in the X-ray energy band. In both cases the planar electrodes faced the source. A valid event required a signal on the strip electrode and a coincident signal on the planar electrode. For the multiple anode strip measurements (pattern 1), the electronic noise for the interconnected anode strips was 9.0 keV FWHM. The leakage current contributed 5.7 keV FWHM. The rest of the electronic noise was caused by stray capacity and pick-up noise in the set-up. The electronic noise for the planar signal corresponded to 6.7 keV FWHM. For the single anode strip detector (pattern 2), the electronic noise on the planar electrode was 4.68 and 3.1 keV FWHM on the anode strip.
Fig. 6. Two-dimensional spectrum of a 137Cs source as a function of ratio R and the photon energy measured on the single anode strip.
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Fig. 7. Spectra of an 137Cs source showing the di!erent improvements that can be achieved. (a) Planar electrode; (b) single anode strip; (c) single anode strip with correction for hole trapping.
single anode measurement shown in Fig. 5d. It should be noted that single anode situation has lower electronic noise and also has lesser sensitivity to the contribution of holes. The measured energy resolutions are given in Table 1. The behaviour of the strip detector at higher photon energies was investigated with the help of a 137Cs source. The planar electrode spectra were completely washed out due to the large variation of the photon interaction depth, x. The cathode strip spectra were dramatically improved and could be corrected for hole trapping as described in the following. The measured correlation between R and q for the single anode con"guration is shown in 453*1 Fig. 6. The logarithmic grey scale indicates the
number of counts. Channel 205 corresponds to an R-value of 1.0. The slope of the R and q curve 453*1 agrees well with the calculations shown in Fig. 3a. Since R is distributed from small values up to 1.0, j must be large. The one-to-one relation between 1 R and q can be used to correct for the hole 453*1 trapping. In Fig. 7, the di!erent stages of improvements that are achieved for the 137Cs peak at 661 keV are shown. No peak could be observed with the planar electrode (Fig. 7a), while the peak has a width of 17.9 keV for the single anode strip detector (Fig. 7b). In Fig. 7c, the resolution is improved further to 6.9 keV after correction for hole trapping. The peak-to-valley ratio is 40:1 and a peak-toCompton ratio is 4:1.
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5. Conclusion It has been demonstrated that strip technology can improve the spectral resolution of the CdZnTe detector at energies above 40 keV drastically. It was also demonstrated that the spectral resolution was improved further by correcting for hole trapping. These corrections were made using both the planar electrode signals and the strip signals. But although the resolution increased considerably, there is still place for further improvement. Since the resolution was mainly limited by the electronic noise and the poor quality of the CdZnTe material, strips will be deposited on spectroscopic grade CdZnTe materials in the near future and more care will be taken to achieve low electronic noise. This will in our opinion lead to very good spectral resolutions over a large dynamic range that is Fano factor limited at high X-ray energies. At DSRI, the next phase of CdZnTe detector development will be the application of ASICs (VA32C from IDE A/S) for the read-out of the
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CdZnTe strip detectors. Such read-out is needed for the eXCALIBur mission and the AXO mission, two candidates in the Danish Micro Satellite Program.
References [1] F.E. Christensen, Multilayer mirrors for future X-ray missions, Leicester Special Report XRA97/02, 1997, p. 133. [2] S.B. Mende, D.D. Sentman, E.M. Wescott, Lightning between earth and space, Sci. American, August 1997, p. 36. [3] V.I. Ivanov et al., IEEE Trans. Nucl. Sci. NS-42 (4) (1995) 258. [4] J.C. Lund et al., IEEE Trans. Nucl. Sci.-NS 43 (3) (Part II of III) (1996) 1411. [5] P.N. Luke, IEEE Trans. Nucl. Sci.-Ns 42 (4) (1995) 207. [6] S. Ramo, Current induced by electron motion, Proceedings of the IRE, Vol. 27, Sept. 1939, pp. 584-585. [7] See for example, T.E. Schlesinger, R.B. James, Semiconductors and Semimetals Vol. 43, Academic Press, New York, 1995. [8] SiR- Simulated Reality, Benutzerhandbuch ELFI, Ein Programm zur numerischen Berechnung zweidimensionaler elektrischer Felder, Siegen, 1993.
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