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First results on the charge collection properties of segmented detectors made with p-type bulk silicon
Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
First results on the charge collection properties of segmented detectors made with p-type bulk silicon G. Casse*, P.P. Allport, T.J.V. Bowcock, A. Greenall, M. Hanlon, J.N. Jackson Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxford street, L69 7ZE Liverpool, UK Received 3 May 2001; received in revised form 11 December 2001; accepted 15 December 2001
Abstract Radiation damage of n-type bulk detectors introduces stable defects acting as effective p-type doping and leads to the change of the conductivity type of the silicon substrate (type inversion) after a fluence of a few times 1013 protons cm 2. The diode junction after inversion migrates from the original side to the back plane of the detector. The migration of the junction can be prevented using silicon detectors with p-type substrates. Furthermore, the use of n-side readout gives higher charge collection efficiency for segmented devices operated below the full depletion voltage. Large area (E 6.4 6.4 cm2) capacitively coupled 80 mm pitch detectors using polysilicon bias resistors have been fabricated on p-type substrates (n-in-p diode structure). These detectors have been irradiated with 24 GeV/c protons to an integrated fluence of 3 1014 cm 2 and kept for 7 days at 251C to reach the broad minimum of the annealing curve. Results are presented on the comparison of their charge collection properties with detectors using p-strip read-out after corresponding dose and annealing. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Wk; 29.40.Gx; 61.82.Fk Keywords: Silicon microstrip detectors; Charge collection efficiency; Radiation hardness
1. Introduction To date, n-type substrate with implanted p strips (p-in-n) is the most commonly used configuration for silicon detectors used in high-energy physics applications. Such devices have the advantage of not requiring dedicated processing steps to isolate each strip from its neighbours. The alternative of n-strips implanted in p-bulk silicon (n-in-p) requires either an isolation implant surrounding the read-out strip (p-stop) or the use of the p-spray *Corresponding author. Tel./fax: +44-151-794-3399. E-mail address: [email protected] (G. Casse).
technique [1]. These prevent shorts between neighbouring strips due to conductive channels induced through positive charge trapped at the dielectric bulk interface. This results in an increased complication for the mask design and additional cost in the production process. However, radiation damage introduces acceptor-like defects, so the use of p-type bulk silicon prevents the migration of the junction after irradiation. This permits read out of the detectors from the junction (i.e. higher electric field) side for the entire life of the detector. The complications and added cost implied by double-side processing mitigate against the alternative n-in-n solution [2,3]. Large
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 2 6 3 - 2
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
area n-in-p detectors with integrated coupling capacitors and polysilicon bias resistors have been fabricated and the pre-irradiation results along with the behaviour of the reverse current and interstrip capacitance have been studied after irradiation [4]. This paper presents the first charge collection efficiency (CCE) studies after irradiation for such n-in-p microstrip detectors.
2. Radiation damage on silicon detectors The damage due to hadron irradiation in the substrate of silicon detectors changes the leakage current, the full depletion voltage Vfd and the concentration of trapping centres [5–8]. The full depletion voltage is proportional to the effective doping concentration, Neff ; which in the case of ntype detectors falls until the detectors invert from n to p-type, with the effective acceptor concentration after heavy irradiation growing approximately linearly with dose [5–8]. For p-type silicon, the effective doping still falls initially but after heavy irradiation, the behaviour is similar to that for initial n-type silicon with Neff showing very similar dependence on dose [3,4]. The high density of radiation-induced trapping centres reduces the charge carrier lifetime. When used as an ionising particle detector, a fraction of the carriers generated will undergo trapping and de-trapping before collection, thereby increasing the overall collection time. Such trapping is different for negative and positive carriers with evidence for less trapping of electrons [9]. In addition, less trapping is expected for charges moving in a high electric field [9,10]. Therefore, the n-in-p diode structure benefits both the reduced trapping for electrons [9] and the high electric field being located at the read-out side [10].
n-in-p diode structure, 80 mm strip pitch and individual p-stop interstrip isolation [3]. These are compared with p-in-n large area capacitively coupled microstrip detectors with polysilicon bias resistors (ATLAS wedge prototypes [11]). The latter were made with both standard and oxygen enriched bulk silicon by high-temperature diffusion [12]. The use of oxygenated material has been shown to reduce the rate of Vfd increase with charged hadron fluence [13]. The n-in-p detectors were irradiated with 24 GeV/c protons in the CERN-PS-T7 irradiation area in a dry N2 atmosphere at 101C and reverse biased at 100 V. The p-in-n detectors were irradiated later under the same conditions as described above. The final fluence in all cases was (370.3) 1014 cm 2. After irradiation, all detectors were annealed for 7 days at 251C to bring them to the broad minimum of the characteristic annealing curve for the depletion voltage [14].
4. Experimental results Fig. 1 shows the characteristic CV curve (measured at 5 kHz) of the p-type detector compared with that for a correspondingly irradiated and annealed oxygenated p-in-n detector. The CV curves are used to estimate Vfd with the fitting technique shown in the figure. The measured 6.0E-07 5.0E-07
1/C2 [pF-2]
466
4.0E-07 3.0E-07 Oxygenated n-type
2.0E-07
Non-oxygenated n-type P-type bulk
1.0E-07 0.0E+00 0
3. p-Type substrate and control n-type microstrip detectors Several large area (E6.4 6.4 cm2) non-oxygenated 784 strip detectors, capacitively coupled with polysilicon bias resistors were fabricated with
100
200
300
400
500
Bias [V]
Fig. 1. Capacitance of all the read-out strips to the backplane in a p-type bulk and an oxygenated and a non-oxygenated ntype bulk detectors after 3 1014 protons cm 2, plotted as 1=C 2 against bias voltage to allow the extraction of Vfd : This is evaluated with the two straight-line interpolations as shown in the figure.
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
467
Fig. 2. Capacitance of central strip to the first two neighbours on each side in a p-type detector after 3 1014 protons cm 2.
values of Vfd as 210710 V for the p-type detector and 185710 V for the oxygenated n-type detector. (The thicknesses are 320710 and 300710 mm for the p-type and n-type detectors, respectively.) Fig. 2 shows the interstrip capacitance (the largest component of the capacitive load seen by the read-out amplifiers) as a function of the applied voltage as measured for the irradiated ptype detector. Fig. 3 shows the corresponding average noise seen with the SCT128 LHC speed read-out (128 channel ASIC, 40 MHz clock rate) [15] as a function of the bias voltage. The noise is higher at very low bias, correlating with the interstrip capacitance behaviour. (The capacitance readings are, however, only at frequencies up to 300 kHz, well below the bandwidth of the SCT128’s amplifiers.) The noise is then constant with voltage from about 50 V until it increases at 400 V, because of micro-discharge [16] setting in at this value. The charge collection properties of the detectors after irradiation with 24 GeV/c protons to a fluence of 3 1014 cm 2 have been studied with a 106 Ru b-source using the SCT128 analogue read-
out chip. The signal is defined as the sum of the pulse heights for strips in a contiguous cluster where at least one channel exceeds 4 times the channel noise (approximately 1300 electrons) and the others exceed 2 times the noise. A lower cut, with one channel exceeding 3 times the noise, has been used for the signal measured at the lowest biases (100, 150 and 200 V for the p-type and oxygenated and unoxygenated n-type, respectively). Fig. 4 shows the energy loss distribution for the signal obtained with the p-type detector biased to 400 V (the value at which the noise begins to rise due to micro-discharges) and for the corresponding oxygenated n-type detector biased at 500 V. Since at its onset the additional noise associated with micro-discharges is localised, the effect on masking out noisy channels is only to reduce the number of channels contributing to the signal distribution for the p-type detector at high voltage. Even though the p-type is at lower voltage, the signal is higher by 573% after allowing for the difference in thickness, in line with the lower charge trapping for electrons when compared to holes [9]. The fitted maxima (Landau
468
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
Fig. 3. Noise (ADC counts) as a function of bias for an irradiated p-type bulk detector. The flat region (B23 counts) corresponds to 1300 e.n.c.
Fig. 4. ADC spectrum of b particles from a 106Ru source as measured using the SCT128 electronics coupled with n-in-p and p-in-n detectors after 3 1014 protons cm 2. The channels showing micro-discharge noise are masked out.
distribution convolved with a Gaussian) are 22,8007700e for the p-type compared with 20,3507600e and 20,1907600e for the oxygenated and the non-oxygenated n-type, respectively, where the error is dominated by the common uncertainty in the overall calibration for this readout chain. These should be compared with expected signals of 24,0007750e (p-type) and 22,5007750e (n-type) for corresponding thickness unirradiated detectors, giving CCE values of 9574%, 9074% and 8974%, respectively. Fig. 5 shows the comparison between the p-type and n-type irradiated detector CCE curves. The ptype detector CCE rises much faster at low biases and starts to plateau at substantially lower bias voltage than either the p-in-n non-oxygenated or oxygenated detectors (e.g. at 250 V, the p-type detector already collects 94% of its maximum charge compared to 81% for the oxygenated p-inn). This is despite being marginally thicker and, with respect to the oxygenated p-in-n (see Fig. 1), having a significantly higher Vfd as determined
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
469
Fig. 5. Comparison of CCE between n-in-p detector (non-oxygenated p-bulk), oxygenated and non-oxygenated p-in-n detectors. The measurements have been taken at 171C.
from CV measurements. Moreover, the p-type substrate was not oxygenated and the comparison with the non-oxygenated p-in-n detector shows an even larger improvement in the CCE characteristics.
the case of irradiation with neutrons [10]. The use of both oxygenation and p-type bulk is proposed to provide the optimal performance for detectors required to operate in the harshest radiation environments.
5. Conclusion
References
The charge collection properties of heavily irradiated silicon microstrip detectors fabricated on p-type substrates (n-in-p) are shown to be significantly better than the more standard p-in-n, since the CCE curve plateaus at a much lower bias voltage (even when compared with an oxygenated p-in-n detector of lower Vfd ) and the plateau CCE value is higher, even after correcting for differences in device thickness. The advantage of using n-in-p is therefore greater than that of using oxygenated substrates, particularly as the latter approach is ineffective in
[1] R.H. Richter, et al., Nucl. Instr. and Meth. A 377 (1996) 412. [2] P.P. Allport, Nucl. Instr. and Meth. A 386 (1997) 109. [3] M. Hanlon, The development of p-type silicon detectors for the high radiation regions of the LHC, Ph.D. Thesis, Department of Physics, University of Liverpool. [4] G. Casse, et al., IEEE Trans. Nucl. Sci. 47 (3) (2000) 527. [5] G. Lindstrom, et al., Nucl. Instr. and Meth. A 426 (1999) 1. [6] G. Lindstrom, et al., Nucl. Instr. and Meth. A 466 (2001) 308. [7] G. Lindstrom, et al., Developments for radiation hard silicon detectors by defect engineeringFresults by the CERN RD48 (ROSE) collaboration, Proceedings of the
470
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
Sixth Workshop on Electronics for the LHC Experiments, Cracow, Poland, September 2000, CERN 2000-010, CERN/LHCC/2000-41, 280–285. [8] A. Chilingarov, et al., Nucl. Instr. and Meth. A 360 (1995) 432. [9] G. Kramberger, et al., Nucl. Instr. and Meth. A 476 (2002) 645. [10] S. Marti, I. Garcia, et al., Nucl. Instr. and Meth. A 473 (2001) 128.
[11] ATLAS Inner Detector Technical Design Report, CERN/ LHCC/97-17. [12] G. Casse, et al., Nucl. Instr. and Meth. A 438 (1999) 429. [13] A. Ruzin, et al., IEEE Trans. Nucl. Sci. 46 (5) (1999) 1310. [14] A. Ruzin, et al., Nucl. Instr. and Meth. A 426 (1999) 94. [15] F. Anghinolfi, et al., IEEE Trans. Nucl. Sci. 44 (3) (1997) 298. [16] T. Oshugi, et al., Nucl. Instr. and Meth. A 342 (1994) 22.
First results on the charge collection properties of segmented detectors made with p-type bulk silicon G. Casse*, P.P. Allport, T.J.V. Bowcock, A. Greenall, M. Hanlon, J.N. Jackson Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxford street, L69 7ZE Liverpool, UK Received 3 May 2001; received in revised form 11 December 2001; accepted 15 December 2001
Abstract Radiation damage of n-type bulk detectors introduces stable defects acting as effective p-type doping and leads to the change of the conductivity type of the silicon substrate (type inversion) after a fluence of a few times 1013 protons cm 2. The diode junction after inversion migrates from the original side to the back plane of the detector. The migration of the junction can be prevented using silicon detectors with p-type substrates. Furthermore, the use of n-side readout gives higher charge collection efficiency for segmented devices operated below the full depletion voltage. Large area (E 6.4 6.4 cm2) capacitively coupled 80 mm pitch detectors using polysilicon bias resistors have been fabricated on p-type substrates (n-in-p diode structure). These detectors have been irradiated with 24 GeV/c protons to an integrated fluence of 3 1014 cm 2 and kept for 7 days at 251C to reach the broad minimum of the annealing curve. Results are presented on the comparison of their charge collection properties with detectors using p-strip read-out after corresponding dose and annealing. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Wk; 29.40.Gx; 61.82.Fk Keywords: Silicon microstrip detectors; Charge collection efficiency; Radiation hardness
1. Introduction To date, n-type substrate with implanted p strips (p-in-n) is the most commonly used configuration for silicon detectors used in high-energy physics applications. Such devices have the advantage of not requiring dedicated processing steps to isolate each strip from its neighbours. The alternative of n-strips implanted in p-bulk silicon (n-in-p) requires either an isolation implant surrounding the read-out strip (p-stop) or the use of the p-spray *Corresponding author. Tel./fax: +44-151-794-3399. E-mail address: [email protected] (G. Casse).
technique [1]. These prevent shorts between neighbouring strips due to conductive channels induced through positive charge trapped at the dielectric bulk interface. This results in an increased complication for the mask design and additional cost in the production process. However, radiation damage introduces acceptor-like defects, so the use of p-type bulk silicon prevents the migration of the junction after irradiation. This permits read out of the detectors from the junction (i.e. higher electric field) side for the entire life of the detector. The complications and added cost implied by double-side processing mitigate against the alternative n-in-n solution [2,3]. Large
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 2 6 3 - 2
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
area n-in-p detectors with integrated coupling capacitors and polysilicon bias resistors have been fabricated and the pre-irradiation results along with the behaviour of the reverse current and interstrip capacitance have been studied after irradiation [4]. This paper presents the first charge collection efficiency (CCE) studies after irradiation for such n-in-p microstrip detectors.
2. Radiation damage on silicon detectors The damage due to hadron irradiation in the substrate of silicon detectors changes the leakage current, the full depletion voltage Vfd and the concentration of trapping centres [5–8]. The full depletion voltage is proportional to the effective doping concentration, Neff ; which in the case of ntype detectors falls until the detectors invert from n to p-type, with the effective acceptor concentration after heavy irradiation growing approximately linearly with dose [5–8]. For p-type silicon, the effective doping still falls initially but after heavy irradiation, the behaviour is similar to that for initial n-type silicon with Neff showing very similar dependence on dose [3,4]. The high density of radiation-induced trapping centres reduces the charge carrier lifetime. When used as an ionising particle detector, a fraction of the carriers generated will undergo trapping and de-trapping before collection, thereby increasing the overall collection time. Such trapping is different for negative and positive carriers with evidence for less trapping of electrons [9]. In addition, less trapping is expected for charges moving in a high electric field [9,10]. Therefore, the n-in-p diode structure benefits both the reduced trapping for electrons [9] and the high electric field being located at the read-out side [10].
n-in-p diode structure, 80 mm strip pitch and individual p-stop interstrip isolation [3]. These are compared with p-in-n large area capacitively coupled microstrip detectors with polysilicon bias resistors (ATLAS wedge prototypes [11]). The latter were made with both standard and oxygen enriched bulk silicon by high-temperature diffusion [12]. The use of oxygenated material has been shown to reduce the rate of Vfd increase with charged hadron fluence [13]. The n-in-p detectors were irradiated with 24 GeV/c protons in the CERN-PS-T7 irradiation area in a dry N2 atmosphere at 101C and reverse biased at 100 V. The p-in-n detectors were irradiated later under the same conditions as described above. The final fluence in all cases was (370.3) 1014 cm 2. After irradiation, all detectors were annealed for 7 days at 251C to bring them to the broad minimum of the characteristic annealing curve for the depletion voltage [14].
4. Experimental results Fig. 1 shows the characteristic CV curve (measured at 5 kHz) of the p-type detector compared with that for a correspondingly irradiated and annealed oxygenated p-in-n detector. The CV curves are used to estimate Vfd with the fitting technique shown in the figure. The measured 6.0E-07 5.0E-07
1/C2 [pF-2]
466
4.0E-07 3.0E-07 Oxygenated n-type
2.0E-07
Non-oxygenated n-type P-type bulk
1.0E-07 0.0E+00 0
3. p-Type substrate and control n-type microstrip detectors Several large area (E6.4 6.4 cm2) non-oxygenated 784 strip detectors, capacitively coupled with polysilicon bias resistors were fabricated with
100
200
300
400
500
Bias [V]
Fig. 1. Capacitance of all the read-out strips to the backplane in a p-type bulk and an oxygenated and a non-oxygenated ntype bulk detectors after 3 1014 protons cm 2, plotted as 1=C 2 against bias voltage to allow the extraction of Vfd : This is evaluated with the two straight-line interpolations as shown in the figure.
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
467
Fig. 2. Capacitance of central strip to the first two neighbours on each side in a p-type detector after 3 1014 protons cm 2.
values of Vfd as 210710 V for the p-type detector and 185710 V for the oxygenated n-type detector. (The thicknesses are 320710 and 300710 mm for the p-type and n-type detectors, respectively.) Fig. 2 shows the interstrip capacitance (the largest component of the capacitive load seen by the read-out amplifiers) as a function of the applied voltage as measured for the irradiated ptype detector. Fig. 3 shows the corresponding average noise seen with the SCT128 LHC speed read-out (128 channel ASIC, 40 MHz clock rate) [15] as a function of the bias voltage. The noise is higher at very low bias, correlating with the interstrip capacitance behaviour. (The capacitance readings are, however, only at frequencies up to 300 kHz, well below the bandwidth of the SCT128’s amplifiers.) The noise is then constant with voltage from about 50 V until it increases at 400 V, because of micro-discharge [16] setting in at this value. The charge collection properties of the detectors after irradiation with 24 GeV/c protons to a fluence of 3 1014 cm 2 have been studied with a 106 Ru b-source using the SCT128 analogue read-
out chip. The signal is defined as the sum of the pulse heights for strips in a contiguous cluster where at least one channel exceeds 4 times the channel noise (approximately 1300 electrons) and the others exceed 2 times the noise. A lower cut, with one channel exceeding 3 times the noise, has been used for the signal measured at the lowest biases (100, 150 and 200 V for the p-type and oxygenated and unoxygenated n-type, respectively). Fig. 4 shows the energy loss distribution for the signal obtained with the p-type detector biased to 400 V (the value at which the noise begins to rise due to micro-discharges) and for the corresponding oxygenated n-type detector biased at 500 V. Since at its onset the additional noise associated with micro-discharges is localised, the effect on masking out noisy channels is only to reduce the number of channels contributing to the signal distribution for the p-type detector at high voltage. Even though the p-type is at lower voltage, the signal is higher by 573% after allowing for the difference in thickness, in line with the lower charge trapping for electrons when compared to holes [9]. The fitted maxima (Landau
468
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
Fig. 3. Noise (ADC counts) as a function of bias for an irradiated p-type bulk detector. The flat region (B23 counts) corresponds to 1300 e.n.c.
Fig. 4. ADC spectrum of b particles from a 106Ru source as measured using the SCT128 electronics coupled with n-in-p and p-in-n detectors after 3 1014 protons cm 2. The channels showing micro-discharge noise are masked out.
distribution convolved with a Gaussian) are 22,8007700e for the p-type compared with 20,3507600e and 20,1907600e for the oxygenated and the non-oxygenated n-type, respectively, where the error is dominated by the common uncertainty in the overall calibration for this readout chain. These should be compared with expected signals of 24,0007750e (p-type) and 22,5007750e (n-type) for corresponding thickness unirradiated detectors, giving CCE values of 9574%, 9074% and 8974%, respectively. Fig. 5 shows the comparison between the p-type and n-type irradiated detector CCE curves. The ptype detector CCE rises much faster at low biases and starts to plateau at substantially lower bias voltage than either the p-in-n non-oxygenated or oxygenated detectors (e.g. at 250 V, the p-type detector already collects 94% of its maximum charge compared to 81% for the oxygenated p-inn). This is despite being marginally thicker and, with respect to the oxygenated p-in-n (see Fig. 1), having a significantly higher Vfd as determined
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
469
Fig. 5. Comparison of CCE between n-in-p detector (non-oxygenated p-bulk), oxygenated and non-oxygenated p-in-n detectors. The measurements have been taken at 171C.
from CV measurements. Moreover, the p-type substrate was not oxygenated and the comparison with the non-oxygenated p-in-n detector shows an even larger improvement in the CCE characteristics.
the case of irradiation with neutrons [10]. The use of both oxygenation and p-type bulk is proposed to provide the optimal performance for detectors required to operate in the harshest radiation environments.
5. Conclusion
References
The charge collection properties of heavily irradiated silicon microstrip detectors fabricated on p-type substrates (n-in-p) are shown to be significantly better than the more standard p-in-n, since the CCE curve plateaus at a much lower bias voltage (even when compared with an oxygenated p-in-n detector of lower Vfd ) and the plateau CCE value is higher, even after correcting for differences in device thickness. The advantage of using n-in-p is therefore greater than that of using oxygenated substrates, particularly as the latter approach is ineffective in
[1] R.H. Richter, et al., Nucl. Instr. and Meth. A 377 (1996) 412. [2] P.P. Allport, Nucl. Instr. and Meth. A 386 (1997) 109. [3] M. Hanlon, The development of p-type silicon detectors for the high radiation regions of the LHC, Ph.D. Thesis, Department of Physics, University of Liverpool. [4] G. Casse, et al., IEEE Trans. Nucl. Sci. 47 (3) (2000) 527. [5] G. Lindstrom, et al., Nucl. Instr. and Meth. A 426 (1999) 1. [6] G. Lindstrom, et al., Nucl. Instr. and Meth. A 466 (2001) 308. [7] G. Lindstrom, et al., Developments for radiation hard silicon detectors by defect engineeringFresults by the CERN RD48 (ROSE) collaboration, Proceedings of the
470
G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 487 (2002) 465–470
Sixth Workshop on Electronics for the LHC Experiments, Cracow, Poland, September 2000, CERN 2000-010, CERN/LHCC/2000-41, 280–285. [8] A. Chilingarov, et al., Nucl. Instr. and Meth. A 360 (1995) 432. [9] G. Kramberger, et al., Nucl. Instr. and Meth. A 476 (2002) 645. [10] S. Marti, I. Garcia, et al., Nucl. Instr. and Meth. A 473 (2001) 128.
[11] ATLAS Inner Detector Technical Design Report, CERN/ LHCC/97-17. [12] G. Casse, et al., Nucl. Instr. and Meth. A 438 (1999) 429. [13] A. Ruzin, et al., IEEE Trans. Nucl. Sci. 46 (5) (1999) 1310. [14] A. Ruzin, et al., Nucl. Instr. and Meth. A 426 (1999) 94. [15] F. Anghinolfi, et al., IEEE Trans. Nucl. Sci. 44 (3) (1997) 298. [16] T. Oshugi, et al., Nucl. Instr. and Meth. A 342 (1994) 22.