- Email: [email protected]
Development of a one-dimensional microstrip germanium detector for Compton scattering experiment at SPring-8
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
Development of a one-dimensional microstrip germanium detector for Compton scattering experiment at SPring-8 H. Toyokawaa, M. Itoua, M. Mizumakia, Y. Sakuraia, M. Suzukia,*, N. Hiraokab, N. Sakaib a
Japan Synchrotron Radiation Research Institute (JASRI) Experimental Facilities Division, SPring-8, Kouto 1-1-1, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan b Himeji Institute of Technology, Kouto 3-2-1, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
Abstract Two prototypes of a one-dimensional microstrip germanium detector were fabricated with seven strips, having different pitches of 200 and 350 mm. Owing to its insensitivity to hole-diffusion process, the latter one has attained a spatial resolution as high as 350 mm, an energy resolution better than 1.4%, and a peak efficiency around 50% at an X-ray energy of 80 keV. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 29.30.Kv; 29.40.Gx; 29.40.Wk; 78.70.Ck Keywords: 1D X-ray detector; Microstrip; Germanium detector; Compton scattering experiment; SPring-8
1. Introduction Japan Synchrotron Radiation Research Institute and Himeji Institute of Technology have been undertaking an R&D project to embody a onedimensional microstrip germanium (1DMSGD) for the Cauchois-type Compton spectrometer at the beamline BL08W of the SPring-8 Facility [1–3]. The spatial resolution, Dx, required for the detector is 350 mm in order to suit the overall momentum resolution of 0.13 atomic unit realized with the spectrometer [4,5]. Also required are a sufficient energy resolution, DE, to reject back-
ground X-rays, and a high peak efficiency, epeak , to maximize beam utilization. Once such a 1DMSGD with 512 channels is realized by satisfying these conditions, a complete data set can be acquired forty times faster than the present system that uses a conventional Germanium detector with four slits in front [5]. The present paper reports the characterization of the two prototypes fabricated for the 1DMSGD, which has been carried out with radioisotopes and a high energy SR beam at the beamline.
2. Experimental *Corresponding author. Tel.: +81-791-58-1842; fax: +81791-58-2752. E-mail address: [email protected] (M. Suzuki).
The prototypes possess high purity germanium crystals with an extension and a thickness of
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 6 1 - 7
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
40 40 mm2 and 5 mm, respectively, with a 1+1+5 microstrip configuration: the first two microstrips function as single electrodes, and the rest as multiple electrodes (see Fig. 1 and Table 1) [3]. Hereafter, the prototype with a pitch of 200 mm will be referred to as No. I, and the one with a pitch of 350 mm as No. II. These microstripformed germanium crystals are housed in vacuum-tight cryostats, entrance windows of which are made of aluminum and beryllium for No. I and No. II, respectively. Each microstrip is connected to an external charge sensitive preamplifier, the output of which is split into two parts, one for a spectroscopy amplifier (SA) and the other for a timing filter amplifier (TFA). The energy signals from the SAs are fed into ADC modules, which are gated with the logic signals generated by processing the timing signals from the TFAs. After being examined with X-rays from 57Co and 241Am isotopes, both prototypes were
1133
mounted on the Compton spectrometer, where a vanadium plate was irradiated as a sample with the primary X-ray beam of 115 keV [4,5]. The scattered X-rays were dispersed onto the Rowland circle defined by the spectrometer, and were detected with the prototypes scanning along the appropriate arc of the circle.
3. Results and discussion The 57Co X-ray energy spectra observed with the central ones, S5, among the five microstrips of No. I and No. II are shown in Figs. 2(a) and (b), respectively. Both No. I and No. II were operated at their nominal voltages of 500 and 1200 V, respectively. Regarding the 121 keV peak, the values of DE are 1.62 keV (fwhm) with No. I and 1.47 keV (fwhm) with No. II. In general, however, epeak is a function of its applied voltage, Vap of the 1DMSGD, since Vap
Fig. 1. Schematic of microstrip pattern formed on the germanium crystals.
Table 1 Major parameters of microstrip structure Prototype No. I No. II
Pitch (mm)
Width (mm)
Interstrip (mm)
Length (mm)
200 350
150 300
50 50
30 30
Fig. 2. (a) 57Co X-ray energy spectra observed with No. I at an applied voltage of 500 V(gray) and 1000 V(line). (b) 57Co X-ray energy spectra observed with No. II at an applied voltage of 1200 V (gray) and 2000 V (line).
1134
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
affects the hole-diffusion processes (charge-split phenomena) [6,7]. In the present work, epeak was increased in No. I with increasing Vap from 500 to 1000 V (see Fig. 2(a)). However, no such increase in epeak was observed in No. II with increasing Vap from 1200 to 2000 V (see Fig. 2(b)). This suggests that, owing to its narrower strip pitch, No. I is more sensitive to the charge split phenomena. If it is the case, one could improve the energy spectrum of No. I, by summing the charge collected on its neighboring strips into a single strip. The energy spectrum thus reconstructed was similar to the one with No. II, concluding that No. II is more suitable to the present application than No. I. Hereafter, both No. I and No. II were operated at their nominal voltages in this paper. Fig. 3 shows an observed energy spectrum of X-rays scattered by the vanadium plate with a scattering energy, Es , of 80 keV, which was obtained with S5 of No. II. It can be seen that the 80 keV peak is associated with a tail towards the lower energy side, which is effectively eliminated by selecting those events where neither left nor right adjacent strip detects any substantial charge while the central one does (single strip event). Since about half of the total cases were found to be single strip event at 80 keV, one could state that hole clouds converge onto single strips 50% of the total cases, and epeak could be 50%
Fig. 3. Energy spectrum of X-rays scattered by vanadium at a scattering energy of 80 keV observed with the central microstrip of No. I (*), and the one obtained by selecting those events where neither left nor right adjacent strip detects any substantial charge while the central one does (*).
when the data acquisition system selects only single-strip events to ensure Dx5350 mm. Fig. 4 shows an observed energy spectrum of X-rays scattered by the vanadium plate as a
Fig. 4. Energy spectrum of X-rays scattered by vanadium as a function of Es , which was observed with the central strip of No. II.
Fig. 5. Vanadium Compton profile observed with prototype No. II.
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
function of Es , which was obtained with S5 of No. II. Fig. 5 shows the intensity of the total energy peak as a function of Es , where the twodimensional energy spectra obtained from all the multiple strips were unified.
1135
type is expected to reduce the data acquisition time by one order of magnitude in comparison to that in the present system.
References 4. Concluding remarks The prototype with a strip pitch of 350 mm was found to attain Dx 350 mm, DE51.4%, and epeak 50% at an X-ray energy of 80 keV. The performance of the prototype with a strip pitch of 200 mm was found to be less satisfactory due to the hole-diffusion process. Having adopted the pitch of 350 mm, there has been a new prototype of the 1DMSGD fabricated with 128 microstrips, which is currently under investigation. The new proto-
[1] SPring-8 Annual Report 1998, SPring-8/JASRI, 1998, 35. [2] M. Suzuki, H. Toyokawa, Oyo Buturi, 69 (2000) 380 (in Japanese). [3] M. Suzuki et al., In: KEK Proceedings of The 14th Workshop on Radiation Detectors and their Use, February 1–3, 2000, Tsukuba, Japan, in press. [4] M. Itou et al., In: Proceedings of 7th International Conference on Synchrotron Radiation Instrumentation, August 21–25, 2000, Berlin, Germany, Nucl. Instr. and Meth. A 467–468 (2001), these proceedings. [5] N. Hiraoka et al., J. Synchrotron Rad. 8 (2001) 26. [6] G. Rossi, Thesis, ESRF, France, 1997. [7] G. Rossi et al., Nucl. Instr. and Meth. A 392 (1997) 264.
Development of a one-dimensional microstrip germanium detector for Compton scattering experiment at SPring-8 H. Toyokawaa, M. Itoua, M. Mizumakia, Y. Sakuraia, M. Suzukia,*, N. Hiraokab, N. Sakaib a
Japan Synchrotron Radiation Research Institute (JASRI) Experimental Facilities Division, SPring-8, Kouto 1-1-1, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan b Himeji Institute of Technology, Kouto 3-2-1, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
Abstract Two prototypes of a one-dimensional microstrip germanium detector were fabricated with seven strips, having different pitches of 200 and 350 mm. Owing to its insensitivity to hole-diffusion process, the latter one has attained a spatial resolution as high as 350 mm, an energy resolution better than 1.4%, and a peak efficiency around 50% at an X-ray energy of 80 keV. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 29.30.Kv; 29.40.Gx; 29.40.Wk; 78.70.Ck Keywords: 1D X-ray detector; Microstrip; Germanium detector; Compton scattering experiment; SPring-8
1. Introduction Japan Synchrotron Radiation Research Institute and Himeji Institute of Technology have been undertaking an R&D project to embody a onedimensional microstrip germanium (1DMSGD) for the Cauchois-type Compton spectrometer at the beamline BL08W of the SPring-8 Facility [1–3]. The spatial resolution, Dx, required for the detector is 350 mm in order to suit the overall momentum resolution of 0.13 atomic unit realized with the spectrometer [4,5]. Also required are a sufficient energy resolution, DE, to reject back-
ground X-rays, and a high peak efficiency, epeak , to maximize beam utilization. Once such a 1DMSGD with 512 channels is realized by satisfying these conditions, a complete data set can be acquired forty times faster than the present system that uses a conventional Germanium detector with four slits in front [5]. The present paper reports the characterization of the two prototypes fabricated for the 1DMSGD, which has been carried out with radioisotopes and a high energy SR beam at the beamline.
2. Experimental *Corresponding author. Tel.: +81-791-58-1842; fax: +81791-58-2752. E-mail address: [email protected] (M. Suzuki).
The prototypes possess high purity germanium crystals with an extension and a thickness of
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 6 1 - 7
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
40 40 mm2 and 5 mm, respectively, with a 1+1+5 microstrip configuration: the first two microstrips function as single electrodes, and the rest as multiple electrodes (see Fig. 1 and Table 1) [3]. Hereafter, the prototype with a pitch of 200 mm will be referred to as No. I, and the one with a pitch of 350 mm as No. II. These microstripformed germanium crystals are housed in vacuum-tight cryostats, entrance windows of which are made of aluminum and beryllium for No. I and No. II, respectively. Each microstrip is connected to an external charge sensitive preamplifier, the output of which is split into two parts, one for a spectroscopy amplifier (SA) and the other for a timing filter amplifier (TFA). The energy signals from the SAs are fed into ADC modules, which are gated with the logic signals generated by processing the timing signals from the TFAs. After being examined with X-rays from 57Co and 241Am isotopes, both prototypes were
1133
mounted on the Compton spectrometer, where a vanadium plate was irradiated as a sample with the primary X-ray beam of 115 keV [4,5]. The scattered X-rays were dispersed onto the Rowland circle defined by the spectrometer, and were detected with the prototypes scanning along the appropriate arc of the circle.
3. Results and discussion The 57Co X-ray energy spectra observed with the central ones, S5, among the five microstrips of No. I and No. II are shown in Figs. 2(a) and (b), respectively. Both No. I and No. II were operated at their nominal voltages of 500 and 1200 V, respectively. Regarding the 121 keV peak, the values of DE are 1.62 keV (fwhm) with No. I and 1.47 keV (fwhm) with No. II. In general, however, epeak is a function of its applied voltage, Vap of the 1DMSGD, since Vap
Fig. 1. Schematic of microstrip pattern formed on the germanium crystals.
Table 1 Major parameters of microstrip structure Prototype No. I No. II
Pitch (mm)
Width (mm)
Interstrip (mm)
Length (mm)
200 350
150 300
50 50
30 30
Fig. 2. (a) 57Co X-ray energy spectra observed with No. I at an applied voltage of 500 V(gray) and 1000 V(line). (b) 57Co X-ray energy spectra observed with No. II at an applied voltage of 1200 V (gray) and 2000 V (line).
1134
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
affects the hole-diffusion processes (charge-split phenomena) [6,7]. In the present work, epeak was increased in No. I with increasing Vap from 500 to 1000 V (see Fig. 2(a)). However, no such increase in epeak was observed in No. II with increasing Vap from 1200 to 2000 V (see Fig. 2(b)). This suggests that, owing to its narrower strip pitch, No. I is more sensitive to the charge split phenomena. If it is the case, one could improve the energy spectrum of No. I, by summing the charge collected on its neighboring strips into a single strip. The energy spectrum thus reconstructed was similar to the one with No. II, concluding that No. II is more suitable to the present application than No. I. Hereafter, both No. I and No. II were operated at their nominal voltages in this paper. Fig. 3 shows an observed energy spectrum of X-rays scattered by the vanadium plate with a scattering energy, Es , of 80 keV, which was obtained with S5 of No. II. It can be seen that the 80 keV peak is associated with a tail towards the lower energy side, which is effectively eliminated by selecting those events where neither left nor right adjacent strip detects any substantial charge while the central one does (single strip event). Since about half of the total cases were found to be single strip event at 80 keV, one could state that hole clouds converge onto single strips 50% of the total cases, and epeak could be 50%
Fig. 3. Energy spectrum of X-rays scattered by vanadium at a scattering energy of 80 keV observed with the central microstrip of No. I (*), and the one obtained by selecting those events where neither left nor right adjacent strip detects any substantial charge while the central one does (*).
when the data acquisition system selects only single-strip events to ensure Dx5350 mm. Fig. 4 shows an observed energy spectrum of X-rays scattered by the vanadium plate as a
Fig. 4. Energy spectrum of X-rays scattered by vanadium as a function of Es , which was observed with the central strip of No. II.
Fig. 5. Vanadium Compton profile observed with prototype No. II.
H. Toyokawa et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1132–1135
function of Es , which was obtained with S5 of No. II. Fig. 5 shows the intensity of the total energy peak as a function of Es , where the twodimensional energy spectra obtained from all the multiple strips were unified.
1135
type is expected to reduce the data acquisition time by one order of magnitude in comparison to that in the present system.
References 4. Concluding remarks The prototype with a strip pitch of 350 mm was found to attain Dx 350 mm, DE51.4%, and epeak 50% at an X-ray energy of 80 keV. The performance of the prototype with a strip pitch of 200 mm was found to be less satisfactory due to the hole-diffusion process. Having adopted the pitch of 350 mm, there has been a new prototype of the 1DMSGD fabricated with 128 microstrips, which is currently under investigation. The new proto-
[1] SPring-8 Annual Report 1998, SPring-8/JASRI, 1998, 35. [2] M. Suzuki, H. Toyokawa, Oyo Buturi, 69 (2000) 380 (in Japanese). [3] M. Suzuki et al., In: KEK Proceedings of The 14th Workshop on Radiation Detectors and their Use, February 1–3, 2000, Tsukuba, Japan, in press. [4] M. Itou et al., In: Proceedings of 7th International Conference on Synchrotron Radiation Instrumentation, August 21–25, 2000, Berlin, Germany, Nucl. Instr. and Meth. A 467–468 (2001), these proceedings. [5] N. Hiraoka et al., J. Synchrotron Rad. 8 (2001) 26. [6] G. Rossi, Thesis, ESRF, France, 1997. [7] G. Rossi et al., Nucl. Instr. and Meth. A 392 (1997) 264.