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μ-Strip photosensors for gas proportional scintillation counters
Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
m-Strip photosensors for gas proportional scintillation counters J.F.C.A. Velosoa,*, D.S.A.P. Freitasa, J.M.F. dos Santosa, R.E. Morgadob a
Physics Department, University of Coimbra, Coimbra P-3004-516, Portugal Los Alamos Nacional Laboratory, Los Alamos, New Mexico 87545, USA
b
Abstract The results of a comparative study of a xenon gas proportional scintillation counter instrumented with photosensors based on two variants of a CsI photocathode and a microstrip plate are reported. The photosensors were isolated from the gas proportional scintillation counter by a quartz window and operated in P-10 gas at one atmosphere. The CsI photocathode is deposited either in the reflective or semitransparent mode. In the reflective mode, the CsI is deposited directly onto the micro strip plate and we observe gain fluctuations due to a geometric factor that degrades the energy resolution. In the semitransparent mode, the CsI is deposited onto the surface of the quartz window. Both modes of operation were evaluated and their performance characteristics reported. r 2003 Elsevier Science B.V. All rights reserved. PACS: 07.85.Nc; 29.40.Cs; 29.40.Mc; 85.60.Ha; 85.60.Gz Keywords: CsI-photocathode; Gas detector; Microstrip; UV photosensor
1. Introduction In earlier works [1,2], we reported on two variants of a xenon gas proportional with CsIcoated microstrip plates (MSP) as photosensors. CsI was deposited directly onto the MSP and operated in a reflective mode. Both results are inferior to the performance of a GPSC instrumented with a PMT [3]. The detectors have shown a fairly low ageing [1].Numerical simulations [4] demonstrated that photoelectrons from a reflective (R) photocathode experience variable gains depending on their point of origin on the MSP surface.To illustrate the effect of the gain varia-
*Corresponding author. Tel.: +351-239410667; fax: +351239829158. E-mail address: jveloso@gian.fis.uc.pt (J.F.C.A. Veloso).
tions, fg ; due to the geometric factor of the photosensor, we will recast the standard expression [1] for the GPSC energy resolution. The number of primary electrons is N ¼ Ex =W ; where Ex is the X-ray energy and W is the mean energy to produce an ion pair. We define the number of photoelectrons produced per primary electron as L ¼ Ne =N [1], where Ne is the total number of photoelectrons. Then, the energy resolution, RE ; is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ f þ fg W RE ¼ 2:35 Fþ ð1Þ Ex L where F is the Fano factor, f is the relative variance of gas multiplication in the MSP anodes, and 2 sG fg ¼ ð2Þ G%
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01057-X
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
224
sG is the standard deviation of the gain due to the geometric effect alone, and G% is its average gain.
A fraction of the photons that reach the CsI photocathodes eject photoelectrons that drift towards the MSP anodes and initiate charge avalanches.
2. Rationale 3. Simulation To reduce the variable photoelectron gain of the R-design, another variant of the detector locates a semitransparent (S) CsI photocathode on the surface of the quartz window that isolates the MSP from the xenon gas envelope of the GPSC [5,6]. In this configuration (Fig. 1(b)), photoelectrons emitted from the S-CsI photocathode drift in P-10 gas towards the MSP anodes where charge multiplication occurs. The S-photocathode, however, will have lower quantum efficiency than Rphotocathode with a subsequent reduction in Ne that could offset any gains achieved in increased sensitive area and effective solid angle. Soft X-rays are absorbed at the beginning of the GPSC scintillation region and produce N primary electrons that drift under the influence of an electric field chosen below the ionization threshold (Fig. 1). Electron-xenon collisions produce scintillation photons in the VUV region.
GPSC/MSGC in
reflective x-ray
mode
Simulation showed that the geometric factor, fg ; depends not only on the potential between the different electrodes but also on the paths followed by the photoelectrons that initiate the avalanche. Only an average MSP gain for all paths between cathodes and anodes is actually measured. The gain, G; of a single photoelectron from the photocathode surface (R or S) was calculated as a function of position by numerical integration. The first Townsend coefficients aðsÞ for P-10 gas were those of Ref. [7]. The simulation geometry and dimensions were the MSP values of this experiment. The variation in gain as a function of photoelectron starting position on the photocathode surface is shown in Fig. 2 for the anode to cathode voltage, Va ¼ 420 V and for 0 V in the drift electrode.
GPSC/MSGC in
semitransparent x-ray
window
mode
window
scintillation region quartz window
Xenon
quartz window grid
Xenon hν (VUV)
grid
CsI MSP
P-10
CsI
hν (VUV)
charge multiplication region
P-10
grid grid+CsI
MSP (a)
(b)
Fig. 1. Schematic of the working principle of both detectors: CsI photo cathode operating in reflective (a), and in semitransparent (b) modes.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
30
from the reflective photocathode (CsI on the MSP)
600
Energy resolution,RE ( % )
Gain
500 400 300 200 from the semitransparent photocathode (CsI on quartz)
100
0
20
25
Semitransparent-cal. Reflective-cal.
20
15
2.3 keV
10
5.9 keV
anode
cathode
0
225
40
60
80
100
Photoelectron starting position (µm) Fig. 2. Simulation results for single photoelectron gain as a function of the starting position, for Va ¼ 420 V.
Photoelectrons from the R-photocathode surface experience much larger gain variations, fg ; than photoelectrons produced in the S-mode. It was also found that, although fg is greater in the R-mode and increases exponentially with Va ; the effect on the gain of the S-mode is negligible. Substituting W ¼ 22 eV for xenon [8], F ¼ 0:17 [9], f ¼ 0:45 for the MSGC [10], and fg values for Va ¼ 420 V from the simulation, detector RE ; Eq. (1), can be determined as a function of L as illustrated in Fig. 3. For small values of L the major factor limiting RE for the R-mode is fg : As L increases, the influence of fg in the factor ð1 þ f þ fg Þ=L in Eq. (1) is less significant and RE of the detector, operating in the R-mode, approaches RE for the S-mode.
4. Experimental description A driftless xenon GPSC was used. The detector consisted of a 6-mm-deep scintillation region separated from the P-10 photosensor chamber by a 1-mm-thick quartz window. A MSP (MS-4 model) is positioned in the P-10 chamber, 3-mm away from the quartz window. Coating either the MSP or the quartz window with a CsI photocathode film produces the two detector operational modes, Fig. 1.
5
0
5
10
15
20
L Fig. 3. Calculated energy resolution as a function of L; for 2.3 and 5.9-keV and for the photosensor operating in R and Smodes.
The photocathodes were produced by vacuum deposition of a CsI crystal. The CsI films were 500 and 18-nm-thick for the R- and S-photocathodes.
5. Experimental results and discussion The relative gains and RE were optimized as functions of the reduced electric field, E=p; in the scintillation region. The RE improves with E=p; reaching a minimum for values just above the ionization threshold [11]. To evaluate the effect of the fg ; RE for the same L value, i.e., RE ðLÞ instead of RE ðE=pÞ; should be compared for each case. Since 22-keV X-rays will interact in both the xenon and P-10 chambers, their respective pulseheight amplitudes, AXe and AP-10 ; can be compared directly. In the R-mode, the amplitude of the 22-keV Xrays that interact in the xenon chamber can be given by AXe ¼ C NXe L G% c ð3Þ where NXe ¼ 22-keV/WXe is the number of primary electrons produced in xenon, G% c is the average photosensor gain for photoelectrons emitted from the photocathode, and C is the electronic signal amplification.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
Similarly, the amplitude of the 22-keV X-rays absorbed in the P-10 chamber is ð4Þ AP-10 ¼ CNP-10 G% d where NP-10 ¼ 22-keV/WP-10 is the number of primary electrons produced in P-10 and G% d is the average photosensor gain for electrons produced in the photosensor drift region. From Eqs. (3) and (4) L is calculated by AXe Gd WXe L¼ : ð5Þ AP-10 Gc WP-10 For the experimental conditions described (E=p ¼ 6 V cm1 Torr1, Va ¼ 420 V, and pP-10 ¼ 800 Torr), G% d =G% c was calculated by numerical simulation, as described in Section 2, and determined to be 0.37. Substituting WXe ¼ 22-eV [8] and WP-10 ¼ 26eV [12] and using the measured values AXe and AP-10 for E=p ¼ 6 V cm1 Torr1, we obtained L ¼ 11 for the R-photocathode. For the S-mode, the amplitude of the 22-keV Xray interactions in the xenon chamber will be given by AXe ¼ CNXe LG% d ð6Þ L is then obtained from Eqs. (4) and (6) AXe WXe L¼ : AP-10 WP-10
ð7Þ
For the same experimental conditions, we obtained L ¼ 4:7 for the S-photocathode. This disappointedly low value is due to the use of an inadequate conducting layer for the CsI. In principle, the L-value for the S-mode should be comparable to the R-mode and an improvement in the energy resolution is expected. In Fig. 4, the measured RE of 2.3-keV X-rays are plotted as a function of L: Prior to the onset of ionization in the scintillation region, the RE for the same L value is noticeably better for the Sphotocathode, in agreement with the simulation.
6. Conclusions The effect of the geometric factor fg ; must be taken into account in determining the RE for CsIcoated MSP photosensors. As a result, the
35
Energy resolution, RE ( % )
226
Reflective Semitransparent
30
Va = 420 V 2.3 keV 25
20 E/p = 6 V cm-1 Torr-1
15
E/p = 6 V cm-1 Torr-1 10
0
5
10
15
20
25
L Fig. 4. Energy resolution as a function of L; for the detector operating in reflective and semitransparent modes.
calculated RE for a reflective photocathode is always worse than that of a photosensor with a semitransparent photocathode. This effect is more pronounced at lower L values. The measured energy resolution is worse than that predicted by simulation due to factors such as solid angle, photocathode non-uniformities, charge buildup, and electronic noise. Experimental results confirm the simulation results for the same L-value when the detectors are operated below the ionization threshold.
Acknowledgements Support is acknowledged to project CERN/FIS/ 43785/01. Travel support is acknowledged for JFCA Veloso to Fund. Calouste Gulbenkian (FCG) and Fund. para a Ci#encia e a Tecnologia, Lisbon, and for JMF dos Santos to Fund. LusoAmericana para o Desenvolvimento and FCG.
References [1] J.F.C.A. Veloso, et al., Nucl. Instr. and Meth. A 457 (2001) 253. [2] J.F.C.A. Veloso, et al., Nucl. Instr. and Meth. A 460 (2001) 297.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227 [3] J.M.F. dos Santos, et al., X-ray Spectrom. 30 (2001) 373. [4] D.S.A.P. Freitas, et al., IEEE Trans. Nucl. Sci. NS-48 (2001) 411. [5] K. Zeitelhack, Nucl. Instr. and Meth. A 371 (1996) 57. [6] A. Breskin, et al., Nucl. Instr. and Meth. A 345 (1994) 205. [7] A. Sharma, et al., Nucl. Instr. and Meth. A 334 (1993) 420. [8] T.H.V.T. Dias, et al., J. Appl. Phys. 82 (1997) 2742.
227
[9] G.D. Alkazov, et al., Nucl. Instr. and Meth. 48 (1967) 1. [10] J. Miyamoto, et al., Nucl. Instr. and Meth. A 399 (1997) 85. [11] F.I.G.M. Borges, et al., Nucl. Instr. and Meth. A 422 (1999) 321. [12] G.F. Knoll, Radiation Detection and Measurement, 2nd Edition, Wiley, New York, 1989, pp. 174–178.
m-Strip photosensors for gas proportional scintillation counters J.F.C.A. Velosoa,*, D.S.A.P. Freitasa, J.M.F. dos Santosa, R.E. Morgadob a
Physics Department, University of Coimbra, Coimbra P-3004-516, Portugal Los Alamos Nacional Laboratory, Los Alamos, New Mexico 87545, USA
b
Abstract The results of a comparative study of a xenon gas proportional scintillation counter instrumented with photosensors based on two variants of a CsI photocathode and a microstrip plate are reported. The photosensors were isolated from the gas proportional scintillation counter by a quartz window and operated in P-10 gas at one atmosphere. The CsI photocathode is deposited either in the reflective or semitransparent mode. In the reflective mode, the CsI is deposited directly onto the micro strip plate and we observe gain fluctuations due to a geometric factor that degrades the energy resolution. In the semitransparent mode, the CsI is deposited onto the surface of the quartz window. Both modes of operation were evaluated and their performance characteristics reported. r 2003 Elsevier Science B.V. All rights reserved. PACS: 07.85.Nc; 29.40.Cs; 29.40.Mc; 85.60.Ha; 85.60.Gz Keywords: CsI-photocathode; Gas detector; Microstrip; UV photosensor
1. Introduction In earlier works [1,2], we reported on two variants of a xenon gas proportional with CsIcoated microstrip plates (MSP) as photosensors. CsI was deposited directly onto the MSP and operated in a reflective mode. Both results are inferior to the performance of a GPSC instrumented with a PMT [3]. The detectors have shown a fairly low ageing [1].Numerical simulations [4] demonstrated that photoelectrons from a reflective (R) photocathode experience variable gains depending on their point of origin on the MSP surface.To illustrate the effect of the gain varia-
*Corresponding author. Tel.: +351-239410667; fax: +351239829158. E-mail address: jveloso@gian.fis.uc.pt (J.F.C.A. Veloso).
tions, fg ; due to the geometric factor of the photosensor, we will recast the standard expression [1] for the GPSC energy resolution. The number of primary electrons is N ¼ Ex =W ; where Ex is the X-ray energy and W is the mean energy to produce an ion pair. We define the number of photoelectrons produced per primary electron as L ¼ Ne =N [1], where Ne is the total number of photoelectrons. Then, the energy resolution, RE ; is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ f þ fg W RE ¼ 2:35 Fþ ð1Þ Ex L where F is the Fano factor, f is the relative variance of gas multiplication in the MSP anodes, and 2 sG fg ¼ ð2Þ G%
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01057-X
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
224
sG is the standard deviation of the gain due to the geometric effect alone, and G% is its average gain.
A fraction of the photons that reach the CsI photocathodes eject photoelectrons that drift towards the MSP anodes and initiate charge avalanches.
2. Rationale 3. Simulation To reduce the variable photoelectron gain of the R-design, another variant of the detector locates a semitransparent (S) CsI photocathode on the surface of the quartz window that isolates the MSP from the xenon gas envelope of the GPSC [5,6]. In this configuration (Fig. 1(b)), photoelectrons emitted from the S-CsI photocathode drift in P-10 gas towards the MSP anodes where charge multiplication occurs. The S-photocathode, however, will have lower quantum efficiency than Rphotocathode with a subsequent reduction in Ne that could offset any gains achieved in increased sensitive area and effective solid angle. Soft X-rays are absorbed at the beginning of the GPSC scintillation region and produce N primary electrons that drift under the influence of an electric field chosen below the ionization threshold (Fig. 1). Electron-xenon collisions produce scintillation photons in the VUV region.
GPSC/MSGC in
reflective x-ray
mode
Simulation showed that the geometric factor, fg ; depends not only on the potential between the different electrodes but also on the paths followed by the photoelectrons that initiate the avalanche. Only an average MSP gain for all paths between cathodes and anodes is actually measured. The gain, G; of a single photoelectron from the photocathode surface (R or S) was calculated as a function of position by numerical integration. The first Townsend coefficients aðsÞ for P-10 gas were those of Ref. [7]. The simulation geometry and dimensions were the MSP values of this experiment. The variation in gain as a function of photoelectron starting position on the photocathode surface is shown in Fig. 2 for the anode to cathode voltage, Va ¼ 420 V and for 0 V in the drift electrode.
GPSC/MSGC in
semitransparent x-ray
window
mode
window
scintillation region quartz window
Xenon
quartz window grid
Xenon hν (VUV)
grid
CsI MSP
P-10
CsI
hν (VUV)
charge multiplication region
P-10
grid grid+CsI
MSP (a)
(b)
Fig. 1. Schematic of the working principle of both detectors: CsI photo cathode operating in reflective (a), and in semitransparent (b) modes.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
30
from the reflective photocathode (CsI on the MSP)
600
Energy resolution,RE ( % )
Gain
500 400 300 200 from the semitransparent photocathode (CsI on quartz)
100
0
20
25
Semitransparent-cal. Reflective-cal.
20
15
2.3 keV
10
5.9 keV
anode
cathode
0
225
40
60
80
100
Photoelectron starting position (µm) Fig. 2. Simulation results for single photoelectron gain as a function of the starting position, for Va ¼ 420 V.
Photoelectrons from the R-photocathode surface experience much larger gain variations, fg ; than photoelectrons produced in the S-mode. It was also found that, although fg is greater in the R-mode and increases exponentially with Va ; the effect on the gain of the S-mode is negligible. Substituting W ¼ 22 eV for xenon [8], F ¼ 0:17 [9], f ¼ 0:45 for the MSGC [10], and fg values for Va ¼ 420 V from the simulation, detector RE ; Eq. (1), can be determined as a function of L as illustrated in Fig. 3. For small values of L the major factor limiting RE for the R-mode is fg : As L increases, the influence of fg in the factor ð1 þ f þ fg Þ=L in Eq. (1) is less significant and RE of the detector, operating in the R-mode, approaches RE for the S-mode.
4. Experimental description A driftless xenon GPSC was used. The detector consisted of a 6-mm-deep scintillation region separated from the P-10 photosensor chamber by a 1-mm-thick quartz window. A MSP (MS-4 model) is positioned in the P-10 chamber, 3-mm away from the quartz window. Coating either the MSP or the quartz window with a CsI photocathode film produces the two detector operational modes, Fig. 1.
5
0
5
10
15
20
L Fig. 3. Calculated energy resolution as a function of L; for 2.3 and 5.9-keV and for the photosensor operating in R and Smodes.
The photocathodes were produced by vacuum deposition of a CsI crystal. The CsI films were 500 and 18-nm-thick for the R- and S-photocathodes.
5. Experimental results and discussion The relative gains and RE were optimized as functions of the reduced electric field, E=p; in the scintillation region. The RE improves with E=p; reaching a minimum for values just above the ionization threshold [11]. To evaluate the effect of the fg ; RE for the same L value, i.e., RE ðLÞ instead of RE ðE=pÞ; should be compared for each case. Since 22-keV X-rays will interact in both the xenon and P-10 chambers, their respective pulseheight amplitudes, AXe and AP-10 ; can be compared directly. In the R-mode, the amplitude of the 22-keV Xrays that interact in the xenon chamber can be given by AXe ¼ C NXe L G% c ð3Þ where NXe ¼ 22-keV/WXe is the number of primary electrons produced in xenon, G% c is the average photosensor gain for photoelectrons emitted from the photocathode, and C is the electronic signal amplification.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227
Similarly, the amplitude of the 22-keV X-rays absorbed in the P-10 chamber is ð4Þ AP-10 ¼ CNP-10 G% d where NP-10 ¼ 22-keV/WP-10 is the number of primary electrons produced in P-10 and G% d is the average photosensor gain for electrons produced in the photosensor drift region. From Eqs. (3) and (4) L is calculated by AXe Gd WXe L¼ : ð5Þ AP-10 Gc WP-10 For the experimental conditions described (E=p ¼ 6 V cm1 Torr1, Va ¼ 420 V, and pP-10 ¼ 800 Torr), G% d =G% c was calculated by numerical simulation, as described in Section 2, and determined to be 0.37. Substituting WXe ¼ 22-eV [8] and WP-10 ¼ 26eV [12] and using the measured values AXe and AP-10 for E=p ¼ 6 V cm1 Torr1, we obtained L ¼ 11 for the R-photocathode. For the S-mode, the amplitude of the 22-keV Xray interactions in the xenon chamber will be given by AXe ¼ CNXe LG% d ð6Þ L is then obtained from Eqs. (4) and (6) AXe WXe L¼ : AP-10 WP-10
ð7Þ
For the same experimental conditions, we obtained L ¼ 4:7 for the S-photocathode. This disappointedly low value is due to the use of an inadequate conducting layer for the CsI. In principle, the L-value for the S-mode should be comparable to the R-mode and an improvement in the energy resolution is expected. In Fig. 4, the measured RE of 2.3-keV X-rays are plotted as a function of L: Prior to the onset of ionization in the scintillation region, the RE for the same L value is noticeably better for the Sphotocathode, in agreement with the simulation.
6. Conclusions The effect of the geometric factor fg ; must be taken into account in determining the RE for CsIcoated MSP photosensors. As a result, the
35
Energy resolution, RE ( % )
226
Reflective Semitransparent
30
Va = 420 V 2.3 keV 25
20 E/p = 6 V cm-1 Torr-1
15
E/p = 6 V cm-1 Torr-1 10
0
5
10
15
20
25
L Fig. 4. Energy resolution as a function of L; for the detector operating in reflective and semitransparent modes.
calculated RE for a reflective photocathode is always worse than that of a photosensor with a semitransparent photocathode. This effect is more pronounced at lower L values. The measured energy resolution is worse than that predicted by simulation due to factors such as solid angle, photocathode non-uniformities, charge buildup, and electronic noise. Experimental results confirm the simulation results for the same L-value when the detectors are operated below the ionization threshold.
Acknowledgements Support is acknowledged to project CERN/FIS/ 43785/01. Travel support is acknowledged for JFCA Veloso to Fund. Calouste Gulbenkian (FCG) and Fund. para a Ci#encia e a Tecnologia, Lisbon, and for JMF dos Santos to Fund. LusoAmericana para o Desenvolvimento and FCG.
References [1] J.F.C.A. Veloso, et al., Nucl. Instr. and Meth. A 457 (2001) 253. [2] J.F.C.A. Veloso, et al., Nucl. Instr. and Meth. A 460 (2001) 297.
J.F.C.A. Veloso et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 223–227 [3] J.M.F. dos Santos, et al., X-ray Spectrom. 30 (2001) 373. [4] D.S.A.P. Freitas, et al., IEEE Trans. Nucl. Sci. NS-48 (2001) 411. [5] K. Zeitelhack, Nucl. Instr. and Meth. A 371 (1996) 57. [6] A. Breskin, et al., Nucl. Instr. and Meth. A 345 (1994) 205. [7] A. Sharma, et al., Nucl. Instr. and Meth. A 334 (1993) 420. [8] T.H.V.T. Dias, et al., J. Appl. Phys. 82 (1997) 2742.
227
[9] G.D. Alkazov, et al., Nucl. Instr. and Meth. 48 (1967) 1. [10] J. Miyamoto, et al., Nucl. Instr. and Meth. A 399 (1997) 85. [11] F.I.G.M. Borges, et al., Nucl. Instr. and Meth. A 422 (1999) 321. [12] G.F. Knoll, Radiation Detection and Measurement, 2nd Edition, Wiley, New York, 1989, pp. 174–178.