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The photomultiplier as a beta detector
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Nuclear Instruments and Methods in Physics Research A320 (1992) 393-395 North-Holland
INSTRUMENTS 1 RES
SeLtimA
Letter to the Editor
The photomultiplier as a beta detector Dinesh ßohra, Arvind Parihar and M.A. Padmanabha Rao
Radiation Safety Group, Defence Laboratory, Jodhpur 342001, India Received 11 March 1992
Investigation on the photomultiplier (PM) 96350B from Thorn EMI connected to a preamplifier revealed that it itself serves as an independent detector for external beta activity of which the performance has not been investigated by others since its introduction half a century ago. The definition of absolute efficiency when applied to the PM exhibited an overall efficiency of 24% in the detection of y"Sr+ y"Y which is mostly due to hard betas from 9') Y (E,3max = 2.27 MeV). Half of the counts observed by a scintillation detector (PM +PS) due to y"Sr+ y"Y comprised of the same PM and a 0.4 mm thin NE102A plastic scintillator (PS) are by the PM alone. The performance of the PM, represented as a percentage of that of the scintillation probe (PM + PS) in the detection of various beta emitters, showed a linear increase with E. ax . Furthermore, in the overall count rate due to the scintillation probe (PM + PS) the individual contributions have been experimentally determined due to the scintillators used in making the NE102A and the PM .
The photomultiplier (PM), since its introduction, is known as a versatile photosensitive device . Detection of gamma rays, X-rays, and energetic particles and measurement of beta energy could also be possible, only in association with plastic scintillators, inorganic scintillators or Cherenkov radiators [1-3]. Plastic scintillator (PS), typically 0.1 to 0.5 mm thin, kept over PM minimises background radiation, and has been used to monitor low energy beta emitters including tritium, and as an excellent alpha particle sensor [4]. In such a case the beta detection is believed to be entirely due to such a thin plastic scintillator but not due to the underneath PM at all . During our long course of research and development of thin plastic scintillators and evaluation of their performance by keeping each of them over the PM led to a suspicion that the PM might also be serving as a beta detector. It is known that decay of traces of naturally occurring isotopes 4"K, 23H U, and 232Th present in all window materials produce light by Cherenkov radiation and electrons by direct interaction with the photocathode that are detected by the I'M [5] . But no documentary evidence is available on the PM as a detector for external beta activity, on our literature survey. Thus the efficiency of a PM for beta detection is never known, since most of its uses are confined by coupling with a scintillator or a Cherenkov medium . Hence on these aspects investigations have been made on a probe comprised of a PM connected to a preamplifier . The purpose of this communication is to disclose the PM as a detector for external beta activity
and to denote its absolute efficiency. Moreover in the overall counts due to a scintillation detector fitted with a 0.4 mm thin NE102A, individual contributions due to the scintillators used in making the NE102A and the PM have been experimentally investigated . A probe consisting of mainly a PM of the type 9635QB from Thorn EMI with high gain (25 x 10') coupled to a preamplifier was employed in the study to investigate the performance of PM as a beta detector. The PM was coupled with a 0.4 mm thin NE102A (PS) to serve as a conventional scintillation detector (PM + PS) for beta detection . The detector was kept in a light tight metal casing with a detachable lid that facilitates frequent replacement of either the source or NE102A over the PM as desired . The detector housed in a lead case for further background reduction was connected through associated electronics to a 8K MCA (Canberra) . The sealed beta sources with aluminium casing and thin aluminium window employed in the study were procured from the Electronics Corporation of India Ltd ., Hyderabad and the Bhabha Atomic Research Centre, Trombay. A similar "Sr +9"Y standard source with absolute activity of 9'Sr (753 Bq) + 9('Y (753 Bq) and an active area with a diameter of 2.2 cm was used for estimation of absolute efficiency of the PM. Counts have been noted with each of the following beta sources kept directly over the PM: 63Ni (E(3max 0.067 MeV), '4C (0.156 MeV), 2+'4TI (0 .766 McV), Ra D&E (1 .16 MeV), and "Sr +"Y ("Y = 2.27 MeV). The counts have been found to be significantly high over the background level, even in case of the weak
0168-9002/92/$05 .0(1 © 1992 - Elsevier Science Publishers B.V . All rights reserved
394
D. Bohra et al. / Photomidtiplier as 13 detector
63 beta (Ejimax = 0.067 MeV) emitter, Ni (22.2 kBq). It shows that those betas and their secondary electrons ultimately detected could penetrate through the quartz window, the thickness of which is unknown to us. The counts with the PM have been improved with the increase in Ma energy. Our results disclose that the PM itself serves as a detector for external beta activity. We have extended the conventional definition of a_hsehite efficiency [6l to PM as a beta detector . The probe fitted with the PM alone exhibited an overall absolute efficiency of nearly 24% in detecting `'()Sr + "Y, and should mostly be due to the energetic betas from "')Y (E(3max = 2.27 MeV) as can be evident from the PM contribution in fig . 1. The PM efficiency is anticipated to be still higher than 24% for betas with Eßm;, x exceeding 2.27 MeV. An attempt has been made to compare the perfomance of the PM connected to a preamplifier as an independent beta probe with that of the conventional scintillation probe (PM + PS) comprised of the same PM coupled to a 0.4 mm thin NE102A . For this purpose counts have been noted again by keeping each of the sources directly over the NE102A of the above probe, compared with those observed earlier with the PM alone and represented the performance of PM as percent of that of the scintillation probe (PM + PS) as shown in fig. 1. The experimentally investigated performance of PM as a percentage of that of the scintillation detector (PM + PS) with each of the beta sources when plotted at their respective Epmax shows a linear increase with Eßmax in the energy range E13max = 0 .067-2.27 MeV. Fig . 1 reveals that the performance of the PM in the detection of "'Sr +"'Y (1'Y, in particular) is nearly half (54%) of that of the scintillation probe (PM + PS) . In other words that half of the counts observed due to "')Sr + 9 'Y by the scintillation detector are by the PM alone.
100 N ar E a 80 0 Z 0 F= 60 Ca
N 100 ar E a 80 0 Z 0 60 m F Z 0 u
40
W
20
F Z
u
z 0 u F u
W
a
Further investigation has been made to know the percentage of betas that penetrate through the NE102A and are detected by the underneath PM. While taking the earlier observations with the PM the sources were placed directly over it, but when coupled with the NE102A (PS) sources were seperated from the PM by 0.4 mm, the thickness of the NE102A. Since the plastic material of the NE102A absorbs some betas while penetrating its 0.4 mm thickness, in order to compensat~; 101 LP's thickness in case of the PM, a 0.4 mm thin transparent plastic sheet was interposed between the PM and source. and counts were once again recorded with each of the sources . Each reading would reflect the numbe: of betas originated from the source that could be detected by the PM having penetrated through the NE102A. These observations with the PM were compared with those taken earlier with the scintillation detector (PM + PS) and plotted as percent of betas detected by the PM after penetration through NE102A in comparison to the overall counts due to the detector (PM + PS) observed with each source at their respective E pm (fig. 2). Fig . 2 shows that in the detection of `'"Sr +'()Y by the scintillation detector (PM + PS) nearly half (46%) of the overall counts are due to the PM alone . The remaining 54% should therefore be due to the scintillators used in making the NE102A, though the nature of the scintillators used by the manufacturers is unknown to us. Likewise the percentage of contributions due to scintillators were evaluated for the rest of the sources and plotted against their respective E)3max (fig. 2). In the detection of beta activity by the detector (PM + PS) the contributions due to PM increases linearly with E pm ;,x , while it falls similarly in the case of scintillators . In case of the weak beta emitters ' 4C and '3 Ni the PM contribution is less than 1%. In other words at very low energies scintillator contribution tends to be the highest .
40 20
W
a
0 E ßmax(MeV)
Fig. 1 . Performance of the PM 9635QB from Thorn EMI as percent of that of the scintillation probe comprised of the same PM coupled to a 0 .4 mm thin NE102A, in the detection of beta emitters at their respective Epm;,x,
0
1 .0
E bmax (MeV)
2 .0
3 .0
Fig. 2. In the overall counts due to the scintillation probe (PM + PS), the individual contributions due to the scintillators used in making the NE102A and the PM in the detection of various beta emitters at their respective Elm ;,,, are shown.
D. Behra et aL / Photomultiplier as ß detector
Though the PM can detect and measure external beta activity it poses a problem to identify the beta emitter involved, as the pulse height spectrum fails to reflect its characteristic beta energy spectrum due to incomplete absorption of beta energy within the thin photocathode layer. The pulse height spectra of 2"4 TI, Ra D& E and "Sr + yu Y can be attributed due to Cherenkov radiation by interaction of their hard betas with the quartz window material of the PM [6]. Our results suggest that in the detection of hard betas by a scintillation detector comprised of a thin plastic scintillator, photoelectrons are generated at the photocathode by both the scintillations produced in the plastic scintillator and by the light due to Cherenkov effect in the window of the PM. In the case of 63 Ni and '4C their weak betas fail to produce Cherenkov radiation light in the quartz window [6] with refractive index of 1 .547 for its faster ray, while that of the slower varies from 1 .547 to 1 .556 [7] as the threshold beta energy [6] required to produce Cherenkov radiation is estimated to be around 0.156 MeV. Since Cherenkov emission is biased towards UV the quartz window variants give the largest pulse height. As this particular PM 9635QB is not ideally suited among others for Cherenkov radiation detection [5] selection of a PM highly efficient for Cherenkov radiation may improve the efficiency further in beta detection . The pulse height spectra can
395
also be due to Iliberation of secondary electrons by direct interaction of betas from an external source with the window of the photocathode which should be the t;ase with 63 Ni and " a C in particular. The contribution due to bremsstrahlung generated by interaction of betas with the quartz window is anticipated to be very much lower than that of the betas . References [l1 RCA Photomultiplier Manual, Technical Series PT-61,
RCA/Electronic Components/Harrison, NJ (1970). [2) G. Zanella and R. Zannoni, Nucl. Instr. and Meth . A-302 (1991) 352. [3] H. Ejiri, K. Higa, T. Kamada, H. Kohiki, K. Matsuoka, K. Okada, H. Sano, T. Shibata, T. Shirna, N. Tanabe, T. Tanaka, T. Taniguchi, T. Waianabe and N. Yamamoto, Nucl . Instr. and Meth . A302 (1991) 304. [4) C.R . Hurlbut, Plastic Scintiilators, a survey, American Nuclear Society Winter Meeting, printed by Bicron Corporation, Newbury, Ohio (November 1985). [5J Thorn EMI Photomultipliers Manual, PMC/86, Thorn EMI Electron Tubes Limited, Middlesex HA4 TTA, UK. [6J G.F . Knoll, Radiation Detection and Measurement (Wiley, USA, 1979). [7] E. Mach, The Principles of Physical Optics (Dover, USA, 1926) p. 221 .
Nuclear Instruments and Methods in Physics Research A320 (1992) 393-395 North-Holland
INSTRUMENTS 1 RES
SeLtimA
Letter to the Editor
The photomultiplier as a beta detector Dinesh ßohra, Arvind Parihar and M.A. Padmanabha Rao
Radiation Safety Group, Defence Laboratory, Jodhpur 342001, India Received 11 March 1992
Investigation on the photomultiplier (PM) 96350B from Thorn EMI connected to a preamplifier revealed that it itself serves as an independent detector for external beta activity of which the performance has not been investigated by others since its introduction half a century ago. The definition of absolute efficiency when applied to the PM exhibited an overall efficiency of 24% in the detection of y"Sr+ y"Y which is mostly due to hard betas from 9') Y (E,3max = 2.27 MeV). Half of the counts observed by a scintillation detector (PM +PS) due to y"Sr+ y"Y comprised of the same PM and a 0.4 mm thin NE102A plastic scintillator (PS) are by the PM alone. The performance of the PM, represented as a percentage of that of the scintillation probe (PM + PS) in the detection of various beta emitters, showed a linear increase with E. ax . Furthermore, in the overall count rate due to the scintillation probe (PM + PS) the individual contributions have been experimentally determined due to the scintillators used in making the NE102A and the PM .
The photomultiplier (PM), since its introduction, is known as a versatile photosensitive device . Detection of gamma rays, X-rays, and energetic particles and measurement of beta energy could also be possible, only in association with plastic scintillators, inorganic scintillators or Cherenkov radiators [1-3]. Plastic scintillator (PS), typically 0.1 to 0.5 mm thin, kept over PM minimises background radiation, and has been used to monitor low energy beta emitters including tritium, and as an excellent alpha particle sensor [4]. In such a case the beta detection is believed to be entirely due to such a thin plastic scintillator but not due to the underneath PM at all . During our long course of research and development of thin plastic scintillators and evaluation of their performance by keeping each of them over the PM led to a suspicion that the PM might also be serving as a beta detector. It is known that decay of traces of naturally occurring isotopes 4"K, 23H U, and 232Th present in all window materials produce light by Cherenkov radiation and electrons by direct interaction with the photocathode that are detected by the I'M [5] . But no documentary evidence is available on the PM as a detector for external beta activity, on our literature survey. Thus the efficiency of a PM for beta detection is never known, since most of its uses are confined by coupling with a scintillator or a Cherenkov medium . Hence on these aspects investigations have been made on a probe comprised of a PM connected to a preamplifier . The purpose of this communication is to disclose the PM as a detector for external beta activity
and to denote its absolute efficiency. Moreover in the overall counts due to a scintillation detector fitted with a 0.4 mm thin NE102A, individual contributions due to the scintillators used in making the NE102A and the PM have been experimentally investigated . A probe consisting of mainly a PM of the type 9635QB from Thorn EMI with high gain (25 x 10') coupled to a preamplifier was employed in the study to investigate the performance of PM as a beta detector. The PM was coupled with a 0.4 mm thin NE102A (PS) to serve as a conventional scintillation detector (PM + PS) for beta detection . The detector was kept in a light tight metal casing with a detachable lid that facilitates frequent replacement of either the source or NE102A over the PM as desired . The detector housed in a lead case for further background reduction was connected through associated electronics to a 8K MCA (Canberra) . The sealed beta sources with aluminium casing and thin aluminium window employed in the study were procured from the Electronics Corporation of India Ltd ., Hyderabad and the Bhabha Atomic Research Centre, Trombay. A similar "Sr +9"Y standard source with absolute activity of 9'Sr (753 Bq) + 9('Y (753 Bq) and an active area with a diameter of 2.2 cm was used for estimation of absolute efficiency of the PM. Counts have been noted with each of the following beta sources kept directly over the PM: 63Ni (E(3max 0.067 MeV), '4C (0.156 MeV), 2+'4TI (0 .766 McV), Ra D&E (1 .16 MeV), and "Sr +"Y ("Y = 2.27 MeV). The counts have been found to be significantly high over the background level, even in case of the weak
0168-9002/92/$05 .0(1 © 1992 - Elsevier Science Publishers B.V . All rights reserved
394
D. Bohra et al. / Photomidtiplier as 13 detector
63 beta (Ejimax = 0.067 MeV) emitter, Ni (22.2 kBq). It shows that those betas and their secondary electrons ultimately detected could penetrate through the quartz window, the thickness of which is unknown to us. The counts with the PM have been improved with the increase in Ma energy. Our results disclose that the PM itself serves as a detector for external beta activity. We have extended the conventional definition of a_hsehite efficiency [6l to PM as a beta detector . The probe fitted with the PM alone exhibited an overall absolute efficiency of nearly 24% in detecting `'()Sr + "Y, and should mostly be due to the energetic betas from "')Y (E(3max = 2.27 MeV) as can be evident from the PM contribution in fig . 1. The PM efficiency is anticipated to be still higher than 24% for betas with Eßm;, x exceeding 2.27 MeV. An attempt has been made to compare the perfomance of the PM connected to a preamplifier as an independent beta probe with that of the conventional scintillation probe (PM + PS) comprised of the same PM coupled to a 0.4 mm thin NE102A . For this purpose counts have been noted again by keeping each of the sources directly over the NE102A of the above probe, compared with those observed earlier with the PM alone and represented the performance of PM as percent of that of the scintillation probe (PM + PS) as shown in fig. 1. The experimentally investigated performance of PM as a percentage of that of the scintillation detector (PM + PS) with each of the beta sources when plotted at their respective Epmax shows a linear increase with Eßmax in the energy range E13max = 0 .067-2.27 MeV. Fig . 1 reveals that the performance of the PM in the detection of "'Sr +"'Y (1'Y, in particular) is nearly half (54%) of that of the scintillation probe (PM + PS) . In other words that half of the counts observed due to "')Sr + 9 'Y by the scintillation detector are by the PM alone.
100 N ar E a 80 0 Z 0 F= 60 Ca
N 100 ar E a 80 0 Z 0 60 m F Z 0 u
40
W
20
F Z
u
z 0 u F u
W
a
Further investigation has been made to know the percentage of betas that penetrate through the NE102A and are detected by the underneath PM. While taking the earlier observations with the PM the sources were placed directly over it, but when coupled with the NE102A (PS) sources were seperated from the PM by 0.4 mm, the thickness of the NE102A. Since the plastic material of the NE102A absorbs some betas while penetrating its 0.4 mm thickness, in order to compensat~; 101 LP's thickness in case of the PM, a 0.4 mm thin transparent plastic sheet was interposed between the PM and source. and counts were once again recorded with each of the sources . Each reading would reflect the numbe: of betas originated from the source that could be detected by the PM having penetrated through the NE102A. These observations with the PM were compared with those taken earlier with the scintillation detector (PM + PS) and plotted as percent of betas detected by the PM after penetration through NE102A in comparison to the overall counts due to the detector (PM + PS) observed with each source at their respective E pm (fig. 2). Fig . 2 shows that in the detection of `'"Sr +'()Y by the scintillation detector (PM + PS) nearly half (46%) of the overall counts are due to the PM alone . The remaining 54% should therefore be due to the scintillators used in making the NE102A, though the nature of the scintillators used by the manufacturers is unknown to us. Likewise the percentage of contributions due to scintillators were evaluated for the rest of the sources and plotted against their respective E)3max (fig. 2). In the detection of beta activity by the detector (PM + PS) the contributions due to PM increases linearly with E pm ;,x , while it falls similarly in the case of scintillators . In case of the weak beta emitters ' 4C and '3 Ni the PM contribution is less than 1%. In other words at very low energies scintillator contribution tends to be the highest .
40 20
W
a
0 E ßmax(MeV)
Fig. 1 . Performance of the PM 9635QB from Thorn EMI as percent of that of the scintillation probe comprised of the same PM coupled to a 0 .4 mm thin NE102A, in the detection of beta emitters at their respective Epm;,x,
0
1 .0
E bmax (MeV)
2 .0
3 .0
Fig. 2. In the overall counts due to the scintillation probe (PM + PS), the individual contributions due to the scintillators used in making the NE102A and the PM in the detection of various beta emitters at their respective Elm ;,,, are shown.
D. Behra et aL / Photomultiplier as ß detector
Though the PM can detect and measure external beta activity it poses a problem to identify the beta emitter involved, as the pulse height spectrum fails to reflect its characteristic beta energy spectrum due to incomplete absorption of beta energy within the thin photocathode layer. The pulse height spectra of 2"4 TI, Ra D& E and "Sr + yu Y can be attributed due to Cherenkov radiation by interaction of their hard betas with the quartz window material of the PM [6]. Our results suggest that in the detection of hard betas by a scintillation detector comprised of a thin plastic scintillator, photoelectrons are generated at the photocathode by both the scintillations produced in the plastic scintillator and by the light due to Cherenkov effect in the window of the PM. In the case of 63 Ni and '4C their weak betas fail to produce Cherenkov radiation light in the quartz window [6] with refractive index of 1 .547 for its faster ray, while that of the slower varies from 1 .547 to 1 .556 [7] as the threshold beta energy [6] required to produce Cherenkov radiation is estimated to be around 0.156 MeV. Since Cherenkov emission is biased towards UV the quartz window variants give the largest pulse height. As this particular PM 9635QB is not ideally suited among others for Cherenkov radiation detection [5] selection of a PM highly efficient for Cherenkov radiation may improve the efficiency further in beta detection . The pulse height spectra can
395
also be due to Iliberation of secondary electrons by direct interaction of betas from an external source with the window of the photocathode which should be the t;ase with 63 Ni and " a C in particular. The contribution due to bremsstrahlung generated by interaction of betas with the quartz window is anticipated to be very much lower than that of the betas . References [l1 RCA Photomultiplier Manual, Technical Series PT-61,
RCA/Electronic Components/Harrison, NJ (1970). [2) G. Zanella and R. Zannoni, Nucl. Instr. and Meth . A-302 (1991) 352. [3] H. Ejiri, K. Higa, T. Kamada, H. Kohiki, K. Matsuoka, K. Okada, H. Sano, T. Shibata, T. Shirna, N. Tanabe, T. Tanaka, T. Taniguchi, T. Waianabe and N. Yamamoto, Nucl . Instr. and Meth . A302 (1991) 304. [4) C.R . Hurlbut, Plastic Scintiilators, a survey, American Nuclear Society Winter Meeting, printed by Bicron Corporation, Newbury, Ohio (November 1985). [5J Thorn EMI Photomultipliers Manual, PMC/86, Thorn EMI Electron Tubes Limited, Middlesex HA4 TTA, UK. [6J G.F . Knoll, Radiation Detection and Measurement (Wiley, USA, 1979). [7] E. Mach, The Principles of Physical Optics (Dover, USA, 1926) p. 221 .