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A new silicon detector with good PSD properties
Nuclear Instruments and Methods in Physics Research A243 (1986) 603-604 North-Holland, Amsterdam
603
Letter to the Editor
A N E W S I L I C O N DETECTOR WITH G O O D P S D PROPERTIES S h n d o r DI~SI Training Reactor of the Technical University, 1521 Budapest, Hungary Received 6 September 1985
A new silicon detector has been constructed which shows very good PSD properties, making it possible to measure the individual components of mixed radiations.
The determination of the individual components in mixed radiation poses a number of problems concerning the measuring instrument. The conventional procedure of surveying different types of radiation is to detect each component using individual detectors with specific sensitivity against the given radiation. This necessitates multiple surveys with a set of instruments increasing the training and maintenance problems [1,2]. To overcome this problem, a variety of combined detectors has been constructed, most of them scintillation types and making use of the pulse shape discrimination (PSD) techniques [3,4]. Scintillation counters, however, are rather bulky, sensitive to mechanical effects and requiring high and stable voltages. To avoid these shortcomings a special type of semiconductor detector has been developed which is small, rugged and sensitive enough to be used either in multipurpose radiacs or in laboratory experiments when the measurement of charged particles is rendered more difficult in the presence of high gamma background radiation. The cross sectional view of the detector is shown in fig. 1. The detector is about 1 mm thick and made of p-type silicon. The entrance window (p + electrode) consists of a thin (less than 0.5 ~ m thick) alloyed aluminium layer which is rugged, stable and can be easily decontaminated if necessary. The n + contact is made of diffused lithium which has been drifted in the usual way to compensate most of the p-type silicon. However, the drifting process is stopped before reaching the entrance electrode, so a thin uncompensated p-type silicon remains between the p+-contact and the compensated
region. The thickness of this layer lies in the range of 10 /~m. By biasing the detector, a low electric field is established in this uncompensated region resulting in relatively slow charge collection as compared to that in the compensated volume. This low field, together with the diffusion of the charge carriers, results in the circumstance that for the detection of heavier charged particles with a range in the order of the uncompensated layer or less, the rise time of the corresponding electric pulses will be much longer than of those belonging to ionisation in the compensated volume and resulting mainly from interactions of electromagnetic radiations. I'he electronics for checking the detector are shown in fig. 2. The detector pulses are shaped using a delay line amplifier (DLA) followed by a pulse shape analyser (PSA). The pulses, according to their rise times, are converted into an amplitude spectrum by a time to pulse height converter (TPHC) and analysed by a multichannel analyser (MCA). At the same time, the selected T P H C pulses are used to gate the delayed linear pulses of the amplifier via the linear gate (LG) to be analysed and counted by a single channel analyser and scaler (SCA and SC, respectively). A 23~Pu alpha and a 6°Co gamma source have been used to investigate the detector characteristics. The count rate from the gamma source was 1500 cps above a 60
...p+{ALLOYED AI ); - "-~ 1 / --UNCOMPENSATED ~-. . . . . . . . q p-TYPe SILIOONi S . . . . . ",(~-...... \COMPENSATED n-"( ~ "-ID FFUSED Li)~ REGION Fig. 1. Cross sectional view of the detector. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 2. Block diagram of the measuring apparatus.
604
S. Dbsi / New silicon detector
10ns/CHANNEL
3000 W Z Z
< ~200C
F
),.Z =) 0
o 100(
20
_J
L0
60 CHANNEL
80 NUMBER
Fig. 3. Alphaandgammarisetimespectrum ofthedetector. keV discrimination level. The rise time spectrum measured by the M C A is shown in fig. 3. The alpha a n d g a m m a peaks are well separated (the alpha peak-to-val-
ley ratio is a b o u t 150) and the rise time of the alpha pulses is a b o u t 350 ns longer than that for gammas at 40 V detector bias. This result is confirmed by simple visual observation on an oscilloscope. The energy resolution of a 12 m m diameter detector is 10 keV for 241Am g a m m a radiation and a b o u t 50 keV for 241pu alphas (fwhm). Next, the detector entrance electrode has been covered with a layer of fine natural b o r o n carbide powder of less than 50 /~m thickness. The detector was irradiated with thermalized neutrons from a 252Cf neutron source and a g a m m a background of a b o u t 300 m R / h was established from a mixed 1~7Cs+6°Co g a m m a source at the site of the detector. The results of the measurements are shown in fig. 4. As can be seen, a g a m m a suppression of a b o u t 1000 was achieved using the PSD gating circuit of fig. 2 at a b o u t 2.5% absolute thermal neutron detection efficiency, by detecting the reaction products of the roB(n, a)Li nuclear reaction. Using b o r o n enriched in l°B, this neutron detection efficiency could be substantially increased. Finally, a simple low cost low power c o n s u m p t i o n PSD circuit has been designed which together with a specially tailored preamplifier and amplifier (total power c o n s u m p t i o n 1.6 m A at 9 V including the PSD circuit) proved very useful in constructing a versatile, cheap, lightweight multipurpose radiac.
LLI Z
;D ;D
E~E UJ n Z 0 0
S.C.A. DISCR
LEVEL V.
Fig. 4. Thermal neutron and gamma detection characteristics.
References
[1] C.J. Umbarger, G.O. Bjarke, B.H. Erkkila, F. Trujillo, D.A. Waechter and M.A. Wolf, IEEE Trans. Nucl. Sci. NS-30 (1983) 528. [2] M.A. Wolf, G.O. Bjarke, J.D. Jarrett and C.J. Umbarger, IEEE Trans. Nucl. Sci. NS-32 (1985) 964. [3] J.H. Kaye and R.A. Warner, IEEE Trans. Nucl. Sci. NS-29 (1982) 812. [4] B.H. Erkkila, M.A. WolL Y. Eisen, W.P. Unruh and R.J. Brake, IEEE Trans. Nucl. Sci. NS-32 (1985) 969.
603
Letter to the Editor
A N E W S I L I C O N DETECTOR WITH G O O D P S D PROPERTIES S h n d o r DI~SI Training Reactor of the Technical University, 1521 Budapest, Hungary Received 6 September 1985
A new silicon detector has been constructed which shows very good PSD properties, making it possible to measure the individual components of mixed radiations.
The determination of the individual components in mixed radiation poses a number of problems concerning the measuring instrument. The conventional procedure of surveying different types of radiation is to detect each component using individual detectors with specific sensitivity against the given radiation. This necessitates multiple surveys with a set of instruments increasing the training and maintenance problems [1,2]. To overcome this problem, a variety of combined detectors has been constructed, most of them scintillation types and making use of the pulse shape discrimination (PSD) techniques [3,4]. Scintillation counters, however, are rather bulky, sensitive to mechanical effects and requiring high and stable voltages. To avoid these shortcomings a special type of semiconductor detector has been developed which is small, rugged and sensitive enough to be used either in multipurpose radiacs or in laboratory experiments when the measurement of charged particles is rendered more difficult in the presence of high gamma background radiation. The cross sectional view of the detector is shown in fig. 1. The detector is about 1 mm thick and made of p-type silicon. The entrance window (p + electrode) consists of a thin (less than 0.5 ~ m thick) alloyed aluminium layer which is rugged, stable and can be easily decontaminated if necessary. The n + contact is made of diffused lithium which has been drifted in the usual way to compensate most of the p-type silicon. However, the drifting process is stopped before reaching the entrance electrode, so a thin uncompensated p-type silicon remains between the p+-contact and the compensated
region. The thickness of this layer lies in the range of 10 /~m. By biasing the detector, a low electric field is established in this uncompensated region resulting in relatively slow charge collection as compared to that in the compensated volume. This low field, together with the diffusion of the charge carriers, results in the circumstance that for the detection of heavier charged particles with a range in the order of the uncompensated layer or less, the rise time of the corresponding electric pulses will be much longer than of those belonging to ionisation in the compensated volume and resulting mainly from interactions of electromagnetic radiations. I'he electronics for checking the detector are shown in fig. 2. The detector pulses are shaped using a delay line amplifier (DLA) followed by a pulse shape analyser (PSA). The pulses, according to their rise times, are converted into an amplitude spectrum by a time to pulse height converter (TPHC) and analysed by a multichannel analyser (MCA). At the same time, the selected T P H C pulses are used to gate the delayed linear pulses of the amplifier via the linear gate (LG) to be analysed and counted by a single channel analyser and scaler (SCA and SC, respectively). A 23~Pu alpha and a 6°Co gamma source have been used to investigate the detector characteristics. The count rate from the gamma source was 1500 cps above a 60
...p+{ALLOYED AI ); - "-~ 1 / --UNCOMPENSATED ~-. . . . . . . . q p-TYPe SILIOONi S . . . . . ",(~-...... \COMPENSATED n-"( ~ "-ID FFUSED Li)~ REGION Fig. 1. Cross sectional view of the detector. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 2. Block diagram of the measuring apparatus.
604
S. Dbsi / New silicon detector
10ns/CHANNEL
3000 W Z Z
< ~200C
F
),.Z =) 0
o 100(
20
_J
L0
60 CHANNEL
80 NUMBER
Fig. 3. Alphaandgammarisetimespectrum ofthedetector. keV discrimination level. The rise time spectrum measured by the M C A is shown in fig. 3. The alpha a n d g a m m a peaks are well separated (the alpha peak-to-val-
ley ratio is a b o u t 150) and the rise time of the alpha pulses is a b o u t 350 ns longer than that for gammas at 40 V detector bias. This result is confirmed by simple visual observation on an oscilloscope. The energy resolution of a 12 m m diameter detector is 10 keV for 241Am g a m m a radiation and a b o u t 50 keV for 241pu alphas (fwhm). Next, the detector entrance electrode has been covered with a layer of fine natural b o r o n carbide powder of less than 50 /~m thickness. The detector was irradiated with thermalized neutrons from a 252Cf neutron source and a g a m m a background of a b o u t 300 m R / h was established from a mixed 1~7Cs+6°Co g a m m a source at the site of the detector. The results of the measurements are shown in fig. 4. As can be seen, a g a m m a suppression of a b o u t 1000 was achieved using the PSD gating circuit of fig. 2 at a b o u t 2.5% absolute thermal neutron detection efficiency, by detecting the reaction products of the roB(n, a)Li nuclear reaction. Using b o r o n enriched in l°B, this neutron detection efficiency could be substantially increased. Finally, a simple low cost low power c o n s u m p t i o n PSD circuit has been designed which together with a specially tailored preamplifier and amplifier (total power c o n s u m p t i o n 1.6 m A at 9 V including the PSD circuit) proved very useful in constructing a versatile, cheap, lightweight multipurpose radiac.
LLI Z
;D ;D
E~E UJ n Z 0 0
S.C.A. DISCR
LEVEL V.
Fig. 4. Thermal neutron and gamma detection characteristics.
References
[1] C.J. Umbarger, G.O. Bjarke, B.H. Erkkila, F. Trujillo, D.A. Waechter and M.A. Wolf, IEEE Trans. Nucl. Sci. NS-30 (1983) 528. [2] M.A. Wolf, G.O. Bjarke, J.D. Jarrett and C.J. Umbarger, IEEE Trans. Nucl. Sci. NS-32 (1985) 964. [3] J.H. Kaye and R.A. Warner, IEEE Trans. Nucl. Sci. NS-29 (1982) 812. [4] B.H. Erkkila, M.A. WolL Y. Eisen, W.P. Unruh and R.J. Brake, IEEE Trans. Nucl. Sci. NS-32 (1985) 969.