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Silicon detectors of nuclear radiation produced by low energy ion implantation
95
Nuclear Instruments and Methods in Physics Research B35 (1988) 95-99 North-Holland, Amsterdam
SILICON DETECTORS OF NUCLEAR RADIATION BY LOW ENERGY ION IMPLANTATION
PRODUCED
D. SUEVA Faculty of Physics,
N. CHIKOV, Institute
University
B. AMOV
of Sofa, 5, A. Ivanoo BIvd II26 So& Bulgaria and N. KALINKOVA
of Nuclear Research and Nuclear Energy, Bulgarian Academ.v of Sciences,
72, Lenin Blvd., 1784 Sofia, Bulgaria
Received 4 November 1987 and in revised form 30 March 1988
Silicon detectors for alpha, gamma, neutron and fission fragment radiation spectrometry have been prepared, by applying low energy (3 and 5 keV) ion implantation techniques together with appropriate mechanical and chemical treatment. The spectrometric measurements were carried out at room temperature.The radiation stability of the detectors was checked in mixed gamma-neutron fields.
Recent achievements in the state-of-the-art of semiconductor production offer a possibility to prepare good quality detectors of nuclear radiation, based on high purity silicon crystals. A wide variety of detectors has been developed [I] with diffused p-n junctions, surface barrier structures and ion implanted. Low energy ion implantation provides the advantage to accurately control the concentration and profile of the implanted dopants, thus ensuring abrupt p-n junctions and extremely thin dead layers. A low temperature treatment is adequate for the annealing of irradiation-induced defects. These requirements are of major importance for the production of high quality detectors of nuclear radiation, especially if they are designed for the spectrometry of charged particles. The low energy ion implantation method has been successfully applied by Kalbitzer et al. [2] They have implanted boron ions onto n-type silicon, using energies within the range 2-2.5 keV. We have succeeded in developing several types of silicon detectors also, by applying the low energy implantation method. The low energy of the implanted ions and the adequate mechanical and chemical treatment of the sample surfaces ensure p-n junctions and ohmic contacts of excellent quality. [3] The starting materials used were n- and p-type silicon wafers with specific resistivity 2000 G cm and loo0 52 em, respectively. The doping was carried out at room temperature, by implanting II, and 31, at an energy of 3 keV for the creation of p-n junctions and 5 keV for the formation of back contacts at a dose rate of 1 x 1Or5 0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
ions cm- *. The electrical conductivity of the surface layers implanted under the described conditions is sufficiently high to allow the use of pressure contacts, thus eliminating the need to deposit supplementary metallic electrodes. After 30 mm thermal treatment in argon atmosphere at 400 ’ C, the surface of the samples was processed chemically as follows: the implanted surface was covered with an acid-resistant foil and the sample was etched in CP-4 etchant. Following ample rinsing and boiling in deionized water, the foil was removed and the sample was immersed in hydrofluoiic acid and washed again with deionized water. This chemical treatment decreased additionally the thickness of the dead layer and the reverse current of the detectors. Detectors for the spectrometric measurements of alpha particles were developed on the basis of both nand p-type silicon wafers with 30 mm2 active area and reverse currents less than 0.5 PA at 450 V. The energy resolution power of these detectors was 16-20 keV at a reverse voltage of 50 V and alpha particles with energy 5.8 MeV (fig. I). The same detectors were used at room temperature and 200 V for the detection and spectrometric measurements of gamma radiation. A neutron spectrometer was designed on the basis of two ion implanted silicon detectors, each having an area of 90 mm2. One of the detectors was coated with a vapour-phase-deposited, 600 pg cm-* thick 6LiF layer. Both detectors were fixed in a single “sandwich’‘-type packing (fig. 2). This spectrometer was used for the measurement of the energy spectrum of neutrons in horizontal channel II of the experimental nuclear reac-
96
D. Sueva et al. / Silicon nuclear radiation detectors
Fig. 2. Design of a neutron spectrometer with two ion implanted detectors: (1,2) Teflon body; (3,4) n-type silicon detectors.
50
250
203
150
Kx)
Channel
The energy spectrum of neutrons formed by the 6Li(n, o)T reaction, measured during the irradation of the detectors with 14 MeV neutrons, is shown in fig. 3. For the calibration, the overall spectrum of alpha particles and tritons was traced during irradiation of the detectors with thermal neutrons. The spectra in fig. 3 show the possibilities of this neutron spectrometer. Accurate energy calibration of the neutron spectra during the reaction with 14 MeV neutrons is a rather complex task, beyond the scope of this study. When semiconductor detectors are used for the spectrometry of fission fragments, various phenomena emerge which interfere with the accuracy of the measurements. This is, above all, the so-called “amplitude effect”, i.e. the difference between the real energy of the incident fragments and the energy registered by the detector. This effect depends on the energy losses of the
Numer
Fig. 1. Spectrum of alpha particles from 239Pu, 24’Am and 244Cm, measured using an ion implanted p-silicon detector with area 30 mm* at room temperatureand 50 V.
tor IRT-2000 and from the neutron generator SAMESD-150. All measurements were carried out at room temperature and normal atmospheric pressure.
...:....d
:‘...:.
.-
~t,_.‘.c
! I
50
100
I
150
Channel
Fig. 3. Neutron
spectra
of the 6Li(n, (Y)reaction
during irradiation
2:..>.:. .,.
I
.x0
250 Number
with (a) 14 MeV neutrons,
(b) thermal
neutrons.
97
D. Sueua et al. / Silicon nuclear radiation detectors
---
---
--
.. 1 .I!
- - ----
vi.
I I
\
I I
0
10
26
30
LO
50
60
Channel
70
a0
90
Number
Fig. 5. Spectrum of fission fragments from 235U measured using an ion implanted p-silicon detector, area 4 cm2 at room temperature and 200 V.
Fig. 4. Alpha spectrum measured with an ion implanted p-silicon detector, area 4 cm* at room temperature and 200 V.
CipFl
I
-
30
720
__,____.,_,_____
.__..__. - ..-..
-..-,
-..-..-.,-..-“-..-.’
-lO
..-.__
d
1 -2L
-22
-26
I _ -r&J
-16
-IL
-12
-10
-8
-6
-L
-2
0
+2
+L
U( VI Capacitance-voltage characteristics of an ion implanted p-silicon detector, area 4 cm2. Detector not (curve a); detector irradiated with dose rates of 5.8 x 10” neutrons cm-* (curve b), 4.3 x 1014 neutrons cm-* neutrons cm-* (curve d); and after annealing at 500 o C during 30 min (curve e). Fig.
6.
exposedto radiation (curve c), 6.3
x
lOI
98
D. Sueoa et al. / Silicon nuclear radiation detectors
fragments in the dead layer of the detector, losses which occur due to elastic scattering by the nuclei in the sensitive layer and failure to register completetly all current carriers of the charge. The main part of the energy losses are in the dead layer, therefore low energy ion implantation is a very promising method for the production of silicon detectors with extremely thin dead layers, designed for spectrometry of fission fragments and other heavy charged particles. Spectrometric measurements of fragments from the fission of heavy nuclei subjected to neutron irradiation were carried out, using p-type silicon detectors with an area of 4 cm’. The reverse current of similar detectors was less than 3 PA at 450 V. The alpha spectrum measured with this type of detectors is shown in fig. 4, while fig. 5 presents the spectrum of ‘?‘IJ fission fragments. Both spectra were measured at 220 V, room temperature and in vacuum. The evaluated ratios NJN,,,, = 13, n,/N,, = 10 and N,/N,, = 1.4 are close to the calibration values given by Schmit [S], suggesting that energy losses of fission fragments in the dead layer are slight. It is well known that some of the basic characteristics of silicon which determine the properties of semiconductor detectors, e.g. lifetime, mobility of current carriers, specific electrical resistance, etc., are abruptly changed when certain critical limits of the integral radiation dose for the given type of radiation are reached. The value of the critical integral dose of the ionizing radiation depends on the properties of the semiconductor material and the method used for the production of the detector. The radiation stability of different types of semiconductor detectors for nuclear radiation has been studied by several authors [6-91. The surface barrier detectors display a short lifetime when used for the measurement of fission fragments of heavy nuclei and other heavy charged particles. The ion implanted detectors are more suitable for similar measurements. They can withstand heating to fairly high temperatures, as used for annealing the radiation defects and recovering the parameters of the detectors. For this purpose pressure contacts are used, thus eliminating the need for additional metal deposition onto the p- and n-type silicon layers. The radiation stability of the detectors prepared by us was checked in mixed gamma-neutron fields of the “V” vertical channel of the experimental nuclear reactor IRT-2000. A cadmium foil was used as a shield against slow neutrons. The neutron irradiation doses varied within the range 3.7 x 109-6.3 x 1014 neutrons cme2 [lo]. Up to doses of 4.3 x lOI neutrons cm-* no noticeable change in the electrical characteristics of the detectors (current-voltage) and capacitance-voltage relationships) was observed (fig. 6). On reaching a dose of 6.3 x 1014 neutrons cme2, the reverse current of the
.I E6kev--_
‘-
,..
‘. . .
..I(’
.,..:
,...,,.:I.’
,
..
kJ
._._, .,
id1
. .
.. -..Y.,~
,
,
(
i
i‘.......l
1
8
I-
Channel Number
Fig. 7. Spectra of alpha particles from 23QPu, 241Am and 2”Cm, measured using an ion implanted p-silicon detector with area 4 cm2. Detector not exposed to radiation (spectrum a); detector irradiated with dose rates of 4.3X10t4 neutrons cm-’ (spectrum b immediately after irradiation and spectrum c after 2 h), 6.3x 1014 neutrons cm-’ (spectrum d); and after annealing at 500 ’ C for 30 min (spectrum e).
detector abruptly increased, while the capacitance of the p-n junction decreased (fig. 6). The resolution of detectors not subjected to irradiation or irradiated with a dose lower than 4.3 X 1014 neutrons cmP2 was approximately identical - 26.3 keV for 5.8 MeV alpha particles from 241Am (fig. 7). On reaching a dose of 4.3 X lOI4 neutrons cm-* a shift of the spectrum toward lower energies was’observed. At dose rates of 6.3 x lOi neutrons cm-’ or higher the detectors no longer retained their spectrometric properties. Annealing at 500° C during 30 min in an argon atmosphere reduced the reverse current of the p-n junction, but its value remained higher than that prior to irradiation. The capacitance-voltage characteristics of a detector irradiated with a dose of 6.3 X 1Or4 neutrons cm-’ (curve d) was improved after annealing (curve e) and was quite similar to that of detectors
D. Sueua et al. / Silicon nuclear radiation detectors
exposed to lower doses (curve c) or not subjected to radiation (curve a). Following annealing the resolution of the detectors also improved, but remained inferior as compared with detectors not subjected to irradiation, e.g. 76 keV against 26 keV in the case of 5.156 MeV alpha particles from 239Pu. These results may be explained by the behaviour of the minority current carriers during the respective treatments. The current-voltage characteristics and the resolution of the detectors were strongly affected by the lifetime of the minority carriers, whereas the capacitance-voltage dependence was only slightly sensitive. It may be concluded that silicon detectors produced by low energy ion implantation are suitable for spectrometric measurements of nuclear radiation and charged particles. Their dead layers are extremely thin and display a good energy resolution not only for alpha and gamma spectrometric measurements, but for neutron and fission fragment spectrometry as well. The detectors are stable against irradiation with fast neutrons at dose rates up to 4.3 x 1014 neutrons cm-2, but at higher doses the spectrometric properties disappear. The thermal treatment of irradiated ion implanted detectors
99
offers a possibility to anneal radiation defects and partially improve the spectrometric properties of the detectors.
References 111 G. Bertolini and A. Cache, Semiconductor Detectors
(North-Holland, 1968). 121 S. Kalbitzer, R. Bacler, H. Herzer and K. Betge, Z. Phys. 203 (1967) 117. 131 D. Sueva, B. Amov, A. Dzhakov and N. Chikov, Bulg. J. Phys. 4/5 (1977) 489. 141 D. Sueva, N. Chikov, N. Kalinkova and Ts. Panteleev, Nucl. Energy 22 (1985) 23. 151 A. Schmith, Nucl. Instr. and Meth. 40 (1966) 204. 161 T. Yang, Nucl. Instr. and Meth. 100 (1972) 533. 171 D. Sachelarie, M. Dragan, M. Stoica and M. Sachelarie, Phys. Status Solidi (a) 65 (1981) 379. VI I. Koitman, K. Muminov and A. Yatasov, Phys. Stat. Sol. (a) 71 (1982) 59. [91 J. Kemmer, P. Burger, R. Henck and E. Heijne, IEEE Trans. Nucl. Sci. NS-29 (1982) 733. [lOI D. Sueva, N. Chikov, N. Kalinkova, I. Ruskov and S. Kaschieva, Nucl. Energy 23 (1986) 3.
Nuclear Instruments and Methods in Physics Research B35 (1988) 95-99 North-Holland, Amsterdam
SILICON DETECTORS OF NUCLEAR RADIATION BY LOW ENERGY ION IMPLANTATION
PRODUCED
D. SUEVA Faculty of Physics,
N. CHIKOV, Institute
University
B. AMOV
of Sofa, 5, A. Ivanoo BIvd II26 So& Bulgaria and N. KALINKOVA
of Nuclear Research and Nuclear Energy, Bulgarian Academ.v of Sciences,
72, Lenin Blvd., 1784 Sofia, Bulgaria
Received 4 November 1987 and in revised form 30 March 1988
Silicon detectors for alpha, gamma, neutron and fission fragment radiation spectrometry have been prepared, by applying low energy (3 and 5 keV) ion implantation techniques together with appropriate mechanical and chemical treatment. The spectrometric measurements were carried out at room temperature.The radiation stability of the detectors was checked in mixed gamma-neutron fields.
Recent achievements in the state-of-the-art of semiconductor production offer a possibility to prepare good quality detectors of nuclear radiation, based on high purity silicon crystals. A wide variety of detectors has been developed [I] with diffused p-n junctions, surface barrier structures and ion implanted. Low energy ion implantation provides the advantage to accurately control the concentration and profile of the implanted dopants, thus ensuring abrupt p-n junctions and extremely thin dead layers. A low temperature treatment is adequate for the annealing of irradiation-induced defects. These requirements are of major importance for the production of high quality detectors of nuclear radiation, especially if they are designed for the spectrometry of charged particles. The low energy ion implantation method has been successfully applied by Kalbitzer et al. [2] They have implanted boron ions onto n-type silicon, using energies within the range 2-2.5 keV. We have succeeded in developing several types of silicon detectors also, by applying the low energy implantation method. The low energy of the implanted ions and the adequate mechanical and chemical treatment of the sample surfaces ensure p-n junctions and ohmic contacts of excellent quality. [3] The starting materials used were n- and p-type silicon wafers with specific resistivity 2000 G cm and loo0 52 em, respectively. The doping was carried out at room temperature, by implanting II, and 31, at an energy of 3 keV for the creation of p-n junctions and 5 keV for the formation of back contacts at a dose rate of 1 x 1Or5 0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
ions cm- *. The electrical conductivity of the surface layers implanted under the described conditions is sufficiently high to allow the use of pressure contacts, thus eliminating the need to deposit supplementary metallic electrodes. After 30 mm thermal treatment in argon atmosphere at 400 ’ C, the surface of the samples was processed chemically as follows: the implanted surface was covered with an acid-resistant foil and the sample was etched in CP-4 etchant. Following ample rinsing and boiling in deionized water, the foil was removed and the sample was immersed in hydrofluoiic acid and washed again with deionized water. This chemical treatment decreased additionally the thickness of the dead layer and the reverse current of the detectors. Detectors for the spectrometric measurements of alpha particles were developed on the basis of both nand p-type silicon wafers with 30 mm2 active area and reverse currents less than 0.5 PA at 450 V. The energy resolution power of these detectors was 16-20 keV at a reverse voltage of 50 V and alpha particles with energy 5.8 MeV (fig. I). The same detectors were used at room temperature and 200 V for the detection and spectrometric measurements of gamma radiation. A neutron spectrometer was designed on the basis of two ion implanted silicon detectors, each having an area of 90 mm2. One of the detectors was coated with a vapour-phase-deposited, 600 pg cm-* thick 6LiF layer. Both detectors were fixed in a single “sandwich’‘-type packing (fig. 2). This spectrometer was used for the measurement of the energy spectrum of neutrons in horizontal channel II of the experimental nuclear reac-
96
D. Sueva et al. / Silicon nuclear radiation detectors
Fig. 2. Design of a neutron spectrometer with two ion implanted detectors: (1,2) Teflon body; (3,4) n-type silicon detectors.
50
250
203
150
Kx)
Channel
The energy spectrum of neutrons formed by the 6Li(n, o)T reaction, measured during the irradation of the detectors with 14 MeV neutrons, is shown in fig. 3. For the calibration, the overall spectrum of alpha particles and tritons was traced during irradiation of the detectors with thermal neutrons. The spectra in fig. 3 show the possibilities of this neutron spectrometer. Accurate energy calibration of the neutron spectra during the reaction with 14 MeV neutrons is a rather complex task, beyond the scope of this study. When semiconductor detectors are used for the spectrometry of fission fragments, various phenomena emerge which interfere with the accuracy of the measurements. This is, above all, the so-called “amplitude effect”, i.e. the difference between the real energy of the incident fragments and the energy registered by the detector. This effect depends on the energy losses of the
Numer
Fig. 1. Spectrum of alpha particles from 239Pu, 24’Am and 244Cm, measured using an ion implanted p-silicon detector with area 30 mm* at room temperatureand 50 V.
tor IRT-2000 and from the neutron generator SAMESD-150. All measurements were carried out at room temperature and normal atmospheric pressure.
...:....d
:‘...:.
.-
~t,_.‘.c
! I
50
100
I
150
Channel
Fig. 3. Neutron
spectra
of the 6Li(n, (Y)reaction
during irradiation
2:..>.:. .,.
I
.x0
250 Number
with (a) 14 MeV neutrons,
(b) thermal
neutrons.
97
D. Sueua et al. / Silicon nuclear radiation detectors
---
---
--
.. 1 .I!
- - ----
vi.
I I
\
I I
0
10
26
30
LO
50
60
Channel
70
a0
90
Number
Fig. 5. Spectrum of fission fragments from 235U measured using an ion implanted p-silicon detector, area 4 cm2 at room temperature and 200 V.
Fig. 4. Alpha spectrum measured with an ion implanted p-silicon detector, area 4 cm* at room temperature and 200 V.
CipFl
I
-
30
720
__,____.,_,_____
.__..__. - ..-..
-..-,
-..-..-.,-..-“-..-.’
-lO
..-.__
d
1 -2L
-22
-26
I _ -r&J
-16
-IL
-12
-10
-8
-6
-L
-2
0
+2
+L
U( VI Capacitance-voltage characteristics of an ion implanted p-silicon detector, area 4 cm2. Detector not (curve a); detector irradiated with dose rates of 5.8 x 10” neutrons cm-* (curve b), 4.3 x 1014 neutrons cm-* neutrons cm-* (curve d); and after annealing at 500 o C during 30 min (curve e). Fig.
6.
exposedto radiation (curve c), 6.3
x
lOI
98
D. Sueoa et al. / Silicon nuclear radiation detectors
fragments in the dead layer of the detector, losses which occur due to elastic scattering by the nuclei in the sensitive layer and failure to register completetly all current carriers of the charge. The main part of the energy losses are in the dead layer, therefore low energy ion implantation is a very promising method for the production of silicon detectors with extremely thin dead layers, designed for spectrometry of fission fragments and other heavy charged particles. Spectrometric measurements of fragments from the fission of heavy nuclei subjected to neutron irradiation were carried out, using p-type silicon detectors with an area of 4 cm’. The reverse current of similar detectors was less than 3 PA at 450 V. The alpha spectrum measured with this type of detectors is shown in fig. 4, while fig. 5 presents the spectrum of ‘?‘IJ fission fragments. Both spectra were measured at 220 V, room temperature and in vacuum. The evaluated ratios NJN,,,, = 13, n,/N,, = 10 and N,/N,, = 1.4 are close to the calibration values given by Schmit [S], suggesting that energy losses of fission fragments in the dead layer are slight. It is well known that some of the basic characteristics of silicon which determine the properties of semiconductor detectors, e.g. lifetime, mobility of current carriers, specific electrical resistance, etc., are abruptly changed when certain critical limits of the integral radiation dose for the given type of radiation are reached. The value of the critical integral dose of the ionizing radiation depends on the properties of the semiconductor material and the method used for the production of the detector. The radiation stability of different types of semiconductor detectors for nuclear radiation has been studied by several authors [6-91. The surface barrier detectors display a short lifetime when used for the measurement of fission fragments of heavy nuclei and other heavy charged particles. The ion implanted detectors are more suitable for similar measurements. They can withstand heating to fairly high temperatures, as used for annealing the radiation defects and recovering the parameters of the detectors. For this purpose pressure contacts are used, thus eliminating the need for additional metal deposition onto the p- and n-type silicon layers. The radiation stability of the detectors prepared by us was checked in mixed gamma-neutron fields of the “V” vertical channel of the experimental nuclear reactor IRT-2000. A cadmium foil was used as a shield against slow neutrons. The neutron irradiation doses varied within the range 3.7 x 109-6.3 x 1014 neutrons cme2 [lo]. Up to doses of 4.3 x lOI neutrons cm-* no noticeable change in the electrical characteristics of the detectors (current-voltage) and capacitance-voltage relationships) was observed (fig. 6). On reaching a dose of 6.3 x 1014 neutrons cme2, the reverse current of the
.I E6kev--_
‘-
,..
‘. . .
..I(’
.,..:
,...,,.:I.’
,
..
kJ
._._, .,
id1
. .
.. -..Y.,~
,
,
(
i
i‘.......l
1
8
I-
Channel Number
Fig. 7. Spectra of alpha particles from 23QPu, 241Am and 2”Cm, measured using an ion implanted p-silicon detector with area 4 cm2. Detector not exposed to radiation (spectrum a); detector irradiated with dose rates of 4.3X10t4 neutrons cm-’ (spectrum b immediately after irradiation and spectrum c after 2 h), 6.3x 1014 neutrons cm-’ (spectrum d); and after annealing at 500 ’ C for 30 min (spectrum e).
detector abruptly increased, while the capacitance of the p-n junction decreased (fig. 6). The resolution of detectors not subjected to irradiation or irradiated with a dose lower than 4.3 X 1014 neutrons cmP2 was approximately identical - 26.3 keV for 5.8 MeV alpha particles from 241Am (fig. 7). On reaching a dose of 4.3 X lOI4 neutrons cm-* a shift of the spectrum toward lower energies was’observed. At dose rates of 6.3 x lOi neutrons cm-’ or higher the detectors no longer retained their spectrometric properties. Annealing at 500° C during 30 min in an argon atmosphere reduced the reverse current of the p-n junction, but its value remained higher than that prior to irradiation. The capacitance-voltage characteristics of a detector irradiated with a dose of 6.3 X 1Or4 neutrons cm-’ (curve d) was improved after annealing (curve e) and was quite similar to that of detectors
D. Sueua et al. / Silicon nuclear radiation detectors
exposed to lower doses (curve c) or not subjected to radiation (curve a). Following annealing the resolution of the detectors also improved, but remained inferior as compared with detectors not subjected to irradiation, e.g. 76 keV against 26 keV in the case of 5.156 MeV alpha particles from 239Pu. These results may be explained by the behaviour of the minority current carriers during the respective treatments. The current-voltage characteristics and the resolution of the detectors were strongly affected by the lifetime of the minority carriers, whereas the capacitance-voltage dependence was only slightly sensitive. It may be concluded that silicon detectors produced by low energy ion implantation are suitable for spectrometric measurements of nuclear radiation and charged particles. Their dead layers are extremely thin and display a good energy resolution not only for alpha and gamma spectrometric measurements, but for neutron and fission fragment spectrometry as well. The detectors are stable against irradiation with fast neutrons at dose rates up to 4.3 x 1014 neutrons cm-2, but at higher doses the spectrometric properties disappear. The thermal treatment of irradiated ion implanted detectors
99
offers a possibility to anneal radiation defects and partially improve the spectrometric properties of the detectors.
References 111 G. Bertolini and A. Cache, Semiconductor Detectors
(North-Holland, 1968). 121 S. Kalbitzer, R. Bacler, H. Herzer and K. Betge, Z. Phys. 203 (1967) 117. 131 D. Sueva, B. Amov, A. Dzhakov and N. Chikov, Bulg. J. Phys. 4/5 (1977) 489. 141 D. Sueva, N. Chikov, N. Kalinkova and Ts. Panteleev, Nucl. Energy 22 (1985) 23. 151 A. Schmith, Nucl. Instr. and Meth. 40 (1966) 204. 161 T. Yang, Nucl. Instr. and Meth. 100 (1972) 533. 171 D. Sachelarie, M. Dragan, M. Stoica and M. Sachelarie, Phys. Status Solidi (a) 65 (1981) 379. VI I. Koitman, K. Muminov and A. Yatasov, Phys. Stat. Sol. (a) 71 (1982) 59. [91 J. Kemmer, P. Burger, R. Henck and E. Heijne, IEEE Trans. Nucl. Sci. NS-29 (1982) 733. [lOI D. Sueva, N. Chikov, N. Kalinkova, I. Ruskov and S. Kaschieva, Nucl. Energy 23 (1986) 3.