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Advantages and limitations of n-type low resistivity cadmium telluride nuclear radiation detectors
NUCLEAR
INSTRUMENTS
AND METHODS
150 (1978)
25-30
; 0
NORTH-HOLLAND
PUBLISHING
CO.
ADVANTAGES AND LIMITATIONS OF n-TYPE LOW RESISTIVITY CADMIUM TELLURIDE NUCLEAR RADIATION DETECTORS A. J. DABROWSKJ, J. IWAfiCZYK, W. M. SZYMCZYK Semiconductor Lletectors Laboratory,
INR. swierk near Warsaw, Poland
P. KOKOSCHINEGG, J. STELZHAMMER Physikinstituur, Oesterreichische Studiengesellschqfi .fiir Atomenergie,
Forschungszentrutn
Seibersdod;
Austria
and R. TRJBOULET Laboratoire de Physique des Solides. 52 Meudon-Beiievue,
France
Cadmium telluride detectors were fabricated from low resistivity (So-500 Q.cm) n-type crystals grown by the sealed-ingotzone refining method. Physical parameters of this kind of detector were discussed and the factors influencing them were analyscd. The main advantage of n-type low resistivity CdTe detectors is high energy resolution for low energy X- and y-rays at room temperature; the best value for the 5.9 keV line from a SSFe source was 1.1 keV fwhm. The main limitation is small active volume (diameter of active surface l-2 mm, thickness of active region below 100 pm), which is relevant to the counting efficiency and the range of registered y-energies. As a result of the performed analysis a miniature room temperature probe with high energy and spatial resolutions was designed.
1. Introduction
The continuous improvement of the technology of growing cadmium telluride single crystals made possible to use them as a starting material for the preparation of X- and y-ray spectrometers. It has been demonstrated that these devices are suitable for spectrometry of radiation without resorting to detector cooling’-‘). However, a complex of problems associated with charge collection, inhomogeneity of crystals, polarisation effects and others in cadmium telluride, still exists and limits many applications of CdTe detectors*-lo). From the point of view of potential applications, the most interesting X- and y-ray detector parameters are usually: energy resolution, counting efficiency, and region of registered radiation energy. These parameters are cross-correlated, and are determined by properties of the starting material and by detector characteristics. The energy resolution is related to the noise level of the detector-preamplifier system as well as to processes of charge creation and collection. The counting efficiency depends on the atomic number of the starting material, the dimensions of the detector’s active volume, and on the charge collection efficiency. The region of registered energies is a consequence of the above mentioned parameters and can be discussed in connection with them.
This paper presents in detail advantages and limitations of low resistivity n-type surface barrier CdTe detectors. The main advantage of the low resistivity n-type CdTe detectors is the high energy resolution for low energy X- and y-rays. Such a high energy resolution can be obtained for the sake of low leakage currents combined with good charge collection efficiency in this kind of detector. 2. Material and detectors n-type cadmium telluride crystals were grown in the Laboratoire de Physique des Solides. Single crystals were obtained from ingots highly purified by the sealed-ingot-zone refining method according to the process described in ref. 11. The resistivity of the material obtained was between 50 and 500 D-cm. The best mobility at room temperature was about 1000 cm*/Vs. The electrical measurement data for several samples are summarized in table 1. Some mobility vs temperature curves are plotted in fig. 1. The first curve corresponds to one of the purest samples, 24A5, with ~32K = 1.46 x 10’ cm*/Vs. The curves 2, 3 and 4 refer to samples of decreasing purity, i.e. samples taken from less pure parts of the ingot. To prepare the detectors, slices of suitable thickness were cut from the above described ingots, lapped, polished, and carefully cleaned mechaniI.
CdTe
26
A. J. DABROWSKI
et al.
TABLE I Name
16A2
24 A 7
24 A 5
CTZV 54
cm-3
1.45 x
10'3
4.95 x
10'3
5.1 x
1013
3.1 x
390
123
130
220
40~
295 K
20~
1100
1 020
950
930
El,0
lpi
60
40
0
0
x0
,”
0.5
* -1
x
7.4 x 10’2 1.65 x 10’3 1.65 x 10’3 1.3 x 10’3 8.3 1.03 x 10s
3.36
2.6
1.13 x 10s
1.46 x 105
5.27 9
x
104
tally and chemically in a bromine-methanol solution for l-2 min, depending on the sample size. After removal from the solution, the samples were kept in an inert gas atmosphere. The back contacts were made of indium. The front surfaces of the slices were covered with evaporated Au dots 20-100pglcm2 thick. The diameter of the front contact was usually made no larger then 2 mm. Finally, the slices were mounted in our standard laboratory holders. The active region of these detectors is formed by a depletion layer created below the crystal surface as a result of reverse bias applied to the surface barrier. In view of the small resistivity of the starting material, the thickness of the active region does not exceed 100 pm for a detector bias voltage below 150 V. The reverse currents of the obtained detectors were in the range of 0.2-2 nA for bias voltages of -1
x UIVI -?
20
n32K
cm-3 P32K Qcm P32K cmz/Vs
6 f
::paQ
P300K
cmz/V s
Cd To DET 54-136 Y Cd To DET 54-137
1013
P300K
Rem
60.
0
n300K
(nA)
Fig. 2. I-V
characteristics for two CdTe detectors at 295 K.
30-150 V. The typical current-voltage characteristic is shown in fig. 2. The junction capacity-voltage characteristic is shown in fig. 3. The C-V characteristic was taken at about 120 kHz. 3. Spectrometric performance The main advantage of the CdTe detectors presented here is the high energy resolution for low energy X- and y-rays. This results from low leakage currents and low capacity of the detectors, combined with good charge collection efficiency. The contribution to the energy resolution due to the capacitance of the junction is considerable, because of the thin active layer. The active surface of the detectors with high spectrometric performance is restricted to several square millimeters. For low energy X-rays, of energies less than about 15 keV, the resolution is determined mainly by the noise level of the detector-preamplifier circuit. The noise sets also the lower limit for the registered photon energy range at about 4 keV6). The best room temperature energy resolution for low resistivity n-type CdTe detectors was 1.1 keV fwhm for 5.9 keV X-rays’) (cf. fig. 4). The noise of the detector-preamplifier system amounted to 0.9 keV fwhm as measured with a pulser. The spectrum of X- and y-rays from an 24’Am
IO:
x0
x Cdk DET 54-136 o CdTe DET 54 - 137 295~ 8 )I
G: a
B
”
f
::s 08 %
10
lea TEMPERATURE(K)
200 300
Fig. 1. Electron mobility vs temperature for samples cut from the head (1) to the end (4) of vertical zone melted ingots.
1 “’
Fig. 3. C-V
28;
1
1
10
100
U(V) characteristics for two CdTe detectors.
n-TYPE
LOW
RESISTlVlTY
27
DETECTORS
abrupt junction. On that basis it was shown that the charge trapping causes two effects: the full energy peak is shifted from the position corresponding to lOO%charge collection, and the peak is deformed. The latter appears either as a long tail on the low energy part of the peak, or as a flattening of its top, depending on the values of the mean trapping times for electrons and holes. The observed shift of peaks in n-type CdTe detectors is small (less than several percent), because of the high electric field in the active volume of this type of detector. The influence of charge trapping on the peak shape decreases with decreasing photon energy. The deformation of the low energy part of the 59.5 keV peak obtained for an 241Am source is clearly visible in fig. 5. On the other hand, the trapping effect is negligible for energies lower than about 15 keV, as compared to the noise of the detector-preamplifier system. This is exemplified by the spectrum of a s5Fe source where the 5.9 keV peak is symmetrical, and its resolution approaches the pulser’s (cf. fig. 4). CHANNEL
Fig. 4. X-ray spectrum tor.
NUMBER
of S5Fe taken with n-type CdTe detec-
source is presented in fig. 5. The resolution for the 59.5 keV line was 1.7 keV fwhm. The observed deterioration of energy resolution is mainly due to charge trapping’*). For higher energies, above about 150 keV, the main contribution to the energy resolution comes from the escape of electrons from the active layer, setting the upper limit for the registered y-energies, from a spectrometric point of view. Detectors with larger active thickness are required for spectrometric registration of y-rays from this energy region. As an example the spectra of 13’Cs and 6oCo taken with a high resistivity CdTe detector produced by us are presented in fig. 6. The results were obtained with a chlorine-doped CdTe detector in a short time after the voltage was applied. The used detector had an active area of 0.3 cm2, a thickness of about 1 mm, and it operated under 800 V bias. 4. Charge collection Charge trapping process is a substantial problem in cadmium telluride detectors8’9). A detailed analysis of the influence of the trapping effects on the spectrometric performance of n-type CdTe detectors was published elsewhere’*). The charge collection efficiency function was derived there for an
5. Efficiency The detection efficiency is defined as the ratio of the number of registered y-quanta to the number of y-quanta entering the active volume. Registration of low energy X-rays is limited rather by the noise level than by absorption in the entrance window, which is extremely thin for these detectors. The detection efficiency decreases from practically 100% for energies near 10 keV to about 20% for E, = 60 keV in the case of our typical detectors, in view of their small active thickness. The characteristic feature of the small active thickness of a detector made of high atomic number material is also the presence of X-ray escape peaks. The X-ray escape peaks located about 23 keV (k, X-ray of cadmium) and about 27 keV (K, X-ray of tellurium) below the full energy peak are clearly observed for low resistivity CdTe detectors (see fig. 5). Generally, the relative height of the escape peak as compared to the full energy peak, can indicate the resistivity of the detector material. In the energy range of interest for n-type CdTe detectors (approximately 4-200 keV), Compton scattering is negligible as compared to the photoeffect. Nevertheless, the number of counts in the full energy peak is less than the number of registered y-quanta, because of escaping X-rays as I.
CdTe
28
A. J. DqBROWSKl
et al.
2rrAm -SOURCE CdTe DET 02905
Np-Ld
13.9 keV
$-$0:2~s 295K
0
s Np-L,g c 17.7 keV
c 1
I
59.5keV v-59.5 X-RAY ESCAPES
CHANNEL Fig. 5. X- and y-ray spectra of 241Am obtained
PUL SER
I
1
ft
%.
NUMBER
with n-type CdTe detector.
well as tailing effects due to escaping photoelectrons from the active layer and charge trapping. 6. Other detector parameters The low resistivity n-type CdTe detectors are able to work over a wide temperature range. The measurements performed down to liquid nitrogen temperature have shown a considerable improvement of energy resolution with decreasing temperature down to about 140 K’). This effect cannot be explained, by merely a decrease of the detector-preamplifier circuit noise with lowering temperature. We attribute this effect to an improvement of charge collection in the detector, associated with an increase of the product ,UZ+ at low temperatures, which indicates the purity of the detector material. The possibility of registration of low energy Xand y-rays at very low bias voltages can be an interesting feature of detectors of this kind. The spectra of a s5Fe source taken at biases of 24, 12,
6 and 3 V are shown in fig. 7. The small deterioration of energy resolution is connected with an increase of detector capacitance. The possibility of operating the detectors with such low biases can be interesting in special medical or industrial applications. In high resistivity material rather high bias voltages are required in order to assure satisfying charge collection from the whole active region. The possibility of decreasing the detector thickness by means of lapping is limited for the reason of mechanical endurance of the crystal. The surface barrier technology used in producing n-type low resistivity CdTe detectors leads to extremely thin entrance windows. The existence of thin dead layers in the detectors was verified by a-measurements; the resolution of 32 keV fwhm was obtained for 212Bi6.05 MeV particles13). The long term measurements showed no change of detector parameters. The polarisation effect was not observed in low resistivity n-type CdTe detectors, in contrast to such an effect appearing
n-TYPE
LOW
RESISTIVITY
r
Fig. 6. y-spectra of 13’Cs and ‘j°Co obtained with a high resistivity of Cl-doped CdTe detector in a short time after applying the bias voltage.
strongly in Cl-doped high resistivity detectors. The effect seriously degraded the spectrometric performance of these detectors after some time. 7. Conclusions The room temperature, low resistivity, n-type surface barrier CdTe detectors are destined for de-
DETECTORS
29
tection of low energy X- and y-rays. The small thickness of the depletion layer, usually below lOOpm, limits the range of y-energies - in regard to full energy peak registration - up to about 200 keV. In the energy range up to about 60 keV these detectors allow registration of more than 20% of the entering photons. The high spectrometric performance at room temperature can only be achieved with small active areas, in practice several square millimeters. Low resistivity surface barrier CdTe detectors ensure an energy resolution for low energy X-rays of better than 1.5 keV fwhm at room temperature (1.1 keV for 5.9 keV X-rays), which is at present the best value for cadmium telluride detectors. The low operating bias, normally below 100 V, and the possibility of applying only several volts in low energy X-rays measurements are the interesting features of these detectors. Analysis of the limitations and advantages of low resistivity n-type surface barrier CdTe detectors gives one the base for the choice of optimal applications of detectors of this kind. It seems that it is possible to find special medical, biological and industrial applications for them as room temperature miniature probes for low energy X-rays and y-rays, with high energy and spatial resolutions. As a result of the performed analysis a miniature room temperature probe was designed. The authors wish to thank Dr. J. Chwaszczewska and Dr. 0. Eder for supporting this work and for helpful suggestions. We are also grateful to Mr. G. Didier for his skilful assistance in preparation of the crystals and to Mrs. MI. Walerian for assisting in the fabrication of the detectors. !
I
1
I
I
-* D
D
I
I
LO CHANNEL
Fig. 7. X-ray spectrum
of ssFe taken with n-type CdTe detector
s
I
a0
NUMBER
at biases of 24, 12, 6 and 3 V.
I.
CdTe
30
A. J.
DABROWSKI
References 1) K. Zanio and W. Akutagawa, Appl. Phys. Lett. 20 (1972) 294. 2) H. B. Serreze, G. Entine, R. 0. Bell and F. V. Wald, IEEE Trans. Nucl. Sci. NS-21, no. 1 (1974) 404. 3) K. Zanio, F. Krajenbrink and H. Montano, IEEE Trans. Nucl. Sci. NS-21, no. 1 (1974) 315. 4) P. Siffert, A. Cornet, R. Stuck, R. Triboulet and Y. Marfaing, IEEE Trans. Nucl. Sci. NS-22, no. 1 (1975) 211. 5) A. J. Dabrowski, J. Iwanczyk and R. Triboulet, Nucl. Instr. and Meth. 126 (1975) 417. 6) A. J. Dabrowski, J. Chwaszczewska, J. Iwanczyk, R. Triboulet and Y. Marfaing, IEEE Trans. Nucl. Sci. NS-23, no. 1 (1976) 171. ‘) A. J. Dabrowski, J. Chwaszczewska, J. Iwanczyk, R. Trib-
et al.
oulet and Y. Marfaing, Rev. Phys. Appl. 12 (1977) 297 [Proc. 2nd Int. Symp. on Cadmium tehride, Strasbourg] (1976). 8) J. M. Mayer, in Semiconductor detectors (eds. G. Bertolini and A. Cache; North-Holland Publ. Co., Amsterdam, 1968) ch. 5. 9, Proc. Int. Symp. on Cadmium tellwide, Strasbourg (1971). to) P. Siffert, J. Berger, C. Scharager, A. Cornet, R. Stuck, R. 0. Bell, H. B. Serreze and F. V. Wald, IEEE Trans. Nucl. Sci. NS-23, no. 1 (1976) 159. It) R. Triboulet and Y. Marfaing, J. Electrochem. Sot. 120 (1973) 1260. 12) J. Iwanczyk and A. J. Dabrowski, Nucl. Instr. and Meth. 134 (1976) 505. 13) A. J. Dabrowski, J. Iwanczyk and R. Triboulet, Nucl. Instr. and Meth. 118 (1974) 531.
INSTRUMENTS
AND METHODS
150 (1978)
25-30
; 0
NORTH-HOLLAND
PUBLISHING
CO.
ADVANTAGES AND LIMITATIONS OF n-TYPE LOW RESISTIVITY CADMIUM TELLURIDE NUCLEAR RADIATION DETECTORS A. J. DABROWSKJ, J. IWAfiCZYK, W. M. SZYMCZYK Semiconductor Lletectors Laboratory,
INR. swierk near Warsaw, Poland
P. KOKOSCHINEGG, J. STELZHAMMER Physikinstituur, Oesterreichische Studiengesellschqfi .fiir Atomenergie,
Forschungszentrutn
Seibersdod;
Austria
and R. TRJBOULET Laboratoire de Physique des Solides. 52 Meudon-Beiievue,
France
Cadmium telluride detectors were fabricated from low resistivity (So-500 Q.cm) n-type crystals grown by the sealed-ingotzone refining method. Physical parameters of this kind of detector were discussed and the factors influencing them were analyscd. The main advantage of n-type low resistivity CdTe detectors is high energy resolution for low energy X- and y-rays at room temperature; the best value for the 5.9 keV line from a SSFe source was 1.1 keV fwhm. The main limitation is small active volume (diameter of active surface l-2 mm, thickness of active region below 100 pm), which is relevant to the counting efficiency and the range of registered y-energies. As a result of the performed analysis a miniature room temperature probe with high energy and spatial resolutions was designed.
1. Introduction
The continuous improvement of the technology of growing cadmium telluride single crystals made possible to use them as a starting material for the preparation of X- and y-ray spectrometers. It has been demonstrated that these devices are suitable for spectrometry of radiation without resorting to detector cooling’-‘). However, a complex of problems associated with charge collection, inhomogeneity of crystals, polarisation effects and others in cadmium telluride, still exists and limits many applications of CdTe detectors*-lo). From the point of view of potential applications, the most interesting X- and y-ray detector parameters are usually: energy resolution, counting efficiency, and region of registered radiation energy. These parameters are cross-correlated, and are determined by properties of the starting material and by detector characteristics. The energy resolution is related to the noise level of the detector-preamplifier system as well as to processes of charge creation and collection. The counting efficiency depends on the atomic number of the starting material, the dimensions of the detector’s active volume, and on the charge collection efficiency. The region of registered energies is a consequence of the above mentioned parameters and can be discussed in connection with them.
This paper presents in detail advantages and limitations of low resistivity n-type surface barrier CdTe detectors. The main advantage of the low resistivity n-type CdTe detectors is the high energy resolution for low energy X- and y-rays. Such a high energy resolution can be obtained for the sake of low leakage currents combined with good charge collection efficiency in this kind of detector. 2. Material and detectors n-type cadmium telluride crystals were grown in the Laboratoire de Physique des Solides. Single crystals were obtained from ingots highly purified by the sealed-ingot-zone refining method according to the process described in ref. 11. The resistivity of the material obtained was between 50 and 500 D-cm. The best mobility at room temperature was about 1000 cm*/Vs. The electrical measurement data for several samples are summarized in table 1. Some mobility vs temperature curves are plotted in fig. 1. The first curve corresponds to one of the purest samples, 24A5, with ~32K = 1.46 x 10’ cm*/Vs. The curves 2, 3 and 4 refer to samples of decreasing purity, i.e. samples taken from less pure parts of the ingot. To prepare the detectors, slices of suitable thickness were cut from the above described ingots, lapped, polished, and carefully cleaned mechaniI.
CdTe
26
A. J. DABROWSKI
et al.
TABLE I Name
16A2
24 A 7
24 A 5
CTZV 54
cm-3
1.45 x
10'3
4.95 x
10'3
5.1 x
1013
3.1 x
390
123
130
220
40~
295 K
20~
1100
1 020
950
930
El,0
lpi
60
40
0
0
x0
,”
0.5
* -1
x
7.4 x 10’2 1.65 x 10’3 1.65 x 10’3 1.3 x 10’3 8.3 1.03 x 10s
3.36
2.6
1.13 x 10s
1.46 x 105
5.27 9
x
104
tally and chemically in a bromine-methanol solution for l-2 min, depending on the sample size. After removal from the solution, the samples were kept in an inert gas atmosphere. The back contacts were made of indium. The front surfaces of the slices were covered with evaporated Au dots 20-100pglcm2 thick. The diameter of the front contact was usually made no larger then 2 mm. Finally, the slices were mounted in our standard laboratory holders. The active region of these detectors is formed by a depletion layer created below the crystal surface as a result of reverse bias applied to the surface barrier. In view of the small resistivity of the starting material, the thickness of the active region does not exceed 100 pm for a detector bias voltage below 150 V. The reverse currents of the obtained detectors were in the range of 0.2-2 nA for bias voltages of -1
x UIVI -?
20
n32K
cm-3 P32K Qcm P32K cmz/Vs
6 f
::paQ
P300K
cmz/V s
Cd To DET 54-136 Y Cd To DET 54-137
1013
P300K
Rem
60.
0
n300K
(nA)
Fig. 2. I-V
characteristics for two CdTe detectors at 295 K.
30-150 V. The typical current-voltage characteristic is shown in fig. 2. The junction capacity-voltage characteristic is shown in fig. 3. The C-V characteristic was taken at about 120 kHz. 3. Spectrometric performance The main advantage of the CdTe detectors presented here is the high energy resolution for low energy X- and y-rays. This results from low leakage currents and low capacity of the detectors, combined with good charge collection efficiency. The contribution to the energy resolution due to the capacitance of the junction is considerable, because of the thin active layer. The active surface of the detectors with high spectrometric performance is restricted to several square millimeters. For low energy X-rays, of energies less than about 15 keV, the resolution is determined mainly by the noise level of the detector-preamplifier circuit. The noise sets also the lower limit for the registered photon energy range at about 4 keV6). The best room temperature energy resolution for low resistivity n-type CdTe detectors was 1.1 keV fwhm for 5.9 keV X-rays’) (cf. fig. 4). The noise of the detector-preamplifier system amounted to 0.9 keV fwhm as measured with a pulser. The spectrum of X- and y-rays from an 24’Am
IO:
x0
x Cdk DET 54-136 o CdTe DET 54 - 137 295~ 8 )I
G: a
B
”
f
::s 08 %
10
lea TEMPERATURE(K)
200 300
Fig. 1. Electron mobility vs temperature for samples cut from the head (1) to the end (4) of vertical zone melted ingots.
1 “’
Fig. 3. C-V
28;
1
1
10
100
U(V) characteristics for two CdTe detectors.
n-TYPE
LOW
RESISTlVlTY
27
DETECTORS
abrupt junction. On that basis it was shown that the charge trapping causes two effects: the full energy peak is shifted from the position corresponding to lOO%charge collection, and the peak is deformed. The latter appears either as a long tail on the low energy part of the peak, or as a flattening of its top, depending on the values of the mean trapping times for electrons and holes. The observed shift of peaks in n-type CdTe detectors is small (less than several percent), because of the high electric field in the active volume of this type of detector. The influence of charge trapping on the peak shape decreases with decreasing photon energy. The deformation of the low energy part of the 59.5 keV peak obtained for an 241Am source is clearly visible in fig. 5. On the other hand, the trapping effect is negligible for energies lower than about 15 keV, as compared to the noise of the detector-preamplifier system. This is exemplified by the spectrum of a s5Fe source where the 5.9 keV peak is symmetrical, and its resolution approaches the pulser’s (cf. fig. 4). CHANNEL
Fig. 4. X-ray spectrum tor.
NUMBER
of S5Fe taken with n-type CdTe detec-
source is presented in fig. 5. The resolution for the 59.5 keV line was 1.7 keV fwhm. The observed deterioration of energy resolution is mainly due to charge trapping’*). For higher energies, above about 150 keV, the main contribution to the energy resolution comes from the escape of electrons from the active layer, setting the upper limit for the registered y-energies, from a spectrometric point of view. Detectors with larger active thickness are required for spectrometric registration of y-rays from this energy region. As an example the spectra of 13’Cs and 6oCo taken with a high resistivity CdTe detector produced by us are presented in fig. 6. The results were obtained with a chlorine-doped CdTe detector in a short time after the voltage was applied. The used detector had an active area of 0.3 cm2, a thickness of about 1 mm, and it operated under 800 V bias. 4. Charge collection Charge trapping process is a substantial problem in cadmium telluride detectors8’9). A detailed analysis of the influence of the trapping effects on the spectrometric performance of n-type CdTe detectors was published elsewhere’*). The charge collection efficiency function was derived there for an
5. Efficiency The detection efficiency is defined as the ratio of the number of registered y-quanta to the number of y-quanta entering the active volume. Registration of low energy X-rays is limited rather by the noise level than by absorption in the entrance window, which is extremely thin for these detectors. The detection efficiency decreases from practically 100% for energies near 10 keV to about 20% for E, = 60 keV in the case of our typical detectors, in view of their small active thickness. The characteristic feature of the small active thickness of a detector made of high atomic number material is also the presence of X-ray escape peaks. The X-ray escape peaks located about 23 keV (k, X-ray of cadmium) and about 27 keV (K, X-ray of tellurium) below the full energy peak are clearly observed for low resistivity CdTe detectors (see fig. 5). Generally, the relative height of the escape peak as compared to the full energy peak, can indicate the resistivity of the detector material. In the energy range of interest for n-type CdTe detectors (approximately 4-200 keV), Compton scattering is negligible as compared to the photoeffect. Nevertheless, the number of counts in the full energy peak is less than the number of registered y-quanta, because of escaping X-rays as I.
CdTe
28
A. J. DqBROWSKl
et al.
2rrAm -SOURCE CdTe DET 02905
Np-Ld
13.9 keV
$-$0:2~s 295K
0
s Np-L,g c 17.7 keV
c 1
I
59.5keV v-59.5 X-RAY ESCAPES
CHANNEL Fig. 5. X- and y-ray spectra of 241Am obtained
PUL SER
I
1
ft
%.
NUMBER
with n-type CdTe detector.
well as tailing effects due to escaping photoelectrons from the active layer and charge trapping. 6. Other detector parameters The low resistivity n-type CdTe detectors are able to work over a wide temperature range. The measurements performed down to liquid nitrogen temperature have shown a considerable improvement of energy resolution with decreasing temperature down to about 140 K’). This effect cannot be explained, by merely a decrease of the detector-preamplifier circuit noise with lowering temperature. We attribute this effect to an improvement of charge collection in the detector, associated with an increase of the product ,UZ+ at low temperatures, which indicates the purity of the detector material. The possibility of registration of low energy Xand y-rays at very low bias voltages can be an interesting feature of detectors of this kind. The spectra of a s5Fe source taken at biases of 24, 12,
6 and 3 V are shown in fig. 7. The small deterioration of energy resolution is connected with an increase of detector capacitance. The possibility of operating the detectors with such low biases can be interesting in special medical or industrial applications. In high resistivity material rather high bias voltages are required in order to assure satisfying charge collection from the whole active region. The possibility of decreasing the detector thickness by means of lapping is limited for the reason of mechanical endurance of the crystal. The surface barrier technology used in producing n-type low resistivity CdTe detectors leads to extremely thin entrance windows. The existence of thin dead layers in the detectors was verified by a-measurements; the resolution of 32 keV fwhm was obtained for 212Bi6.05 MeV particles13). The long term measurements showed no change of detector parameters. The polarisation effect was not observed in low resistivity n-type CdTe detectors, in contrast to such an effect appearing
n-TYPE
LOW
RESISTIVITY
r
Fig. 6. y-spectra of 13’Cs and ‘j°Co obtained with a high resistivity of Cl-doped CdTe detector in a short time after applying the bias voltage.
strongly in Cl-doped high resistivity detectors. The effect seriously degraded the spectrometric performance of these detectors after some time. 7. Conclusions The room temperature, low resistivity, n-type surface barrier CdTe detectors are destined for de-
DETECTORS
29
tection of low energy X- and y-rays. The small thickness of the depletion layer, usually below lOOpm, limits the range of y-energies - in regard to full energy peak registration - up to about 200 keV. In the energy range up to about 60 keV these detectors allow registration of more than 20% of the entering photons. The high spectrometric performance at room temperature can only be achieved with small active areas, in practice several square millimeters. Low resistivity surface barrier CdTe detectors ensure an energy resolution for low energy X-rays of better than 1.5 keV fwhm at room temperature (1.1 keV for 5.9 keV X-rays), which is at present the best value for cadmium telluride detectors. The low operating bias, normally below 100 V, and the possibility of applying only several volts in low energy X-rays measurements are the interesting features of these detectors. Analysis of the limitations and advantages of low resistivity n-type surface barrier CdTe detectors gives one the base for the choice of optimal applications of detectors of this kind. It seems that it is possible to find special medical, biological and industrial applications for them as room temperature miniature probes for low energy X-rays and y-rays, with high energy and spatial resolutions. As a result of the performed analysis a miniature room temperature probe was designed. The authors wish to thank Dr. J. Chwaszczewska and Dr. 0. Eder for supporting this work and for helpful suggestions. We are also grateful to Mr. G. Didier for his skilful assistance in preparation of the crystals and to Mrs. MI. Walerian for assisting in the fabrication of the detectors. !
I
1
I
I
-* D
D
I
I
LO CHANNEL
Fig. 7. X-ray spectrum
of ssFe taken with n-type CdTe detector
s
I
a0
NUMBER
at biases of 24, 12, 6 and 3 V.
I.
CdTe
30
A. J.
DABROWSKI
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