This document evaluates monocrystalline ZnSe as a high-temperature radiation detector. It finds that undoped ZnSe crystals have an extremely low dark conductivity of around 200 pA at 150°C. Spectrometric measurements using a 57Co source show ZnSe can effectively detect gamma and X-rays with a counting efficiency close to maximum in the range of 500-750V between 20-130°C. The results indicate ZnSe can be used as an effective radiometric detector for gamma and X-rays over a wide temperature range up to at least 130°C.

1. Evaluation of monocrystalline ZnSe as a high-temperature
radiation detector
Andrii Sofiienko1
, Volodymyr Degoda2
, David M. Ponce-Marquez3
, Geir A. Johansen4
1
University of Bergen, Allegaten 55, PO Box 7803, 5020 Bergen, Norway. E-mail: asofienko@gmail.com
2
Taras Shevchenko National University of Kyiv, 64 Volodymyrs'ka Str., 01601 Kyiv, Ukraine. e-mail: degoda@univ.kiev.ua
3
Visuray AS, Strandbakken 10, 4070 Randaberg, Norway. E-mail: david.ponce@visuray.com
4
University of Bergen, Allegaten 55, PO Box 7803, 5020 Bergen, Norway. E-mail: geiranton.johansen@ift.uib.no
Introduction
Nowadays solid-state radiation detectors are in widespread use, but one limitation on them is that they
need to be operated at low or close to room temperatures [1-6]. Conversely, inorganic scintillators [1-
2] and detectors based on semiconductors with energy gaps between 1.4 eV and 1.9 eV, such as
CdTe, CZT, AlGaAs or GaAs [3-6], can be operated in a wide temperature range up to 100 °C, but
they usually require additional cooling to operate at higher temperatures that limits their application
in metallurgy [7], petroleum [8] and many other fields of science and technology. In this work we
show the results of experimental investigations into the electrical and spectrometric properties of
undoped monocrystalline ZnSe samples in a temperature range from 20 0
C to 167 0
C in order to
estimate the efficiency of the use of this semiconductor as a high temperature X-ray detector. The
dense semiconductor ZnSe (5.26 g/cm3
) has a band gap of 2.8 eV (300 K) and good absorption
efficiency for X-ray photons (3 cm-1
for 100 keV [9]). The physical features of ZnSe allow for single
crystals to be grown with a large bulk volume (∆V > 50 cm3
, [10, 11]), which is a significant
advantage over wide-band gap semiconductors such as SiC, GaN/ InGaN and diamond [12-14].
Experimental methods
The samples under study consisted of undoped ZnSe crystals that have been grown in a highly clean
environment to obtain crystals with a minimum concentration of impurities and a maximum specific
resistance between 1013
-1014
Ohm•cm. The measurements were performed within a temperature
range of 20 0
C to 167 0
С (293-440 K). The conductivity of the ZnSe samples was analysed through
excitation with different X-ray sources. Initially, X-ray irradiation was achieved by radiation from an
X-ray tube (Re - anode, 20 kV, 5-25 mA, distance to the sample is 150 mm) through a cryostat
beryllium window. The maximal energy flux of the generated X-rays was of 0.42 mJ/s∙cm2
in the
sample plane at the accelerating potential of 20 kV and tube current of 25 mA. Subsequently, a 57
Co
point source with activity of 1.47 MBq was used to investigate the spectrometric properties of ZnSe
samples. This source has the dominant X-ray emission line at 122 keV (85.6%). Schematics for the
X-ray measurements with ZnSe samples are shown in Figure 1.
ZnSe: dark current, X-ray and noise spectra
Conclusions
Temperature measurements of the dark conductivity of monocrystalline ZnSe samples show that
specially undoped ZnSe has an extremely low dark conductivity of approximately 200 pA at the
temperature of 150 °C (423 K, 330 V/cm), and the estimated concentration of deep donors is
approximately 1013
cm-3
. Spectrometric measurements of X-rays with a 57
Co point source show that a
range of optimal values for the bias voltages of the ZnSe samples allow the influence of the noise to
be minimised and simultaneously give a counting efficiency close to the maximum measured value.
For the ZnSe sample explored, this range is from 500 V (2000 V/cm) to 750 V (3000 V/cm) in a
temperature range from 20 °C to 130 °C. Under these conditions and with a low energy threshold of
approximately 35-40 keV, the maximal noise count rate can be reduced to approximately 10 cps. The
obtained results allow to assume that monocrystalline ZnSe can be effective used as a radiometric
detector of gamma and X-ray radiation in a wide temperature range up to at least 130 °C.
This work was partially funded by the Research Council of Norway under contract 200888. All tested
samples were manufactured by Scientific & Production Centre “Arvina” Ltd (Ukraine) in
collaboration with Taras Shevchenko National University of Kyiv.
References
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Figure 1: The schematic for the measurements of the dark current or the X-ray-induced conductivity of ZnSe (а) and the schematic of the measurements of the
spectrometric properties of ZnSe with a 57
Со point source (b)
0 50 100 150 200 250 300 350 400 450 500 550 600
0,01
0,1
1
10
20
0
C
90
0
C
130
0
C
noise measurements
Countrate,cps
Channel
1
2
3
Figure 3: The amplitude spectra of the detector noise measured at temperatures of 20
0
C (1), 90 0
C (2) and of 130 °C (3) and an electric field strength of 4000 V/cm
50 100 150 200 250 300 350 400
0,1
1
10
20
0
C
130
0
C
X-ray spectra of
57
Co
Countrate,cps
Channel
1
2
Figure 5: The amplitude spectra of X-rays from 57
Co measured at temperatures of 20
0C (1) and 130 °C (2) and an electric field strength of 4000 V/cm. The noise spectra
measured separately were extracted. The detected count rate increases from 1600 cps
at 20 0
C to 2400 cps at 130 0
C (1.5 times)
0 1000 2000 3000 4000
10
0
10
1
10
2
10
3
20
0
C
130
0
C
noise measurements
Countrate,cps
E, V/cm
1
2
Figure 4: The dependence of the noise count rate of one ZnSe sample on the electric
field strength in the sample at different temperatures: 20 °C (1) and 130 °C (2)
320 340 360 380 400 420 440
10
-1
10
0
10
1
10
2
10
3
i(T) ~ exp(-E/kT)
E = 1.09 eV
idark
,pA
T, K
Figure 2: The temperature dependence of the dark current of monocrystalline ZnSe for
an electric field strength of 330 V/cm