Evaluation of Antimony Tri-Iodide Crystals for Radiation Detectors.
Numerous compound semiconductors have been actively investigated for use in radiation detectors [1-9]. High detection efficiency for gamma-rays as well as low-noise operation at room temperature and high charge collection efficiency are important characteristics for radiation detectors with high spectroscopic performance. Although CdTe, CdZnTe , and Hg[I.sub.2] , have recently been considered as ideal detector materials in this application, these materials are more expensive than traditional detector materials such as Si and Ge because of the difficulty associated with their crystal growth.
Antimony tri-iodide (Sb[I.sub.3]) is a compound semiconductor with an As[I.sub.3] -type crystal structure; it has been reported to be a potential semiconductor material for radiation detectors . The characteristics of Sb[I.sub.3] include high atomic number (Sb: 51, I: 53), high density (4.92 g/cm), and wide bandgap energy (2.2 eV). The physical properties of Sb[I.sub.3] suggest that it can be used in radiation detectors with high detection efficiency and low-noise operation at room temperature without any cooling. Figure 1 shows the attenuation coefficients for Ge, CdTe, and Sb[I.sub.3] . As shown in this figure, Sb[I.sub.3] exhibits high gamma-ray stopping power equivalent to that of CdTe. In addition, Sb[I.sub.3] melts at a low temperature (171[degrees] C) and exhibits no phase transition between room temperature and its melting point . Thus, a single Sb[I.sub.3] crystal can be grown from the molten material via the conventional crystal growth technique such as the Bridgman method or the traveling molten-zone method.
The aforementioned attractive properties of Sb[I.sub.3] suggest that Sb[I.sub.3] detectors can be fabricated at low cost compared with CdTe and CdZnTe detectors. Bi[I.sub.3]  and Pb[I.sub.2]  crystals were also layered structure crystals similar to Sb[I.sub.3] and have been studied as a new radiation detector material. However, gamma-ray energy resolution obtained from these layered materials was poor than CdTe and CdZnTe due to low charge transport properties.
Although Sb[I.sub.3] is a promising material, no previous studies have reported the performance of Sb[I.sub.3] detectors. The objective of this work was to evaluate the potential of Sb[I.sub.3] as a radiation detector.
2.1. Crystal Growth. Commercially available Sb (nominal purity of 99.9999%, Kojundo chemical laboratory Co., Ltd) and [I.sub.2] (nominal purity of 99.999%, Kojundo chemical laboratory Co., Ltd) were used as starting materials for growing Sb[I.sub.3] crystals. Stoichiometric amounts of the starting materials were loaded into a quartz ampoule ([phi]10 mm) that was subsequently vacuum sealed at approximately 1 Pa. For the synthesis of Sb[I.sub.3], the ampoule was kept in a furnace above the melting point for 24 h. After the synthesis, multipass zone refining was used to reduce the impurities in the synthesized material. The length of the molten zone was approximately 10 mm. The furnace was moved at a speed of 5 cm/h, and the purification was repeated 100 times. After the material had been purified, Sb[I.sub.3] crystals were grown by the Bridgman method  at the middle section of the purified material. Figure 2 shows a schematic of the Bridgman furnace and the temperature distribution used in this study. Crystal growth was carried out in a temperature gradient of approximately 3.4[degrees]C/cm. The ampoule was located in the furnace as shown in the figure, and the material in the ampoule was completely melted before the ampoule was lowered. After the material had melted, the ampoule was lowered at a speed of 1 mm/h using a stepping motor. Figure 3 shows cleaved Sb[I.sub.3] crystals grown in this study. The main cleavage plane was parallel to the growth direction and contained several grain boundaries. The crystal color was metallic red by reflected light and red by transmitted light, consistent with the band-gap of Sb[I.sub.3] (2.2 eV).
2.2.1. Optical Transmittance Spectra. A UV-Vis spectrophotometer (Shimadzu UV-1800) was used to evaluate the bandgap energy of the grown Sb[I.sub.3] crystal. After the crystal growth, the Sb[I.sub.3] crystal was cleaved into thin plates (thickness of 0.1-0.3 mm) using a cutter blade. Flat transparent Sb[I.sub.3] plates with a thickness of 0.1-0.2 mm were selected as samples for this evaluation.
2.2.2. X-Ray Diffraction. X-ray diffraction (XRD) analysis (Rigaku 2500HF) was used to confirm the crystallinity of the grown crystal. The voltage and current of the X-ray tube (CuK[alpha]) were 40 kV and 200 mA, respectively. Cleaved plates and powdered Sb[I.sub.3] were prepared from the grown crystal. Commercially available Sb[I.sub.3] powder was used to obtain reference data for the Sb[I.sub.3] phase.
2.2.3. Current-Voltage Characteristics. The current-voltage characteristics of the Sb[I.sub.3] detectors were measured to evaluate the resistivity of the Sb[I.sub.3] detectors. The bias voltage was changed from 0 V to 200 V, and the leakage current of the detectors was measured at room temperature.
2.2.4. Fabrication of an Sb[I.sub.3] Detector. The grown crystal was cleaved into thin plates (thickness of 0.1-0.3 mm), and gold electrodes (3 mm in diameter) were deposited onto both sides of the Sb[I.sub.3] crystals by the vacuum evaporation method. After electrode deposition, the Sb[I.sub.3] crystals were placed on a stage used for detector fabrication and palladium wires were connected to each electrode from a terminal on the stage. The detector was enclosed in an aluminum case, and the signal from the detector was taken from a BNC terminal on the case.
2.2.5. Radiation Response. For initial operation before evaluating gamma-ray detector performance of the Sb[I.sub.3] detectors, [sup.241]Am alpha-particle energy spectra were recorded using the Sb[I.sub.3] detectors in a conventional measurement system comprising a preamplifier, bias supply, shaping amplifier, and a multichannel pulse-height analyzer. The [sup.241]Am alpha-particle source (185 kBq) and the detector were enclosed in the aluminum case used for the current-voltage measurements. Distance between the alpha-particle source and the Sb[I.sub.3] crystal was approximately 2 mm and the measurement was carried out in the atmosphere. After the initial evaluation, gamma-ray response was also evaluated using the same measurement system and [sup.137]Cs gamma-ray source (3.7 MBq).
3. Results and Discussion
3.1. Characterization. Figure 4 shows the transmittance spectrum as a function of wavelength. The cutoff wavelength was approximately 576 nm, suggesting that the band-gap energy of the grown crystal was approximately 2.15 eV, consistent with a previously reported value .
Figure 5 shows the XRD patterns obtained from an Sb[I.sub.3] plate, the powder, and the commercial Sb[I.sub.3] powder. All of the diffraction peaks in the pattern of the powdered crystal match the JCPDS data , indicating that Sb[I.sub.3] was successfully synthesized using the starting materials and that single-crystalline Sb[I.sub.3] was grown by the Bridgman method.
Figure 6 shows the extended XRD patterns in the region of the (006) plane. The peaks of the (006) plane in the pattern of the grown crystal and the powdered crystal were slightly shifted toward higher angles compared with the corresponding peak in the pattern of the commercial powder. This result implies that the lattice constant of the grown and powdered crystal samples was somewhat smaller than that of the commercial Sb[I.sub.3] powder. The origin of this small difference requires further investigation beyond the scope of the present work.
3.2. Detector Performance. Figure 7 shows the leakage current as a function of the bias voltage obtained from the Sb[I.sub.3] detectors. The thickness of the Sb[I.sub.3] crystal was 0.339 mm. The resistivity of the Sb[I.sub.3] detectors was estimated to be approximately 1.0 x [10.sup.10] [ohm] x cm.
Figures 8 and 9 show the alpha-particle energy spectra obtained from a typical Sb[I.sub.3] detector at room temperature. The thickness of the detector was 0.229 mm. The spectra were obtained by changing the polarity of the bias voltage to evaluate the difference in charge transport properties between electrons and holes. The bias voltage was 200 V, and the amplifier shaping time was 30 [micro]s. Both energy spectra recorded under alpha-particle irradiation show increased counts above the noise counts. Penetration depth of 5.48 MeV alpha-particles is calculated to be less than 40 [micro]m in water and the range for the alpha-particles is much smaller than the crystal thickness. Because the penetration depth of alpha-particles into the crystals was very shallow, the maximum count channel in the energy spectra depends on the transport properties of holes or electrons. As shown in Figures 8 and 9, the maximum count channel was independent of the polarity of the bias voltage. Thus, the charge transport properties of electrons and holes in the Sb[I.sub.3] detector were approximately equivalent. In addition, because the Sb[I.sub.3] detector exhibited no full-energy peaks corresponding to 5.48 MeV of alpha-particles, the [micro]t products for both carriers in the Sb[I.sub.3] detector were substantially smaller than those for other compound semiconductor detectors such as CdTe, CdZnTe, and TlBr.
Figure 10 was the alpha-particle energy spectra acquired for 30 s and 180 s and total counts were 15,834 counts and 50,969 counts for 30 s and 180 s, respectively. Although the count rate linearity could not be observed from the Sb[I.sub.3] detector, the linearity may be improved by reducing charge trapping and detector noise during operation.
Figure 11 shows the [sup.137]Cs gamma-ray energy spectra obtained from the Sb[I.sub.3] detector at room temperature. Counts from the detectors were slightly increased by the irradiation of gamma-rays. These results suggested that almost gamma-rays from the source were penetrating through the detector because the thickness of the Sb[I.sub.3] crystal was 0.229 mm.
3.3. Detector Stability. Stable detector performance is an important characteristic for radiation detectors. During operation of semiconductor detectors, temporal changes in detector performance known as the polarization phenomenon are usually observed as a decrease in pulse height. This phenomenon depends on the semiconductor material used for the detector and has been previously observed for CdTe detectors and TlBr detectors. Methods for improving or suppressing this phenomenon have been reported [17-20]. As previously mentioned, confirmation of the polarization phenomenon and research overcoming the phenomenon are necessary for realizing semiconductor detectors suitable for applications such as medical devices and industrial imaging.
In the present study, the Sb[I.sub.3] detector was continuously operated at room temperature and alpha-particle energy spectra were recorded as a function of time to evaluate the polarization phenomenon in the detector. Figure 12 shows the operation-time dependence of the [sup.241]Am alpha-particle energy spectra obtained using the Sb[I.sub.3] detector. The spectral response of the Sb[I.sub.3] detector was stable for almost 24 h under the bias voltage (200 V). Although a reduction in pulse height was observed after 90 h of operation, the pulse height immediately recovered when the bias voltage was cutoff. Recovery from the polarization phenomenon after cutoff of the applied bias voltage has also been reported for CdTe detectors , whose recovery behavior differs from that of TlBr detectors. TlBr detectors required a long time to recover their performance because the accumulation of ionic charge at the electrodes caused the polarization phenomenon. Therefore, the polarization phenomenon observed for the Sb[I.sub.3] detector is similar to that of CdTe detectors and is speculatively attributed to trapped charges.
Sb[I.sub.3] crystals were grown by the Bridgman method, and the performance of Sb[I.sub.3] detectors was evaluated on the basis of alpha-particle energy spectra recorded at room temperature. Although the spectra recorded using the fabricated Sb[I.sub.3] detectors showed no full-energy peak of the alpha-particles, the detector responded to alpha-particle irradiation. The Sb[I.sub.3] detector exhibited a decrease in pulse height during long-term operation, similar to other semiconductor detectors based on CdTe and TlBr; however, stable operation for 24 h was observed and the pulse height immediately recovered when the applied bias was cut off. Thus, Sb[I.sub.3] is a promising material for use in radiation detectors that exhibit stable operation. Further research on purification of the starting material and on improving carrier transport is required to improve the spectroscopic performance of the Sb[I.sub.3] detectors.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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Toshiyuki Onodera (ID), (1) Koei Baba, (1) and Keitaro Hitomi (2)
(1) Department of Electrical and Electronic Engineering, Tohoku Institute of Technology, Sendai 982-8577, Japan
(2) Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578, Japan
Correspondence should be addressed to Toshiyuki Onodera; email@example.com
Received 23 May 2018; Revised 5 November 2018; Accepted 14 November 2018; Published 5 December 2018
Academic Editor: Michael I. Ojovan
Caption: Figure 1: Attenuation coefficients of semiconductor detector materials as functions of the photon energy.
Caption: Figure 2: Schematic of a Bridgman furnace and the temperature distribution used in the present study.
Caption: Figure 3: Cleaved Sb[I.sub.3] crystals.
Caption: Figure 4: Transmittance spectrum of a cleaved Sb[I.sub.3] crystal.
Caption: Figure 5: X-ray diffraction patterns of the grown Sb[I.sub.3] crystal and a commercially available Sb[I.sub.3] powder.
Caption: Figure 6: Magnification of the X-ray diffraction patterns in the region of the (006) reflection of Sb[I.sub.3].
Caption: Figure 7: Current-voltage characteristics of an Sb[I.sub.3] detector at room temperature.
Caption: Figure 8: Alpha-particle energy spectrum of the Sb[I.sub.3] detector under cathode irradiation (electrons mainly transport in the crystal).
Caption: Figure 9: Alpha-particle energy spectrum obtained from the Sb[I.sub.3] detector under anode irradiation (holes mainly transport in the crystal).
Caption: Figure 10: Alpha-particle energy spectrum obtained from the Sb[I.sub.3] detector under cathode irradiation for 30 s and 180 s.
Caption: Figure 11: [sup.137]Cs gamma-ray energy spectrum obtained from the Sb[I.sub.3] detector.
Caption: Figure 12: Alpha-particle energy spectra obtained from the Sb[I.sub.3] detector as a function of operation time at room temperature.
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|Title Annotation:||Research Article|
|Author:||Onodera, Toshiyuki; Baba, Koei; Hitomi, Keitaro|
|Publication:||Science and Technology of Nuclear Installations|
|Date:||Jan 1, 2018|
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