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김영학,신기창,Bolotnikov Aleksey,이원호 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.5
The virtual Frisch-grid method for room-temperature radiation detectors has been widely used because of its simplicity and high performance. Recently, side electrodes were separately attached to each surface of the detectors instead of covering the entire detector surface with a single electrode. The side-electrode structure enables the measurement of the three-dimensional (3D) gamma-ray interaction in the detector. The positional information of the interaction can then be utilized to precisely calibrate the response of the detector for gamma-ray spectroscopy and imaging. In this study, we developed a 3D position-sensitive 5 5 12 mm3 cadmium-zinc-telluride (CZT) detector and applied a flattening method to correct detector responses. Collimated gamma-rays incident on the surface of the detector were scanned to evaluate the positional accuracy of the detection system. Positional distributions of the radiation interactions with the detector were imaged for quantitative and qualitative evaluation. The energy spectra of various radioisotopes were measured and improved by the detector response calibration according to the calculated positional information. The energy spectra ranged from 59.5 keV (emitted by 241Am) to 1332 keV (emitted by 60Co). The best energy resolution was 1.06% at 662 keV when the CZT detector was voxelized to 20 20 10.
New Virtual Frisch-Grid CdZnTe Detector Design With Sub-Millimeter Spatial Resolution
Kisung Lee,Bolotnikov, Aleksey,Seungbin Bae,Roy, Utpal,Camarda, Giuseppe,Petryk, Matthew,Yonggang Cui,Hossain, Anwar,Ge Yang,Dedic, Vaclav,Kihyun Kim,James, Ralph B. Professional Technical Group on Nuclear Science 2014 IEEE transactions on nuclear science Vol.61 No.5
<P>We evaluated the performance of a position-sensitive virtual Frisch-grid (VFG) CdZnTe detector, 6 mm ×6 mm ×15 mm, via sensing strips on its side surfaces. Once the signals were collected from the anode, and from four or eight strips attached to the detector's sides, we assessed the anode's energy spectra and derived histograms from the side electrodes to evaluate the feasibility of achieving sub-millimeter spatial resolution in the X-Y plane. Using a highly collimated 30-keV X-ray beam at the National Synchrotron Light Source, and applying corrections to the raw data, we determined the photon-interaction points by conventional Anger logic and via a more sophisticated statistics-based positioning (SBP) algorithm. With the VFG detector's current configuration, we achieved a resolution below 1 mm, even for low-energy X-rays.</P>
Overcoming Zn segregation in CdZnTe with the temperature gradient annealing
Kim, K.,Bolotnikov, A.E.,Camarda, G.S.,Hossain, A.,James, R.B. Elsevier 2016 Journal of crystal growth Vol.442 No.-
<P><B>Abstract</B></P> <P>The availability of large volume crystals with the same energy gap in melt-grown CdZnTe (CZT) is restricted due to the Zn segregation in CdTe hosts. We observed the migration of Zn in the solid phase along the positive temperature gradient direction both <I>in situ</I> and post-growth temperature gradient annealing (TGA) of CZT. Diffusivity of Zn obtained from the <I>in situ</I> TGA was approximately 10<SUP>−5</SUP> cm<SUP>2</SUP>/s order and completely different mechanism with that of post-growth. The CZT ingots obtained through <I>in situ</I> TGA have uniform Zn and resistivity of <SUP> 10 10 </SUP> Ω cm orders. The CZT detectors fabricated from <I>in situ</I> TGA applied ingots exhibit 10% of energy resolution for 59.5keV peak of <SUP>241</SUP>Am.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Zn migrates in the solid phase along the positive temperature gradient. </LI> <LI> Zn migration mechanism is different in <I>in situ</I> and post-growth CdZnTe annealing. </LI> <LI> Zn segregation in CdZnTe can be solved by temperature gradient annealing. </LI> <LI> <I>In situ</I> CdZnTe annealing is a useful method to obtain highly uniform bandgap detector. </LI> </UL> </P>
Evaluation of sequence tracking methods for Compton cameras based on CdZnTe arrays
이준,김영학,Aleksey Bolotnikov,이원호 한국원자력학회 2021 Nuclear Engineering and Technology Vol.53 No.12
In this study, the performance of sequence tracking methods for multiple interaction events in specificCdZnTe Compton imagers was evaluated using Monte Carlo simulations. The Compton imager consistedof a 6 6 array of virtual Frisch-grid CZT crystals, where the dimensions of each crystal were 5 512 mm3. The sequence tracking methods for another Compton imager that consists of two identical CZTcrystals arrays were also evaluated. When 662 keV radiation was incident on the detectors, the percentages of the correct sequences determined by the simple comparison and deterministic methods fortwo sequential interactions were identical (~80%), while those evaluated using the minimum squareddifference method (55e59%) and Three Compton method (45e55%) for three sequential interactions,differed from each other. The reconstructed images of a 662 keV point source detected using single anddouble arrays were evaluated based on their angular resolution and signal-to-noise ratio, and the resultsshowed that the double arrays outperformed single arrays
New Insight into the 1.1-eV Trap Level in CdTe-based Semiconductor
김기현,최종학,A. E. Bolotnikov,G. S. Camarda,A. Hossain,G. Yang,Y. Cui,R. B. James 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.4
We investigated trap levels in detector-grade CdZnTe (CZT) material grown by using three different methods, viz, the Bridgman, traveling heater method (THM), and high-pressure Bridgman method (HPB), by current deep-level transient spectroscopy (I-DLTS). All CZT detectors contained deep trap levels located at around 1.1 eV (which we designated DE1), which has been attributed to Te vacancies induced one. However, our crystal growth and characterization results indicated that dislocations induced by Te secondary defects (inclusions/precipitates)were a more probable origin than Te vacancies. Also, a theoretical calculation of the electron de-trapping time associated with DE1 can explain well the abnormal residual current behavior at temperatures slightly above room temperature. Our results show better control of the concentrations and the sizes of Te secondary defects is critical to improving the detector’s performance at room temperature by reducing lagging effects.
Detector performance and defect densities in CdZnTe after two-step annealing
Kim, Eunhye,Kim, Yonghoon,Bolotnikov, A.E.,James, R.B.,Kim, Kihyun Elsevier BV * North-Holland 2019 Nuclear Instruments & Methods in Physics Research. Vol. No.
<P><B>Abstract</B></P> <P>Defects both microscale and nanoscale are play an important role in CdZnTe (CZT) device performance. Typical micro-scale defects such as Te inclusions were removed via a two-step annealing process, and their concentration was analyzed via IR transmission microscopy. In addition, transmission electron microscopy (TEM) measurement was employed to investigate the evolution of nano-scale defects after the annealing process. Dislocation and stacking faults were commonly observed defects in as-grown and annealed CZT. The line shape defects, which are possibly related to the stress field around dislocations, disappeared during the in-situ the annealing at 200–220 ° C. A Frisch-grid CZT detector made via the two-step annealing process exhibited improved energy resolution and low backscattering counts in Cs-137 gamma spectra.</P>
Roy, U.N.,Bolotnikov, A.E.,Camarda, G.S.,Cui, Y.,Hossain, A.,Lee, K.,Lee, W.,Tappero, R.,Yang, Ge,Cui, Y.,Burger, A.,James, R.B. Elsevier 2015 Journal of crystal growth Vol.411 No.-
<P><B>Abstract</B></P> <P>We grew CdTe<SUB> <I>x</I> </SUB>Se<SUB>1−<I>x</I> </SUB> crystals with nominal Se concentrations of 5%, 7%, and 10% by the vertical Bridgman technique, and evaluated their compositional homogeneity and structural quality at the NSLS’ X-ray fluorescence and white beam X-ray topography beam lines. Both X-ray fluorescence and photoluminescence mapping revealed very high compositional homogeneity of the CdTe<SUB> <I>x</I> </SUB>Se<SUB>1−<I>x</I> </SUB> crystals. We noted that those crystals with higher concentrations of Se were more prone to twinning than those with a lower content. The crystals were fairly free from strains and contained low concentrations of sub-grain boundaries and their networks.</P> <P><B>Highlights</B></P> <P> <UL> <LI> CdTeSe crystals were grown by the vertical Bridgman technique. </LI> <LI> Very high compositional homogeneity. </LI> <LI> Low concentration of sub-grain boundaries. </LI> <LI> Almost free from sub-grain boundary network. </LI> </UL> </P>
Ge Yang,Genda Gu,Aleksey E. Bolotnikov,Yonggang Cui,Giuseppe S. Camarda,Anwar Hossain,Utpal N. Roy,Nicholas Kivi,Tiansheng Liu,Ralph B. James 대한금속·재료학회 2015 ELECTRONIC MATERIALS LETTERS Vol.11 No.3
Cadmium manganese telluride (CdMnTe or CMT), a compound semiconductor, is considered a promising material for the fabrication of high-performance room-temperature x-ray and gamma-ray detectors. The presence of material defects, e.g., high density of Te inclusions, has been a long-standing issue in CMT crystals grown by various Bridgman methods, since these defects degrade the device performance via charge-trapping. To address this issue, we employed the modified floating-zone method (MFZ) to grow CMT crystals and obtained as-grown crystals free of Te inclusions. This represents a new and distinct feature, absence of Te inclusions, compared to CMT crystals grown by Bridgman methods. White-beam x-ray diffraction topography (WBXDT) measurements demonstrated the existence of a high stress field within the MFZ-grown CMT crystals, which originates from the steep temperature gradient near the growth interface. Furthermore, we achieved a resistivity of 109 Ωcm for the MFZ-grown CMT crystals. The low-temperature photoluminescence (PL) measurements show that the intensity of the dislocation-related Y band is much higher than that of the principal exciton peaks, (D0,X) and (A0,X), confirming that the crystalline quality is affected by the high stress field. A long-term in-situ or post-growth thermal annealing will help to release such stress to improve the crystalline quality.
Tellurium Secondary-Phase Defects in CdZnTe and their Association With the 1.1-eV Deep Trap
Suh, Jonghee,Hong, Jinki,Franc, J.,Bolotnikov, A. E.,Hossain, A.,James, Ralph. B.,Kim, Kihyun Professional Technical Group on Nuclear Science 2016 IEEE transactions on nuclear science Vol.63 No.5
<P>Defects located at the EC - 1.1 (eV) level, which are electro-optically active deep traps, generally have been overlooked since their presence cannot be detected except for those in highly resistive CdTe compounds. The origin of this trap is still debated on whether it is from Te vacancies or from dislocations induced by secondary phase defects in Te. We have grown high-resistivity Te-rich CZT ingots to clarify the origin of the 1.1 eV defects and to analyze the defect levels in the CZT samples by current deep level transient spectroscopy (I-DLTS) and photoluminescence (PL). From the analysis, defect levels such as shallow acceptor/donor, A-centers, Cd vacancies and Te antisite appeared to be in both high and low concentrations of Te inclusions, but the level of 1.1-eV defects exhibited dependence on the density of Te inclusion in the CZT samples. We also evaluated the effect of the 1.1-eV deep-level defects on the detector's performance in point view of carrier trapping and de-trapping.</P>