Gamma-ray Detector Materials Research Articles

A new Excel-based Monte Carlo transport code for simulating energy response spectra and efficiencies in gamma-ray detection materials

This study presents and assesses a new, user-friendly, and Excel-based Monte Carlo photon transport code, specifically designed to simulate Gaussian energy broadened response spectra of gamma-ray detector materials. This vectorized code utilizes the EPICS2017 photoatomic data library, with unionized energy grid construction, to facilitate precise and efficient photon transport simulations in any user-defined materials composition. Performance benchmarks were conducted using PHITS 3.31, with both basic and detailed models of a gamma-ray scintillator system, and detection crystals of NaI(Tl) and LaBr3(Ce). These benchmarks spanned a wide array of photon source monoenergetic emissions. Detector model efficiency was determined through automated peak area analysis, employing algorithms from the Ortec GammaVision software. The results indicated only minor spectral differences between the Excel-based code and the PHITS models, particularly within the range of practical gamma-ray energies and near-zero source-detector distances. Efficiency curves demonstrated good agreement between the Excel-based code and PHITS models, especially within the energy range of approximately 200 keV to a few MeV, with increasing agreement as detector crystal size increased. Comparisons were conducted with the existing data in literature at source-detector distances of 2 cm and 5 cm, using a detector crystal size of 2”. These comparisons support the code’s validity and its implementation of a basic geometry. This new Excel-based code appears as a promising tool with potential applications in radiation detector characterization and materials research.

Bridgman-Grown (Cd,Mn)Te and (Cd,Mn)(Te,Se): A Comparison of Suitability for X and Gamma Detectors.

This study explores the suitability of (Cd,Mn)Te and (Cd,Mn)(Te,Se) as room-temperature X-ray and gamma-ray detector materials, grown using the Bridgman method. The investigation compares their crystal structure, mechanical and optical properties, and radiation detection capabilities. Both crystals can yield large-area single crystal samples measuring approximately 30 × 30 mm2. In low-temperature photoluminescence analysis, both materials showed defect states, and annealing in cadmium vapors effectively eliminated donor-acceptor pair luminescence in (Cd,Mn)Te but not in (Cd,Mn)(Te,Se). Moreover, harder (Cd,Mn)(Te,Se) exhibited a higher etch pit density compared to softer (Cd,Mn)Te. X-ray diffraction examination revealed uniform lattice constant distribution in both compounds, with variations at a part per million level. (Cd,Mn)Te crystals demonstrated excellent single crystal properties with narrower omega scan widths, while (Cd,Mn)(Te,Se) exhibited a high contribution of block-like structures with significantly larger misorientation angles. Spectroscopic evaluations revealed better performance of a pixelated (Cd,Mn)Te detector, in comparison to (Cd,Mn)(Te,Se), achieving a mean full width at half maximum of 14% for the 122 keV gamma peak of Co-57. The reduced performance of the (Cd,Mn)(Te,Se) detector may be attributed to deep trap-related luminescence or block-like structures with larger misorientation angles. In conclusion, Bridgman-grown (Cd,Mn)Te emerges as a more promising material for X-ray and gamma-ray detectors when compared to (Cd,Mn)(Te,Se).

Gamma‐Ray Detection Using Bi‐Poor Cs2AgBiBr6 Double Perovskite Single Crystals (Advanced Optical Materials 8/2021)

Ge Yang and co-workers (article number 2001575) report the gamma energy-resolving performance of a new class of all-inorganic and lead-free Cs2AgBiBr6 double perovskite single crystals. The authors grow the crystals from Bi-normal and Bi-poor precursor solutions, respectively, and demonstrate that tuning the stoichiometry of Cs2AgBiBr6 could directly affect its electrical pro perties and enable its response to gamma-ray radiation. These efforts provide a basis for further development of all-inorganic and environmental-friendly perovskites-based gamma-ray detector materials.

Onset of Fogging and Degradation in Polyvinyl Toluene-Based Scintillators

Plastic-based gamma-ray detector materials such as polyvinyl toluene (PVT) occasionally suffer from loss of optical light transmission due to internal defects or “fogging” when exposed to some environmental conditions over time. Fogging results in reduced scintillation light propagated to the photomultiplier tube for collection. Investigations of the physical cause of these defects and their associated environmental conditions are ongoing. The goal of this article was to characterize the signs and effects of fogging over a simulated 22-year lifespan of the PVT in outdoor environments. Two potential mitigation methods were also evaluated, and one successfully mitigated the phenomena.

Recent advancements in using perovskite single crystals for gamma-ray detection

Perovskites in the form of polycrystalline thin films have been extensively studied in solar cell research over the past few years. Recently, monocrystalline perovskites have been discovered with great potentials for the use in radiation detection, in which higher energy photons (X-ray and gamma ray) or heavy charged alpha particles compared with low-energy visible light in solar cell research need to be detected. In this review, we summarize the state-of-the-art perovskite single crystals which have been investigated specifically for gamma photon detection. The physical, optical, and electrical properties will be reviewed and compared with today’s market-dominating gamma-ray detector materials, such as cadmium zinc telluride (CdZnTe) and its low-cost alternative cadmium zinc telluride selenium (CdZnTeSe) single crystals. Lastly, suggestions are proposed for the future development in this research direction.

Role of selenium addition to CdZnTe matrix for room-temperature radiation detector applications

Because of its ideal band gap, high density and high electron mobility-lifetime product, cadmium zinc telluride (CdZnTe or CZT) is currently the best room-temperature compound-semiconductor X- and gamma-ray detector material. However, because of its innate poor thermo-physical properties and above unity segregation coefficient for Zn, the wide spread deployment of this material in large-volume CZT detectors is still limited by the high production cost. The underlying reason for the low yield of high-quality material is that CZT suffers from three major detrimental defects: compositional inhomogeneity, high concentrations of dislocation walls/sub-grain boundary networks and high concentrations of Te inclusions/precipitates. To mitigate all these disadvantages, we report for the first time the effects of the addition of selenium to the CZT matrix. The addition of Se was found to be very effective in arresting the formation of sub-grain boundaries and its networks, significantly reducing Zn segregation, improving compositional homogeneity and resulting in much lower concentrations of Te inclusions/precipitates. Growth of the new quaternary crystal Cd1−xZnxTe1−ySey (CZTS) by the Traveling Heater Method (THM) is reported in this paper. We have demonstrated the production of much higher yield according to its compositional homogeneity, with substantially lower sub-grain boundaries and their network, and a lower concentration of Te inclusions/precipitates.

Study on electric field in Au/CdZnTe/In detectors under high fluxes of X-ray and laser irradiation

We used simultaneous Pockels effect and photoconductivity measurements to study of processes in high-resistivity CdZnTe semiconductor X-ray and gamma ray detector material at steady state high flux conditions. We have proven, that with an appropriate choice of monochromatic laser light, the profile of the electric field is nearly the same as with a polychromatic X-ray source. We observed that the electric field is strongly deformed in the case of both above and below bandgap laser radiation. This leads to a decrease of the photocurrent when the sample is illuminated with above bandgap radiation. When below bandgap illumination is used the photocurrent values are not affected by electric field weakening. This is explained by increased filling of the midgap trap with trapped electrons leading to an increase of the lifetime.

Improvements and problems of Bridgman–Stockbarger method for fabrication of TlBr single crystal detectors

TlBr is a promising material for gamma-ray detectors; however, it has crystal quality problems originated from the crystal growth itself and further treatments during detector fabrication. The effect of technological improvements on crystal quality has been studied in this work. Low performance of studied detectors was shown to be associated with residual stress or/and small-angle grain boundaries in these crystals. Several crystals failed as detectors were proved to contain impurities.

A theoretical approach to calibrate radiation portal monitor (RPM) systems

Radiation portal monitor (RPM) systems are widely used at international border crossings, where they are applied to the task of detecting nuclear devices, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, the former being required to rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce [Chambers, W.H., Atwater, H.F., Fehlau, P.E., Hastings, R.D., Henry, C.N., Kunz, W.E., Sampson, T.E., Whittlesey, T.H., Worth, G.M., 1974. Portal Monitor for Diversion Safeguards. LA-5681, Los Alamos Scientific Laboratory, Los Alamos, NM]. In the present work, compact analytical formulae are derived and used to calibrate two RPM systems with isotropic radiating sources: (i) polyvinyltoluene (PVT) or plastic and (ii) thallium-doped crystalline sodium iodide, NaI(Tl), gamma-ray detector materials. The calculated efficiencies are compared to measured values reported in the literatures, showing very good agreement.

Determination of absolute detection efficiencies for detectors of interest in homeland security

The absolute total and absolute peak detection efficiencies of gamma ray detector materials NaI:Tl, CdZnTe, HPGe, HPXe, LaBr 3:Ce and LaCl 3:Ce were simulated and compared to that of polyvinyltoluene (PVT). The dimensions of the PVT detector were 188.82 cm×60.96 cm×5.08 cm, which is a typical size for a single-panel portal monitor. The absolute total and peak detection efficiencies for these detector materials for the point, line and spherical source geometries of 60Co (1332 keV), 137Cs (662 keV) and 241Am (59.5 keV) were simulated at various source-to-detector distances using the Monte Carlo N-Particle software (MCNP5-V1.30). The comparison of the absolute total detection efficiencies for a point, line and spherical source geometry of 60Co and 137Cs at different source-to-detector distance showed that the absolute detection efficiency for PVT is higher relative to the other detectors of typical dimensions for that material. However, the absolute peak detection efficiency of some of these detectors are higher relative to PVT, for example the absolute peak detection efficiency of NaI:Tl (7.62 cm diameter×7.62 cm long), HPGe (7.62 cm diameter×7.62 cm long), HPXe (11.43 cm diameter×60.96 cm long), and LaCl 3:Ce (5.08 cm diameter×5.08 cm long) are all greater than that of a 188.82 cm×60.96 cm×5.08 cm PVT detector for 60Co and 137Cs for all geometries studied. The absolute total and absolute peak detection efficiencies of a right circular cylinder of NaI:Tl with various diameters and thicknesses were determined for a point source. The effect of changing the solid angle on the NaI:Tl detectors showed that with increasing solid angle and detector thickness, the absolute efficiency increases. This work establishes a common basis for differentiating detector materials for passive portal monitoring of gamma ray radiation.

Signal variance in gamma-ray detectors—A review

Signal variance in gamma-ray detector materials is reviewed with an emphasis on intrinsic variance. Phenomenological models of electron cascades are examined and the Fano factor ( F) is discussed in detail. In semiconductors, F is much smaller than unity and charge carrier production is nearly proportional to energy. Based on a fit to a number of semiconductors and insulators, a new relationship between the average energy for electron–hole pair production and band gap energy is proposed. In scintillators, the resolution is governed mainly by photoelectron statistics and proportionality of light yield with respect to energy.

Analysis of gamma ray spectra measured by Mars Odyssey

Gamma ray spectra measured by the Mars Odyssey gamma ray spectrometer while in orbit around Mars were analyzed to identify the sources of 334 observed peaks and features. Most peaks were of a standard Gaussian shape with a low‐energy tail. However, Doppler‐broadened and sawtooth‐shaped peaks were also observed in the spectra. The sources of most peaks were identified. Many peaks come from the gamma ray detector material or materials around the detector, particularly Ti and Mg. Identifications were often confirmed by analyzing other spectra, such as those measured during the cruise to Mars, before and after the boom deployment, at the end of solar particle events, and over thick frozen CO2 polar caps during winters.

Monte Carlo simulation of induced radioactive background in gamma-ray detector materials

Induced activation in detector materials constitutes a major source of background for spaceborne /spl gamma/-ray instruments. Future detector systems will utilise a variety of background reduction techniques and novel detector materials to maximise signal-to-noise and resolution of the measurements taken, and assessment of the merits of different designs will require detailed attention to a wide range of physical processes. The Integrated Radiation Transport Suite (IRTS) has been applied to simulate the effects of spacecraft shielding against Van Allen and galactic cosmic-ray protons, the production of secondary hadrons, and induced radioactivity in and the response of /spl gamma/-ray detector materials. This paper discusses the physics requirements for these simulations, and the degree of success of the models through comparisons with experimental data obtained from Space Shuttle flights and proton-beam irradiation of detector materials.

The use of pulse processing techniques to improve the performance of Cd/sub 1-x/Zn/sub x/Te gamma-ray spectrometers

Cd/sub 1-x/Zn/sub x/Te (CZT) has recently shown great promise for use as a room temperature gamma-ray detector material. The availability of large volume (>1 cm/sup 3/) high resistivity CZT crystals has allowed the demonstration of detectors much larger than can be built with the similar material CdTe. However, CZT-like many other room-temperature materials-suffers from the poor transport properties of holes. The poor hole drift properties of CZT cause the characteristic "hole tailing" in gamma-ray pulse height spectra. We have applied pulse processing methods to reduce the hole tailing effects and improve the energy resolution of CZT detectors. We have used two signal processing methods to reduce hole tailing in CZT detectors: digital rise-time compensation and dual time-constant sampling. We discuss the implementation of these techniques, demonstrate results obtained in the laboratory, and compare the performance obtained with other detector systems.

The Shuttle activation monitor: a system for direct comparison of gamma-ray detector materials in a space environment

An experimental system which was used to compare gamma-ray detector materials in a space environment is described. Two 3-in*3-in scintillator detectors, NaI and BGO, were flown on the Space Shuttle Columbia as part of the Shuttle Activation Monitor (SAM) experiment. The goals of this experiment were to compare the performance of the two detector materials in the same environment and to measure the variations in dynamic radiation background as a function of geomagnetic coordinates, amount of shielding, and type of detector material. Twenty-four hours of data in 5-min time bins were recorded with each detector in both high-shielding and low-shielding locations in the Orbiter middeck. The high-inclination orbit (57 degrees , 160 nmi) provided exposure to the trapped charged particles in the South Atlantic Anomaly as well as the electrons in the polar regions. The system used specially adapted off-the-shelf hardware for data acquisition and storage. The multichannel analyzer was a commercially available unit with added circuitry to support an active particle shield. Data during the flight were stored using a cassette tape recorder. A preliminary look at some of the data from the first flight is included. >