Material For Radiation Detectors Research Articles

Kinetics of Schottky defect formation and annihilation in single crystal TlBr

The kinetics for Schottky defect (Tl and Br vacancy pair) formation and annihilation in ionically conducting TlBr are characterized through a temperature induced conductivity relaxation technique. Near room temperature, defect generation-annihilation was found to take on the order of hours before equilibrium was reached after a step change in temperature, and that mechanical damage imparted on the sample rapidly increases this rate. The rate limiting step to Schottky defect formation-annihilation is identified as being the migration of lower mobility Tl (versus Br), with an estimate for source-sink density derived from calculated diffusion lengths. This study represents one of the first investigations of Schottky defect generation-annihilation kinetics and demonstrates its utility in quantifying detrimental mechanical damage in radiation detector materials.

Electronic structure and defect properties of Tl6SeI4: Density functional calculations

We report density functional calculations of electronic structure, phase diagram, and dielectric, optical, and defect properties of Tl${}_{6}$SeI${}_{4}$. We discuss how electronic structure and defect properties affect resistivity and carrier mobility-lifetime (\ensuremath{\mu}\ensuremath{\tau}) products in Tl${}_{6}$SeI${}_{4}$. We find large Born effective charges due to covalency involving Tl-6$p$ states. High Born charges generally enhance the static dielectric constant. This provides a mechanism for effective screening of charged defects and impurities. We find that high resistivity can be obtained under near-stoichiometric growth conditions via Fermi level pinning near the middle of the band gap by shallow donors and acceptors, as opposed to deep traps that can give high resistivity, but at the expense of short carrier drift lengths. Defect calculations also reveal the presence of deep native donors that may cause electron trapping. The experimentally observed good \ensuremath{\mu}\ensuremath{\tau} products may be explained by a combination of small effective masses and effective screening of charged defects. High resistivity and good \ensuremath{\mu}\ensuremath{\tau} products make Tl${}_{6}$SeI${}_{4}$ a promising room-temperature radiation detector material. We also show the calculated defect diffusion barriers, which affect defect migration under external bias in a detector.

First-principles study of impurities in TlBr

TlBr is a promising semiconductor material for room-temperature radiation detection. Material purification has been the driver for the recent improvement in the TlBr detector performance, mainly reflected by the significant increase in the carrier mobility-lifetime product. This suggests that impurities have significant impact on the carrier transport in TlBr. In this paper, first-principles calculations are used to study the properties of a number of commonly observed impurities in TlBr. The impurity-induced gap states are presented and their effects on the carrier trapping are discussed.

Monte Carlo simulation for the electron cascade due to gamma rays in semiconductor radiation detectors

A Monte Carlo code was developed for simulating the electron cascade in radiation detector materials. The electron differential scattering cross sections were derived from measured electron energy-loss and optical spectra, making the method applicable for a wide range of materials. The detector resolution in a simplified model system shows dependence on the bandgap, the plasmon strength and energy, and the valence band width. In principle, these parameters could be optimized to improve detector performance. The intrinsic energy resolution was calculated for three semiconductors: silicon (Si), gallium arsenide (GaAs), and zinc telluride (ZnTe). Setting the ionization thresholds for electrons and holes is identified as a critical issue, as this strongly affects both the average electron-hole pair energy w and the Fano factor F. Using an ionization threshold from impact ionization calculations as an effective bandgap yields pair energies that are well matched to measured values. Fano factors of 0.091 (Si), 0.100 (GaAs), and 0.075 (ZnTe) were calculated. The Fano factor calculated for silicon using this model was lower than some results from past simulations and experiments. This difference could be attributed to problems in simulating inter-band transitions and the scattering of low-energy electrons.

Quantum Dot-Organic Polymer Composite Materials for Radiation Detection and Imaging

Colloidal semiconductor nanocrystals exhibit physical properties that are characteristic of intermediate size scales between molecular states and solid state materials, and are often called quantum dots. Solid state semiconductor materials have been used extensively as scintillation detectors for ionizing radiation. We describe the use of semiconductor quantum dot-organic polymer composites for use as scintillation detectors and report the use of quantum dot-polymer composite thin films for X-ray imaging.

Lead Iodide Crystals as Input Material for Radiation Detectors

Lead iodide is an important inorganic solid for fundamental research and possible technological applications and is considered to be a potential room temperature nuclear radiation detector. In lead iodide the phenomenon of polytypism is posing an interesting problem of phase transformations amongst its various polytypic modifications. The transformations have also been observed even when the crystals are stored for few months. It causes deterioration in functioning of PbI2 devices. Taking into account the known structures of PbI2 and the data available on the mode of growth and storage of crystals, it has been concluded that purified melt grown crystals of PbI2 are the best suited for nuclear radiation detectors.

Dimensional Reduction: A Design Tool for New Radiation Detection Materials

John Androulakis , Sebastian C. Peter , Hao Li , Christos D. Malliakas , John A. Peters , Zhifu Liu , Bruce W W. essels , Jung-Hwan Song , Hosub Jin , Arthur J. reeman , F and Mercouri G. Kanatzidis *

High-Throughput Combinatorial Database of Electronic Band Structures for Inorganic Scintillator Materials

For the purpose of creating a database of electronic structures of all the known inorganic compounds, we have developed a computational framework based on high-throughput ab initio calculations (AFLOW) and an online repository (www.aflowlib.org). In this article, we report the first step of this task: the calculation of band structures for 7439 compounds intended for the research of scintillator materials for γ-ray radiation detection. Data-mining is performed to select the candidates from 193,456 compounds compiled in the Inorganic Crystal Structure Database. Light yield and scintillation nonproportionality are predicted based on semiempirical band gaps and effective masses. We present a list of materials, potentially bright and proportional, and focus on those exhibiting small effective masses and effective mass ratios.

A transport-based model of material trends in nonproportionality of scintillators

Electron-hole pairs created by the passage of a high-energy electron in a scintillator radiation detector find themselves in a very high radial concentration gradient of the primary electron track. Since nonlinear quenching that is generally regarded to be at the root of nonproportional response depends on the fourth or sixth power of the track radius in a cylindrical track model, radial diffusion of charge carriers and excitons on the ∼10 picosecond duration typical of nonlinear quenching can compete with and thereby modify that quenching. We use a numerical model of transport and nonlinear quenching to examine trends affecting local light yield versus excitation density as a function of charge carrier and exciton diffusion coefficients. Four trends are found: (1) nonlinear quenching associated with the universal “roll-off” of local light yield versus dE/dx is a function of the lesser of mobilities μe and μh or of DEXC as appropriate, spanning a broad range of scintillators and semiconductor detectors; (2) when μe ≈ μh, excitons dominate free carriers in transport, the corresponding reduction of scattering by charged defects and optical phonons increases diffusion out of the track in competition with nonlinear quenching, and a rise in proportionality is expected; (3) when μh ≪ μe as in halide scintillators with hole self-trapping, the branching between free carriers and excitons varies strongly along the track, leading to a “hump” in local light yield versus dE/dx; (4) anisotropic mobility can promote charge separation along orthogonal axes and leads to a characteristic shift of the “hump” in halide local light yield. Trends 1 and 2 have been combined in a quantitative model of nonlinear local light yield which is predictive of empirical nonproportionality for a wide range of oxide and semiconductor radiation detector materials where band mass or mobility data are the determinative material parameters.

First principles study of native defects in InI

Heavy-metal halide semiconductors have attracted much interest recently for their potential applications in radiation detection because the large atomic numbers (high Z) of their constituent elements enable efficient radiation absorption and their large band gaps allow room temperature operation. However, defect properties of these halides and their connection to carrier transport are little known. In this paper, we present first-principles calculations on native defects in InI, which is a promising material for applications in room temperature radiation detection. The important findings are: (1) anion and cation vacancies (Schottky defects) form the dominant low-energy defects that can pin the Fermi level close to midgap, leading to high resistivity that is required for a good radiation detector material; (2) the anion vacancy in InI induces a deep electron trap, which should reduce electron mobility-lifetime product in InI; (3) low diffusion barriers of vacancies could be responsible for the observed polarization phenomenon at room temperature.

Scintillation characteristics of Cs2LiCeBr6 crystal

Abstract This work investigates the scintillation properties of Cs 2 LiCeBr 6 crystal as a new material for radiation detection. This scintillation material is grown by the vertical Bridgman method. Under X-ray excitation the sample crystal shows a broad cerium based emission band between 390 and 450 nm wavelength range. Energy resolution for 662 keV γ-rays is measured to be 7.4% (FWHM). At room temperature Cs 2 LiCeBr 6 crystal exhibits three exponential decay time components. The fast and major component of scintillation time profile of Cs 2 LiCeBr 6 emission decays with a 86 ns time constant. Absolute light yield for the sample crystal is estimated to be 27,000 and 29,000 photons/MeV using APD and photomultiplier tube, respectively. The sample crystal shows good proportionality of 5% in the measured energy range from 31 to 1333 keV. This study showed that this new scintillation crystal can be a good candidate for radiation detection and medical imaging. The sample crystal is highly hygroscopic.

Designing metal-organic frameworks for radiation detection

Five metal-organic frameworks (MOFs) were synthesized and investigated via steady-state photoluminescence and radioluminescence measurements. Unique spectral features were observed in the 2.5 MeV proton spectra, corresponding to differences in the electronic and crystalline structures of each material. Targeted structural transformations and infiltration with extrinsic dopants were also employed to modify the luminescence of these frameworks, establishing MOFs as a platform to design new radiation detection materials.

Application of solution techniques for rapid growth of organic crystals

Single crystals of a pure hydrocarbon, 1,3,5-triphenylbenzene, with properties suitable for high-energy neutron detection were grown from toluene solutions using slow evaporation and temperature reduction methods with growth rates up to 10 mm/day. Application of the rapid growth technique developed earlier for growth of large water-soluble crystals shows that crystals of aromatic compounds can be successfully grown from solutions in large volumes required for their use as scintillator materials for radiation detection.

Performance of novel materials for radiation detection: Tl3AsSe3, TlGaSe2, and Tl4HgI6

Abstract In this paper we report on the electrical characteristics of three novel ternary compounds, Tl 3 AsSe 3 (TAS), TlGaSe 2 (TGS), and Tl 4 HgI 6 (THI), pertaining to their use as radiation detectors. The details for growth and material characterization are not presented. A semiconductor based gamma ray detector requires a material with high Z , high density, high resistivity, appropriate bandgap (1.5–2 eV), low energy/electron–hole pair, and a high μτ product. CZT is currently the best semiconductor material for room temperature gamma ray spectroscopy; however, it is extremely difficult to produce large volumes of detector grade material, making it expensive and in limited supply. DNDO/DHS began searching for other materials that might perform as well as CZT but be easier to grow and in the end lower the cost. For this purpose, we investigated the above three materials as possible replacements for CZT as gamma ray detectors. The bulk resistivity, I – V curves, X-ray response, and gamma ray response measurements for doped and undoped crystals are presented and discussed. TAS shows good X-ray and gamma ray response, but has poor resistivity, which results in large dark current and poor spectral response. TGS has good resistivity, but shows poor X-ray and gamma ray response. THI has excellent resistivity, shows some X-ray and gamma ray response, and has great potential as a gamma ray detector.

The role of hole mobility in scintillator proportionality

Abstract Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE / dx and high dE / dx ends of electron light-yield curves and their respective contributions to nonproportionality. In CsI:Tl, it is shown that electron confinement toward the end of the track accentuates high-order quenching such as Auger recombination or dipole–dipole transfer, while in HPGe extremely rapid ( dE / dx .

Study on Crystal Growth and Scintillation Characteristics of Cs$_{2}$ LiCeCl$_{6}$

In this work, we present the crystal growth and scintillation properties of our newly developed scintillation crystal, Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiCeCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , for a γ-ray spectroscopy. This scintillation crystal is grown by using the vertical Bridgman method. The crystal of this material belongs to the elpasolite family characterized by a cubic structure and potentially can be easily grown in large volumes. Under the X-ray excitation, cerium emission band is observed to peak at 385 nm and 405 nm. An energy resolution (full width at half maximum over the peak position) of 5.5% was observed for the 662 keV full absorption peak. We measured an absolute light yield of 22 000 photons/MeV of absorbed η -ray energy. The crystal shows three main scintillation decay time components of 101 ns (42%), 557 ns (35%) and 2.9 μs (23%). This material is highly hygroscopic and special attention was paid during data taking and handling processes. We believe that the Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiCeCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> crystal can be a promising material for medical imaging and radiation detection. Moreover due to the presence of Li ions, this scintillation crystal can also be a possible candidate for thermal neutron detection.

Atmospheric Effects on the Performance of CdZnTe Single-Crystal Detectors

The production of high-quality ternary single-crystal materials for radiation detectors has progressed over the past 15 years. One of the more common materials being studied is CdZnTe (CZT), which can be grown using several methods to produce detector-grade materials. The work presented herein examines the effects of environmental conditions including temperature and humidity on detector performance [full-width at half-maximum (FWHM)] using the single pixel with guard detector configuration. The effects of electrical probe placement, reproducibility, and aging are also presented.

Single crystal artificial diamond detectors for VUV and soft X-rays measurements on JET thermonuclear fusion plasma

Diamond appears to be a promising material for VUV and soft X-ray radiation detection. Its wide band-gap (5.5 eV) results in a very low leakage current (it can operate above room temperature) and its electronic properties (high carrier mobility) allow a fast time response. More importantly, it is optimally suited for harsh environment applications, like those in the JET Tokamak located at the Culham laboratory (UK). Its extreme radiation hardness is well known and another interesting feature, again related to the wide band-gap, is its selective sensitivity to radiation with wavelengths shorter than 225 nm (visible-blind detectors).We report on the performances of two photodetectors based on Chemical Vapor Deposition (CVD) single crystal diamonds, one optimized for extreme UV detection, the other for soft X-ray radiation detection in the 0.8–8 keV range. These detectors have been fabricated at Roma “Tor Vergata” University using a p-type/intrinsic/metal configuration and they behave like photodiodes allowing operation with no external applied voltage. They have been installed on JET inside a vacuum chamber with a direct horizontal view of JET plasma without any wavelength selection. Their low thickness, low sensitivity to gamma ray and the unbiased operation mode make both detectors ideal for a Tokamak environment. The measurements routinely performed at JET show a low intrinsic dark current (∼0.01 pA) and very high signal to noise ratio (50 dB). Both detectors show a fast response and their signals are acquired using an electronic chain and ADC able to operate at 200 kHz, providing very interesting results for MHD and Edge Localized Modes (ELMs) instability studies on fusion plasmas.

Chemical etching and surface oxidation studies of cadmium zinc telluride radiation detectors

AbstractCdZnTe (CZT) is commonly used as a radiation detector material and the surface properties are important as they influence detector performance. The surface chemistry is generally controlled through chemical etching and oxidation processes. In this paper, X‐ray photoelectron spectroscopy (XPS) is employed to investigate changes in the surface composition of single‐crystal Cd0.95Zn0.05Te samples after exposure to bromine in methanol (BM) chemical etching treatments and subsequent oxidation in air or 30% H2O2. BM treatment of CZT is found to result in a graded Te‐rich surface layer which increases in thickness as a function of BM concentration. Room‐temperature air oxidation of 0.2% BM‐treated CZT follows a logarithmic rate law. BM‐treated CZT exposed to 30% H2O2 for 30 s shows a linear increase in the TeO2 oxide thickness with increasing BM concentration up to a BM concentration of 1.5%. Above 2% BM concentration, the Te enrichment in the CZT surface region has reached saturation and is effectively pure Te. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The fabrication and evaluation of CdS sensor for diagnostic x-ray detector application

Recently, various semiconductor compounds as radiation detection material have been researched for a diagnostic x-ray detector application. In this paper, we have fabricated the CdS detecton sensor that has good photosensitivity and high x-ray absorption efficiency among other semiconductor compounds, and evaluated the application feasibility by investigating the detection properties about energy range of diagnostic x-ray generator. We have fabricated the line voltage selector(LCV) for a signal acquisition and quantities of CdS sensor, and designed the voltage detection circuit and rectifying circuit. Also, we have used a relative relation algorithm according to x-ray exposure condition, and fabricated the interface board with DAC controller. Performance evaluation was investigated by data processing using ANOVA program from voltage profile characteristics according to resistive change obtained by a tube voltage, tube current, and exposure time that is a exposure condition of x-ray generator. From experimental results, an error rates were reduced according to increasing of a tube voltage and tube current, and a good properties of 6%(at 90 kVp) and 0.4%(at 320 mA) ere showed. and coefficient of determination was 0.98 with relative relation of 1:1. The error rate according to x-ray exposure time showed exponential reduction because of delayed response velocity of CdS material, and the error rate has 2.3% at 320 msec. Finally, the error rate according to x-ray dose is below 10%, and a high relative relation was showed with coefficient of determination of 0.9898.