Keitaro Hitomi

Characterization of thallium bromide crystals for radiation detector applications

In this study, thallium bromide (TlBr) crystals have been investigated for radiation detector applications. Thallium bromide crystals have been grown by the traveling molten zone method using materials purified by the conventional zone refining method. The grown crystals have been characterized in terms of their structural properties, stoichiometry and surface properties. Charge transport properties of the grown crystals have also been evaluated by estimating the carrier mobilities and lifetimes of electrons and holes. Room temperature radiation detectors have been fabricated from the grown crystals by depositing gold electrodes on the surfaces that had been treated by mechanical polishing and chemical etching. An energy resolution of 37.2 keV full-width at half-maximum (5.6%) has been recorded for 662 keV γ-rays with a TlBr detector at room temperature.

Bismuth tri-iodide crystal for nuclear radiation detectors

Bismuth tri-iodide (BiI/sub 3/) is an attractive material for room temperature radiation detectors because of its wide bandgap energy and high photon stopping power. In this paper, BiI/sub 3/ crystals have been grown by the vertical Bridgman technique using commercially available powder. The grown crystals have been characterized in terms of their structural properties and stoichiometry. Room temperature radiation detectors have been fabricated from the crystals and tested by measuring their leakage currents and spectral responses. A clear peak corresponding to 5.48 MeV /spl alpha/-particles (/sup 241/Am) was recorded with an 82 /spl mu/m thick BiI/sub 3/ detector with Pd electrodes of 1 mm in diameter. The energy resolution of the 5.48 MeV peak was found to be 2.2 MeV FWHM.

Polarization Phenomena in TlBr Detectors

A comparison of long-term stabilities of TlBr detectors with Au and Tl electrodes was performed in this study, in order to reveal polarization phenomena in TlBr detectors. Au/TlBr/Au detectors exhibited polarization phenomena at room temperature, observed as degradation of spectral performance and reduction of leakage current with elapsed time. Applying reverse bias voltage to the detectors recovered the detector performance temporarily. No polarization phenomena were observed in Tl/TlBr/Tl detectors at room temperature. The detector exhibited stable spectral performance for 24 hours.

Fabrication of a nuclear radiation detector using the PbI/sub 2/ crystal and its response characteristics for gamma-rays

We have been studying a /spl gamma/-ray detector which uses a lead iodide PbI/sub 2/ crystal. The detector was fabricated from a PbI/sub 2/ crystal grown by the 10-pass zone travelling molten zone (TMZ) method. The /spl mu//spl tau/ product of this crystal is estimated to be about 1/spl times/10/sup -6/ cm/sup 2//V for electrons and about 6/spl times/10/sup -7/ cm/sup 2//V for holes. The resistivity of the PbI/sub 2/ crystal is estimated to be about 10/sup 12/ /spl Omega/cm. The FWHM energy resolution of the detector for a photoelectric peak for the 59.5 keV /spl gamma/-rays is found to be about 5 keV.

Energy-Discriminating Gadolinium K-Edge X-ray Computed Tomography System

An energy-discriminating K-edge X-ray computed tomography (CT) system is useful for increasing the contrast resolution of a target region utilizing contrast media and for reducing the absorbed dose for patients. The CT system is of the first-generation type of detector using cadmium telluride (CdTe). CT is performed by repeated translations and rotations of an object. Penetrating X-ray photons from the object are detected by a CdTe detector, and event signals of X-ray photons are produced using charge-sensitive and shaping amplifiers. Both photon energy and energy width are selected out using a multichannel analyzer, and the number of photons is counted by a countercard. To perform energy discrimination, a low-dose-rate X-ray generator for photon counting was developed. Its maximum tube voltage and minimum tube current were 110 kV and 1 µA, respectively. In energy-discriminating CT, the tube voltage and tube current were 100 kV and 20 µA, respectively, and the X-ray intensity was 2.98 µGy/s at a distance of 1.0 m from the source and a tube voltage of 100 kV. The demonstration of enhanced gadolinium K-edge X-ray CT was carried out by selecting photons with energies just beyond the gadolinium K-edge energy of 50.3 keV.

Equilibrium and kinetic studies of selective adsorption and separation for strontium using DtBuCH18C6 loaded resin

A crown ether loaded resin was prepared by successive impregnation and fixing the 4′,4′(5″)-di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and its molecule modifier, 1-dodecanol, onto the porous silica/polymer composite support (SiO2-P particles). The characterization of DtBuCH18C6 loaded resin was examined by thermal gravimetry and differential thermal analysis and electron probe microanalysis. The adsorption behavior of Sr(II), Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) was investigated by the batch method. Furthermore, the column test for Sr (II) was performed. The batch experiments were carried out by varying the shaking times, HNO3 concentration, and initial concentration of metal ions. A relatively large K d value above 182 cm3/g for Sr(II) was obtained in the presence of 3 M HNO3. In contrast, the K d values of Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) were considerably lower than 10 cm3/g. The adsorption of Sr(II) was found to be controlled by chemisorption mechanism, and followed a Langmuir-type adsorption equation. The breakthrough curve of Sr(II) had S-shaped profile, and the elution percentage was estimated to be 99.9% by using the eluent of H2O.

Energy discriminating x-ray camera utilizing a cadmium telluride detector

An energy-discriminating x-ray camera is useful for performing monochromatic radiography using polychromatic x rays. This x-ray camera was developed to carry out K-edge radiography using iodine-based contrast media. In this camera, objects are exposed by a cone beam from a cerium x-ray generator, and penetrating x-ray photons are detected by a cadmium telluride detector with an amplifier unit. The optimal x-ray photon energy and the energy width are selected out using a multichannel analyzer, and the photon number is counted by a counter card. Radiography was performed by the detector scanning using an x-y stage driven by a two-stage controller, and radiograms obtained by energy discriminating are shown on a personal computer monitor. In radiography, the tube voltage and current were 60 kV and 36 µA, respectively, and the x-ray intensity was 4.7 µGy/s. Cerium K-series characteristic x rays are absorbed effectively by iodine-based contrast media, and iodine K-edge radiography was performed using x rays with energies just beyond iodine K-edge energy 33.2 keV.

Thallium bromide optical and radiation detectors for X-ray and gamma-ray spectroscopy

Compound semiconductor, thallium bromide (TlBr), has been investigated as an optical and radiation detector material for use in X- and /spl gamma/-ray spectroscopy. Single crystals of TlBr have been grown by the traveling molten zone method using zone-purified materials. Direct X- and /spl gamma/-ray detectors have been fabricated from the TlBr crystals. The direct TlBr detectors have exhibited good spectrometric performances at room temperature. Polarization in TlBr detectors has been observed to deteriorate detector performance. Optical detectors for scintillation spectroscopy have been fabricated from the crystals by depositing an optically transparent electrode of indium-tin-oxide (ITO) on the front surface of the crystals. Quantum efficiency of the TlBr optical detectors has been high in wavelength region below /spl sim/460 nm where scintillation emissions of LSO and GSO occur.

TlBr Gamma-Ray Spectrometers Using the Depth Sensitive Single Polarity Charge Sensing Technique

Pixellated thallium bromide (TlBr) gamma-ray spectrometers were fabricated and evaluated with the depth sensitive single polarity charge sensing technique. The detectors exhibited energy resolutions of 2.22%, 1.22% and 1.11% for 356 keV, 662 keV and 1.33 MeV, respectively, at room temperature. Direct measurements of electron mobility-lifetime products in TlBr detectors were performed. An electron mobility-lifetime product of 4.1times10-3 cm2/V was obtained from a TlBr detector.

Polarization phenomena in TlBr detectors

A comparison of long-term stabilities of TlBr detectors with Au and Tl electrodes was performed in this study in order to reveal polarization phenomena in TlBr detectors. Au/TlBr/Au detectors exhibited polarization phenomena at room temperature observed as degradation of spectral performance and reduction of leakage current with elapsed time. Applying reverse bias voltage to the detectors recovered the detector performance temporary. No polarization phenomena were observed in Tl/TlBr/Tl detectors at room temperature. The detector exhibited stable spectral performance for 24 hours.