Tadayoshi Shoji

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.

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.

Fabrication of radiation detector using PbI/sub 2/ crystal

Radiation detectors have been fabricated from lead iodide (PbI/sub 2/) crystals grown by two methods: zone melting and Bridgman methods. In response characteristics of the detector fabricated from crystals grown by the zone melting method, a photopeak for /spl gamma/-rays from an /sup 241/Am source (59.5 keV) has been clearly observed with applied detector bias of 500 V at room temperature. The hole drift mobility is estimated to be about 5.5 cm/sup 2//Vs from measurement of pulse rise time for 5.48 MeV /spl alpha/-rays from /sup 241/Am. By comparing the detector bias versus saturated peak position of the PbI/sub 2/ detector with that of the CdTe detector, the average energy for producing electron-hole pairs is estimated to be about 8.4 eV for the PbI/sub 2/ crystal. A radiation detector fabricated from PbI/sub 2/ crystals grown by the Bridgman method, however, exhibited no response for /spl gamma/-rays. >

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.

Evaluation of CdTe(Cl) crystal growth with THM and application to a multichannel detector

Abstract The relationship between the properties of a CdTe radiation detector and its crystal growth condition has been studied. The crystals were grown from an excess Te solution doped with chlorine using radio frequency (RF-THM) and ordinary travelling heater methods (OR-THM). The concentration of chlorine atoms in the CdTe crystal grown by the RF-THM method is estimated to be about 1017 /cm3 and is the same as the concentration in the crystal grown by the OR-THM method. However, the CdTe detector characteristics show excellent properties in the detector fabricated by the RF-THM technique. The depth profile of Cl dopants has been measured by using the SIMS technique and homogeneity of the Cl distribution in the crystal grown by the RF-THM is recognized. It is suggested that the characteristics of the detector depend strongly on the chlorine distribution. In order to investigate the Cl-related emission bands, edge and bound-exciton emission spectra were measured at 4.2 K by photoluminescence. It is suggested that the 1.554 eV emission band is related to the Cl impurity level and that the characteristics of the radiation detector are affected strongly by the appearance of this level. A 46 channel array detector can be produced using high-resistivity CdTe:Cl crystals grown by the RF-THM technique.

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.

Characterization of PbI/sub 2/ radiation detectors using the response of /spl alpha/-rays

An improved method is proposed for the production of single crystals of lead iodide (PbI/sub 2/). For the work reported here a modified Bridgman method is used. Bulk-type nuclear radiation detectors have been fabricated by use of crystals grown by this method. The characteristics of the detectors, particularly the response for 5.48 MeV /spl alpha/-particles from /sup 241/Am source, have been measured and compared with characteristics of other detectors fabricated with crystals grown by the ordinary Bridgman method. An /spl alpha/-peak has been clearly observed in the pulse-height spectrum. On the other hand, no peak has been observed in the detectors made from crystals grown by the ordinary Bridgman method. From the relations of peak position versus applied bias voltage, the average energy to produce electron-hole pairs and the /spl mu//spl tau/ product of holes are estimated to be about 30 eV and 10/sup -7/ cm/sup 2//V, respectively. >