Young Scientist
Dr. Keitaro Hitomi
Department of Electronics, Tohoku Institute of Technology
35-1 Yagiyama Kasumi-cho, Taihaku-ku, Sendai 982-8577, JapanE-mail: hitomi@tohtech.ac.jp

In the autumn of 1995, when I was 19years old, I first visited Prof. Shoji’s laboratory
at the Tohoku Institute of Technology. Afterwards, I started to study room-temperature
semiconductor X- and gamma-ray detectors,
and after finishing my Doctorate of Engineering in the spring of 2001, I became a postdoctoral fellow at his laboratory. For thesepast six years, I have worked on the development of TlBr semiconductor detectors. I was
honored to receive the Radiation Award (encouragement Award) awarded by the Radiation Science Division of the Japanese Society
for Applied Physics for my research into TlBrdetectors.
The room-temperature semiconductor
X- and gamma-ray detector is a device inwhich X- and gamma-rays are converted into
electronic pulses. By analyzing the pulses,
one can obtain information on the incident
photons, particularly the energy, the numberof incidences, and the photon arrival time.
Semiconductor X- and gamma-ray detectorsthat can operate at room-temperature are
now being increasingly applied in medicine,industry, astronomy, and nuclear science.
Conventional semiconductor X- and gammaray detectors based on Ge and Si are wellestablished and widely used in various fields.
They offer considerable advantages over other kinds of radiation detector, such as scintillation detectors and gas counters, especially
their high energy resolution. The applicationof these detectors, however, has been somewhat restricted since Ge detectors provide
high-resolution capabilities only at a cryogenic temperature (77 K) because of their lowresistivity at room temperature, and Si detectors exhibit sufficient detection efficiency
for only low energy X-rays (less than 20 keV)because of their low photon stopping power
at higher energies. Thus, there has been astrong interest in developing semiconductor detectors that have high photon stopping
power and can operate at room temperature,
without sacrificing the advantages of Ge and
Si detectors. The main physical semiconductor properties requisite for fabrication ofroom-temperature semiconductor detectorsare
1. high atomic number and density for highphoton stopping power,
2. a bandgap large enough to keep leakage
currents low at room temperature,and3. large electron and hole mobility-lifetime
products for efficient charge collection.
The compound semiconductors CdTe,CdZnTe (CZT), and HgI2 have been investigated as detector materials that meet theserequirements. Although devices and instruments based on CdTe, CZT, and HgI2 detectors are commercially available, widespreaduse of these devices is limited because ofdrawbacks such as their small sizes, insufficient energy resolution, or high cost. Therefore, research into other semiconductor materials for use in X- and gamma-ray detectors
continues. Although many semiconductormaterials have been promising or were expected to show suitable physical properties, none of these have been established aspractical X- and gamma-ray detector materials. At the start of my research, I selected acompound semiconductor, thallium bromide,
from among the candidate materials because
it has a high atomic number (Tl: 81, Br: 35),high density (7.56 g/cm3), and a wide bandgap (2.68 eV). Because of its high atomic
number and density, TlBr has greater photon
stopping power for hard X- and gamma-rays
than other semiconductors commonly used
for detector fabrication (e.g., Ge, Si, CdTe,and HgI2). Also, the wide bandgap of TlBr

implies a low intrinsic carrier concentration-i.e., low leakage currents- and accordingly
permits low-noise device operation at andabove room temperature. Because of these
attractive properties, TlBr has been studied as a detector material by several groups.
However, the TlBr detectors reported so far
have exhibited satisfactory detector performance only for low energy X-rays because of
their low mobility-lifetime products for electrons (1.3 10-5 cm2/V) and holes (1.5 10-6cm2/V).1)

The development of semiconductor Xand gamma-ray detectors requires extensive
work that extends into many fields involving
material science, crystal growth, electronics, and nuclear science. I have continuously
worked on the development of a TlBr detector from the initial crystal growth throughto detector evaluation. In this work, the firststep of TlBr detector fabrication was furtherpurification of the TlBr starting material for
crystal growth using the conventional zone
refining method. The traveling molten zone(TMZ) method was used to grow TlBr crystals.
To fabricate detectors, the as-grown crystals
were then cut into several wafers whose surfaces were treated mechanically and chemically. The resultant wafers were transparent
and had a good surface finish (see the photograph). The treated wafers were charac-

JSAP International No.6 (July 2002)

29

Young Scientist

The performance of our TlBr detectors is
close to that of commercially available CdTe,CZT, and HgI2 detectors, and this makes theTlBr detector a promising candidate for Xand gamma-ray applications. We foresee
potential applications in various fields. However, further investigation regarding the crystal growth, semiconducting properties, andlong- term stability is needed to permit practical use of these TlBr detectors.

30JSAP International No.6 (July 2002)

221000Counts/Channelterized in terms of their structural properties,
stoichiometry, and surface properties. Finally,TlBr detectors with a planar structure werefabricated by depositing gold electrodes onboth surfaces of the wafers. Electrical properties of the TlBr, including the resistivity andthe charge transport properties for electronsand holes, were measured using the fabricated detectors. The performance of the TlBr
detectors was characterized by acquiring Xand gamma-ray energy spectra and coincidence timing spectra. By continuously repeating the processes from crystal growth todetector evaluation, I improved the detector
performance step by step. Finally, I obtainedTlBr crystals exhibiting mobility-lifetime products for electrons (2.6 10-4 cm2/V) and holes(3.7 10-5 cm2/V) that were about one orderof magnitude greater than previously reported values. With these crystals, I was able tofabricate TlBr detectors that could adequately
detect high energy photons as well as low
energy X-rays (see the figure).2) These TlBr
detectors can be applied, for example, as flux
detectors for X-ray computed tomography
and photodetectors for scintillation detection.3, 4)

511 keVNa Spectrum

800600
FWHM= 29.8 keV(5.8%)40020001275 keV

0100200300

Channel Number

References
1 ) K.S. Shah, F. Olschner, L.P. Moy, J.C. Lundand M.R. Squillante, Nucl. Instrum. andMethods A299 (1990) 57.
2 ) K. Hitomi, O. Muroi, M. Matsumoto, R. Hirabuki, T. Shoji, T. Suehiro and Y. Hiratate, Nucl. Instrum. and Methods A458 (2001) 365.
3 ) K. Hitomi, O. Muroi, M. Matsumoto, R. Hirabuki, T. Shoji and Y. Hiratate, IEEE Trans.
Nucl. Sci. NS-47 (2000) 777.
4 ) K. Hitomi, O. Muroi, T. Shoji, Y. Hiratate, H.
Ishibashi and M. Ishii , Nucl. Instrum. andMethods A448 (2000) 571.

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