The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

Semiconductor pixel detectors: Si, CdTe, and TlBr

World's First New Technology to Reduce Patient Burden
Using High-Performance Hybrid Semiconductor PixelDetector with Thallium Bromide
Japan Synchrotron Radiation Research Institute1, Tohoku University2, Clear-Pulse Co., Ltd3, Howa Sangyo Co., Ltd4
Hidenori Toyokawa1, Keitaro Hitomi2, Mitsuhiro Nogami2, Shin Kubo2,3, Atsushi Suenaga4

Summary• The world's first hybrid semiconductor pixel detector have been developed with a highquality thallium bromide (TlBr) sensor that operates stably at room temperature using
thallium electrodes.
• The groundbreaking achievement was made possible by deep institutional expertise andexperience in developing hybrid semiconductor pixel detectors and by collaborating withjoint researchers to harness their technology to produce highly pure thallium bromide
crystals.
• The high-performance thallium-bromide detector that can operate at room temperature

can potentially reduce cancer patients’ burden by monitoring irradiated areas inparticle radiation therapy and lowering radiation doses. A clinical research plan iscurrently underway at Gunma University Heavy Ion Medical Center.
A research group led by Hidenori Toyokawa of the Japan Synchrotron Radiation Research Institute and Keitaro Hitomi ofTohoku University has built a high-performance two-dimensional detector equipped with thallium bromide, having thehighest gamma-ray absorption coefficient among semiconductor materials. Radiation imaging allows noninvasive internal
observation and is an indispensable technology for medical applications and industrial development; improvements indetector performance will greatly advance these fields. The hybrid pixel detector with the thallium bromide sensor
presented in this presentation has already been prototyped and heading for commercialization. This promisingtechnology is the culmination of many years of research by this research group and is expected to be widely used in
medical applications to reduce the burden on patients. It will also be available in overseas markets as a Japanesetechnology.

DetailTechnological innovation of radiation measurements by hybrid pixel detectors
Hybrid pixel detector technology has revolutionized radiation imaging measurement technology,
allowing the observation of an object’s interior without destroying it, and driving the development ofmedicine, engineering, and physics. In medicine, observing the internal organs of the human bodyby radiation has become indispensable for diagnosis and treatment. In industry, radiationtransmission testing using X-rays and gamma rays is widely used in the nondestructive
observation of the inside of materials.
Hidenori Toyokawa of the Japan Synchrotron Radiation Research Institute (JASRI) is a researcher
at SPring-8, a large synchrotron radiation facility generating the world’s highest levels ofsynchrotron radiation. Toyokawa has long been working to improve detector performance bysampling the irradiation of diffracted and scattered X-ray to increase the accuracy of radiation
imaging. One of the innovations led by Toyokawa is the hybrid semiconductor pixel detector
developed in the 1990s. The name"hybrid" originates from how the sensor and readout integrated
circuit are made and assembled separately. Using semiconductor sensors with microelectrodes
processed in a two-dimensional array the hybrid detector has both higher spatial resolution andenergy measurement, which had been a challenge in previous research.
In 2001, the Pilatus detector using silicon as a sensor was put into practice. This detector later
became the standard two-dimensional detector in synchrotron radiation facilities and is now usedworldwide. The Pilatus detector was developed at the Paul Scherrer Institute (PSI) in Switzerland,
which also developed the CMS pixel detector for the LHC experiment at the European
Organization for Nuclear Research (CERN), sharing the same basic technology as the Pilatus
detector.
In 2009, a two-dimensional detector with extremely high sensitivity over a wide energy range using
cadmium telluride was developed. This work progressed to the Wide Energy Range Pixel Array
Detector detector (WERPAD), commercialized in 2018. The readout circuit used a large-scale
integrated circuit capable of discriminating wavelengths and extracting their respective diffraction
images; the combination of the two functions resulted in a detector capable of simultaneously
measuring multiple wavelengths with high precision.
Aiming for even higher-performance detectors, Toyokawa has long dreamed of developing aradiation detector using thallium bromide, a compound semiconductor. The atomic numbers ofthallium and bromine are very high, 35 and 81, respectively, and their density is 7.56 g/cm3, about
the same as iron, meaning that the detection efficiency of highly penetrating γ-rays is much higher
than other materials (Figure 1). Furthermore, thallium bromide can stably measure the X-ray to γray region at room temperature. This characteristic is useful for practical use as a detector.
Compared to cadmium telluride, which requires a cooling system, a thallium bromide detector,
which is stable at room temperature, is low-cost and can be installed under any condition, thereby
expanding its range of applications.
Meeting the world’s leading authority on thallium-bromide research
Thallium bromide is a semiconductor material that was used long ago. In addition to Toyokawa,
many researchers aim to develop detectors using thallium bromide. However, the difficulty ingrowing thallium-bromide crystals has made it challenging to realize this goal because of the
difficulty of producing pure crystals, and impurities in thallium-bromide crystals limit the detector’sperformance. For this reason, developing a thallium-bromide detector at a practical level seemedimpossible.
However, Toyokawa's group was still able to develop a thallium bromide detector. This was due toan encounter at the Applied Physics Society of Japan presentation in 2022.
"I had long dreamed of developing a detector using thallium bromide, and when I heard Dr. Hitomi,
a world authority on thallium bromide, present his research last year, I was convinced that thedetector I was dreaming of could be realized by combining my technology with his work. As soon
as the presentation was over, I approached him with an invitation to collaborate with him," saidToyokawa.
Toyokawa's conviction came true in December 2022 when the first thallium-bromide hybrid pixel
detector was completed (Figure 2). Only nine months had passed since the day it was approached— an exceptionally fast realization. The research team included Clear-Pulse Co., Ltd, whichdevelops and manufactures radiation measuring instruments, and Howa Sangyo Co., Ltd, which
has microfabrication and application technologies for various materials. They have already begun
working toward social implementation. Toyokawa expresses his enthusiasm as follows:"We are also preparing a manufacturing system in collaboration with a semiconductor-manufacture
venture company. We hope to develop this technology as a Japanese technology that cancompete with the rest of the world through an all-Japan system to reach practical fruition incollaboration with private enterprise."
More effective and less burdensome cancer treatment
Currently, Gunma University Heavy Ion Medical Center, in collaboration with the "RI Imaging
Research" project at the Takasaki Advanced Radiation Research Institute of the National Institutesfor Quantum Science and Technology (QST), is conducting basic research to visualize theirradiated area of the treatment beam in real-time using the cadmium telluride hybrid pixel detector
developed by Toyokawa. Heavy-ion beam therapy is a form of radiation therapy in which carbon
beams are focused on the patient, striking the cancerous cells intensively, deep within the body. It
is a treatment method that does not use a scalpel; does not damage surrounding normal cells,such as those in front of and behind cancer; and has few side effects. However, if the area to be
irradiated with heavy ion beams is outside the cancerous lesion, the desired outcome cannot bereached. Currently, the irradiation area is calculated at the time of treatment planning, but if thearrival position of the treatment beam can be observed, a more patient-friendly treatment method
can be developed.
Introducing cadmium telluride detectors has solved this problem because the detector can capture
the secondary electron bremsstrahlung generated after heavy particle irradiation. Thus theeffectiveness of the treatment can be determined in real-time.
The results of introducing the cadmium telluride detector are already beginning to be seen, but thecompany is also looking to improve performance with the thallium bromide detector that Toyokawa
has now developed.
The thallium bromide detector, which does not require a cooling system, is not only more powerful
but also more compact than the cadmium-telluride detector. Eliminating the need for cumbersome
equipment reduces the psychological pressure on patients. In addition, since it can be operated at

a lower cost than cadmium telluride, expanding its use to general hospitals will be possible in thefuture," says Toyokawa.
Furthermore, Toyokawa says that if thallium bromide detectors are implemented with X-ray
examination and computer tomographic equipment, they may reduce the examination X-ray doseand patient radiation exposure. The range of applications will not be limited to the medical field.
“New instruments open up new fields. We believe that the thallium bromide detector we developed
will not only improve the performance of existing methods but also trigger the birth of further
research and industry. We are at the starting point now, but over the next few years, we would liketo expand the scale of the system and further improve its performance,” says Toyokawa.
FiguresFigure 1: Properties of thallium bromide

Figure 2 Flip-chip mounted thallium-bromide hybrid pixel detector prototype