Minoru Funaki

Growth and characterization of CdTe single crystals for radiation detectors

To improve the productivity of CdTe radiation detectors, the crystal growth by traveling heater method (THM) as well as the quality of the fabricated detectors were investigated. In the THM growth, optimization of the solvent volume was found to be essential because it affects the shape of the growth interface. The use of the slightly tilted seed from 〈1 1 1〉B was also effective to limit the generation of twins having different directions. Single-crystal (1 1 1) wafers, larger than 30×30 mm2 were successfully obtained from a grown crystal of 50 mm diameter. Pt/CdTe/Pt detectors of dimensions 4×4×2 mm3, fabricated from the whole crystal ingot, showed an energy resolution (FWHM of 122 keV peak from a 57Co source) between 6% and 8%. Similarly, Pt/CdTe/In detectors of dimensions 2×2×0.5 mm3 showed a resolution better than 3%. These characteristics encourage the practical applications of various types of CdTe detectors.

THM Growth and Characterization of 100 mm Diameter CdTe Single Crystals

The THM growth technology for 100 mm diameter CdTe single crystals has been intensively developed. In consequence of the optimization of growth conditions, we have succeeded in controlling growth interface shape and growing 100 mm diameter CdTe single crystals with 300 mm in length. Concerning the behavior of Te inclusions in the grown crystal, their size and density were investigated by IR transmission microscopy. The size distribution of Te inclusions was divided into two groups, and the density of them was less than 1 × 105 cm−3. Charge transport properties of the grown crystal were investigated by using the “μτ-model” spectral fitting method, and were found to be quite uniform all over the wafer. To investigate the homogeneity of radiation detector performances, Ohmic type and Schottky type detectors with 4 mm × 4 mm × 1 mm were fabricated from the left half and the right half of the 100 mm diameter wafer, respectively. Standard deviations of their energy resolutions for the 122 keV line from 57Co were less than 6 %. This excellent uniformity is essential for the room temperature semiconductor detectors in the major application areas, such as medical imaging, non-destructive inspection and homeland security.

Improvement of the Productivity in the THM Growth of CdTe Single Crystal as Nuclear Radiation Detector

The effect of the THM growth rate on the CdTe crystalline quality and the detector performance was intensively investigated. The maximum growth rate for the single crystal growth was found to be approximately 15 mm/day which was 3 times greater than the conventional one. By optimizing other growth conditions, 90% of every ingot volume has become a single crystal. Te inclusions in the CdTe single crystal grown at various growth rates were also investigated by IR transmission microscopy. There was no correlation between the behavior of Te inclusions and the growth rate. The detector performance was also independent of the growth rate. Taking advantage of the large volume CdTe single crystals, about 700 000 Schottky detectors with 4 mm × 7.5 mm × 1 mm were fabricated for the research and development of the new positron emission tomography (PET) system using CdTe detectors. The average FWHM for the 662 keV line from 137Cs and its standard deviation were 2.24% and 0.48%, respectively. This uniformity was essential for the development of the new PET system.

THM growth and characterization of 100 mm diameter CdTe single crystals

The THM growth technology for 100 mm diameter CdTe single crystals has been intensively developed. In consequence of the optimization of growth conditions, we have succeeded in controlling growth interface shape and growing 100 mm diameter CdTe single crystals with 300 mm in length. Concerning the behavior of Te inclusions in the grown crystal, their size and density were investigated by IR transmission microscopy. The size distribution of Te inclusions was divided into two groups, and the density of them was less than 1 times 105 cm-3. Charge transport properties of the grown crystal were investigated by using the ldquomutau-modelrdquo spectral fitting method, and were found to be quite uniform all over the wafer. To investigate the homogeneity of radiation detector performances, Ohmic type and Schottky type detectors with 4 mm times 4 mm times 1 mm were fabricated from the left half and the right half of the 100 mm diameter wafer, respectively. Standard deviations of their energy resolutions for the 122 keV line from 57Co were less than 6%. This excellent uniformity is essential for the room temperature semiconductor detectors in the major application areas, such as medical imaging, non-destructive inspection and homeland security.

Improvement of the productivity in the growth of CdTe single crystal by THM for the new PET system

The effect of the THM growth rate on the CdTe crystalline quality and the detector performance was intensively investigated. The maximum growth rate for the single crystal growth was found to be approximately 15 mm/day which was 3 times greater than the conventional one. By optimizing other growth conditions, 90% of every ingot volume has become a single crystal. Te inclusions in the CdTe single crystal grown at various growth rates were also investigated by IR transmission microscopy. There was no correlation between the behavior of Te inclusions and the growth rate. The detector performance was also independent of the growth rate. Taking advantage of the large volume CdTe single crystals, about 700,000 Schottky detectors with 4 mm times 7.5 mm times 1 mm were fabricated for the research and development of the new PET system using CdTe detectors. The average FWHM for the 662 keV line from 137Cs and its standard deviation were 2.24% and 0.48%, respectively. This uniformity was essential for the development of the new PET system. For the further improvement of productivity, the growth technology of a 100 mm diameter crystal by THM is in the development and is presented.

High resolution fourier synthesis hard X-ray imaging based on CdTe strip detectors

Employing Fourier-synthesis optics and one-dimensional position-sensitive detectors, we are developing a novel hard X-ray imager which can work in the ~10 keV to ~200 keV range either as a telescope or a microscope. As the detection part of our imager, we have developed a strip detector made of Schottky CdTe diode, with its cathode divided into 64 channels of 150 mum pitch. Electrodes of all channels are gold-stud bonded to a fanout board, and connected to low noise analog ASIC. We read out signals from all channels simultaneously. As the grid optics elements, one-dimensional modulation collimator grids of 1 mm thick tungsten have been manufactured, with 10 grid pitches ranging from 0.2 mm to 2 mm with harmonic ratios. Combining the CdTe strip detector and the modulation collimators, we have verified hard X-ray imaging performance of this system. Specifically, by observing an 241Am source, we have successfully obtained an image in the 10-70 keV range

Performance of a small CdTe gamma camera for radio-guided surgery

Sentinel lymph node biopsy has been shown to be highly accurate for detecting metastatic diseases, such as melanoma and breast cancer. Gamma probes that measure only the relative presence of radioactivity are commonly used to identify sentinel lymph nodes. We have developed a small semiconductor gamma camera (SSGC) that allows the size, shape, and location of the target tissues to be visualized. The purpose of this study is to characterize the performance of the SSGC for radioguided surgery of metastatic lesions and for diagnosing other diseases amenable to the smaller- format associated with this prototype imaging system. Methods & Design: The detector head was comprised of a 32 x 32 Cadmium Telluride semiconductor array and application- specific integrated circuit (ASIC) with a tungsten collimator. The entire assembly was encased in a lead housing measuring 152 mm x 166 mm x 65 mm. The effective visual field was 44.8 mm x 44.8 mm. Two spherical 5 mm diameter Tc-99m radioactive sources having activities of 0.15 MBq and 100 MBq were used to simulate sentinel lymph nodes and injection site. The relative detectability of these foci was compared using the new detector and a conventional scintillation camera. Use of the prototype was also explored on patients in a variety of clinical applications. Results: the SSGC provided better spatial resolution on phantom studies than the conventional gamma camera control. Both foci could be visualized clearly by the SSGC using a 15 second acquisition time, whereas they could not be readily identified using the conventional system under comparable conditions. Preliminary clinical tests of the SSGC were found to be successful in imaging diseases in a variety of tissues including salivary and thyroid glands, temporomandibular joints, and sentinel lymph nodes. Conclusion: The SSGC has significant potential for use in diagnosing diseases and for facilitating subsequent radioguided surgery. (This project was supported by a Grant- in-aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan).