Tetsuro Ohtsuchi

Fabrication and characteristics of a submillimeter detector element formed on a CdTe single-crystal wafer as a multichannel detector array applicable to diagnostic imaging with energy information

A feasibility study to realize a minimal-sized X-ray detector element applicable to a multichannel array for diagnostic imaging with energy information is attempted. A thirty-two-channel radiation detector is fabricated by arranging thirty-two detector elements, each having an effective detection volume of 1×0.25×0.3 mm3 with a pitch of 0.25 mm, on a CdTe single-crystal wafer measuring 1(width)×8(length)×0.3(thickness) mm3. Thirty-two divided electrodes are arranged on its surface, and a common electrode is provided on its rear surface. A radiation spectrum measurement conducted at room temperature for one of the elements with 241Am γ-ray (59.54 keV) shows an energy resolution of 12% full width at half maximum (FWHM). Furthermore, by providing radiation shielding grids between electrodes, a reduction of K-shell characteristic X-ray escape from 40% to 30%, as well as a resultant reduction in channel crosstalk against the total counts, is accomplished.

X-ray energy separation method using a CdTe semiconductor X-ray imaging sensor and photon counting method

A multichannel X-ray imaging sensor using a CdTe compound semiconductor radiation detector in photon counting mode was developed and tested for digital X-ray imaging and an X-ray energy separation capability. The X-ray imaging sensor was constructed of CdTe detector elements at a pitch of 0.25 mm. High band amplifiers, discriminators, and counters attached to each element formed the pulse counting circuit. Charge pulses generated by absorbed X-ray photons were directly counted and separated into two energy regions. Digitized X-ray images containing energy information were thus obtained. Using this sensor two separate X-ray images of different energy can be obtained simultaneously. A 256-channel X-ray imaging sensor was prepared and used to provide spatial resolution measurement for an X-ray charge. Low and high energy images of a hand phantom were thus obtained and both a soft tissue image and a bone image were produced using an energy subtraction method. >

CdTe semiconductor X-ray imaging sensor and energy subtraction method using X-ray energy information

A multichannel X-ray imaging sensor using a CdTe compound semiconductor radiation detector was developed. Both the digital X-ray imaging and energy-information-generating analyzing method were studied. The X-ray imaging sensor consisted of 512-channel CdTe detector elements at a pitch of 0.2 mm. X-ray photons were directly detected using a photon-counting method and high- and low-energy images were obtained simultaneously. The specific resolution was obtained over 2.5 line pairs/mm in the channel direction and 1.6 line pairs/mm in the scanning direction with a scanning pitch of 0.2 mm. The energy subtraction method was found to be effective in distinguishing an objects component materials. >

X-ray imaging sensor using CdTe crystals for dual energy X-ray absorptiometry

The authors have designed and built a multi-channel cadmium telluride detector array to test its suitability as an X-ray imaging sensor for dual energy X-ray absorptiometry. The X-ray imaging sensor was constructed of 64 CdTe detector elements with high frequency current amplifiers, discriminators, and counters. The detector elements were operated in the photon counting mode and output pulses induced by the X-ray photons were separated into two energy levels according to their height. The energy resolution of the detector elements was 18.7% to 59.54 keV gamma-rays. In combination with a K-edge filter to produce dual energy X-rays, the X-ray imaging sensor was able to generate two energy X-ray images simultaneously over a short time. By applying a simple energy subtraction method to these images, bone phantoms were distinguished from the overlying tissue phantoms and their densities could be successfully measured. It was clearly demonstrated that this X-ray imaging sensor using CdTe crystals has good potential for high speed bone densitometry. >

Measurement of X-ray spectrum using a small size CdTe multichannel detector

A multichannel detector was fabricated with 32 elements, each 1 mm wide*0.25 mm long*0.3 mm thick, on a single CdTe crystal wafer whose overall size was 1*8*0.3 mm. One of the elements was subjected to an energy analysis using diagnostic wavelength X-rays. It is noted that the energy spectrum was distorted because of the small size of the element. Errors caused by X-ray escape and by the energy dependency of the photoelectric absorption process were appreciable; however, these errors are specific to narrow energy ranges and can be corrected. The response function of the detector to monochromatic 59.54 keV /sup 241/Am gamma-rays was measured and, by fitting the results to a calibration curve, this small-size CdTe detector was able to detect X-ray spectra reliably. >

Evaluation of K X-ray escape and crosstalk in CdTe detectors and multi-channel detectors

The simple structure of CdTe semiconductor detectors facilitates their downsizing, and their possible application to radiographic sensors has been studied. The escape of K X-rays from these detectors increases with reduction of their dimensions and affects the measurements of X- and gamma-ray spectra. K X-rays also produce crosstalk in multi-channel detectors with adjacent channels. Therefore, K X-rays which escape from the detector elements degrade both the precision of energy spectra and spatial resolution. The ratios of escape peak integrated counts to total photon counts for various sizes of CdTe single detectors were calculated for gamma rays using the Monte Carlo method. Also, escape and crosstalk ratios were simulated for the CdTe multi-channel detectors. The theoretical results were tested experimentally for 59.54-keV gamma rays from a /sup 241/Am radioactive source. Results showed that escape ratios for single detectors were strongly dependent on element size and thickness. The escape and crosstalk ratios increased with closer channel pitch. Our calculated results showed a good agreement with the experimental data. The calculations made it clear that K X-rays which escaped to neighboring channels induced crosstalk more frequently at smaller channel pitch in multi-channel detectors. A radiation shielding grid which blocked incident photons between the boundary channels was also tested by experiment and by calculation. It was effective in reducing the probability of escape and crosstalk. >

A high-speed cadmium telluride radiation detector

A small CdTe detector capable of high repetition pulse rate with possibilities for applications in radiation monitoring and similar uses was developed. The hole transit time through the crystal was shortened by decreasing the crystal thickness of 0.3 mm (1*1 mm) and by applying fields as high as 10 kV/cm. The electron transit time, although short, was electrically elongated by applying appropriate capacitance to the amplifier, and the output pulse waveform was designed to be similar to that for the holes. High, uniform output pulses with widths of 30 ns were thus obtained. The energy resolution was 9 keV for 59.4-keV photons from /sup 241/Am using a 100-ns-response amplifier. >