S. Izumi

High energy X-ray computed tomography for industrial applications

A high-energy X-ray computed tomography system with an electron linear accelerator was developed to image cross-sections of large-scale and high-density materials. An electron linear accelerator is used for the X-ray source. The maximum X-ray energy is 12 MeV and the average energy is around 4 MeV. The intensity of an X-ray fan beam passing through the test object is measured by a 15-channel detector array. CWO (CdWO/sub 4/) scintillators and photodiodes are employed for the X-ray detectors. The crosstalk noise due to scattering at X-ray photons by adjacent detectors is reduced to less than 1.6% by installing a tungsten shield between the scintillators. Extra channels are used to compensate for the baseline shift of the circuits. These techniques allowed attainment of a dynamic range of more than 85 dB and a noise level comparable to the signal amplitude of X-ray transmitted in a 420 mm-thick iron block. A spatial resolution of 0.8 mm was confirmed with an iron test piece 200 mm in diameter.<<ETX>>

Silicon semiconductor detectors for various nuclear radiations

Silicon semiconductor detectors for a nuclear radiation monitor of various rays (/spl gamma/ rays, neutrons and charged particles) have been developed. These detectors are diffused p-n junction type devices with low leakage current. The /spl gamma/ ray detector is produced, using high resistivity p-type silicon (2-10 k /spl Omega//spl middot/cm), to increase thickness of the depletion layer. This detector has been applied as an area monitor and a computerized personal dosimeter with an IC card. For the neutron detector, proton radiators and boron convertors are coated on the surface of the detector element. Neutrons are detected as recoil protons by interaction of the proton radiator and, as /spl alpha/-particles generated by the nuclear reaction /sup 10/B(n,/spl alpha/)/sup 7/Li. The energy response of this detector meets the ICRP recommendation for a radiation monitor. The charged particles (/spl alpha/, /spl beta/ rays) detector having a large sensitive area (up to 6000 mm/sup 3/) is coated with double passivation layers (Si/sub 3/N/sub 4//SiO/sub 2/) on the surface of the detector element in order to reduce leakage current due to surface stress. The leakage current of less than 220 nA is achieved for the sensitive area by application of the double layers. The charged particles detector can be applied as a radioactive dust monitor and a contamination monitor, and the neutron detector is suited as an area monitor and a personal dosimeter.

Response of silicon detector for high energy X-ray computed tomography

The suitability of an Si-SSD as a detector for high energy X-ray CT has been investigated. Two types of Si-SSDs which can take the place of ordinary scintillation detectors were proposed. Their predicted responses were compared with that of a CWO scintillation detector of almost the same dimensions by using the EGS4 Monte Carlo simulation code. The sensitivities of the sole-silicon detector and the metal-sandwiched detector were about ten times higher than that of the CWO scintillation detector at X-ray energies above 1 MeV. The metal-sandwiched detector was judged more useful than the sole Si-SSD when the width of the X-ray beam was less than about 0.2 mm. The silicon detector for high energy X-ray CT was fabricated and actual measurements of the depletion layer width and the electronic noise contribution were made. >

A computerized personal dosimeter with an IC card

A microprocessor-based personal dosimeter with a silicon solid-state gamma-ray detector has been designed and built to provide reliable personal dosimetry. An IC (integrated circuit) card equipped in the dosimeter can exchange data with the microprocessor. Personal data concerning radiation dose control such as personal dose history and personal ID data are previously stored in the IC card. The dosimeter provides alarms on exceeding dose and working period limits in controlled areas and records the time differential dose data which can be used for the analysis of working procedures to minimize the personal dose in the controlled areas. The data stored in the IC card while the dosimeter is being used can be read by an external computer and used in preparing documents for personal dose control. These functions were confirmed by trial use in a nuclear power plant. >

Characteristics of a silicon detector for industrial X-ray computed tomography

Abstract We have evaluated a silicon solid state detector (Si-SSD) as a detector for Industrial X-ray Computed Tomography (IXCT) from the viewpoints of sensitivity and radiation damage. The detector which we used in this work, is a diffused junction type Si-SSD (43 × 3.8 × 0.5 mm) fabricated on high purity n-type silicon. We measured pulse height distributions for narrow pencil beam 60 Co gamma rays parallelly incident to the detector. The expected deposit energy was 0.146 ± 0.001 MeV (11.7% of the incident photon energy). The calculated pulse height distribution using the general purpose EGS4 code was confirmed to be in good agreement with the measured one. The radiation damage was evaluated using the 60 Co gamma ray source. Exposure of up to 88 kGy(Si) was achieved over a period of 20 hours, with an exposure rate of 4.4 kGy(Si)/h (36 mC/kg s). It was confirmed that the Si-SSDs could be used at least up to 88 kGy(Si) with X-rays equivalent to 60 Co gamma rays.

A high energy X-ray computed tomography using silicon semiconductor detectors

Second- and third-generation high energy industrial X-ray computed tomography systems using silicon semiconductor detectors were developed. Both systems consist of a high energy X-ray source using an electron linear accelerator with the maximum energy of 6 MeV and an array detector unit using silicon semiconductor detectors. This unit also includes a collimator with slits 0.2 mm wide and 1.4 mm high, located in front of the detectors. To increase sensitivity, detector elements were placed parallel to the X-ray beams, The third-generation system had 512 detector elements arrayed with 1.3 mm pitch. To reduce detector pitch, each detector chip was mounted on a tungsten base plate using a thin film circuit board. The plate acts as shield to absorb incoming scattered X-rays and secondary electrons generated at neighbor detectors. Performance tests were carried out in the second- and third-generation systems. The spatial resolution of 0.30 mm was confirmed for an iron test piece of 70 mm diameter using the second-generation system. The scans for a slice were completed within less than 10 seconds in the third-generation system.

Correction of cross-talk noise in high energy X-ray computed tomography

Effect of cross-talk noise on reconstructed images of high energy X-ray computed tomography is described. Cross-talk noise caused by secondary electrons and/or scattered photons of high energy X-rays generated from adjacent detectors is evaluated using the EGS4 code. Distortion of the projection (Radon transform) is expressed as simultaneous equations based on cross-talk noise and it can be corrected by solving the equations. Random noise contained in the projection, however, is amplified by the correction process. This amplification can be estimated from the determinant of the cross-talk matrix of the simultaneous equations. It is seen that noise amplification does not matter in practical applications when cross-talk noise is less than 0.1.<<ETX>>

Energy Response of Neutron Area Monitor with Silicon Semiconductor Detector.

Abstract A prototype neutron area monitor with a silicon semiconductor detector has been developed which has the energy response of 1 cm dose equivalent recommended by the 1CRP-26. Boron and proton radiators are coated on the surface of the silicon semiconductor detector. The detector is set at the center of a cylindrical polyethylene moderator. This moderator is covered by a porous cadmium board which serves as the thermal neutron absorber. Neutrons are detected as α-particles generated by the nuclear reaction 10B(n, α)7Li and as recoil protons generated by the interaction of fast neutrons with hydrogen. The neutron energy response of the monitor was measured using thermal neutrons and monoenergetic fast neutrons generated by an accelerator. The response was consistent with the 1cm dose equivalent response required for the monitor within ±34% in the range of 0.025–15 MeV.

Application of region-of-interest (ROI) method to industrial X-ray computed tomography

Summary form only given, as follows. Numerical simulations and experiments have confirmed that the shape of an objects cross section is correctly reflected on the reconstructed image obtained by ROI mode computed tomography (CT). The shape information can be described as the second derivative of the projection data. It can be mathematically proved that any contribution from outside the ROI to this derivative is very small, and shape information is not affected by it. Experiments were carried out using a prototype industrial X-ray CT system. An iron test phantom of 200-mm diameter was used to compare the CT images of the ROI mode and entire mode. The ROI area was 1/64 of the entire area of the phantom, and the measuring time of the ROI mode was 1/8 of the entire mode. The reconstructed images binarized to intensify contours of the holes on the phantom show that the ROI mode CT can correctly reproduce the objects shape in the ROI, and ROI mode CT can be applied to industrial CT without correction or interpolation of projection data.<<ETX>>