Experiments are made at the second thermal neutron radiography field (TNRF-2) of the Japan Research Reactor 3M in Japan Atomic Energy Research Institute. Its thermal neutron intensity was 1.5×108 n/cm2/s. The n/γ ratio of the field was 62.5 n/cm2/μSv and so excellent for our purpose. Fig. 5 shows a raw color image obtained at the field. As shown in Fig. 6 objects were an acrylic resin step wedge (left), an iron step wedge (middle) and three kinds of ASTM indicators (right). As the neutron component was dominant, the image was tinged with red. On the other hand an image shown in Fig. 7 was obtained with the last neutron beam shutter was closed. It means the image was composed of gamma component and the tinge of green can be recognized. For our purpose of the simultaneous neutron and gamma ray radiography the neutron dose had to be decreased by inserting an acrylic resin plate. The thickness of 10 mm was experimentally determined for optimum S/N condition.
A set of dark image data, field shading image data and object image data was required for the neutron and gamma ray radiography. Eight frame data were taken with the exposure time of about 22 s for each image and digital image processing was performed using these eight frames for the reduction of impulse noises produced by direct gamma ray reaction with the CCD chip. After the noise reduction processing, data correction for dark current and shading distortion was carried out. The pre-processed image is shown in Fig. 8. Then selecting color pixels, red image (Fig. 9) and green image (Fig. 10) were obtained. The red image was mostly made up of neuron component. In the green image gamma ray component could be observed. To obtain neutron and gamma radiographs the optimum data processing was examined. The neutron and gamma ray images could be derived by these red and green images using the following formulae:
equation(1)
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