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The Forefront of Space Science

Experiment demonstrating the Viability of Easy, On-site Visualization of the Distribution of Radioactive Materials by the “Ultra-Wide-Angle Compton Camera”
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On-Site Check, Prototype Model Production and On-Site Imaging Experiment

After receiving the request from Tokyo Electric Power Company, we reviewed our approach. Even though we were able to predict the detection sensitivity of gamma rays from a huge black hole at the center of the galaxy, we had no idea how hot spots spreading on the ground would be seen with our detector. Moreover, we had none of the information required to set the detector performance. We needed to know the ambient dose on the site, the energy of the gamma rays actually emitted there, etc. First, since we had no knowledge of the situation in Fukushima, we were unsure whether the Compton camera we had developed could function there. So, taking time out from our development work on ASTRO-H and obtaining permission from the relevant authorities, we visited the site carrying dosimeters and portable gamma-ray detectors.

In our on-site check, we found that there were places with a strong ambient dose as publicly reported and, at 5 cm above those areas, there were hot spots with a dose approximately 10 times the ambient dose. Our on-site observations also revealed that, in addition to the constant-energy gamma-rays directly emitted from cesium-134 or cesium-137, the sites were dominated by low-energy gamma rays (gamma rays that had been scattered by the ground, buildings, etc., and changed to lower-energy ones). Finally, we came to the positive conclusion that it was quite possible for us to locate the sources of radioactive materials with our Compton camera and to contribute to solving the problem.

Unfortunately, there was no camera available on site with sufficient capability to perform the measurements. The ASTRO-H program was in the detailed design phase at that time and the design of its Compton camera was complete. Furthermore, we had begun partial fabrication of the prototype model. It was technically difficult to use the model in its existing condition in Fukushima, however, so we had to build a new model dedicated to observations there. We decided to secure the research budget and manufacture the dedicated model. This was considered to be a project that could drive our future astronomy research and, eventually, also drive the development of space technology that would contribute to the national disaster. We promptly established a team for prototype-model manufacture and on-site imaging. The model assembly started in October 2011 and was completed in January 2012 (Fig. 1). In February, we performed an imaging experiment at Iidate Village in Fukushima in cooperation with the Japan Atomic Energy Agency (JAEA) and Tokyo Electric Power Company (Fig. 2).

Figure 1
Figure 1. Prototype model of Ultra-Wide-Angle Compton Camera

Figure 2
Figure 2. Imaging test in Iidate Village, Fukushima
Shown on the left is the camera with fisheye lens attached. On the right is an image shot by the Ultra-Wide-Angle Compton Camera showing the intensity (flux) distribution of gamma rays with energies of 605, 662, 796, and 802 keV directly emitted from cesium-134 and 137. Red is high intensity while blue is low.

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