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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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Principle of Ultra-Wide Angle Compton Camera

The Compton camera utilizing the Compton scattering effect was proposed in the 1970s. It was soon incorporated into a space mission as the COMPTEL Gamma-Ray Telescope onboard NASA's CGRO satellite (1991 to 2000). COMPTEL then accomplished high-sensitivity astronomical observations in the MeV gamma ray regime. COMPTEL was a huge system 2m in size with a total mass of 1.5t, however, so it could not be used for ground applications. Later, along with rapid advances in semiconductor detectors and gas detectors, new generation Compton cameras were proposed. In this environment, about 15 years ago we proposed and developed a Si/CdTe Compton camera that combined semiconductor imaging elements made of silicon (Si) and cadmium telluride (CdTe). A very compact, high-angular resolution (fineness of image) camera could be realized if we were able to successfully fabricate semiconductor imaging elements made of Si and CdTe, which had excellent performance in position resolution, high-energy resolution, and high-temporal resolution, and then combine the elements appropriately. The Hard X-ray Imager (HXI) and Soft Gamma-ray Detector (SGD) designed for ASTRO-H combine two types of semiconductor imaging elements developed to meet the above requirements. HXI measures energies of 100 keV or less while SGD covers those above 100 keV.

At the request of Tokyo Electric Power Company and based on our on-site checks, we decided to quickly fabricate a hybrid camera. It utilized the ASTRO-H HXI structure (i.e., a 5-layer sensor), but a change was made to the mix of layers: from 4 layers of Si and 1 of CdTe used in HXI to 2 and 3 layers, respectively (Fig. 3). For the analog read-out LSI (Large Scale Integrated circuit), we employed the one developed for SGD with large dynamic range. The camera has a wide field of view, about 180 deg., and was thus named the Ultra-Wide-Angle Compton Camera. Our Si/CdTe Compton camera has an angular resolution of several degrees in the energy range of 500 to 800 keV. This means that we can localize hot spots to several tens cm size at a distance of 10m.

Figure 3
Figure 3. Principle of Ultra-Wide-Angle Compton Camera
Gamma rays are scattered through the Si semiconductor detector and then absorbed by the CdTe semiconductor detector. Based on the values measured by semiconductor detectors, we can retrace the initial direction of incoming gamma rays.

The challenge with future decontamination work is to improve the sensitivity of the camera several times over, up to 10-fold, in order to enable fast imaging in a few minutes, and so facilitate the operator's work at on-site decontamination. The SGD for ASTRO-H has a multiple-layer structure (32-layer Si detector and 8-layer CdTe detector) to detect faint gamma-ray signals coming from celestial bodies. If we can apply that structure to the gamma-ray camera used in Fukushima, the sensitivity should be improved by about 30 times. We expect that the 20 to 30 minutes required for "gamma-ray imaging" with the current prototype could be significantly shortened. To introduce a new model to visualize the distribution of radioactive materials in Fukushima, a plan to create a new camera based on the design of ASTRO-H SGD is underway with the support of the Japan Science and Technology Agency (JST).

Contributing to Decontamination with the Most Advanced Space Observation Technology

The keys to realization of the prototype Ultra-Wide Angle Compton Camera were detector technology employing CdTe semiconductors with excellent energy resolution and extremely high-density packaging technology. These were jointly developed with Acrorad Co. Ltd. and Nagoya Guidance Systems Works of Mitsubishi Heavy Industries, Ltd. over 15 years. The silicon detector and analog read-out LSI were based on joint research with Prof. Tajima, Nagoya University, while the data collection unit using SpaceWire is based on joint research with Prof. Nomachi, Osaka University. The HXI and SGD for ASTRO-H are under incessant development toward launch in 2014 by the same team that developed the world's top class hard X-ray/gamma-ray observation instruments for and since the SUZAKU program. The team consists of JAXA's ISAS, University of Tokyo, Nagoya University, Hiroshima University, Waseda University, et al. Space science observations require the most advanced technology. By realizing our own cutting-edge space observation technology, we were able to contribute to current and future radioactive decontamination, and widely expand the role of ASTRO-H from space to the earth.

Tadayuki TAKAHASHI, Shin WATANABE, Shinichiro TAKEDA

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