Spent fuels and high level radioactive wastes which emit high doze of gamma rays could be a promising and long-lasting power source, if the gamma ray energy was effectively converted other forms of energy. In the present study, we have tried to convert gamma ray to electricity directly, with using silicon semiconductor cells made of p-type Si single crystal wafers with various specific resistivities ranging from 0.01 to 1000 Ohm∙cm. On both surfaces of the cell (20×20×0.5mm3), Al and Sb were deposited in vacuum to make electrodes at room temperature. The voltage-current measurement of the cells showed a rectification effect, and Al side was found to work a cathode. This suggests a Schottky junction was formed at the interface between the deposited Al and Si wafer. The cell irradiated by gamma ray in Co-60 irradiation facility in Kyushu Univ. with an absorbed dose of about 200Gy/h, and output voltage and current generated by the irradiation with external resistances varying from 200 to 100,000 Ohm were measured. The maximum electric power obtained for each cell ranged from 0.002 to 200 micro-W/m2, and clearly increased with increasing the specific resistivity of Si wafers. For comparison, a single crystal Si solar cell (2400mm2×0.5m, 0.5V×450mA in AM1.5 condition) was also exposed to the gamma ray, and its maximum electric power was 2 micro-W/m2. The output power of the present cell with high resistivity was two orders of magnitude higher than that of the Si solar cell.
Energy deposition in the Si cell during gamma irradiation was evaluated with the Monte Carlo Simulation for N Particles (MCNP) code. For Si with 0.5 mm thickness, the deposited energy was calculated to be 17000 micro-W/m2 for 200Gy/h. Comparing the output energy by the gamma irradiation, the energy conversion efficiency of the present Si cells reached about 1%. Unfortunately, the present cells were unstable even in ambient atmosphere, the conversion ratio of which decreased to less than one tenth in six months. Further development of the cells with higher conversion ratio and improvement of its stability will be discussed.