Characteristics of proton beam scanning dependent on Li target thickness from the viewpoint of heat removal and material strength for accelerator-based BNCT

This study demonstrates the characterization of proton spot scanning on a Li target assembly for accelerator-based BNCT from the viewpoint of heat removal and material strength. These characteristics are investigated as to their dependence on the Li target thickness, considering that the Cu backing plate has more suitable heat removal properties than Li. Two situations are considered in this paper, i.e. the cyclic operation of the spot scanning, and a stalled spot scanning cycle where the proton beam stays focused on a single position on the Li target. It was found that the maximum of the Li temperature and the strain of the Cu backing increase as the cycle period increases. A cycle period less than 120 ms (over 8.3 Hz of frequency) enables the Li temperature to be kept below 150 °C and a cycle of less than 115 ms (8.7 Hz) keeps the Cu strain below the critical value for a 230 μm thick Li target, though the values are evaluated conservatively. Against expectation, the Li temperature and Cu strain are larger for a 100 μm thick target than for a 230 μm target. The required cycle period in this case is 23 ms (43 Hz) for maintaining a reasonable Li temperature and 9 ms (110 Hz) to prevent Cu fatigue fracture. For a stall in the spot scanning cycle, the Cu temperature increases as the beam shutdown time increases. The time for Cu to reach its melting point is estimated to be 4.2 ms at the surface, 20 ms at 1 mm depth, for both of 100 and 230 μm thick targets. At least 34 ms is estimated to be enough to make a hole on Cu backing plate. A beam shutdown mechanism with a response time of about 20 ms is therefore required.