Remote-controlled non-destructive measurement for thermal conductivity of highly radioactive isotropic graphite used as the muon production target at J-PARC/MUSE

The most intense pulsed muon beam in the world will be generated by a 3-GeV 333-μA proton beam on a muon target made of 20-mm thick isotropic graphite at J-PARC/MUSE (Japan Proton Accelerator Research Complex /Muon Science Establishment). The first muon beam was successfully produced on September 26th, 2008, and the most intense pulsed muon beam has continuously been produced with a 200-kW proton beam since November of 2010. To measure the radiation damage to thermal conductivity of the graphite, we adopted a modified Thermal Imaging Scope, TSI (Bethel Co., Ltd.), based on a laser-spot-heating technique. In this technique, the material is heated by a periodic heating laser diode on a two-dimensional movable stage. The propagation of a thermal wave around the heated spot is measured with a two-dimensional infrared thermometer. The thermal conductivity can be locally evaluated through a delay of propagation, which depends on the distance between the laser-heated spot, or the maximum amplitude on the heated-spot. Because the measurements can be performed non-destructively and without any contact to the radioactive materials in this method, the muon target after the measurement can be used again. Furthermore the thermal conductivity can be measured with a high spatial resolution. In summer of 2011, the 3-GeV proton irradiation effect to the thermal conductivity of the graphite was measured for the used radioactive target. The accumulated radiation damage by the protons was evaluated to be approximately 0.3 dpa at the center of the beam spot. The temperature during the proton irradiation could be evaluated to be 470 K. In the vicinity of the interface between the graphite and the surrounding titanium frame, the graphite had been irradiated with the radiation damage of approximately 0.002 dpa at 350 K. Consequently, it was found out that a decrement of thermal conductivity under a low temperature from 350 K to 430 K takes place drastically even by a low radiation damage up to 0.1 dpa. Then a decrement above the radiation damage will be saturated.