Evaluation of heat removal performance of a sealed Li target for an accelerator-driven neutron source of Boron Neutron Capture Therapy at Nagoya University

Abstract

An accelerator-driven neutron source has been developed in Nagoya University for boron neutron capture therapy using a new concept of thick lithium (Li) target in which protons are stopped in a Li layer. A sealed Li target developed consists of a Li sheet placed on a copper (Cu) base plate equipped with water-cooled channels and a titanium (Ti) foil seal to cover Li surface. The proton beam injected into the target is produced through the proton–Dynamitron accelerator (IBA) with a maximum energy of 2.8 MeV and a current of 15 mA. The results of our previous study confirmed that the target was not damaged when irradiated with an acceleration energy of 1.8 MeV and a current of 0.5 mA (heat input of 4.6 MW/m2). To keep the soundness of the target up to 6–7 MW/m2, it is considered to be essential to improve the bonding between the Ti foil and Li interface, which will contribute the heat removal. It was suggested from our previous experiences that the bonding property may be improved by forming a Cu–Li alloy at the Ti–Li interface. In this paper, we evaluate the heat removal performance of a sealed Li target with Cu–Li alloy formation to improve the target durability. The sealed Li target was produced using a “Cu-coated pure Ti foil”, in which Cu was deposited on the surface of a Ti foil with a thickness of 0.2μm. The use of Cu-coated pure Ti foil actively forms Cu–Li alloys at the Ti–Li interface. For comparison, sealed Li targets were produced with pure Ti foil, and irradiation tests were conducted on each target. The heat removal performance was evaluated by electron beam irradiation using the active cooling teststand 2 (ACT2), a high heat flux test facility, and proton beam irradiation using the Dynamitron accelerator. We obtain a result that the Cu-coated pure Ti foil-sealed Li target has a heat removal capacity of 11 MW/m2, about five times higher than that of the pure Ti foil-sealed Li target. This confirmed that the Cu–Li alloy contributed to the improvement of the heat removal performance. For the proton beam irradiation test, a Cu-coated pure Ti foil target was produced and irradiated for 26 h at an acceleration energy of 1.8 MeV and an average current of 3.5 mA, where no neutrons were generated. The tests confirmed that the Cu-coated pure Ti foil target could withstand 26 h of irradiation at a heat input of 9.0 MW/m2. About anticipated degradation of the heat removal performance due to stopping the protons in the Li layer, no change was observed during this long irradiation test, in which 2.15 × 1024 protons were injected into the Li layer. Although further long-time irradiation test will be needed in the future, at present we have confirmed that there is no problem with stopping protons in the Li layer.