Fixed measurement of thermal neutron intensity with gold-foil activation method at the J-PARC spallation neutron source

Harada, Masahide   ; Oikawa, Kenichi   ; Tsuchikawa, Yusuke ; Yamaguchi, Yuji   ; Haga, Katsuhiro  

In the spallation neutron source at Materials and Life Science Experimental Facility in J-PARC, a 3-GeV and 1-MW proton beam induces into a mercury target and the neutrons beam is provide to neutron instruments. The target vessels are regularly replaced due to pitting damage. The proton beam power is gradually increased to 800 kW as of June 2022, and the optimum proton beam profile is used depending on the beam power. Based on these observations, fixed-point measurements have been periodically carried out to observe the intensity of the neutrons supplied from the spallation neutron source under the same measurement conditions at NOBORU. In the fixed-point measurements, the activation method with gold foils (15x15x0.1mm) was used to observe the absolute intensity of thermal neutrons with and without cadmium foil. The irradiation time was set to about 10 to 30 minutes in accordance with the proton beam intensity. The gamma-ray from irradiated foil was measured with a germanium detector, and the amount of activation was identified. As a result of the measurement, the thermal neutron intensity was slightly dependent on the proton beam intensity.