Precise measurement of 3He absorption cross sections for neutrons using the J-PARC pulsed neutron source

The decay lifetime of neutrons is an important parameter used in Big Bang elemental synthesis theory and in the unitarity calculation of the CKM matrix. Decay lifetimes have been measured by two methods, the UCN storage method and the proton count method, but there is a discrepancy of 9.5 s (4.5σ) between the results of each method. To solve this problem, lifetime measurements are being performed using a new method at J-PARC in Tokai-mura, Ibaraki Prefecture. This experiment aims to achieve a measurement accuracy of less than 0.1%.

In the above experiment, the 3He neutron absorption cross section is used to determine the neutron flux. The current value is 5333±7 barn, which is problematic because there are only two old measurement points. To update this value, a precise measurement of the absorption cross section was performed at J-PARC in June 2023 with a target accuracy of 0.06%.

Absorption cross sections are inversely proportional to neutron velocity. In order to measure absorption cross sections precisely, calibration experiments of the TOF method were performed at BL05 with a coupled moderator and at BL10 with an uncoupled moderator. Sharp spectra were obtained at BL10, and we decided to use BL10 as the beamline for the absorption cross section measurement experiments.

Absorption cross sections were measured at J-PARC BL10 using the transmission method. This method is based on counting the number of neutrons transmitted through a gas-filled cell and a vacuum cell.

I report on these experiments.