Study of spin-dipole transitions from ¹⁶O via the (p,n) reaction and improvement of position resolution of two-dimensional neutron detector NPOL2

The ¹⁶O(ν,e^-)¹⁶F reaction is caused by weak interaction and the quantitative understanding of the transition strength is essential for the neutrino detection efficiencies of water Cherenkov detectors such as the Super-Kamiokande. In order to evaluate this transition strength, the ¹⁶O(p,n)¹⁶F reaction is useful since the relevant ¹⁶F states are strongly excited, and thus we can obtain the information based on the data with reasonable statistical accuracy. Since ¹⁶O is a doubly magic nucleus, spin-dipole (SD) states are strongly excited at small momentum transfers whereas Gamow-Teller ones are weakly excited. The SiO2 and Si targets were each bombarded with protons at 230 MeV and 8 degrees where the SD states are most prominently excited. The ¹⁶O(p,n) spectrum has been successfully obtained by subtracting Si contribution from the spectrum for SiO₂. The data were compared with the distorted wave impulse approximation plus random phase approximation calculation to assess the SD strength.

Our group plans to measure the polarization observable for the same reaction to separate the observed SD states into each spin-parity state. Our neutron detector NPOL2 has the two-dimensional position sensitivity and its resolution affects the efficiency of the polarization measurement. Therefore I have been trying to improve the position resolution using the correlation of timing information of NPOL2. The current status of this development will be presented.