Measurement of $\gamma$-ray production via the neutron-${}^{16}\mathrm{O}$ reaction using a 77 MeV quasimonoenergetic neutron beam

Understanding of $\gamma$-ray production via neutron interactions on oxygen is essential for the study of neutrino neutral-current quasielastic interactions in water Cherenkov detectors. A measurement of $\gamma$-ray production from such reactions was performed using a 77 MeV quasimonoenergetic neutron beam. Several $\gamma$-ray peaks, which are expected to come from neutron-${}^{16}\mathrm{O}$ reactions, are observed and production cross sections are measured for nine $\gamma$-ray components of energies between 2 and 8 MeV. These are the first measurements at this neutron energy using a nearly monoenergetic beam.

Figure 1

Schematic illustration of primary and secondary $\gamma$-ray productions via neutrino and subsequent nuclear reactions in water. The figure is taken from Ref. [14].

Figure 2

Schematic drawing of the RCNP facility and the neutron time-of-flight beamline. The dotted box shows a magnified depiction of the experimental setup.

Figure 3

PSD parameter as a function of the total deposited charge ($Q_{\mathrm{total}}$) value. The blue line represents the neutron selection criterion.

Figure 4

Deposited energy in the BC-501A detector for all (black), neutron-like (blue), and $\gamma$-like (red) events.

Figure 5

TOF distributions of all (black), neutron-like (blue), and $\gamma$-like (red) events. The TDC was operated in common stop mode.

Figure 6

Neutron kinetic energy distribution reconstructed from the TOF distribution.

Figure 7

Neutron detection efficiencies of the BC-501A detector calculated with scinful-qmd. The attenuation factor of 0.008 ${\mathrm{cm}}^{-1}$ and the light output function from Ref. [27] are used as the nominal setting.

Figure 8

Neutron flux normalized by the detector covering the solid angle and the incident protons. The red bars indicate the peak region used for the cross section measurement.

Figure 9

Neutron beam profile measured by the BC-501A detector. The results are for the $72–82$ MeV region.

Figure 10

TOF distribution of observed events in the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detector. The red and green bars indicate the “on-timing” and “off-timing” regions, respectively. The TDC was operated in common stop mode.

Figure 11

Neutron kinetic energy distribution inferred from timing information of observed events in the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detector. The red bars indicate the peak region used for the cross section measurement.

Figure 12

Energy spectra of the HPGe detector with water (red) and without water (blue). The bottom panel gives the region between 3 and 8 MeV.

Figure 13

Energy spectra of the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detector with water (red) and without water (blue) before the TOF cut.

Figure 14

Energy spectra of the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detector with three different conditions: on-timing with water, on-timing without water, and off-timing with water.

Figure 15

Distribution of the ratio of the integrated tail to total signal pulse of the CsI(Tl) pulse height as a function of deposited energy. Three populations are seen: (a) $\gamma$ rays, (b) neutrons, and (c) pileup events.

Figure 16

Schematic illustration of the continuous background caused by $\gamma$ rays from scattered fast neutron reactions with the surrounding materials for a water-filled vessel (left) and an empty vessel (right).

Figure 17

Comparison of the spectra for the neutron energy regions $72–77$ and $77–82$ MeV (left) and comparison between the shape difference of these two and the nonwater spectrum for the $72–82$ MeV region (right).

Figure 18

Energy spectrum of the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ scintillator after the TOF cut and after subtracting nonwater and off-timing spectra (black points). The best-fit spectrum (red dashed line), and the spectra of both the signal and background components are shown in a linear (a) and a logarithmic (b) scale. Different colored spectra correspond to the fitted signals and continuous background: 7.12 MeV (gray), 6.92 MeV (brown), 6.32 MeV (magenta), 6.13 MeV (orange), 5.27 MeV (cyan), 5.18 MeV (dark red), 4.44 MeV (green), 3.68 MeV (yellow), 2.30 MeV (violet), and continuous background (blue).

Figure 19

Comparison of the measured 6.13 MeV $\gamma$-ray production cross sections. This work is shown by a red square while the results of Ref. [38] appear as black circles and those of Ref. [39] are shown by blue triangles. Note that the latter is based on measurements of ${}^{16}\mathrm{O}(p,p^{\prime }\gamma )$.

Figure 20

Comparison of the measured 4.44 MeV $\gamma$-ray production cross sections on oxygen. The present result is shown by a red square while the results of Ref. [38] appear as black circles.