Neutron multiplicity measurement in muon capture on oxygen nuclei in the Gd-loaded Super-Kamiokande detector

Abstract

In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with the muon capture events followed by gamma rays to be $50.2^{+2.0}_{-2.1}\%$. By fitting the observed multiplicity considering the detection efficiency, we measure neutron multiplicity in muon capture as $P(0)=24\pm3\%$, $P(1)=70^{+3}_{-2}\%$, $P(2)=6.1\pm0.5\%$, $P(3)=0.38\pm0.09\%$. This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold.

Figures (3)

• Figure 1: The observed and simulated energy spectrum of decay electrons and de-excitation gamma rays following stopping muons. The time difference between the decay electron or the gamma ray and the preceding stopping muon is required to be within [1.1, 5] $\mathrm{\mu s}$ . The simulation includes only decay electrons and is normalized with the number of stopping muons. The energy in MC is scaled by $+2.0\%$ to match the spectrum in data. The fit includes events with energies greater than 15 MeV.
• Figure 2: The detection time of neutrons with respect to the preceding stopping muons in those events with de-excitation gamma ray candidates. The search window is [18, 535] $\mathrm{\mu s}$ after the stopping muons to avoid after-pulse of PMTs, which happens 12--18 $\mathrm{\mu s}$ after a large hit Abe2022a. The red line shows the fitting result by Eq. \ref{['eq:captue_time_fit']}. The hatched box represents the $B$ component, corresponding to the false tagged signals.
• Figure 3: $\Delta\chi^2$ contours for the number of events with each neutron multiplicity obtained by unfolding the observed distribution. Blue lines with dot markers show the Feldman-Cousins critical values Feldman1998 for $1\sigma$, 90%, and $2\sigma$ confidence level from bottom to top. Green dashed lines represent $\Delta \chi^2 = 1,\ 2.71$, and $4$, roughly corresponding to $1\sigma$, 90%, and $2\sigma$ confidence levels, from bottom to top.