Speaker
Description
Super-Kamiokande has been operating with gadolinium loading (SK-Gd) to improve the sensitivity to the diffuse supernova neutrino background (DSNB) search. The detector has been running with a gadolinium (Gd) concentration of 0.011 wt% since July 2020 and 0.033 wt% since June 2022. The addition of Gd significantly enhances the detection efficiency of neutrons produced by inverse beta decay of electron antineutrinos. On the other hand, systematic uncertainties associated with the neutron tagging (ntag) algorithm have a significant impact on the DSNB search sensitivity. Thus, we have evaluated ntag efficiency using 241Am-Be (Am/Be) neutron source and BGO scintillator. In the ntag analysis, a delayed coincidence method is employed, in which a prompt and a delayed event are identified. The Am/Be source simultaneously emits a neutron and 4.4 MeV gamma ray. The 4.4 MeV gamma ray deposits energy in BGO scintillator, leading to the emission of scintillation light, which is detected as the prompt event. The neutron is subsequently thermalized and captured on Gd, emitting a cascade of gamma rays with a total energy of about 8 MeV. These gamma rays also interact with electrons in water, and resulting recoil electrons emit Cherenkov light, which is observed as the delayed event. For a Gd concentration of 0.033 wt%, the neutron capture time constant is observed to be approximately 60 μs.
During a part of the SK-Gd period, a failure occurred in the geomagnetic compensation coils installed around the detector tank. Until the coils were repaired, performance changes were observed in part of the photomultiplier tubes (PMTs) due to residual magnetic fields, which affected the energy reconstruction. To account for the effect, the impact of residual magnetic fields was treated as a change in the photon collection efficiency (CE) of the PMTs, and the energy reconstruction was corrected using tuned CE tables. Validation using calibration data with the Am/Be source shows that the data taken during the coil failure period are corrected to a level comparable to that of the pre-failure period. Furthermore, a comparison between the measured ntag efficiency and results from Geant4-based simulator (SKG4) demonstrates that the simulation reproduces the calibration data throughout the entire SK-Gd period.
In this poster, we present the details of the analysis, our findings on variations in the ntag efficiency during SK-Gd phases VI to VIII, where Gd concentration and detector conditions differ, and an estimation of the systematic uncertainty of the ntag efficiency that affects to the DSNB search sensitivity.