スーパーカミオカンデへのガドリウム導入と超新星ニュートリノの観測

2016 Fiscal Year Annual Research Report

• Project/Area Number: 16F16803
• Research Institution: The University of Tokyo
• Principal Investigator: ヴァギンズ マーク 東京大学, カブリ数物連携宇宙研究機構, 教授 (90509902)
• Co-Investigator(Kenkyū-buntansha): MURDOCH MATTHEW 東京大学, カブリ数物連携宇宙研究機構, 外国人特別研究員
• Project Period (FY): 2016-11-07 – 2019-03-31
• Keywords: Spallation / supernova / neutrino / neutron / EGADS / Super-Kamiokande

Outline of Annual Research Achievements

The main purpose of my research is to maximise Super-Kamiokande’s sensitivity to supernova neutrinos through reduction and characterisation of detector backgrounds, particularly those from cosmic ray spallation.
Cosmic ray spallation interactions produce many neutrons. A few microseconds after their creation, the neutrons are observed when they capture on a gadolinium nucleus. Developing and understanding effective neutron identification methods is therefore vital to selecting spallation events. To do this, simulation tools were developed and detector calibration was performed
Data was taken with an AmBe source to produce a data sample of neutrons with minimal background. This data was analysed using an initial set of neutron selection criteria. The neutron detection efficiency was measured using these baseline criteria and found to be in good agreement with simulation and design specifications. This data will be used in developing neutron identification methods.
The GEANT4 simulation of the detector was improved to provide more utility and accuracy. The simulation was tuned using radioactive sources so that its output resembles real detector data. Although further development is needed, the simulation now functions well enough to be used in initial physics studies. An accurate Monte Carlo simulation will allow the generation of data samples with large statistics that will be required for the spallation analysis among others

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

Work has begun on developing the software and tools for the cosmic ray background investigation using the EGADS detector as proposed. The simulation is suitable for initial baseline cosmic ray studies but still needs some improvement.

Strategy for Future Research Activity

Development of detector simulation will be continued, including updating the electronics simulation to match the upgraded detector readout system. The simulation will be used in the training of an enhanced neutron selection algorithm and the development of a cosmic ray spallation tag. This cosmic ray spallation tag will then be applied to the EGADS data-set to measure the cosmic spallation rate and the neutron multiplicity of spallation events.
It has been found that the reduction of the gadolinium’s intrinsic radioactivity is best handled by the companies at the production stage. Therefore, the assay/resin portion of the project has been reduced. The gadolinium will be tested as it is shipped from the companies to ensure it meets radioactivity requirements. I plan to contribute to these assay efforts as needed at Boulby/Canfranc/Kamioka. The neutron tagging and reconstruction portion of the project has been expanded to account for this reduced scope.
The AmBe neutron source currently used provides only a single neutron at a time. Another source that produces several neutrons at a time will be investigated. This new source will more closely replicate spallation signals in the detector and allow assessment of systematic errors caused by high neutron multiplicity.