Towards a Nuclear Recoil Calibration of XENON1T

Jacques Pienaar, Purdue University

Abstract

This thesis was aimed at developing the tools necessary to deliver a well calibrated nuclear recoil calibration source, in the form of a deuterium-deuterium fusion generator, for the XENON1T detector. This source will be used to perform an in situ calibration of the detector response to low energy single scatters of neutrons, as well as to define the nuclear recoil band in the detector for use in dark matter searches. The neutron generator characterized in this work produced neutrons with energy (2.2 - 2.7) MeV. To measure fast neutrons such as these, we used an organic liquid scintillator with the ability to discriminate between particle types by looking at the pulse shapes of scintillation light produced when a particle interacts in the scintillator. The pulse shape discrimination ability of these scintillators was improved upon in this work through the implementation of a new algorithm utilising Laplace transforms of the recorded pulses to separate the events in frequency domain. Additionally this allowed us to significantly reduce the energy threshold for discrimination in these detectors and quantify the efficiency of the detector to detect neutrons as a function of the applied threshold. Due to the degeneracy between the acceptance roll-off experienced at low energies in the detector, and the absolute neutron flux of a neutron source, the neutron generator had to be characterized with as high precision as possible. The well-characterized neutron detectors allowed us to, for the first time, map out the dependence of the produced neutron flux on the operating parameters of an inertial electrostatic confinement neutron generator; namely applied current and voltage. We measured the energy spectrum, and have been able to match it to the predicted energy spectrum from a detailed GEANT4 Monte Carlo simulation of the generator. The measured anisotropy of the emitted neutron flux has also been reproduced in the same simulation. Finally, preliminary studies of nuclear recoil calibrations on XENON1T have been done. The calibration data, using a AmBe source, are analysed to determine the ratio of multiple to single scattering events of neutrons within the detector. These scattering ratios are compared to GEANT4 Monte Carlo predictions. The simulations show that the effect of micro physics processes in the signal production within the detector after an interaction still need to be included to fully match with data. Nevertheless I am able to make a prediction of the expected ratio of multiple to single scatters in dark matter searches, using the known radioactivity of the detector materials. Additionally I can extract the number of multiple scatter events which pass data quality cuts, and could be included in the final search data. I show that this rate would double the expected nuclear recoil background of the XENON1T detector.

Degree

Ph.D.

Advisors

Lang, Purdue University.

Subject Area

Astrophysics

COinS