A Study of Nuclear Recoils in Liquid Argon Time Projection Chamber for the Direct Detection of WIMP Dark Matter

Abstract

Robust results of WIMP direct detection experiments depend on firm understandings of nuclear recoils in the detector media. This thesis documents the most comprehensive study to date on nuclear recoils in liquid argon - a strong candidate for the next generation multi-ton scale WIMP detectors. This study investigates both the energy partition from nuclear recoil energy to secondary modes (scintillation and ionization) and the pulse shape characteristics of scintillation from nuclear recoils.

Our collaboration, SCENE, acquired the scintillation and ionization signals of recoiling nuclei in liquid argon as a function of applied electric field by exposing a dual-phase Liquid Argon Time Projection Chamber (LAr-TPC) to a low energy pulsed narrowband neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. I present measurements of the scintillation yield and the scintillation pulse shape for nuclear recoils with energies from 10.3 to 57.2 keV and for applied electric fields from 0 to 1000 V/cm. For the ionization yield, I report measurements from 16.9 to 57.2 keV and for electric fields from 50 to 500 V/cm. I also report the observation of an anti-correlation between scintillation and ionization from nuclear recoils, which is similar to the anti-correlation between scintillation and ionization from electron recoils. With an assumption that the energy partition in excitons and ion pairs of Kr-83m internal conversion electrons is comparable to that of Bi-207 conversion electrons, the numbers of excitons and ion pairs and their ratio produced by nuclear recoils from 16.9 to 57.2 keV are calculated. Motivated by arguments suggesting direction sensitivity in LAr-TPC signals due to columnar recombination, a comparison of the light and charge yield of recoils parallel and perpendicular to the applied electric field is presented for the first time.