Measurement of Scintillation and Ionization Yield and Scintillation Pulse Shape from Nuclear Recoils in Liquid Argon

TL;DR

This study measures the scintillation and ionization yields from nuclear recoils in liquid argon, revealing their dependence on electric field and recoil energy, and explores potential directional sensitivity in LAr-TPC detectors.

Contribution

It provides the first detailed measurements of scintillation and ionization yields for nuclear recoils in liquid argon across various energies and electric fields, including directional dependence insights.

Findings

Observed anticorrelation between scintillation and ionization yields.

Quantified exciton and ion pair production ratios for nuclear recoils.

Identified potential direction sensitivity in liquid argon TPC signals.

Abstract

We have measured the scintillation and ionization yield 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 narrow band neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We report measurements of the scintillation yields for nuclear recoils with energies from 10.3 to 57.3 keV and for median applied electric fields from 0 to 970 V/cm. For the ionization yields, we report measurements from 16.9 to 57.3 keV and for electric fields from 96.4 to 486 V/cm. We also report the observation of an anticorrelation between scintillation and ionization from nuclear recoils, which is similar to the anticorrelation between scintillation and ionization from electron recoils. Assuming that the energy loss partitions into excitons and ion pairs from $^{83m}$Kr internal conversion electrons is comparable to that from $^{207}$Bi conversion electrons, we obtained the numbers of excitons ($N_{ex}$) and ion pairs ($N_i$) and their ratio ($N_{ex}/N_i$) produced by nuclear recoils from 16.9 to 57.3 keV. 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.