Quenching of Nuclear Recoil Scintillation Signals in Liquid Argon

A novel approach to detection of low-energy astrophysical particles, such as the hypothesized WIMP dark matter, relies on measurement of scintillation light in noble liquids such as argon and neon. It is necessary to understand the differing response of such a detector to nuclear and electron recoil events. In order to measure this quenching of nuclear recoil signals, a D-D neutron generator was used to produce neutrons that scattered from a liquid argon cell into another detector at a series of known angles, so that the energies of the nuclear recoils in the liquid argon were kinematically determined. After calibrating the liquid argon detector with electron recoil events, the quenching factor for nuclear recoils was determined from the ratio of the measured electron-equivalent recoil energy at a given scattering angle to the expected nuclear recoil energy at that angle. Details of this measurement in the 4-kg liquid argon microCLEAN detector will be presented, along with quenching factor results for nuclear recoil energies of tens to hundreds of keV.