Studies of the responses of liquid and solid targets for direct dark matter searches.

Abstract

The quenching factor for sodium recoils in a 2-inch Nal(Tl) scintillating crystal has been measured at room temperature. The crystal has been exposed to 2.45 MeV mono-energetic neutrons generated by a deuterium-deuterium fixed target accelerator in the energy range 10 to 100 keV nuclear recoil energy. A BC501A liquid scintillator detector has been used to tag neutrons that scatter off sodium nuclei in the crystal. Cuts on pulse shape discrimination in BC501A and neutron time of flight have been performed on pulses recorded by an Acqiris DC265 digitiser with a 2 ns sampling time. A quenching factor of 25.2 ± 6.4% has been determined for 10 keV sodium recoils. Measured quenching factors range from 19% to 26% in good agreement with other experiments. From pulse shape analysis, the mean time of pulses from electron and nuclear recoils have been compared down to 2 keV electron equivalent energy. Photon reponse measurements with the same crystal have also been taken by exposing it to gamma-ray sources with a range of energies. In agreement with results from other experiments, a dip in the photon response is seen at energies coincident with the iodine K-shell electron binding energy. This is a direct consequence of the energy of the photoelectron emitted during photoelectric absorption. An attempt to measure the non-linear electron response is also made with the Compton Coincidence Technique. Implications of this result on the quenching factor for sodium recoils in Nal(Tl) are also discussed. The commissioning of passive neutron and gamma shielding, together with an active veto system, around the ZEPLIN-II dark matter detector is described. The preparation of neutron shielding has required the impregnation of paraffin wax with gadolinium, and the casting of this into cuboids. Tests of the veto trigger with source data yield gamma and neutron background rejection efficiencies of 21.13% and 50.44% respectively. The veto efficiency is 99.15%