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

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%

Demonstration of surface electron rejection with interleaved germanium detectors for dark matter searches

The following article appeared in Applied Physics Letters 103.16 (2013): 164105 and may be found at http://scitation.aip.org/content/aip/journal/apl/100/26/10.1063/1.4729825The SuperCDMS experiment in the Soudan Underground Laboratory searches for dark matter with a 9-kg array of cryogenic germanium detectors. Symmetric sensors on opposite sides measure both charge and phonons from each particle interaction, providing excellent discrimination between electron and nuclear recoils, and between surface and interior events. Surface event rejection capabilities were tested with two 210 Pb sources producing ∼130 beta decays/hr. In ∼800 live hours, no events leaked into the 8–115 keV signal region, giving upper limit leakage fraction 1.7 × 10−5 at 90% C.L., corresponding to < 0.6 surface event background in the future 200-kg SuperCDMS SNOLAB experiment.This work is supported in part by the National Science Foundation (Grant Nos. AST-9978911, NSF-0847342, PHY-1102795,NSF-1151869, PHY-0542066, PHY-0503729, PHY-0503629, PHY-0503641, PHY-0504224, PHY-0705052,PHY-0801708, PHY-0801712, PHY-0802575, PHY-0847342, PHY-0855299, PHY-0855525, and PHY-1205898), by the Department of Energy (Contract Nos. DE-AC03-76SF00098, DE-FG02-92ER40701, DE-FG02-94ER40823,DE-FG03-90ER40569, DE-FG03-91ER40618, and DESC0004022),by NSERC Canada (Grant Nos. SAPIN 341314 and SAPPJ 386399), and by MULTIDARK CSD2009-00064 and FPA2012-34694. Fermilab is operated by Fermi Research Alliance, LLC under Contract No. De-AC02-07CH11359, while SLAC is operated under Contract No. DE-AC02-76SF00515 with the United States Department of Energy