Demonstration of Surface Electron Rejection with Interleaved Germanium Detectors for Dark Matter Searches
R. AgneseA. J. AndersonD. BalakishiyevaR. Basu ThakurD. BauerA. W. BorglandD. BrandtP. L. BrinkR. BunkerBlas CabreraD. O. CaldwellD. G. CerdeñoH. ChaganiM. CherryJ. CooleyBruce CornellC. H. CrewdsonP. CushmanM. DaalP. C. F. Di StefanoE. do Couto e SilvaT. DoughtyLuis EstebanS. FallowsE. Figueroa‐FelicianoJ. FoxM. FrittsG. L. GodfreyS. R. GolwalaJ. HallH. R. HarrisJ. HasiS. A. HertelB. A. HinesT. HöferD. HolmgrenL. HsuM. E. HuberA. JastramO. KamaevBurhan KaraM. H. KelseyS. Al KenanyA. KennedyC. KenneyM. KiveniK. KochB. LoerE. Lopez AsamarR. MahapatraV. MandicC. Moreno MartinezK. A. McCarthyN. MirabolfathiR. A. MoffattD. MoorePhilippe NadeauR. NelsonL. NovákK. PageR. PartridgeM. PepinA. PhippsK. PrasadM. PyleH. QiuRoxanne RadpourW. RauP. RedlA. ReisetterR. ReschY. RicciT. SaabB. SadouletJ. SanderR. SchmittK. SchneckR. W. SchneeS. ScorzaD. N. SeitzB. SerfassB. ShankD. SpellerA. TomadaA. N. VillanoB. WelliverD. H. WrightS. YellinJ. J. YenBetty YoungJ. Zhang
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The 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 $\sim$130 beta decays/hr. In $\sim$800 live hours, no events leaked into the 8--115 keV signal region, giving upper limit leakage fraction $1.7 \times 10^{-5}$ at 90% C.L., corresponding to $< 0.6$ surface event background in the future 200-kg SuperCDMS SNOLAB experiment.Keywords:
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A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O2 high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. It is then applied to study the temporal behavior of the discharge plasma parameters such as electron density, the neutral and ion composition, the ionization fraction of the sputtered vapor, the oxygen dissociation fraction, and the composition of the discharge current. We study and compare the discharge properties when the discharge is operated in the two well established operating modes, the metal mode and the poisoned mode. Experimentally, it is found that in the metal mode the discharge current waveform displays a typical non-reactive evolution, while in the poisoned mode the discharge current waveform becomes distinctly triangular and the current increases significantly. Using the R-IRM we explore the current increase and find that when the discharge is operated in the metal mode Ar+ and Ti+ -ions contribute most significantly (roughly equal amounts) to the discharge current while in the poisoned mode the Ar+ -ions contribute most significantly to the discharge current and the contribution of O+ -ions, Ti+ -ions, and secondary electron emission is much smaller. Furthermore, we find that recycling of atoms coming from the target, that are subsequently ionized, is required for the current generation in both modes of operation. From the R-IRM results it is found that in the metal mode self-sputter recycling dominates and in the poisoned mode working gas recycling dominates. We also show that working gas recycling can lead to very high discharge currents but never to a runaway. It is concluded that the dominating type of recycling determines the discharge current waveform.
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The discharge voltage was measured for 15 different metallic target materials at constant current before and after plasma oxidation in order to understand its behavior during reactive magnetron sputtering. Plasma oxidation of the target surface was achieved by sputtering the target in pure oxygen. The discharge voltage measured in pure argon is characteristic for each kind of metallic target and is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. Based on this relation a value for the ISEE coefficient of the oxidized target surface can be calculated. Two distinct groups can be discerned: for one group the ISEE coefficient of the oxidized target surface is larger than the ISEE coefficient of the metal, while the opposite behavior is noticed for the second group. This difference seems to find its origin in the reduction behavior of the oxides under ion bombardment, since the ISEE coefficient of the oxide can be related to the simulated degree of reduction of the oxide. It is shown that the ISEE coefficient of the reduced oxides decreases with increasing oxygen content in the target. This is confirmed experimentally by sputtering in pure argon reduced titanium oxide targets with a known composition.
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The atomic oxygen metastable dynamics in a Reactive High-Power Impulse Magnetron Sputtering (R-HiPIMS) discharge has been characterized using time-resolved diode laser absorption in an Ar/O2 gas mixture with a Ti target. Two plasma regions are identified: the ionization region (IR) close to the target and further out the diffusion region (DR), separated by a transition region. The μs temporal resolution allows identifying the main atomic oxygen production and destruction routes, which are found to be very different during the pulse as compared to the afterglow as deduced from their evolution in space and time.
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