Browsing by Subject "Cryogenics"
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Item Bicep Array: Searching for Signals of Inflation From The South Pole(2022-01) Crumrine, MichaelThe $\Lambda$CDM cosmological model posits a universe that began with a big bang - like singularity, which contains mostly cold dark matter, and which is experiencing an accelerating expansion due to a dark energy component. This model has experienced resounding success and is consistently upheld by experiments across the globe. The model is incomplete however, it cannot describe how the specific initial conditions required to create the universe we see today came about. Inflation is an extension to the $\Lambda$CDM model which hypothesizes that the universe underwent a period of exponential expansion just after the big bang, sufficient to set up the required initial conditions. Most inflationary theories predict a stochastic gravitational wave background generated as a result of this expansion which would have imprinted a characteristic B-mode signal into the polarization pattern of the Cosmic Microwave Background. Detecting this primordial gravitational wave signal would provide direct evidence for inflation The \textsc{Bicep}/{\it Keck} program constitutes a series of polarization sensitive microwave telescopes situated at the geographic South Pole targeting the degree-angular scale $B$-modes and searching for a primordial signal. Over the last two decades this program has consistently reported the tightest constraints on this signal, with the most recent analysis of data through $2018$ providing an upper limit on the tensor-to-scalar ratio $r<0.036$ at $95\%$ confidence. {\sc Bicep} Array is the latest experiment in the series and replaces the {\it Keck Array}, expanding the frequency coverage to two new low-frequency bands and, once fully operational, increasing the detector count by over an order of magnitude. {\sc Bicep} Array is expected to achieve $\sigma(r) = 0.002 - 0.004$ depending on foreground complexity and the degree of lensing removal. In this dissertation I cover the design of this new experiment -- with a focus on the design and performance of the cryogenics down to $4$\,K -- and the first year's observations. I analyze the first year of new low frequency data in combination with the recently release BK18 results and find that the new data provides no improvement on $\sigma(r)$. However, it provides significant constraining power on galactic synchrotron radiation resulting in a factor of two decrease in the uncertainty on the amplitude of this foreground signal.Item Development of CDMS-II surface event rejection techniques and their extensions to lower energy thresholds(2014-12) Hofer, Thomas JamesThe CDMS-II phase of the Cryogenic Dark Matter Search, a dark matter direct-detection experiment, was operated at the Soudan Underground Laboratory from 2003 to 2008. The full payload consisted of 30 ZIP detectors, totaling approximately 1.1 kg of Si and 4.8 kg of Ge, operated at temperatures of 50 mK. The ZIP detectors read out both ionization and phonon pulses from scatters within the crystals; channel segmentation and analysis of pulse timing parameters allowed effective fiducialization of the crystal volumes and background rejection sufficient to set world-leading limits at the times of their publications. A full re-analysis of the CDMS-II data was motivated by an improvement in the event reconstruction algorithms which improved the resolution of ionization energy and timing information. The Ge data were re-analyzed using three distinct background-rejection techniques; the Si data from runs 125 - 128 were analyzed for the first time using the most successful of the techniques from the Ge re-analysis. The results of these analyses prompted a novel "mid-threshold" analysis, wherein energy thresholds were lowered but background rejection using phonon timing information was still maintained. This technique proved to have significant discrimination power, maintaining adequate signal acceptance and minimizing background leakage. The primary background for CDMS-II analyses comes from surface events, whose poor ionization collection make them difficult to distinguish from true nuclear recoil events. The novel detector technology of SuperCDMS, the successor to CDMS-II, uses interleaved electrodes to achieve full ionization collection for events occurring at the top and bottom detector surfaces. This, along with dual-sided ionization and phonon instrumentation, allows for excellent fiducialization and relegates the surface-event rejection techniques of CDMS-II to a secondary level of background discrimination. Current and future SuperCDMS results hold great promise for mid- to low-mass WIMP-search results.