arXiv subjects

S. R. Golwala

Publications and source records attributed to S. R. Golwala.

At least 55 records · Page 3Linked to original sources

arXiv2014

The BetaCage, an ultra-sensitive screener for surface contamination

Material screening for identifying low-energy electron emitters and alpha-decaying isotopes is now a prerequisite for rare-event searches (e.g., dark-matter direct detection and neutrinoless double-beta decay) for which surface radiocontamination has become an increasingly important background. The BetaCage, a gaseous neon time-projection chamber, is a proposed ultra-sensitive (and nondestructive) screener for alpha- and beta-emitting surface contaminants to which existing screening facilities are insufficiently sensitive. Sensitivity goals are 0.1 betas per keV-m$^2$-day and 0.1 alphas per m$^2$-day, with the former limited by Compton scattering of photons in the screening samples and (thanks to tracking) the latter expected to be signal-limited; radioassays and simulations indicate backgrounds from detector materials and radon daughters should be subdominant. We report on details of the background simulations and detector design that provide the discrimination, shielding, and radiopurity necessary to reach our sensitivity goals for a chamber with a 95$\times$95 cm$^2$ sample area positioned below a 40 cm drift region and monitored by crisscrossed anode and cathode planes consisting of 151 wires each.

physics.ins-det↗

arXiv2014

Search for Low-Mass WIMPs with SuperCDMS

We report a first search for weakly interacting massive particles (WIMPs) using the background rejection capabilities of SuperCDMS. An exposure of 577 kg-days was analyzed for WIMPs with mass < 30 GeV/c2, with the signal region blinded. Eleven events were observed after unblinding. We set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2e-42 cm2 at 8 GeV/c2. This result is in tension with WIMP interpretations of recent experiments and probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6 GeV/c2.

hep-ex↗

arXiv2014

A Comparison of Fundamental Noise in Kinetic Inductance Detectors and Transition Edge Sensors for Millimeter-wave Applications

Kinetic inductance detectors (KIDs) show promise as a competitive technology for astronomical observations over a wide range of wavelengths. We are interested in comparing the fundamental limitations to the sensitivity of KIDs with that of transition edge sensors (TESs) at millimeter wavelengths, specifically over the wavelengths required for studies of the Cosmic Microwave Background (CMB). We calculate the total fundamental noise arising from optical and thermal excitations in TESs and KIDs for a variety of bath temperatures and optical loading scenarios for applications at millimeter wavelengths. Special consideration is given to the case of ground-based observations of 100 GHz radiation with a 100 mK bath temperature, conditions consistent with the planned second module of the QUBIC telescope, a CMB instrument. Under these conditions, a titanium nitride KID with optimized critical temperature pays a few percent noise penalty compared to a typical optimized TES.

astro-ph.IM↗

arXiv2013

CDMSlite: A Search for Low-Mass WIMPs using Voltage-Assisted Calorimetric Ionization Detection in the SuperCDMS Experiment

SuperCDMS is an experiment designed to directly detect Weakly Interacting Massive Particles (WIMPs), a favored candidate for dark matter ubiquitous in the Universe. In this paper, we present WIMP-search results using a calorimetric technique we call CDMSlite, which relies on voltage- assisted Luke-Neganov amplification of the ionization energy deposited by particle interactions. The data were collected with a single 0.6 kg germanium detector running for 10 live days at the Soudan Underground Laboratory. A low energy threshold of 170 eVee (electron equivalent) was obtained, which allows us to constrain new WIMP-nucleon spin-independent parameter space for WIMP masses below 6 GeV/c2.

physics.ins-det↗

arXiv2013

A prototype low-background multiwire proportional chamber for measuring alphas and low-energy betas

A prototype multiwire proportional chamber (MWPC) was developed to demonstrate the feasibility of constructing a radiopure time projection chamber with MWPC track readout to assay materials for alpha- and beta-emitting surface contaminants for future rare-event-search experiments as well as other scientific fields. The design features and assembly techniques described here are motivated by the position and energy resolution required to reconstruct alpha and beta tracks while efficiently rejecting backgrounds. Results from a test setup using an $^{55}$Fe x-ray source indicate excellent operational stability and a near-ideal energy resolution of 15.8% FWHM at 5.89 keV and a gas gain of $\sim$10$^{4}$.

physics.ins-det↗

arXiv2013

Silicon Detector Dark Matter Results from the Final Exposure of CDMS II

We report results of a search for Weakly Interacting Massive Particles (WIMPS) with the silicon detectors of the CDMS II experiment. This blind analysis of 140.2 kg-days of data taken between July 2007 and September 2008 revealed three WIMP-candidate events with a surface-event background estimate of 0.41^{+0.20}_{-0.08}(stat.)^{+0.28}_{-0.24}(syst.). Other known backgrounds from neutrons and 206Pb are limited to < 0.13 and <0.08 events at the 90% confidence level, respectively. The exposure of this analysis is equivalent to 23.4 kg-days for a recoil energy range of 7-100 keV for a WIMP of mass 10 GeV/c2. The probability that the known backgrounds would produce three or more events in the signal region is 5.4%. A profile likelihood ratio test of the three events that includes the measured recoil energies gives a 0.19% probability for the known-background-only hypothesis when tested against the alternative WIMP+background hypothesis. The highest likelihood occurs for a WIMP mass of 8.6 GeV/c2 and WIMP-nucleon cross section of 1.9e-41 cm2.

hep-ex↗

arXiv2013

Demonstration of Surface Electron Rejection with Interleaved Germanium Detectors for Dark Matter Searches

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.

physics.ins-det↗

arXiv2013

Silicon detector results from the first five-tower run of CDMS II

We report results of a search for Weakly Interacting Massive Particles (WIMPs) with the Si detectors of the CDMS II experiment. This report describes a blind analysis of the first data taken with CDMS II's full complement of detectors in 2006-2007; results from this exposure using the Ge detectors have already been presented. We observed no candidate WIMP-scattering events in an exposure of 55.9 kg-days before analysis cuts, with an expected background of ~1.1 events. The exposure of this analysis is equivalent to 10.3 kg-days over a recoil energy range of 7-100 keV for an ideal Si detector and a WIMP mass of 10 GeV/c2. These data set an upper limit of 1.7x10-41 cm2 on the WIMP-nucleon spin-independent cross section of a 10 GeV/c2 WIMP. These data exclude parameter space for spin-independent WIMP-nucleon elastic scattering that is relevant to recent searches for low-mass WIMPs.

astro-ph.CO↗

arXiv2013

High Spectral Resolution Measurement of the Sunyaev-Zel'dovich Effect Null with Z-Spec

The Sunyaev-Zel'dovich (SZ) effect spectrum crosses through a null where dT_CMB = 0 near nu_0 = 217 GHz. In a cluster of galaxies, nu_0 can be shifted from the canonical thermal SZ effect value by corrections to the SZ effect scattering due to the properties of the inter-cluster medium. We have measured the SZ effect in the hot galaxy cluster RX J1347.5-1145 with Z-Spec, an R ~ 300 grating spectrometer sensitive between 185 and 305 GHz. These data comprise a high spectral resolution measurement around the null of the SZ effect and clearly exhibit the transition from negative to positive dT_CMB over the Z-Spec band. The SZ null position is measured to be nu_0 = 225.8 \pm 2.5 (stat.) \pm 1.2 (sys.) GHz, which differs from the canonical null frequency by 3.0 sigma and is evidence for modifications to the canonical thermal SZ effect shape. Assuming the measured shift in nu_0 is due only to relativistic corrections to the SZ spectrum, we place the limit T_e = 17.1 \pm 5.3 keV from the zero-point measurement alone. By simulating the response of the instrument to the sky, we are able to generate likelihood functions in {y_0, T_e, v_pec} space. For v_pec = 0 km/s, we measure the best fitting SZ model to be y_0 = 4.6 (+0.6, -0.9) x 10^-4, T_e,0 = 15.2 (+12,-7.4) keV. When v_pec is allowed to vary, a most probable value of v_pec = +450 \pm 810 km/s is found.

astro-ph.CO↗

arXiv2012

Search for annual modulation in low-energy CDMS-II data

We report limits on annual modulation of the low-energy event rate from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Such a modulation could be produced by interactions from Weakly Interacting Massive Particles (WIMPs) with masses ~10 GeV/c^2. We find no evidence for annual modulation in the event rate of veto-anticoincident single-detector interactions consistent with nuclear recoils, and constrain the magnitude of any modulation to <0.06 event [keVnr kg day]^-1 in the 5-11.9 keVnr energy range at the 99% confidence level. These results disfavor an explanation for the reported modulation in the 1.2-3.2 keVee energy range in CoGeNT in terms of nuclear recoils resulting from elastic scattering of WIMPs at >98% confidence. For events consistent with electron recoils, no significant modulation is observed for either single- or multiple-detector interactions in the 3.0-7.4 keVee range.

astro-ph.CO↗

arXiv2012

Antenna-coupled TES bolometers for the Keck Array, Spider, and Polar-1

Between the BICEP2 and Keck Array experiments, we have deployed over 1500 dual polarized antenna coupled bolometers to map the Cosmic Microwave Background's polarization. We have been able to rapidly deploy these detectors because they are completely planar with an integrated phased-array antenna. Through our experience in these experiments, we have learned of several challenges with this technology- specifically the beam synthesis in the antenna- and in this paper we report on how we have modified our designs to mitigate these challenges. In particular, we discus differential steering errors between the polarization pairs' beam centroids due to microstrip cross talk and gradients of penetration depth in the niobium thin films of our millimeter wave circuits. We also discuss how we have suppressed side lobe response with a Gaussian taper of our antenna illumination pattern. These improvements will be used in Spider, Polar-1, and this season's retrofit of Keck Array.

astro-ph.IM↗

arXiv2012

BICEP2 and Keck Array operational overview and status of observations

The BICEP2 and Keck Array experiments are designed to measure the polarization of the cosmic microwave background (CMB) on angular scales of 2-4 degrees (l=50-100). This is the region in which the B-mode signal, a signature prediction of cosmic inflation, is expected to peak. BICEP2 was deployed to the South Pole at the end of 2009 and is in the middle of its third year of observing with 500 polarization-sensitive detectors at 150 GHz. The Keck Array was deployed to the South Pole at the end of 2010, initially with three receivers--each similar to BICEP2. An additional two receivers have been added during the 2011-12 summer. We give an overview of the two experiments, report on substantial gains in the sensitivity of the two experiments after post-deployment optimization, and show preliminary maps of CMB polarization from BICEP2.

astro-ph.IM↗

arXiv2012

Optical Characterization of the Keck Array Polarimeter at the South Pole

The Keck Array (SPUD) is a set of microwave polarimeters that observes from the South Pole at degree angular scales in search of a signature of Inflation imprinted as B-mode polarization in the Cosmic Microwave Background (CMB). The first three Keck Array receivers were deployed during the 2010-2011 Austral summer, followed by two new receivers in the 2011-2012 summer season, completing the full five-receiver array. All five receivers are currently observing at 150 GHz. The Keck Array employs the field-proven BICEP/BICEP2 strategy of using small, cold, on-axis refractive optics, providing excellent control of systematics while maintaining a large field of view. This design allows for full characterization of far-field optical performance using microwave sources on the ground. We describe our efforts to characterize the main beam shape and beam shape mismatch between co-located orthogonally-polarized detector pairs, and discuss the implications of measured differential beam parameters on temperature to polarization leakage in CMB analysis.

astro-ph.IM↗

arXiv2012

Optimization and sensitivity of the Keck Array

The Keck Array (SPUD) began observing the cosmic microwave background's polarization in the winter of 2011 at the South Pole. The Keck Array follows the success of the predecessor experiments Bicep and Bicep2, using five on-axis refracting telescopes. These have a combined imaging array of 2500 antenna-coupled TES bolometers read with a SQUID-based time domain multiplexing system. We will discuss the detector noise and the optimization of the readout. The achieved sensitivity of the Keck Array is 11.5 μK_(CMB)*sqrt{s} in the 2012 configuration.

astro-ph.IM↗

arXiv2012

Position and energy-resolved particle detection using phonon-mediated microwave kinetic inductance detectors

We demonstrate position and energy-resolved phonon-mediated detection of particle interactions in a silicon substrate instrumented with an array of microwave kinetic inductance detectors (MKIDs). The relative magnitude and delay of the signal received in each sensor allows the location of the interaction to be determined with < 1 mm precision at 30 keV. Using this position information, variations in the detector response with position can be removed, and an energy resolution of σ_E = 0.55 keV at 30 keV was measured. Since MKIDs can be fabricated from a single deposited film and are naturally multiplexed in the frequency domain, this technology can be extended to provide highly-pixelized athermal phonon sensors for ~1 kg scale detector elements. Such high-resolution, massive particle detectors would be applicable to next-generation rare-event searches such as the direct detection of dark matter, neutrinoless double-beta decay, or coherent neutrino-nucleus scattering.

astro-ph.IM↗

arXiv2011

Combined Limits on WIMPs from the CDMS and EDELWEISS Experiments

The CDMS and EDELWEISS collaborations have combined the results of their direct searches for dark matter using cryogenic germanium detectors. The total data set represents 614 kg.d equivalent exposure. A straightforward method of combination was chosen for its simplicity before data were exchanged between experiments. The results are interpreted in terms of limits on spin-independent WIMP-nucleon cross-section. For a WIMP mass of 90 GeV/c^2, where this analysis is most sensitive, a cross-section of 3.3 x 10^{-44} cm^2 is excluded at 90% CL. At higher WIMP masses, the combination improves the individual limits, by a factor 1.6 above 700 GeV/c^2. Alternative methods of combining the data provide stronger constraints for some ranges of WIMP masses and weaker constraints for others.

astro-ph.CO↗

arXiv2011

Thermal architecture for the SPIDER flight cryostat

We describe the cryogenic system for SPIDER, a balloon-borne microwave polarimeter that will map 8% of the sky with degree-scale angular resolution. The system consists of a 1284 L liquid helium cryostat and a 16 L capillary-filled superfluid helium tank, which provide base operating temperatures of 4 K and 1.5 K, respectively. Closed-cycle helium-3 adsorption refrigerators supply sub-Kelvin cooling power to multiple focal planes, which are housed in monochromatic telescope inserts. The main helium tank is suspended inside the vacuum vessel with thermally insulating fiberglass flexures, and shielded from thermal radiation by a combination of two vapor cooled shields and multi-layer insulation. This system allows for an extremely low instrumental background and a hold time in excess of 25 days. The total mass of the cryogenic system, including cryogens, is approximately 1000 kg. This enables conventional long duration balloon flights. We will discuss the design, thermal analysis, and qualification of the cryogenic system.

astro-ph.IM↗

arXiv2011

Search for inelastic dark matter with the CDMS II experiment

Results are presented from a reanalysis of the entire five-tower data set acquired with the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory, with an exposure of 969 kg-days. The analysis window was extended to a recoil energy of 150 keV, and an improved surface-event background-rejection cut was defined to increase the sensitivity of the experiment to the inelastic dark matter (iDM) model. Three dark matter candidates were found between 25 keV and 150 keV. The probability to observe three or more background events in this energy range is 11%. Because of the occurrence of these events the constraints on the iDM parameter space are slightly less stringent than those from our previous analysis, which used an energy window of 10-100 keV.

astro-ph.CO↗