arXiv subjects

A. Scarff

Publications and source records attributed to A. Scarff.

44 records · Page 3Linked to original sources

arXiv2017

Characterisation of Large Area THGEMs and Experimental Measurement of the Townsend Coefficients for CF$_4$

Whilst the performance of small THGEMs is well known, here we consider the challenges in scaling these up to large area charge readouts. We first verify the expected gain of larger THGEMs by reporting experimental Townsend coefficients for a 10 cm diameter THGEM in low-pressure CF$_4$. Large area 50 cm by 50 cm THGEMs were sourced from a commercial PCB supplier and geometrical imperfections were observed which we quantified using an optical camera setup. The large area THGEMs were experimentally characterised at Boulby Underground Laboratory through a series of gain calibrations and alpha spectrum measurements. ANSYS, Magboltz and Garfield++ simulations of the design of a TPC based on the large area THGEMs are presented. We also consider their implications for directional dark matter research and potential applications within nuclear security.

physics.ins-det↗

arXiv2017

Measurement of directional range components of nuclear recoil tracks in a fiducialised dark matter detector

We present results from the first measurement of axial range components of fiducialized neutron induced nuclear recoil tracks using the DRIFT directional dark matter detector. Nuclear recoil events are fiducialized in the DRIFT experiment using temporal charge carrier separations between different species of anions in 30:10:1 Torr of CS$_2$:CF$_4$:O$_2$ gas mixture. For this measurement, neutron-induced nuclear recoil tracks were generated by exposing the detector to $^{252}$Cf source from different directions. Using these events, the sensitivity of the detector to the expected axial directional signatures were investigated as the neutron source was moved from one detector axis to another. Results obtained from these measurements show clear sensitivity of the DRIFT detector to the axial directional signatures in this fiducialization gas mode.

physics.ins-det↗

arXiv2017

Low Threshold Results and Limits from the DRIFT Directional Dark Matter Detector

We present results from a 54.7 live-day shielded run of the DRIFT-IId detector, the world's most sensitive, directional, dark matter detector. Several improvements were made relative to our previous work including a lower threshold for detection, a more robust analysis and a tenfold improvement in our gamma rejection factor. After analysis, no events remain in our fiducial region leading to an exclusion curve for spin-dependent WIMP-proton interactions which reaches 0.28 pb at 100 GeV/c^2 a fourfold improvement on our previous work. We also present results from a 45.4 live-day unshielded run of the DRIFT-IId detector during which 14 nuclear recoil-like events were observed. We demonstrate that the observed nuclear recoil rate of 0.31+/-0.08 events per day is consistent with detection of ambient, fast neutrons emanating from the walls of the Boulby Underground Science Facility.

astro-ph.IM↗

arXiv2017

First search for a dark matter annual modulation signal with NaI(Tl) in the Southern Hemisphere by DM-Ice17

We present the first search for a dark matter annual modulation signal in the Southern Hemisphere conducted with NaI(Tl) detectors, performed by the DM-Ice17 experiment. Nuclear recoils from dark matter interactions are expected to yield an annually modulated signal independent of location within the Earth's hemispheres. DM-Ice17, the first step in the DM-Ice experimental program, consists of 17 kg of NaI(Tl) located at the South Pole under 2200 m.w.e. overburden of Antarctic glacial ice. Taken over 3.6 years for a total exposure of 60.8 kg yr, DM-Ice17 data are consistent with no modulation in the energy range of 4-20 keV, providing the strongest limits on weakly interacting massive particle dark matter from a direct detection experiment located in the Southern Hemisphere. The successful deployment and stable long-term operation of DM-Ice17 establishes the South Pole ice as a viable location for future dark matter searches and in particular for a high-sensitivity NaI(Tl) dark matter experiment to directly test the DAMA/LIBRA claim of the observation of dark matter.

physics.ins-det↗

arXiv2016

First measurement of nuclear recoil head-tail sense in a fiducialised WIMP dark matter detector

Recent computational results suggest that directional dark matter detectors have potential to probe for WIMP dark matter particles below the neutrino floor. The DRIFT-IId detector used in this work is a leading directional WIMP search time projection chamber detector. We report the first measurements of the detection of the directional nuclear recoils in a fully fiducialised low-pressure time projection chamber. In this new operational mode, the distance between each event vertex and the readout plane is determined by the measurement of minority carriers produced by adding a small amount of oxygen to the nominal CS$_{2}$ + CF$_{4}$ target gas mixture. The CS$_2$ + CF$_4$ + O$_2$ mixture has been shown to enable background-free operation at current sensitivities. Sulfur, fluorine, and carbon recoils were generated using neutrons emitted from a $^{252}$Cf source positioned at different locations around the detector. Measurement of the relative energy loss along the recoil tracks allowed the track vector sense, or the so-called head-tail asymmetry parameter, to be deduced. Results show that the previously reported observation of head-tail sensitivity in pure CS$_{2}$ is well retained after the addition of oxygen to the gas mixture.

physics.ins-det↗

arXiv2015

First background-free limit from a directional dark matter experiment: results from a fully fiducialised DRIFT detector

The addition of O2 to gas mixtures in time projection chambers containing CS2 has recently been shown to produce multiple negative ions that travel at slightly different velocities. This allows a measurement of the absolute position of ionising events in the z (drift) direction. In this work, we apply the z-fiducialisation technique to a directional dark matter search. In particular, we present results from a 46.3 live-day source-free exposure of the DRIFT-IId detector run in this completely new mode. With full-volume fiducialisation, we have achieved the first background-free operation of a directional detector. The resulting exclusion curve for spin-dependent WIMP-proton interactions reaches 1.1 pb at 100 GeV/c2, a factor of 2 better than our previous work. We describe the automated analysis used here, and argue that detector upgrades, implemented after the acquisition of these data, will bring an additional factor of >3 improvement in the near future.

hep-ex↗

arXiv2014

Radon in the DRIFT-II directional dark matter TPC: emanation, detection and mitigation

Radon gas emanating from materials is of interest in environmental science and also a major concern in rare event non-accelerator particle physics experiments such as dark matter and double beta decay searches, where it is a major source of background. Notable for dark matter experiments is the production of radon progeny recoils (RPRs), the low energy (~100 keV) recoils of radon daughter isotopes, which can mimic the signal expected from WIMP interactions. Presented here are results of measurements of radon emanation from detector materials in the 1 metre cubed DRIFT-II directional dark matter gas time projection chamber experiment. Construction and operation of a radon emanation facility for this work is described, along with an analysis to continuously monitor DRIFT data for the presence of internal 222Rn and 218Po. Applying this analysis to historical DRIFT data, we show how systematic substitution of detector materials for alternatives, selected by this device for low radon emanation, has resulted in a factor of ~10 reduction in internal radon rates. Levels are found to be consistent with the sum from separate radon emanation measurements of the internal materials and also with direct measurement using an attached alpha spectrometer. The current DRIFT detector, DRIFT-IId, is found to have sensitivity to 222Rn of 2.5 μBq/l with current analysis efficiency, potentially opening up DRIFT technology as a new tool for sensitive radon assay of materials.

physics.ins-det↗

arXiv2014

Long-term study of backgrounds in the DRIFT-II directional dark matter experiment

Low-pressure gas Time Projection Chambers being developed for directional dark matter searches offer a technology with strong particle identification capability combined with the potential to produce a definitive detection of Galactic Weakly Interacting Massive Particle (WIMP) dark matter. A source of events able to mimic genuine WIMP-induced nuclear recoil tracks arises in such experiments from the decay of radon gas inside the vacuum vessel. The recoils that result from associated daughter nuclei are termed Radon Progeny Recoils (RPRs). We present here experimental data from a long-term study using the DRIFT-II directional dark matter experiment at the Boulby Underground Laboratory of the RPRs, and other backgrounds that are revealed by relaxing the normal cuts that are applied to WIMP search data. By detailed examination of event classes in both spatial and time coordinates using 5.5 years of data, we demonstrate the ability to determine the origin of 4 specific background populations and describe development of new technology and mitigation strategies to suppress them.

physics.ins-det↗