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 0.28pb at 100GeV/c2, 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.
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. We present results from a 46.3 live-day source-free exposure of the DRIFT-IId detector run in this 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.
► DRIFT is only one of a handful of directional dark matter detectors. ► A method of rejecting Radon Progeny Recoils, DRIFT’s main background, is discussed. ► Additionally DRIFT-IId was found to run ...stably with a mixture of CS2 and CF4. ► DRIFT’s sensitivity is orders of magnitude better than other directional detectors.
Data are presented from the DRIFT-IId detector operated in the Boulby Underground Science Facility in England. A 0.8m3 fiducial volume, containing partial pressures of 30Torr CS2 and 10Torr CF4, was exposed for a duration of 47.4 live-time days with sufficient passive shielding to provide a neutron free environment within the detector. The nuclear recoil events seen are consistent with a remaining low-level background from the decay of radon daughters attached to the central cathode of the detector. However, charge from such events must drift across the entire width of the detector, and thus display large diffusion upon reaching the readout planes of the device. Exploiting this feature, it is shown to be possible to reject energy depositions from these Radon Progeny Recoil events while still retaining sensitivity to fiducial-volume nuclear recoil events. The response of the detector is then interpreted, using the F nuclei content of the gas, in terms of sensitivity to proton spin-dependent WIMP–nucleon interactions, displaying a minimum in sensitivity cross section at 1.8pb for a WIMP mass of 100GeV/c2. This sensitivity was achieved without compromising the direction sensitivity of DRIFT.
Reducing DRIFT backgrounds with a submicron aluminized-mylar cathode Battat, J.B.R.; Daw, E.; Dorofeev, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2015, Letnik:
794
Journal Article
Recenzirano
Odprti dostop
Background events in the DRIFT-IId dark matter detector, mimicking potential WIMP signals, are predominantly caused by alpha decays on the central cathode in which the alpha particle is completely or ...partially absorbed by the cathode material. We installed a 0.9μm thick aluminized-mylar cathode as a way to reduce the probability of producing these backgrounds. We study three generations of cathode (wire, thin-film, and radiologically clean thin-film) with a focus on the ratio of background events to alpha decays. Two independent methods of measuring the absolute alpha decay rate are used to ensure an accurate result, and agree to within 10%. Using alpha range spectroscopy, we measure the radiologically cleanest cathode version to have a contamination of 3.3±0.1 ppt 234U and 73±2 ppb 238U. This cathode reduces the probability of producing an RPR from an alpha decay by a factor of 70±20 compared to the original stainless steel wire cathode. First results are presented from a texturized version of the cathode, intended to be even more transparent to alpha particles. These efforts, along with other background reduction measures, have resulted in a drop in the observed background rate from 500/day to 1/day. With the recent implementation of full-volume fiducialization, these remaining background events are identified, allowing for background-free operation.
PDF
