The DRIFT Dark Matter Project: Directionality, Sensitivity, and Environmental Backgrounds
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Date
2008Author
Plank, Steven J S
Metadata
Abstract
It is now largely accepted that dark matter, and more specifically, Weakly Interacting
Massive Particles (WIMPs), constitute the majority of the mass in our
Universe. Within this thesis are presented: (i) an overview of the motivation
and evidence for the existence of dark matter; (ii) a detailed discussion of direct
detection techniques and a worldwide review of WIMP search experiments;
and (iii) new experimental measurements and complementary detailed numerical
simulations, carried out by the author, to determine the performance of DRIFT
experimental technology. Collectively, this work explores the capability of DRIFT
technology to detect dark matter, and in doing so, to resolve one of the key open
questions of contemporary science.
The DRIFT programme consists of an array of direct dark matter search
detectors located in the Boulby mine. An important limitation to the experiment
is the neutron and gamma-ray background. Experimental work presented
here has determined the U and Th content of the cavern rock to be 66±6 ppb
and 145±13 ppb respectively, clarifying ambiguities in previous estimations.
Through the use of a Monte Carlo simulation the neutron and gamma-ray background
experienced by DRIFT has been determined and the experimental implications
assessed. In addition, the activity of the main neutron calibration
source used to calibrate DRIFT modules has been measured and was found to be
11600 n s−1±5% on the date of exposure, resolving an earlier discrepancy.
Analysis of experimental data has confirmed that the technology employed by
DRIFT detectors has the capability to provide directional information of recoiling
nuclei at the low energies of interest to dark matter searches. A Monte Carlo
simulation has then been employed to determine the WIMP-nucleon sensitivity
achievable using DRIFT detectors of the present performance, also examining
what would be achievable if this was supplemented by a realistic active neutron
veto detector. It is found that a CS2-filled DRIFT type detector running at
a 500 NIP threshold ( 16 keV and 27 keV for C and S recoils respectively)
for 300 kg years, and surrounded by the proposed veto scheme, would expect to
observe a background of six un-vetoed events. The minimum positive signal above
this background (90% C.L.) would correspond to a WIMP-nucleon sensitivity
limit of 1.75×10−9 pb. This identifies the realistic limit of what can be achieved
using gaseous CS2 as a target medium. An investigation into the limits achievable
using a similar array in which DRIFT modules act as self-vetoing detectors is
also examined providing insight into the future development and operation of the
DRIFT programme.