Collaborators: UKDMC Rutherford Appleton Laboratory, University of Sheffield, Imperial College Londo

1
CollaboratorsUKDMC (Rutherford Appleton
Laboratory, University of Sheffield, Imperial
College London),Occidental College (Los
Angeles), Temple University (Philadelphia)
Directional Recoil Identification From Tracks

Introduction
DRIFT I Results
It is well known that we do not know what 90 of
our galaxy is composed of. There are many pieces
of observational evidence for our galaxy
containing dark matter. Answering the question
what is the nature of the dark matter in our
galaxy? is one of the most important challenges
in physics of our time.
With the DRIFT I detector installed it was
decided to run an engineering and research
development phase before beginning the intended
dark matter runs. This preparatory phase was
essential to understand the detectors
characteristics and responses to raw backgrounds
underground, as well as addressing operational
questions.
There are several different theories as to what
characteristics this unusual new matter may
possess, but the exact nature of dark matter is
still elusive and will continue to be disputed
until it is determined by experiment.
Surface view of Boulby mine, run by Cleveland
Potash Ltd.
Reducing Background Triggers Examining the gamma
rejection capability in DRIFT I has exhibited
such a high efficiency that it is entirely
unnecessary to add any passive gamma shielding to
the detector. This almost entire elimination of
the major gamma background is a first for any
fully recoil sensitive WIMP detector a great
progression for dark matter detection technology.
One major candidate is the Weakly Interacting
Massive Particle (WIMP), which is predicted to be
a high mass (10 1000 GeV), neutral particle
that interacts only very weakly with other
matter. It is these exotic WIMPs that the DRIFT
project hopes to discover.
It is also possible to eliminate any remaining
gamma-induced triggers by raising the threshold
voltage on the MWPC wires. This has the effect
of preventing the small voltage pulses caused by
electrons (from gammas) from triggering the wires
whilst the higher voltage pulses from nuclear
recoils do trigger them.
WIMPs
It is believed that the Milky Way, along with all
other galaxies, is surrounded by an enormous halo
of dark matter WIMPs that, at least for our
galaxys case, account for 90 of its mass.
The Earth and the entire solar system move
through this halo and so, it is predicted, must
feel a WIMP wind from the direction in which
the solar system travels around the Milky
Way. Due to the Earths motion in space, at any
point on the Earth fluctuations in the WIMP wind
should be observed. During each day the
direction from which the incoming WIMPs are
expected should change as the Earth rotates.
a) Monte carlo results showing expected
distribution
b) DRIFT neutron data from a Cf252 source.
c) Plot from a dark matter run on DRIFT I (1
day).
Neutron calibration background runs in DRIFT I
Employing the higher wire threshold to optimise
gamma rejection meant that a competent neutron
calibration run could be successfully performed
and the detectors response recorded and
analysed. The results confirmed that no passive
gamma shielding is required for DRIFT I.
Visualisation of the changing flux of WIMPs
impacting on the Earth
Also, the flux of WIMPs on the Earth should vary
throughout the year as the Earths orbit takes it
around the Sun. These daily and annual
fluctuations could provide a simple way to prove
that any directional detection is, or is not, the
dark matter searched for. Predictions of the
forward-backward ratios are more than 41 at low
energies and over 201 above 100keV.
With the knowledge that gamma backgrounds can be
disregarded lower flux backgrounds are
consequently more significant. Other backgrounds
to be considered for DRIFT include alphas and
noise. Underground tests have, however, shown
that these too can be efficiently rejected,
leaving only rock neutron events, which
themselves can be removed effectively by using CH
shielding.
A typical neutron event from DRIFT I.
? Signal recorded on all hit wires.
The direction from which WIMPs are expected
varies throughout the day.
A detector sensitive to this ratio, a
directionally sensitive detector, has the
potential to identify any dark matter detection
with very high confidence.
2-dimensional ? reconstruction of track.
Detecting WIMPs
The DRIFT project attempts to detect nuclear
recoils produced from collisions with incoming
WIMPs. The number of events expected is as low
as 0.01 to 1 per kg per day. This is due to the
very low rate of WIMP interactions. The low
event rate means that the collaboration has had
to consider carefully the design of detector that
will be most effective.
The engineering and RD phase has led to new
operational procedures for the running of DRIFT
I, particularly as regards safety and reliability
of this and future detectors, which is vital when
working within the mine facilities.
Along with remote monitoring, automated data
collection and new safety procedures, it has also
been possible to achieve the automation of
MWPC/chamber gain and sensitivity calibration as
well as verifying the directional sensitivity to
neutrons.
Two typical 55Fe x-ray ? calibrations of DRIFT I
showing the 5.9keV spectrum taken two months
apart.
It was decided that a detector employing a Time
Projection Chamber (TPC) containing a low
pressure gas was the most suitable to use as a
directional detector.
Schematic of the DRIFT technique.
It is essential to reduce the background as much
as possible to enhance the resulting data and
TPCs have a remarkable capability of doing this.
? Raw test of directional sensitivity of neutrons
DRIFT I
Simulations
The first full scale DRIFT detector, DRIFT I, was
introduced underground in the summer of 2001. It
has been installed 1.1km below the Earths
surface in
It is important for the running of any detector
to have a computer simulation in order to
understand the detector and gain useful
information about relevant backgrounds,
distributions and possible problems.
Boulby mine, North Yorkshire. The reason for
positioning the detector so far below the ground
is to reduce the number of background events due
to cosmic rays.
Various simulations of the DRIFT I detector are
already available and can produce useful data for
the project. The most recent is a very promising
monte carlo made using
Image of the DRIFT I detector in the underground
lab using Geant4 and OpenGL
Geant4, a powerful toolkit for computer
simulation of particles, physics processes and
interactions in detectors.
Running DRIFT I.
Geant4 simulation of DRIFT I.
Cosmic rays can be stopped be the large volume of
rock above the detector while WIMPs, which rarely
interact with matter, can penetrate through the
rock and into the cavern where DRIFT I is
situated.
At present the monte carlo is still being
developed and
improved, but a basic simulation of DRIFT I in
the underground lab is up and running and
producing some useful data on neutron energy
distributions passing into the detector from the
surrounding rock as well as recoil spectra.
Preliminary spectrum of nuclear recoils.
An impression of the Boulby mine facility.
The detector itself consists of two field cages
back to back on a common cathode plane placed
within a vacuum vessel filled with carbon
disulfide (CS2) gas at low pressure (40 torr).
The field cages drift electronegative ions
(produced by interactions in the gas) away from
the central cathode and towards the readout
planes at the top and bottom of the vessel.
Preliminary energy distribution of neutrons
entering the detector
Future
Indications so far show that DRIFT I is on
target to reach its operational goals and
sensitivity predictions as expected.
The next stage of the DRIFT project is currently
undergoing planning. It has been proposed that
DRIFT II have an increased volume and gas
pressure to that of DRIFT I to improve
sensitivity. This higher sensitivity, however,
means that an improved readout device is also
required and it is on this that the research and
development of the DRIFT project is focussed at
present.
The DRIFT I inner detector.
The track data is read out using Multi-Wire
Proportional Chambers (MWPCs) consisting of two
grid planes of wires sandwiching an anode plane
of 512 wires at 2mm pitch. All the materials
used in and around the detector are radio-pure or
screened by shielding around the detector to
reduce the number of unwanted background events.
Sensitivity predictions for DRIFT I, II and III.
Poster produced by J. C. Davies on behalf of the
DRIFT collaborators. Background picture Arial
view of the Cleveland Potash mine at Boulby,
North Yorkshire