Transcript D R I F

Directional Recoil Identification From Tracks
Collaborators:
UKDMC (Rutherford Appleton Laboratory, University of Sheffield, Imperial College London),
Occidental College (Los Angeles), Temple University (Philadelphia)
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.
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.
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 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 galaxy’s case,
account for ~90% of it’s 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 Earth’s motion in space, at any point
on the Earth fluctuations in the WIMP wind should
be observed. During each day the direction from
Visualisation of the changing flux
which the incoming WIMPs are expected should
of WIMPs impacting on the Earth
change as the Earth rotates.
Also, the flux of WIMPs on the Earth should vary
throughout the year as the Earth’s 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
The direction from which WIMPs are
forward-backward ratios are more than 4:1 at low expected varies throughout the day.
energies and over 20:1 above 100keV.
A detector sensitive to this ratio, a directionally sensitive detector, has the
potential to identify any dark matter detection with very high confidence.
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.
It was decided that a detector employing a
Time Projection Chamber (TPC) containing
a low pressure gas was the most suitable to
Schematic of the DRIFT technique.
use as a directional detector.
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.
The first full scale DRIFT detector, DRIFT I, was introduced underground in
the summer of 2001. It has been installed 1.1km below the Earth’s surface in
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.
Running 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.
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
The DRIFT I ‘inner detector’.
of the vessel.
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.
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 detector’s
characteristics and responses to raw backgrounds
underground, as well as addressing operational questions.
Reducing Background Triggers
Surface view of Boulby
mine, run by Cleveland
Potash Ltd.
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.
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.
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
detector’s response recorded and analysed. The results confirmed that no
passive gamma shielding is required for DRIFT I.
With
the
knowledge
that
gamma
A typical neutron event from DRIFT I.
backgrounds can be disregarded lower flux
← Signal recorded
backgrounds are consequently more
on all hit wires.
significant.
Other backgrounds to be
considered for DRIFT include alphas and
noise. Underground tests have, however,
shown that these too can be efficiently
2-dimensional →
rejected, leaving only rock neutron events,
reconstruction of
which themselves can be removed
track.
effectively by using CH shielding.
The engineering and R&D 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.
← Raw test of directional
sensitivity of neutrons
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.
Various simulations of the DRIFT I detector
of the DRIFT I
are already available and can produce Image
detector in the
useful data for the project. The most recent underground lab using
is a very promising monte carlo made using Geant4 and OpenGL
Geant4, a powerful toolkit for computer simulation of particles,
physics processes and interactions in detectors.
Geant4 simulation
At present the monte carlo is still being developed and
of DRIFT I.
Preliminary spectrum
improved, but a basic simulation of
of nuclear recoils.
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 energy
distribution of neutrons
entering the detector
Indications so far show that DRIFT I is on target to reach it’s 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
Sensitivity predictions this that the research and development of the DRIFT project is
for DRIFT I, II and III. focussed at present.
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