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Particle Physics and Astronomy Research Council
Swindon, U.K.

Embargoed until: 15.00hrs, Tuesday 29th April 2003

90% of the Universe is Missing!

Part 2 of 3

When a WIMP collides with the nucleus of an atom, it will knock it 
backwards and the recoil energy released by the atom can be detected
in one of three ways, depending on the detector material. Either
there will be a slight rise in temperature (phonon based detection),
or a slight electric charge is released (ionisation) or a photon of
light is released (scintillation). It is possible for more than one
of these effects to occur.

The UK Dark Matter Collaboration

(UKDMC) operates three different kinds of detector: NAIAD is a 
scintillation detector; DRIFT is an ionisation detector; and ZEPLIN
uses both methods.

As WIMPS so rarely collide with matter, it is important to screen out
as much background noise in the small signal produced by WIMP
collisions as possible. The UKDMC screens out particles from space
(such as cosmic rays) and from radioactive substances by installing
its detectors 1100 metres underground in Boulby salt and potash mine.
The salt mine has a low natural radioactivity and absorbs most of the
particles coming in from space. Encasing the detectors in lead or
copper 'castles' provides more protection, reducing the radiation by
a factor of a million. High purity materials are used in every stage
of constructing the detector and careful analysis is carried out on
all signals recorded to screen out those that are caused by other
particles, for example electron recoils caused by gamma rays.

NAIAD -- NaI (sodium iodide) Advanced Array Detector

NAIAD is an array of 8 sodium iodide capsules, which produce 
scintillation signals when a nucleus recoils.

The outcome of a neutron-nucleus interaction in NaI (sodium iodide)
is very similar to that expected from a WIMP-nucleus interaction, so
neutrons and gamma rays are used to calibrate the experiments so that 
scientists can distinguish possible WIMP signals from other sources
of signal. Electron recoils can be removed from the data by measuring
the decay time of the scintillation light, typically 30% slower than
that caused by a neutron or WIMP.

ZEPLIN I

The ZEPLIN programme (originally 'ZonEd Proportional scintillation in 
LIquid Noble gases') makes use of the scintillation properties of
liquid xenon.

Liquid xenon has a number of properties, which make it very suitable
for searching for dark matter particles:

* It is an efficient scintillator, emitting UV photons when
  nuclei recoil through the medium. This gives a low energy
  threshold and hence improved sensitivity to dark matter.

* It has a large quenching factor, meaning that much of the
  energy of recoiling nuclei is converted into observable
  forms, again improving the energy threshold.

* It contains heavy nuclei giving enhanced dark matter
  interaction rates and hence a better chance of seeing a
  signal.

* It can be purified by distillation to remove radioactive
  contaminants. This reduces the rate of background electron
  recoils, which could be confused for evidence of dark
  matter.

* It allows discrimination between nuclear recoil signals
  and background electron recoils. This very important as
  it allows us to discover rather than just set limits on
  dark matter particles.

ZEPLIN-I began operating underground at Boulby during 2001. The lead 
shielding eliminates most of the background pulses, which result from 
natural radioactivity and surviving cosmic-ray particles. The liquid 
scintillator veto is used to reject most of the remaining background 
(xenon pulses 'simultaneous' with pulses in the veto are ignored). As 
with the NaI detectors, ZEPLIN-I uses the different time-dependence
of scintillation pulses from nuclear recoils and those from
photon/electron scattering to discriminate against background that
eludes both shield and veto.

DRIFT -- Directional Recoil Identification from Tracks

DRIFT is the first experiment to be installed in the new JIF area of
the laboratory and is unique because its aim is not only to detect
WIMPS, but to also determine what direction they come from. The Earth
is subject to a steady stream of WIMPS from space as it moves through
the Galaxy, blowing from the direction of the motion. As the Earth
rotates on its axis, there should be a daily modulation of the signal
direction. DRIFT is the world's first experiment designed to look for
this modulation. 

What is JIF?
[http://www.wellcome.ac.uk/en/1/biosfgjifbacori.html]
 
Launched in 1998, the Joint Infrastructure Fund (JIF) is a 750
million partnership between the Wellcome Trust, the Office of Science
and Technology, and the Higher Education Funding Council for England.
The purpose of the JIF is to enhance and modernise the research
infrastructure of the UK University sector.

Joint Infrastructure Fund award -- £3,140,366

The aim is to transform the existing UK underground facilities for
dark matter and neutrino studies, thereby accelerating the present
world-class UK programme searching for new sub-atomic dark matter 
particles believed to constitute 90-99% of the Universe and boosting 
prospects for UK led international experiments to study fundamental 
properties of neutrinos.

The proposal involves: complete refurbishment of the existing UK deep 
site at Boulby mine, North Yorkshire, full upgrade of the main UK 
University laboratories and facilities involved in developing
technology for detectors for Boulby and upgrade of closely related
computational research programmes at the same institutes.

Overseas Collaborators

Occidental College
Prof. D. P. Snowden-Ifft
Physics Department, Occidental College, 1600 Campus Road, Los
Angeles, CA 90041, USA
Email:ifft{at}oxy.edu
Tel: 1-323-259-2793
Fax: 1-323-259-2704
Web: http://departments.oxy.edu/physics/dan.htm

 - Continued -

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