Speaker
Mr
Alex Lindote
(University of Coimbra)
Description
ZEPLIN III is a xenon detector for direct dark matter searches soon
to be deployed underground at the Boulby mine (North Yorkshire, UK).
This two-phase (liquid/gas) system will look for the rare nuclear
recoils that should be produced by elastic scattering of Weakly
Interacting Massive Particles (WIMPs) off xenon atoms.
Neutron interactions can also cause nuclear recoils and therefore
constitute an irreducible background in this type of detector. This
is mitigated by the use of radio-pure construction materials, by
surrounding the detector with extensive hydrocarbon shielding and by
deploying the system deep underground, where a large rock overburden
protects it from the effects of cosmic rays. Another key challenge
is to identify the very rare nuclear recoil events due to WIMPs or
neutrons in a background of beta and gamma-rays from residual
internal and external radioactivity. These electron-recoil events
are some ~106 times more abundant and must be effectively
discriminated from nuclear recoils.
ZEPLIN III measures both the scintillation and ionisation signals
produced in liquid xenon by the interacting particle. The prompt
scintillation is measured by an array of 31 photomultipliers
immersed in the liquid xenon. A strong electric field also extracts
the ionisation from the interaction site, which drifts up to the
liquid surface and is extracted into the gas phase. A large number
of electroluminescence photons is produced in the gas and detected
by same photomultiplier array. A sensitivity down to one electron
extracted from the liquid is thus easily achieved. The ratio of the
two response channels is different for electron and nuclear recoils,
allowing effective discrimination between the two types of
interaction down to ~keV deposited energies (electron-equivalent).
In order to achieve the excellent discrimination ability required to
fully separate the electron and nuclear-recoil populations, a very
uniform response is necessary for both channels across the active
volume. In most detectors, this fiducialisation is achieved by
physically delimiting the active region. In ZEPLIN III, this is
accomplished by reconstructing the 3-dimensional interaction point
and rejecting events occurring in out-lying regions, where the light
collection and the electric field may not be uniform. We will
describe the position reconstruction methods to be used in ZEPLIN
III and how we hope to achieve a positional accuracy of a few
millimetres in the horizontal plane and sub-millimetre in the
vertical coordinate.
Primary author
Mr
Alex Lindote
(University of Coimbra)
