13 research outputs found
Quenching Factor for Low Energy Nuclear Recoils in a Plastic Scintillator
Plastic scintillators are widely used in industry, medicine and scientific
research, including nuclear and particle physics. Although one of their most
common applications is in neutron detection, experimental data on their
response to low-energy nuclear recoils are scarce. Here, the relative
scintillation efficiency for neutron-induced nuclear recoils in a
polystyrene-based plastic scintillator (UPS-923A) is presented, exploring
recoil energies between 125 keV and 850 keV. Monte Carlo simulations,
incorporating light collection efficiency and energy resolution effects, are
used to generate neutron scattering spectra which are matched to observed
distributions of scintillation signals to parameterise the energy-dependent
quenching factor. At energies above 300 keV the dependence is reasonably
described using the semi-empirical formulation of Birks and a kB factor of
(0.014+/-0.002) g/MeVcm^2 has been determined. Below that energy the measured
quenching factor falls more steeply than predicted by the Birks formalism.Comment: 8 pages, 9 figure
The ZEPLIN-III anti-coincidence veto detector
The design, optimisation and construction of an anti-coincidence veto
detector to complement the ZEPLIN-III direct dark matter search instrument is
described. One tonne of plastic scintillator is arranged into 52 bars
individually read out by photomultipliers and coupled to a gadolinium-loaded
passive polypropylene shield. Particular attention has been paid to
radiological content. The overall aim has been to achieve a veto detector of
low threshold and high efficiency without the creation of additional background
in ZEPLIN-III, all at a reasonable cost. Extensive experimental measurements of
the components have been made, including radioactivity levels and performance
characteristics. These have been used to inform a complete end-to-end Monte
Carlo simulation that has then been used to calculate the expected performance
of the new instrument, both operating alone and as an anti-coincidence detector
for ZEPLIN-III. The veto device will be capable of rejecting over 65% of
coincident nuclear recoil events from neutron background in the energy range of
interest in ZEPLIN-III. This will reduce the background in ZEPLIN-III from ~0.4
to ~0.14 events per year in the WIMP acceptance region, a significant factor in
the event of a non-zero observation. Furthermore, in addition to providing
valuable diagnostic capabilities, the veto is capable of tagging over 15% for
gamma-ray rejection, all whilst contributing no significant additional
background. In conjunction with the replacement of the internal ZEPLIN-III
photomultiplier array, the new veto is expected to improve significantly the
sensitivity of the ZEPLIN-III instrument to dark matter, allowing spin
independent WIMP-nucleon cross sections below 1E-8 pb to be probed