Background Considerations
To maximize the benefit of the various background rejection factors available
for the XENON experiment, the absolute count rate itself must be minimized.
This is a challenge for any WIMP experiment, as the possible sources of
background are numerous and of different origin. Here, we limit the discussion
to the dominant sources specific to a LXe experiment. Natural Xe has no
longlived isotopes. However, radioactive impurities in the gas, most notably
Kr-85, must be reduced by a large amount.
Gamma and Beta induced background
- Kr-85 and Radon
- Xe-136 double beta dacay
Neutrons are a major source of background because their nuclear recoils render them indistinguishable from WIMP events. Nuclear recoils in the 10 keV range arise from the elastic scattering of 0.1- 10 MeV neutrons on Xe. With an active anti-coincidence, neutrons recoiling both in the LXe shield and the TPC LXe target are effectively identified and rejected. Nevertheless, underground operation and a sufficient neutron shield to absorb or at least thermalize the environment neutrons are essential for a WIMP experiment. The main sources of neutrons are:
- Muon Induced Neutrons
- Neutrons from U/Th contamination in the detector and surrounding materials
Gamma-ray background from the same U/Th and K in most detector materials will
dominate the overall count rate. PMTs, in particular, are typically a copious
source of gamma-rays. The K/U/Th content is highest in the HV divider chain.
The baseline XENON LXeTPC uses special PMTs, stripped of the standard divider
chain and with selected compact metal envelopes and quartz windows. The
radioactivity is estimated at ~ 100 cts/d.
