The last decade has seen the discovery that neutrinos are massive. Large experiments, housed deep underground, have unmistakably observed neutrinos oscillate from one flavour to another as they travelled through the atmosphere and from the suns core, indicating that their mass eigenstates could not all be vanishing. Detectors such as SuperKamiokaNDE and SNO were able to detect and identify a signal of just a few events per day. The detection of solar neutrinos especially, requires extremely low levels of radioactive contamination. Current and future experiments for neutrino, double beta decay, and WIMP dark matter detection are pushing the frontier of low background detectors even further.
I will illustrate results and expectations for two such experiments: Borexino and EXO. Borexino, installed at the Gran Sasso laboratory in Italy, has recently made the first real time observation of 7Be solar neutrinos, demonstrating unprecedented levels of radio-purity. It will attempt at measuring most of the low-energy part of the solar neutrino spectrum in the next few years. Such measurement will test the current neutrino oscillations solution while shedding light on some important aspects of solar physics.
EXO will search for the neutrinoless double beta decay (0nbb) of 136Xe. If observed, this decay would prove that neutrinos are their own antiparticle and provide the most sensitive means to measure their absolute mass in the laboratory. Only a lower bound on neutrino mass can be set by oscillation experiments. In its final configuration, EXO is planning on detecting the appearance of 136Ba ions following double beta decays of 136Xe, drastically reducing the impact of radioactive background on the measurement. The first phase of the experiment, EXO-200 at the WIPP mine in New Mexico, employs 200 kg of enriched liquid xenon (80% 136Xe) in an ultra-low background time projection chamber with scintillation light readout. EXO-200 will be completed in 2008 and is designed to reach a half-life sensitivity for 136Xe
0nbb of 6 x 1025 years, equivalent to an effective neutrino mass sensitivity of 150 meV.