J. D. Lewin

New detectors for neutrinos and dark matter

Hypothetical particles in the GeV mass range and with typical Galactic velocity 10-3 c would have MeV range momentum, similar to that of solar or supernova neutrinos. Thus elastic collisions with target nuclei would in each case give keV range nuclear recoils. There would also be a cross-section enhancement arising from full or partial coherence over the constituent nucleons. Detectors for these low energy nuclear recoils can be based on ionization, scintillation or low temperature phonon techniques. Radioactive background in the detector materials provides the main obstacle to detecting low event rates and significant effort is now being made to develop more advanced ideas which will distinguish the nuclear recoil events from background. Examples are simultaneous measurement of ionization and phonon energy in semiconductors, and photon timing or wavelength filtering in scintillators. Several groups are actively constructing underground dark matter detectors with targets in the 1-100 kg range. Solar and supernova neutrino detectors based on coherent scattering would have much lower target masses (by factors 20—100) than conventional detectors but would still require a substantial scale-up of these new techniques. Experiments with reactor neutrinos will provide a first step in verifying coherent neutrino scattering. Further scale-up to allow extra-galactic neutrino detection is feasible in principle and a possible challenge for the 21st century. Macroscopic coherent detection of the relic neutrino background may also become possible with foreseeable new technology.