In such a collision a WIMP transfers part of its energy to a nucleus. Such nuclear recoils induced by WIMPs are the signals dark matter searches look for. Since these events are very rare, experiments have to fight against the background of natural radioactivity and cosmic rays. This is the reason why dark matter searches are usually in laboratories deep underground, as the L.N.G.S., and heavily shielded against radioactivity. These searches are conducted by two classes of detectors, conventional and cryogenic.
Conventional detectors measure the ionization or scintillation caused by a particle.
But this reveals only a small fraction of the energy of a nuclear recoil. They consist of an absorber crystal and a thermometer on it. A particle interacting in the absorber deposits some energy which will, after a thermalization process, be converted into heat and result in a temperature rise. The temperature rise of the thermometer can be read-out electronically and is proportional to the energy deposited. The maximum achievable sensitivity depends strongly on the amount of radioactive background.