The aim of a direct dark matter search is to detect the elastic scattering of WIMPs by atomic nuclei. This is done by measuring the energy deposited in an absorber material by a recoiling nucleus, either as light (in scintillation detectors); as ionization (in semiconductor detectors); or as phonons (in cryogenic detectors). We also need to be able to discriminate between nuclear recoils events (caused by WIMPs) and electron recoil events (caused by β and γ particles); otherwise our signal would be swamped by the radioactive background. This can be done by measuring multiple signals; either phonon and light (used by CRESST), or phonons and ionization (used by EDELWEISS). Nuclear recoil events produce much less scintillation light and ionization than electron recoils.
The detector technology to be used by EURECA has yet to be decided. It is likely that during the early stages we will install more than one type of detectors, to assess the performance of different designs.
Why use cryogenic detectors?
Cryogenic detectors have a very low energy threshold, making them ideal for dark matter experiments where the expected energy spectrum is dominated by low energy events. They also have excellent energy resolution, and can be used to discriminate electron recoils and recoils on an event by event basis.
In the CRESST design of Cryogenic Phonon Scintillation Detectors (CPSDs) a particle interaction inside a 300g CaWO4 crystal creates a large number of phonons, which propagate throughout the crystal and become trapped in a thermometer deposited on the surface of the crystal; where they thermalise leading to a rise in the temperature. The thermometer consists of a tungsten film biased in the centre of its superconducting transition (at ~15mK) so a small temperature rise causes a large change in resistance. A second detector measures the scintillation light produced by the particle collision. This allows us to reject the vast majority of radioactive background. The tungsten thermometers are read out using SQUIDs.
The EDELWEISS detectors work by measuring the phonon and ionization signals (instead of phonon and scintillation signals as in CRESST). The detectors are 320 g germanium crystals with two electrodes to measure the ionization. The phonon signal is measured using thermometers of neutron-transmutation doped germanium, read out with FETs.