CDMS II Detectors

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CDMSII detectors are designed to detect the minute phonon and ionization signals generated within a detector crystal by elastic collisions between detector nuclei and WIMPs. The energy deposited in a detector by an incident WIMP through the weak interaction may be as low as a few tens of keV, and event detection at such energy levels requires a sensitive experimental apparatus. The foremost requirement is that the detector be maintained at a very low temperature to distinguish the deposited energy from the thermal energy of the detector's nuclei. The CDMSII project and associated test facilities employ helium3-helium4 dilution refrigerator techniques which, with the appropriate cryostat apparatus, are able to achieve detector base temperatures as low as 10mK.

The detectors themselves, known as ZIP detectors, feature state-of-the-art thin film superconducting technology. Each 250g germanium or 100g silicon crystal provides two sets of information about interactions with incident particles. On one surface of the detectors are charge-collection plates which record the amount of electrical charge displaced within the detector's body by the incident particle. On the opposite detector surface is an array of tiny superconducting transition edge sensors (TES) consisting of micro strips of tungsten coupled to aluminum "fins" which collect phonon energy from the crystal.

Phonon Sensors

The phonon sensor array works as follows. An incident particle, perhaps a WIMP, collides with a nucleus in the detector generating vibrations in its crystal lattice. These vibrations are called "phonons". These phonons propagate through the crystal and some reach the surface. There, they are absorbed by the aluminum collector fins. In the aluminum, the phonons convert their energy into "quasi-particles", which are basically just electrons which had been in a superconducting "Cooper pair". The incident phonon energy breaks these Cooper pairs and gives energy to the electrons. These "quasi-particle" electrons migrate (or "diffuse") to the tiny strip of tungsten which is attached to each aluminum fin. This is the important step. The tungsten strips are "biased" with some electrical energy already which pushes them right near the brink of going through a transition from being a superconductor to being "normal". When the tungsten strips receive the energy from the "quasi-particles" which were made in the aluminum by the phonons, they go through the transition. This means their electrical resistance changes dramatically with the addition of a very small amount of energy (funneled to it from the aluminum). The tungsten strips are thus called "transition edge sensors" since we exploit their transition from superconducting to normal as a way to sense a small input of energy. This change in electrical resistance caused by the transition is amplified first by a SQUID circuit down within the cryostat itself and then by a sophisticated series of amplifiers at room temperature. This amplified change in resistance makes the "pulse" which we observe.

The tungsten TES array (and accompanying aluminum fins) on phonon-detecting side of the detector is divided into 4 "channels" (like 4 slices of pie), each containing over 1000 tungsten sensors.

Ionization Sensors

The charge sensor is conceptually simpler. An incident particle interaction disrupts the crystal band structure, generating free electrons and holes. A small electric field (3 V/cm in Ge) is applied across the crystal to drift the charge carriers to the crystal faces, where they are measured by a sensitive JFET charge amplifier. The ground for the charge bias is provided by the phonon sensors, while the bias voltage is applied from two concentric electrodes on the charge-amplifier face.

Discrimination

Particle events come in two main classes: electron recoils and nuclear recoils. Photons, electrons and alpha particles (and thus nearly all radioactive backgrounds) generate the former, while WIMP and neutron interactions generate the latter. CDMS thus seeks to reject background events by discriminating between electron and nuclear recoils on an event-by-event basis.

The ratio of charge and phonon signals depends on the class of particle interaction. Nuclear recoils generate only one-third as much ionization as electron recoils of equivalent energy.ZIP detectors can thus use the ratio of these two signals to discriminate signal from background at very high accuracy. Further discrimination against surface electron recoils (which may have reduced charge collection) is achieved using the detailed shape of the phonon pulse.


Last updated April 28, 2007