CDMS and SuperCDMS Experiments

CDMS

The Cryogenic Dark Matter Search (CDMS) experiment uses cryogenic germanium detectors to search for direct interactions of WIMPs with ordinary matter. In the previous section we have learned that the biggest challenge is the low interaction rate in connection with the high rate of background events. We have also learned that we should look for nuclear recoils from WIMP interactions, while most interactions from background radiation lead to electron recoils.

The CDMS detectors master the challenge of background discrimination by detecting two distinct signals for each interaction. The detectors are operated at a temperature of ~40 mK (-273.11°C = -459.6 F or 4/100 of a degree (C or K) above the coldest possible temperature). At this temperature a WIMP interaction would cause a measurable (even though tiny) increase in temperature. The thermal signal from an interaction is used in CDMS to determine the energy deposited.

A particle interaction in a semiconductor (such as Ge or Si) produces a charge signal if a voltage is applied. The magnitude of this signal depends both on the energy deposition and on the type of interaction: an electron recoils produces a ~3 times a large a signal as a nuclear recoil of the same energy. CDMS uses the ratio of these two signals as primary discrimination parameter between signal and background.
CDMS detector. The pattern on the surface is the phonon sensor, separated in four individual sensors which allows us to located where in the detector each event happened.


Electrical resistance as function of temperature for a superconductor around the critical temperature. The large change in resistance for a small change in temperature makes this a very sensitvie thermal sensor.

The CDMS detectors are germanium disks with a diameter of 3 inches and a thickness of 1 cm (~250 g each). The thermal sensor is a superconducting thin film at the crystal surface (a superconductor is a material which loses its electrical resistance below a certain critical temperature). At the critical temperature the electrical resistance shows a very strong dependence on the temperature. The resistance is constantly monitored and a sudden change indicates a particle interaction.

On a microscopic level, the thermal energy in a crystal is nothing else than the vibration of the individual atoms about their average position. The vibrations can mathematically be described as small energy packages that move through the crystal. These packages are called phonons. Therefore the thermal sensors are called phonon sensors. Each detector is equipped with four individual phonon sensors which each covers one quadrant of one of the flat surfaces of the germanium crystal.


The back side of the detector is covered with a thin aluminum film through which the voltage is applied which is needed to readout the charge signal. The detectors are put together in stacks of six called tower. The towers are equipped with amplifiers for the signals and mounted in the innermost of a set of copper cans (ice box). The cans are connected via a cold finger to the different temperature stages of a dilution refrigerator which provides the low temperature needed to operate the detectors. The ice box is surrounded by lead and polyethylene shielding against radioactive radiation from the immediate environment. The whole setup is placed in the Soudan Underground Laboratory to reduce the background from cosmic radiation.


View into the icebox with five towers installed.
 		From 2006 to 2009 CDMS operated a total of 30 detectors (5 towers) at the Soudan Underground Laboratory. For most of its lifetime CDMS provided the the best sensitivity for spin independent interaction of WIMPs with atomic nuclei compared to all other experiments.


Tower with 6 detectors.

SuperCDMS

So far no WIMP interaction has been observed, so the sensitivity needs to be improved further. This will be achieved by increasing the total detector mass (and with this the probability that a WIMP interacts in the detector) and at the same time reducing the background and improving the discrimination power. This effort started in 2009 under the name SuperCDMS.


New SuperCDMS detector with larger mass and improved sensor

The basic idea of the experiment is identical to the CDMS experiment. The main differences are an increased mass for the individual detectors (the new modules are 2.54 cm thick instead of 1 cm) and an improved sensor design which leads to an even better discrimination between a potential WIMP signal and background radiation.

Two different detector prototypes were tested in the experimental setup at Soudan in 2009 to 2011. The one with the superior performance takes advantage of a new geometry for the electrodes, which are now interleaved with the thermal sensors. Also the new detectors have thermal sensors on both sides of the crystal. This doubles the number of readout channels per detector and since we presently cannot increase the channel count overall, the number of detectors has to be reduced. Since fall 2011, 15 new detectors with a total mass of roughly 10 kg are operating at Soudan.


First SuperTower

However, this is only a first step: the next phase of SuperCDMS aims at a target mass of 200 kg. This will increase the sensitivity by another order of magnitude, which has as one consequence that the experiment would become sensitive to the small fraction of cosmic radiation which makes it down through 700 m of rock into the Soudan underground laboratory. Therefore it is planned to install this next phase of SuperCDMS in the much deeper SNOLAB near Sudbury ON (Canada).

To take full advantage of the larger mass, not only the cosmic radiation needs to be suppressed further, but also the requirements for discrimination power of the detectors against background from radioactivity are much higher. In fact, the requirements are so strict that it becomes very difficult if not impossible to demonstrate the performance of these detectors in an ordinary laboratory on the earth surface. Therefore it is planned to install a detector testing facility at SNOLAB where the discrimination power of the detectors will be demonstrated before they are implemented into the experiment.