SuperCDMS - Activities of the Queen's Group

Detector Testing Facility at Queen's

Figure 1: Cryostat mounted in its frame inside a clean-room. The outside (red) is at room temperature; the next layer (reflective can) is at -200 °C during normal operation. There are several more layers inside with the coldest stage at only ~ 20 mK (or 0.02 degree above the lowest possible temperature).

Each new detector being produce for SuperCDMS needs to be tested and characterized before it can be installed in the final experimental setup. We have installed a detector testing facility at Queen's University to contribute to this testing effort. The core of this facility is a VeriCold dilution refrigerator (cryostatwhich is based on a novel technology which makes it possible to reach the necessary low temperatures without the use of liquid nitrogen or helium.

Since the detectors to be tested will eventually be used in the SuperCDMS experiment, great care needs to be taken to keep the detectors clean and avoid contamination. Therefore the cryostat is mounted inside a clean room (better than class 1000). Fig. 1 shows a picture of the cryostat mounted inside the cleanroom. In Fig. 2 you see the inside of the cryostat with the different temperature stages (left) and a CDMS detector mounted with a mechanical structure on the lowest temperature stage of the cryostat.

While detectors await being tested or shipped they are stored in a special cabinet flushed with nitrogen which has a significantly lower concentration of the radioactive noble gas radon than room air.

Figure 2. Left: All thermal shields removed from the cryostat; the 5 plates we see are from top to bottem at about -210 °C (60 K), -270 °C (3 K), 700 mK, 60 mK and 20 mK. Right: CDMS detector setup mounted on the lowest temperature stage in the cryostat.

Besides the production testing for SuperCDMS this facility will also be used to test new detector concepts. The main developments for the next generation detectors are the further increase of the mass per detector module, new detector designs which improve the discrimination power or simplify the production process.

Detector Testing Facility at SNOLAB

The newest detector concepts being developed by the SuperCDMS collaboration show a very good discrimination power - in fact, the discrimination power is so high that measurements in an ordinary laboratory above the earth's surface are limited by the neutron flux produced by cosmic radiation. Therefore we plan to install a detector testing facility at SNOLAB where these discrimination measurements can be performed thanks to the very effective shielding against cosmic radiation. The setup still requires shielding against neutrons from radioactivity in the laboratory walls which will be accomplished by placing the facility inside a water tank of approximately 3 m in diameter. This new facility will be installed at SNOLAB over the course of the next year. Some more details regarding this facility can be found here.

Cryogenic Scintillation

Many materials behave quite differently at low temperatures than they do at room temperature and often these differneces are not well known or understood. Some behaviours can only be studied at low temperatures.

One particular property of materials often used for particle detection is scintillation, the production of small amounts of light when a particle interacts in the material. The properties of the emitted light change with temperature. We have a second cryogenic testing facility at Queen's which is specifically desinged to allow us to test scintillation and some other optical properties of materials at low temperature (down to ~3 Kelvin)

Operations at Soudan and Data Analysis

SuperCDMS is a running experiment. This requires a continuous presence of scientists, graduate students and technicians at the expeirmental site in Soudan, Minnesota. All scientists (including graduate students and postdocs) participating in the experiment travel from time to time to Soudan to take their turn in operating the experiment, performing calibration measurements and taking data.

The continuously produced data need to be analysed. This is also an activity in which most of the participating groups, including the SuperCDMS group at Queen's, are invovled.