Underground Detector Test Facility at SNOLAB

Motivation

SuperCDMS plans to install a 100-200 kg experiment at SNOLAB. When going to a large detector mass we need at the same time to reduce the primary background rate and improve the discrimination capability of our detectorsbetween background and potential WIMP events. One step in this direction is the a new detector design. A prototype detector with this design shows an excellent discrimination. However, we are not able to quantify the discrimination power, since the measurements done at a testing facility at the surface are subject to neutron background (see Fig. 1). In order to qualify any new detector design for a large scale experiment as planned for SNOLAB (order of 100-200 kg) we need a way to measure the actual discrimination power, which is only possible in a well shielded testing facility.

Another problem appears when we go to larger individual detector modules: the event rate from environmental radiation in a testing facility at the surfaces will become too large, resulting in constant pile-up, so that it becomes impossible to operate the detector. We could still measure operational parameters of the detectors, but not test the actual performance as particle detector. This will already be an important issue if we go from our present detector modules with a size of 3" × 1" (≅ 620 g) to the next generation with 100 mm × 33 mm (≅ 1.4 kg), and even more so once we further increase the detector volume to 6" × 2" as under discussion for a future ton-scale phase of the experiment. To avoid pile-up and be able to operate the detector we again need a shielded testing facility.

Figure 1:Ionization yield vs. Recoil energy for the prototype detector. WIMP events would lay between the two green lines (nuclear recoil band or signal region). The events above are artificialy induced surface events. Most of the events inside the nuclear recoil band are likely from neutrons, but we have no means of discriminating event by event between real neutron events and surface events leaking into the nuclear recoil band. So we cannot determine how likely it is that a surface event shows up in the signal region, unless we have a testing facility which is well shielded from neutrons.

The Facility


Figure 2: Sketch of the SNOLAB testing facility in the ladder lab. The cryostat is hanging inside a water tank for shielding against environmental neutrons and gammas. The PE and Pb lids of the SuperCDMS experimental setup can be stored above the test facility.

The core of this new facility will be a dilution refrigerator (cryostat). To shield the facility from environmental neutrons (as well as gammas) it will be surrounded by a water shield. Fig. 2 shows a sketch of the cross section of the facility, consisting of the cryostat attached to a frame, hanging inside a water shielding tank. The facility will be installed in the so called ladder lab are at SNOLAB. Fig. 3 shows a rough layout of the space. The plan is to build a structure which holds the cryostat and at the same time provides an mezzanine level where parts of the full SuperCDMS experiment (to be installed later) can be stored. The facility will be able to operate at least two, perhaps three detectors at a time.


Figure 3: Rough layout of the ladder lab area. The TF will be positioned next to where the SuperCDMS experiment will go, in the space that is dedicated to the storage of the PE and Pb lids of the experiment. The lids will be stored at a mezzanine level above the test facility.