Kristin Poinar with Richard Schnee

Characterizing Electron Background with the CDMS Beta Cage – A Predecessor to a Screener of Ultra-Low-Level Radiation: the Prototype Beta Cage

The beta cage is a proposed multi-wire proportional chamber that will be the most sensitive device available to screen low-energy (200 keV or less) betas emitted at very low rates (down to $10^{-5}$ counts/keV/$cm^{2}$/day).  Shielding and radiopure components will allow this sensitivity to be achieved in the full-size beta cage, which will be used to assess beta-emitting impurities on the surface of detectors for use in the Cryogenic Dark Matter Search.  These impurities currently limit the experiment’s sensitivity to the dark matter candidate WIMPs.

The project consists of the construction and commissioning of a smaller, prototype chamber.  The prototype beta cage will enclose a 40x40x20 cm drift region and two horizontal multi-wire proportional counters, all contained within a vessel of argon gas.  Samples will be placed beneath the bottom grid; emitted betas will ionize the gas atoms and produce a shower of secondary electrons, which the high-voltage anode wires of each grid will multiply and collect.  Their readouts will allow discrimination of events from background and determination of the energy of each beta event.  The use of the prototype chamber will assess the accuracy of isotope identification by reconstructing the beta energy spectrum.