PNNL

Abstract

SIMPLE (superheated instrument for massive particle search) is an experiment(1,2) to search for evidence of spin-dependent dark matter using fluorine-loaded superheated droplet detectors (SDDs). The application of the detector is based on the presence of fluorine, which possesses the highest figure-of-merit for spin-dependent interactions. Furthermore, these devices are virtually insensitive to the majority of backgrounds associated with such searches. For dark matter applications, less than 10 events kg-1 d-1 are expected. The sensitivity to backgrounds is therefore an important issue for detector use. Following the thermal spike model of Seitz(3), there are two thresholds for bubble nucleation: (1) the deposited energy must be larger than the work of formation of a critical nucleus (Ec) and (2) Ec must be deposited over a distance of the order of a critical radius (rc). Both thresholds can be tuned by modifying the operating conditions as to render the SDD insensitive to radiations depositing less than ~200 keV mm-1. Although the SDD can be made insensitive to energetic muons, gamma rays, X rays and electrons with linear energy transfer (LET) below this threshold, response to neutrons and alpha particles remains problematic. These originate from radioactivity in the rock, detector shielding and the detector itself. We describe the detector fabrication and response studies required by the large volume and concentration of the SIMPLE device.