Simulation of special bubble detectors for PICASSO

Abstract

The PICASSO project is a cold dark matter (CDM) search experiment relying on the superheated droplet technique. The detectors use superheated freon liquid droplets (active material) dispersed and trapped in a polymerized gel. This detection technique is based on the phase transition of superheated droplets at about room temperature and ambient pressure. The phase transition is induced by nuclear recoils when an atomic nucleus in the droplets interacts with incoming subatomic particles. This includes CDM particles candidate as the neutralino (a yet-to-discover particle predicted in extensions of the Standard Model of particle physics).Simulations performed to understand the detector response to neutrons and alpha particles are presented along with corresponding data obtained at the Montreal Laboratory. INTRODUCTIONSuperheated droplet detectors, here referred to as special bubble detectors (SBD's), use superheated liquid droplets (active medium) dispersed and suspended in a polymerized gel (1,2) . Presently, these droplet detectors consist of an emulsion of droplets of a superheated liquid (such as C 3 F 8 , C 4 F 10 , CF 3 Br, CCl 2 F 2 ) which are metastable at about room temperature and ambient pressure. During the fabrication process, the droplets are dispersed in an aqueous solution to which a heavy salt (e.g. CsCl) is added to equalize densities so that the droplets neither sink nor raise to the surface. As a result of the fabrication process, a gel matrix with an isotropic distribution of droplets is obtained. The operation of the detector can be controlled by applying an adequate pressure which makes the boiling temperature of the liquid raise, preventing bubble formation. Under this external pressure, the detectors are insensitive to radiation.By removing the external pressure, the liquid becomes superheated and sensitive to incoming particles and radiation. Bubble formation occurs through liquid-to-vapour phase transitions, triggered by the energy deposited by incoming particles and radiation. The energy deposition can be the result of nuclear recoils (through interactions with neutrons or other particles), or gamma and alpha particles. Bubble detectors are threshold detectors as a minimal energy deposition is needed to induce a phase transition. Their sensitivity to various types of radiation strongly depends on the operating temperature and pressure. The liquid-to-vapour transition is explosive in nature and is accompanied by an acoustic shock wave which can be detected with piezoelectric sensors (3)