PICO-500: A tonne scale bubble chamber for the search of dark matter

The PICO-500 experiment will search for dark matter by first operating a 260 liters bubble chamber filled with C3F8, with a target sensitive to the spin-independent and spin-dependent WIMP-nucleon cross-sections of approximately 10(-44) cm(2) and 10(-42) cm(2), respectively. To guarantee a low background rate and therefore find a WIMP signal or set strong exclusion limits, the geometry and radio-purity of the materials to be used must be optimized. A detailed model of the detector has been developed in CEANT4 to perform the Monte Carlo simulations. The main sources of background signals in this detector are from (alpha, n) reactions and spontaneous fission. Contributions from internal components, exposure to radon present in SNOLAB's Cube Hall as well as photonuclear reactions have been considered in this work. Preliminary results show a very low estimated rate of background events, less than one single bubble event per year.