The SIMPLE Phase II dark matter search

Abstract

Phase II of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) searched for astroparticle dark matter using superheated liquid C2ClF5 droplet detectors. Each droplet generally requires an energy deposition with linear energy transfer (LET) > ∼ 150 keV/µm for a liquid-to-gas phase transition, providing an intrinsic rejection against minimum ionizing particles of order 10 −10 , and reducing the backgrounds to primarily α and neutron-induced recoil events. The droplet phase transition generates a millimetric-sized gas bubble which is recorded by acoustic means. We describe the SIMPLE detectors, their acoustic instrumentation, and the characterizations, signal analysis and data selection which yield a particle-induced, "true nucleation" event detection efficiency of better than 97% at a 95% C.L. The recoil-α event discrimination, determined using detectors first irradiated with neutrons and then doped with alpha emitters, provides a recoil identification of better than 99%; it differs from those of COUPP and PICASSO primarily as a result of their different liquids with lower critical LETs. The science measurements, comprising two shielded arrays of fifteen detectors each and a total exposure of 27.77 kgd, are detailed. Removal of the 1.94 kgd Stage 1 installation period data, which had previously been mistakenly included in the data, reduces the science exposure from 20.18 to 18.24 kgd and provides new contour minima of σp = 4.3 × 10 −3 pb at 35 GeV/c 2 in the spin-dependent sector of WIMP-proton interactions and σN = 3.6 × 10 −6 pb at 35 GeV/c 2 in the spin-independent sector. These results are examined with respect to the fluorine spin and halo parameters used in the previous data analysis.

Simple improvements to classical bubble nucleation models

“…(ii) When the liquid pressure is slightly below the equilibrium pressure, the deviations in the free energy for bubble formation from the CNT become large. For example, in dark matter detection experiments which use superheated liquids as targets [6][7][8][9], the degree of nonequilibrium is very small. For such a near-equilibrium case, the PCNT should be used instead of CNT.…”

“…Bubble nucleation in liquid is a liquid-to-vapor transition [1] phenomenon, and plays an important role in many areas of science and technology, e.g., degassification of steel [1], bubble jet printers [2], volcanism [3][4][5], the direct detection of dark matter [6][7][8][9], and medicine [10,11]. Bubble nucleation occurs in a metastable liquid under a pressure below its equilibrium vapor pressure.…”

Detection of alpha particle contamination on ultra low activity-grade integrated circuits

“…Underground testing of the standard DM SDD demonstrated an intrinsic background in the order of 10 −4 count/h and 10 −3 count/h for n and , respectively [9] with a detection efficiency of ∼95% [10]. The background signal being the critical parameter to qualify an instrument's ability for low-level measurements, our SDD has an intrinsic -background signal 100 times smaller than that of the ionisation counter, and a comparable detection sensitivity.…”

“…At signal amplitudes of ∼100 mV there is a 20 mV gap between the and n-distributions which is clearly resolved by the signal acquisition system (resolution 0.3 mV) [9].…”

“…In this work we propose a new approach to measure -emission rates, based on the superheated droplet detectors (SDD) developed by our team towards the direct search for dark matter (DM) [9]. Similarly to IC, SDD are sensitive to 's and n's.…”