We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, the second stage including an improved neutron shielding. The analyses includes a refined signal analysis, and revised nucleation efficiency based on reanalysis of previously-reported monochromatic neutron irradiations. The combined results yield a contour minimum of σp = 5.7 × 10 −3 pb at 35 GeV/c 2 in the spin-dependent sector of WIMP-proton interactions, the most restrictive to date for MW ≤ 60 GeV/c 2 from a direct search experiment and overlapping for the first time results previously obtained only indirectly. In the spin-independent sector, a minimum of 4.7 × 10 −6 pb at 35 GeV/c 2 is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.The search for weakly interacting massive particle (WIMP) dark matter remains at the forefront of modern physics activity. Estimated to comprise ∼ 23% of the Universe mass, it is the role of direct detection efforts to elaborate its nature, and whether its interaction with nucleons is spin-independent (SI) or spin-dependent (SD). SIMPLE (Superheated Instrument for Massive ParticLe Experiments) [1] is a direct search activity using superheated liquid detectors, and one of only a few in the international panorama with sensitivity to the WIMPproton sector of the SD phase space. It is operated at the 1500 mwe level of the Low Noise Underground Laboratory (LSBB) in southern France.In [1], we reported the first results of a two stage Phase II measurement, comprising a 14.1 kgd Stage 1 exposure of 15 superheated droplet detectors (SDDs) [2-4] with a total active mass of 0.208 kg. We here provide the results of the full Phase II measurement, including a 13.67 kgd Stage 2 exposure of a second 15 SDD set, together with improved neutron shielding and a refined analysis of the individual detector run signals, sensitivities, and nucleation efficiency.A SDD consists of a dispersion of superheated liquid droplets homogeneously distributed within a gel matrix, which may undergo a transition to the gas phase upon energy deposition by incident radiation. Two conditions are required for the nucleation of the gas phase of the superheated droplets [5]: (i) the energy deposited must be greater than a thermodynamic minimum, and (ii) this * criodets@cii.fc.ul.pt energy must be deposited within a thermodynamicallydefined minimum distance (Λr c ) inside the droplet, where Λ is the nucleation parameter and r c = the thermodynamic critical bubble radius. Adjustment of the two conditions results in the necessity of depositions of order ≥ 150 keV/µm for a bubble nucleation, rendering the SDD effectively insensitive to the majority of traditional detector backgrounds (including electrons, γ's and cosmic muons) which complicate more conventional dark matter search detectors, leaving only α-and neutron-induced events.The 15 Stage 2 SDDs were fabricated as described in [1], each containing between 11-19 g of C 2 ...

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We report results of a 14.1   kg d measurement with 15 superheated droplet detectors of total active mass 0.208 kg, comprising the first stage of a 30 kg d Phase II experiment. In combination with the results of the neutron-spin sensitive XENON10 experiment, these results yield a limit of |a(p)|<0.32 for M(W)=50 GeV/c² on the spin-dependent sector of weakly interacting massive particle-nucleus interactions with a 50% reduction in the previously allowed region of the phase space, formerly defined by XENON, KIMS, and PICASSO. In the spin-independent sector, a limit of 2.3×10⁻⁵ pb at M(W)=45 GeV/c² is obtained.

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We report on the fabrication aspects and calibration of the first large active mass (ϳ15 g) modules of SIMPLE, a search for particle dark matter using superheated droplet detectors (SDDs). While still limited by the statistical uncertainty of the small data sample on hand, the first weeks of operation in the new underground laboratory of Rustrel -Pays d'Apt already provide a sensitivity to axially coupled weakly interacting massive particles (WIMPs) competitive with leading experiments, confirming SDDs as a convenient, low-cost alternative for WIMP detection. PACS numbers: 95.35.+d, 05.70.Fh, The rupture of metastability by radiation has been historically exploited as a method for particle detection. Perhaps its most successful application is the bubble chamber, where ionizing particles deposit enough local energy in a superheated liquid to produce vaporization along their wake. Apfel extended this concept in the form of superheated droplet detectors [1] (SDDs, aka bubble detectors), in which small drops (radius ϳ10 mm) of the liquid are uniformly dispersed in a gel or viscoelastic medium. In a SDD the gel matrix isolates the fragile metastable system from vibrations and convection currents, while the smooth liquid-liquid interfaces impede the continuous triggering on surface impurities that occurs in bubble chambers. The lifetime of the superheated state is extended, allowing for new applications: SDDs are increasingly popular as neutron dosimeters, where the nucleated visible bubbles provide a reading of the radiation exposure. SIMPLE (superheated instrument for massive particle searches) aims to detect particle dark matter using SDDs. We report here on the sensitivity attained at the early prototype stage, already comparable to the best achieved with competing technologies.In the moderately superheated industrial refrigerants used in SDDs, bubbles are produced only by particles having elevated stopping powers (dE͞dx * 200 keV͞mm) such as nuclear recoils. This is understood in the framework of the "thermal spike" model [2], common to bubble chambers: for the transition to occur, a vapor nucleus of radius .r c must be created, while only the energy deposited along a distance comparable to this critical radius r c is available for its formation. Hence, a double threshold is imposed: the deposited energy E must be larger than the work of formation of the critical nucleus, E c , and this energy must be lost over a distance O͑r c ͒, i.e., a minimum dE͞dx is required. More formally [3,4] E . E c 4pr 2 c g͞3e ,where r c 2g͞DP, g͑T ͒ is the surface tension, DP P V 2 P, P V ͑T ͒ is the vapor pressure, P and T are, respectively, the operating pressure and temperature, e varies in the range ͓0.02, 0.06͔ for different liquids [4,5], and a͑T ͒ ϳ O͑1͒ [6]. Both thresholds can be tuned by changing the operating conditions: keV nuclear recoils like those expected from scattering of weakly interacting massive particles (WIMPs) (currently the favored galactic dark matter candidates [7]) are detectable at room T and atmospheric ...