M. Felizardo

Final Analysis and Results of the Phase II SIMPLE Dark Matter Search

We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, with the second stage including an improved neutron shielding. The analyses include a refined signal analysis, and revised nucleation efficiency based on a reanalysis of previously reported monochromatic neutron irradiations. The combined results yield a contour minimum of σp=5.7×10(-3)u2009u2009pb at 35u2009u2009GeV/c2 in the spin-dependent sector of weakly interacting massive particle (WIMP) proton interactions, the most restrictive to date for MW}≤60u2009u2009GeV/c2 from a direct search experiment and overlapping, for the first time, with results previously obtained only indirectly. In the spin-independent sector, a minimum of 4.7×10(-6)u2009u2009pb at 35u2009u2009GeV/c2 is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.

First Results of the Phase II SIMPLE Dark Matter Search

We report results of a 14.1 u2009 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.

The SIMPLE Phase II Dark Matter Search

Phase II of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) searched for astroparticle dark matter using superheated liquid C 2 ClF 5 droplet detectors. Each droplet generally requires an energy deposition with linear energy transfer (LET) ≳ 150 keV / μ m 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 that 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%xa0C.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 27.77xa0kgd, are detailed. Removal of the 1.94xa0kgd Stage 1 installation period data, which had previously been mistakenly included in the data, reduces the science exposure from 20.18 to 18.24xa0kgd and provides new contour minima of σ p = 4.3 × 10 − 3 pb at 35 GeV / c 2 in the spin-dependent sector of astroparticle dark matter–proton interactions and σ N = 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.

Recent results from the SIMPLE dark matter search

We present the results of the recent Phase II SIMPLE search effort, comprising two stages each of ~14 kgd exposure of 15 Superheated Droplet Detectors with ~0.2 kg active mass and recoil energy threshold of 8 keV. In the second stage, the neutron shielding was increased to reduce the on-detector rate by a factor 3.5. Combined with an improved nucleation efficiency and analysis of the detector pressure evolution during the measurements, the results yield improved results in the phase space of both spin -dependent and -independent WIMP interactions.

Neutron background signal in superheated droplet detectors of the Phase II SIMPLE dark matter search

Abstract The simulation of the neutron background for Phase II of the SIMPLE direct dark matter search experiment is fully reported with various improvements relative to previous estimates. The model employs the Monte Carlo MCNP neutron transport code, using as input a realistic geometry description, measured radioassays and material compositions, and tabulated (α,n) yields and spectra. Developments include the accounting of recoil energy distributions, consideration of additional reactions and materials and examination of the relevant (α,n) data. A thorough analysis of the simulation results is performed that addresses an increased number of non-statistical uncertainties. The referred omissions are seen to provide a net increase of 13% in the previously-reported background estimates whereas the non-statistical uncertainty rises to 25%. The final estimated recoil event rate is 0.372xa0±xa00.002 (stat.)xa0±xa00.097 (non-stat.) evt/kgd resulting in insignificant changes over the results of the experiment.

Fabrication and response of high concentration SIMPLE superheated droplet detectors with different liquids

Abstract The combined measurement of dark matter interactions with different superheated liquids has recently been suggested as a cross-correlation technique in identifying WIMP candidates. We describe the fabrication of high concentration superheated droplet detectors based on the light nuclei liquids C3F8, C4F8, C4F10 and CCl2F2, and investigation of their irradiation response with respect to C2ClF5. The results are discussed in terms of the basic physics of superheated liquid response to particle interactions, as well as the necessary detector qualifications for application in dark matter search investigations. The possibility of heavier nuclei SDDs is explored using the light nuclei results as a basis, with CF3I provided as an example.

Doping of Ga2O3 bulk crystals and NWs by ion implantation

Ga2O3 bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×1013 to 4×1015 at/cm2. The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×1015 at/cm2 and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga2O3 nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.

New limits on WIMP interactions from the SIMPLE dark matter search

The SIMPLE project uses superheated C2ClF5 liquid detectors to search for particle dark matter candidates. We report the results of the first stage exposure (14.1 kgd) of its latest two-stage, Phase II run, with 15 superheated droplet detectors of total active mass 0.208 kg. In combination with the results of the neutron-spin sensitive XENON10 experiment, these results yield a limit of |ap| < 0.32, |an| < 0.17 for MW = 50 GeV/c 2 on the model-independent, spin-dependent sector of weakly interacting massive particle (WIMP)-nucleus interactions, and together yield a 50% reduction in the previously allowed region of the phase space. The result provides a contour minimum of σp ~ 2.8 x 10 -2 pb at MW = 45 GeV/c 2 , constituting the most restrictive direct detection limit to date against a spin-dependent WIMP-proton coupling. In the spin-independent sector, the result is seen to offer the prospect of contributing to the question of light mass WIMPs with an improvement in the current understanding of its nucleation efficiency.

GaN:Pr3+ nanostructures for red solid state light emission

The photoluminescence of praseodymium implanted and annealed GaN films, quantum wells, nanowires and quantum dots was studied. After implantation and annealing, Pr3+ intra-shell luminescence was achieved for all the analysed samples. In the trivalent charge state the ions luminescence was found to be dominated by the red lines of the 3P0 → 3F2 transition. In the case of GaN films, an intense red emission is observed with the naked eye at room temperature. Photoluminescence excitation indicates that the preferential population mechanisms of this emission are achieved by using excitation above the GaN band gap. A correlation of the optically active ions luminescence spectral shape and peak position in the different structures is established. For the GaN nanowires the 3P0 → 3F2 lines of the Pr3+ ions are in good agreement with those identified in GaN films. In the case of GaN quantum dots, the ions emission was found to be similar to that observed in AlN layers. For AlN/GaN/AlN quantum wells a similar behaviour was identified with the sharp ionic luminescence lines superimposed as a broad band, likely generated by the overlap of multiple Pr-centres. The ionic luminescence stability was analysed and discussed for all the studied samples.

SIMPLE limits on spin-dependent WIMP interactions

An improved SIMPLE experiment comprising four superheated droplet detectors with a total exposure of 0.42 kg.d yields ~factor 10 improvement in the previously-reported results. Despite the low exposure, the result provides restrictions on the allowed phase space of spin-dependent coupling strengths almost equivalent to those from the significantly larger exposure NAIAD-CDMS/ZEPLIN searches.