D. Limagne

SIMPLE dark matter search results

We report an improved SIMPLE experiment comprising four superheated droplet detectors with a total exposure of 0.42 kgd. The result yields similar to factor 10 improvement in the previously-reported results, and-despite the low exposure-is seen to provide restrictions on the allowed phase space of spin-dependent coupling strengths almost equivalent to those from the significantly larger exposure NAIAD-CDMS/ZEPLIN searches. (c) 2005 Elsevier B.V. All rights reserved.

Long-Range Nanometer-Scale Organization of Semifluorinated Alkane Monolayers at the Air/Water Interface

We have determined the structure formed at the air-water interface by semifluorinated alkanes (C(8)F(17)C(m)H(2m+1) diblocks, F8Hm for short) for different lengths of the molecule (m = 14, 16, 18, 20) by using surface pressure versus area per molecule isotherms, Brewster angle microscopy (BAM), and grazing incidence x-ray experiments (GISAXS and GIXD). The behavior of the monolayers of diblocks under compression is mainly characterized by a phase transition from a low-density phase to a condensed phase. The nonzero surface pressure phase is crystalline and exhibits two hexagonal lattices at two different scales: a long-range-order lattice of a few tens of nanometers lateral parameter and a molecular array of about 0.6 nm parameter. The extent of this organization is sufficiently large to impact larger scale behavior. Analysis of the various compressibilities evidences the presence of non organized molecules in the monolayer for all 2D pressures. At room temperature, the self-assembled structure appears generic for all the F8Hm investigated.

Expulsion of magnetic flux in a type-I superconducting strip

Abstract Flux expulsion in a type-I superconducting strip, induced by decrease of a perpendicular magnetic field following field increase-induced flux penetration, is observed by real time detection of the flux-induced voltage pulses. Three different expulsion regimes are identified, corresponding in general to the three initial regimes of flux penetration. The crossover field between two expulsion regimes is found to be associated with the change of nucleation mechanism at Hc2. Possible association of the expulsion onset with surface superconductivity and Hc3 is discussed. The results support the idea of existence of an energy barrier against flux expulsion.

Irreversible Flux Entry in Tin Superconducting Strips: Geometrical Metastability

The nucleation of the normal state in a superconducting tin strip is observed in real time with a low-noise pulse pre-amplifier. For flat strips, the magnetic field at which the flux entry occurs is higher than the thermodynamical critical-field value. This is due to an edge pinning effect against the penetration of normal zones into the bulk. In this letter we establish that using our technique, the determination of the first irreversible flux entry (FIFE) is possible and provides a direct evaluation of the height of the energy barrier against flux penetration. The irreversible nucleation process is discussed.

Self-Organization of Polystyrene-b-polyacrylic Acid (PS-b-PAA) Monolayer at the Air/Water Interface: A Process Driven by the Release of the Solvent Spreading

We present an in situ structural study of the surface behavior of PS-b-PAA monolayers at the air/water interface at pH 2, for which the PAA blocks are neutral and using N,N-dimethyformamide (DMF) as spreading solvent. The surface pressure versus molecular area isotherm shows a perfectly reversible pseudoplateau over several cycles of compression/decompression. The width of such plateau enlarges when increasing temperature, conversely to what is classically observed in the case of an in-plane first order transition. We combined specular neutron reflectivity (SNR) experiments with contrast variation to solve the profile of each block perpendicular to the surface with grazing-incidence small-angle scattering (GISAXS) measurements to determine the in-plane structure of the layer. SNR experiments showed that both PS and PAA blocks remain adsorbed on the surface for all surface pressure probed. A correlation peak at Q(xy)* = 0.021 Å(-1) is evidenced by GISAXS at very low surface pressure which intensity first increases on the plateau. When compressing further, its intensity decays while Q(xy)* is shifted toward low Q(xy). The peak fully disappears at the end of the plateau. These results are interpreted by the formation of surface aggregates induced by DMF molecules at the surface. These DMF molecules remain adsorbed within the PS core of the aggregates. Upon compression, they are progressively expelled from the monolayer, which gives rise to the pseudoplateau on the isotherm. The intensity of the GISAXS correlation peak is set by the amount of DMF within the monolayer as it vanishes when all DMF molecules are expelled. This result emphizes the role of the solvent in Langmuir monolayer formed by amphiphilic copolymers which hydrophobic and hydrophilic parts are composed by long polymer chains.

Irreversible flux penetration regimes in type-I superconducting strips

Abstract Flux penetration into superconducting tin strips, induced by incrementation of a perpendicular magnetic field, is observed by the detection in real time of flux variations while the field is increased up to H c . Three different flux penetration regimes are observed, well separated by two limiting fields. The limits of the different flux penetration regimes are determined by the sample geometry. These results allow a reinterpretation of previous magneto-optic observations, providing a simple picture associating the energy barrier against flux penetration into superconducting strips with the presence of entirely diamagnetic bands at the strip borders.

MCNP optimization of filtered neutron beams for calibration of the SIMPLE detector

We report an MCNP study of filtered monochromatic neutron beams of energies 25, 54 and 149 keV for response studies of a superheated droplet detector for the SIMPLE experiment. The results identify the importance of the detector temperature stabilizing water bath and the aqueous gel of the detector on the beam quality, in general agreement with recent measurements made on the thermal port of the Portuguese research reactor.

Beta Irradiation of a Geometrically Metastable Superconducting Strip Detector with a Magnetic Flux Penetration Read-Out.

Geometrical metastability, observed in superconducting type I tin flat strips, has been previously proposed as a principle for particle detection. The energy deposition of an incoming beta-particle induces the rupture of the metastability and consequently the penetration of multiquantum flux tubes into a superconducting tin strip. We present here the first absorption spectra from two beta sources, which demonstrate the linearity and energy-resolution of these detectors (presented at the 6th International Workshop on Low Temperature Detectors for Dark Matter and Neutrinos (LTD-6), Interlaken, Switzerland, Sept. 1995)Geometrical metastability, observed in superconducting type I tin flat strips, has been previously proposed as a principle for particle detection. The energy deposition of an incoming beta-particle induces the rupture of the metastability and consequently the penetration of multiquantum flux tubes into a superconducting tin strip. We present here the first absorption spectra from two beta sources, which demonstrate the linearity and energy-resolution of these detectors (presented at the 6th International Workshop on Low Temperature Detectors for Dark Matter and Neutrinos (LTD-6), Interlaken, Switzerland, Sept. 1995)

Response of a metastable superconducting grains suspension to irradiation by 35S decay electrons

Abstract We report a measurement of the energy loss spectrum of electrons in the beta decay of 35 S using a prototype detector composed of a suspension of 10–25 μm diameter tin grains at 2.3 K. Preliminary analyses suggests the capability of achieving sub-eV resolutions in such measurements, and that—contrary to conventional wisdoms—this is possible with both relatively large grains and a distribution of the grain sizes.

A new description of the behavior of metastable superconducting grains suspensions under irradiation

Intense or prolonged irradiation of suspensions of disordered Type I superconducting grains maintained in a superheated state creates a “hot border zone” below the normal superheated transition boundary. Subsequent increase of the applied magnetic field moves superconducting grains across the border into the zone, for which only thermal nucleation is possible until the highest local fields have again reached the usual superheated-normal transition value. This model suggests that, contrary to conventional wisdoms, energy resolution is possible with such detectors using both relatively large grains and size distributions.