High Precision CTE-Measurement of SiC-100 for Cryogenic Space-Telescopes

We present the results of high precision measurements of the thermal expansion of the sintered SiC, SiC-100, intended for use in cryogenic space-telescopes, in which minimization of thermal deformation of the mirror is critical and precise information of the thermal expansion is needed for the telescope design. The temperature range of the measurements extends from room temperature down to $\sim$ 10 K. Three samples, #1, #2, and #3 were manufactured from blocks of SiC produced in different lots. The thermal expansion of the samples was measured with a cryogenic dilatometer, consisting of a laser interferometer, a cryostat, and a mechanical cooler. The typical thermal expansion curve is presented using the 8th order polynomial of the temperature. For the three samples, the coefficients of thermal expansion (CTE), \bar{\alpha}_{#1}, \bar{\alpha}_{#2}, and \bar{\alpha}_{#3} were derived for temperatures between 293 K and 10 K. The average and the dispersion (1 $\sigma$ rms) of these three CTEs are 0.816 and 0.002 ($\times 10^{-6}$/K), respectively. No significant difference was detected in the CTE of the three samples from the different lots. Neither inhomogeneity nor anisotropy of the CTE was observed. Based on the obtained CTE dispersion, we performed an finite-element-method (FEM) analysis of the thermal deformation of a 3.5 m diameter cryogenic mirror made of six SiC-100 segments. It was shown that the present CTE measurement has a sufficient accuracy well enough for the design of the 3.5 m cryogenic infrared telescope mission, the Space Infrared telescope for Cosmology and Astrophysics (SPICA).Comment: in press, PASP. 21 pages, 4 figure

Chiral three-nucleon interaction and the carbon-14 dating beta decay

We present a shell model calculation for the beta decay of 14-C to the 14-N ground-state, treating the relevant nuclear states as two 0p-holes in an 16-O core. Employing the universal low-momentum nucleon-nucleon potential V(low-k) only, one finds that the Gamow-Teller matrix element is too large to describe the known (very long) lifetime of 14-C. As a novel approach to the problem, we invoke the chiral three-nucleon force (3NF) at leading order and derive from it a density-dependent in-medium NN interaction. Including this effective in-medium NN interaction, the Gamow-Teller matrix element vanishes for a nuclear density close to that of saturated nuclear matter. The genuine short-range part of the three-nucleon interaction plays a particularly important role in this context, since the medium modifications to the pion propagator and pion-nucleon vertex (due to the long-range 3NF) tend to cancel out in the relevant observable. We discuss also uncertainties related to the off-shell extrapolation of the in-medium NN interaction. Using the off-shell behavior of V(low-k) as a guide, we find that these uncertainties are rather small.Comment: 22 pages, 11 figure

Dominant g(9/2)^2 neutron configuration in the 4+1 state of 68Zn based on new g factor measurements

The $g$ factor of the $4_1^+$ state in $^{68}$Zn has been remeasured with improved energy resolution of the detectors used. The value obtained is consistent with the previous result of a negative $g$ factor thus confirming the dominant $0g_{9/2}$ neutron nature of the $4_1^+$ state. In addition, the accuracy of the $g$ factors of the $2_1^+$, $2_2^+$ and $3_1^-$ states has been improved an d their lifetimes were well reproduced. New large-scale shell model calculations based on a $^{56}$Ni core and an $0f_{5/2}1pg_{9/2}$ model space yield a theoretical value, $g(4_1^+) = +0.008$. Although the calculated value is small, it cannot fully explain the experimental value, $g(4_1^+) = -0.37(17)$. The magnitude of the deduced B(E2) of the $4_1^+$ and $2_1^+$ transition is, however, rather well described. These results demonstrate again the importance of $g$ factor measurements for nuclear structure determination s due to their specific sensitivity to detailed proton and neutron components in the nuclear wave functions.Comment: 7 pages, 3 figs, submitted to PL

Shell-Model Effective Operators for Muon Capture in ^{20}Ne

It has been proposed that the discrepancy between the partially-conserved axial-current prediction and the nuclear shell-model calculations of the ratio $C_P/C_A$ in the muon-capture reactions can be solved in the case of ^{28}Si by introducing effective transition operators. Recently there has been experimental interest in measuring the needed angular correlations also in ^{20}Ne. Inspired by this, we have performed a shell-model analysis employing effective transition operators in the shell-model formalism for the transition $^{20}Ne(0^+_{g.s.})+\mu^- \to ^{20}F(1^+; 1.057 MeV) + \nu_\mu$. Comparison of the calculated capture rates with existing data supports the use of effective transition operators. Based on our calculations, as soon as the experimental anisotropy data becomes available, the limits for the ratio $C_P/ C_A$ can be extracted.Comment: 9 pages, 3 figures include

TREX-DM: a low background Micromegas-based TPC for low-mass WIMP detection

Dark Matter experiments are recently focusing their detection techniques in low-mass WIMPs, which requires the use of light elements and low energy threshold. In this context, we describe the TREX-DM experiment, a low background Micromegas-based TPC for low-mass WIMP detection. Its main goal is the operation of an active detection mass $\sim$0.3 kg, with an energy threshold below 0.4 keVee and fully built with previously selected radiopure materials. This work describes the commissioning of the actual setup situated in a laboratory on surface and the updates needed for a possible physics run at the Canfranc Underground Laboratory (LSC) in 2016. A preliminary background model of TREX-DM is also presented, based on a Geant4 simulation, the simulation of the detector's response and two discrimination methods: a conservative muon/electron and one based on a neutron source. Based on this background model, TREX-DM could be competitive in the search for low-mass WIMPs. In particular it could be sensitive, e.g., to the low-mass WIMP interpretation of the DAMA/LIBRA and other hints in a conservative scenario.Comment: Proceedings of the XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015), 7-11 September 2015, Torino, Ital

Parasitémies à Plasmodium falciparum ou P. malariae chez les porteurs du trait drépanocytaire dans différents biotopes du Bénin

La prévalence du paludisme en fonction du phénotype de l'hémoglobine et la fréquence du gène S ont été recherchées dans deux différents biotopes du Bénin. Dans aucun d'eux, la prévalence du paludisme n'est significativement différente entre les porteurs du trait drépanocytaire et les autres. De même, la prévalence du trait drépanocytaire n'est pas modifiée par l'âge. En revanche, en région holoendémique, la densité parasitaire moyenne à #P. falciparum$ est significativement plus faible chez les sujets AS que chez les sujets AA. La présence du trait drépanocytaire ne réduit pas le risque d'infection palustre mais semble diminuer l'importance de la parasitémie. (Résumé d'auteur

Lowering the background level and the energy threshold of Micromegas x-ray detectors for axion searches

Axion helioscopes search for solar axions by their conversion in x-rays in the presence of high magnetic fields. The use of low background x-ray detectors is an essential component contributing to the sensitivity of these searches. In this work, we review the recent advances on Micromegas detectors used in the CERN Axion Solar Telescope (CAST) and proposed for the future International Axion Observatory (IAXO). The actual setup in CAST has achieved background levels below 10$^{-6}$ keV$^{-1}$ cm$^{-2}$ s$^{-1}$, a factor 100 lower than the first generation of Micromegas detectors. This reduction is based on active and passive shielding techniques, the selection of radiopure materials, offline discrimination techniques and the high granularity of the readout. We describe in detail the background model of the detector, based on its operation at CAST site and at the Canfranc Underground Laboratory (LSC), as well as on Geant4 simulations. The best levels currently achieved at LSC are low than 10$^{-7}$ keV$^{-1}$ cm$^{-2}$ s$^{-1}$ and show good prospects for the application of this technology in IAXO. Finally, we present some ideas and results for reducing the energy threshold of these detectors below 1 keV, using high-transparent windows, autotrigger electronics and studying the cluster shape at different energies. As a high flux of axion-like-particles is expected in this energy range, a sub-keV threshold detector could enlarge the physics case of axion helioscopes.Comment: Proceedings of 3rd International Conference on Technology and Instrumentation in Particle Physics (TIPP 2014