MACHe3, a prototype for non-baryonic dark matter search: KeV event detection and multicell correlation

Superfluid He3 at ultra-low temperatures (100 microKelvins) is a sensitive medium for the bolometric detection of particles. MACHe3 (MAtrix of Cells of Helium 3) is a project for non-baryonic dark matter search using He3 as a sensitive medium. Simulations made on a high granularity detector show a very good rejection to background signals. A multicell prototype including 3 bolometers has been developed to allow correlations between the cells for background event discrimination. One of the cells contains a low activity Co57 source providing conversion electrons of 7.3 and 13.6 keV to confirm the detection of low energy events. First results on the multicell prototype are presented. A detection threshold of 1 keV has been achieved. The detection of low energy conversion electrons coming from the Co57 source is highlighted as well as the cosmic muon spectrum measurement. The possibility to reject background events by using the correlation among the cells is demonstrated from the simultaneous detection of muons in different cells

Excitation energy and deformation of the 1/2+[431] intruder band in 107Tc^{107}Tc

The already detailed study of $^{107}Tc$ nucleus was complemented by a search for microsecond isomers at very low energy. For this purpose, this neutron-rich nucleus was produced by thermal-neutron-induced fission of $^{241}Pu$. We have found a new 30.1 keV microsecond isomeric state which deexcites to the ground state by a strongly-hindered E1 transition. This isomer was identified as the 3/2+ level of the 1/2+[431] intruder band in $^{107}Tc$ and is also the lowest-lying member of the band. The very low energy of the band head suggests a large quadrupole deformation. From a comparison with $^{105}Tc$,where more information is known about the intruder band, it is deduced that the 1/2+[431] band has a quadrupole deformation, $\epsilon_{2} \geq$ 0.35 and a possible triaxial shape, $\gamma \approx$ 20°

First observation of low-lying excited states in the very neutron-rich 95Kr^{95}Kr

Microsecond isomers of neutron-rich nuclei in the mass A=95 chain were investigated at the ILL reactor, Grenoble. These nuclei were produced by thermal-neutron induced fission of $^{241}Pu$. The detection is based on time correlation between fission fragments selected by the Lohengrin mass spectrometer and the $\gamma$-rays from the isomers. In this paper the decay scheme of an isomer of half life 1.4(2) micro second in $^{95}Kr$ is reported. It is the first time that excited states of this nucleus have been measured. These data show that this neutron rich N = 59 isotone is spherical at low excitation energy

High spin microsecond isomers in 129^{129}In and 129^{129}Sb

In this work the microsecond isomers in 129In and 129Sb were investigated. These nuclei were produced by the thermal-neutron-induced fission of 241Pu. The detection is based on a time correlation between the fission fragments selected by the LOHENGRIN spectrometer at the ILL (Grenoble) and the γ rays or conversion electrons from the isomers. The decay schemes of the new 17/2− isomer in 129In and 23/2+ isomer in 129Sb are reported. A shell-model study of these two nuclei was performed using a realistic effective interaction derived from the CD–Bonn nucleon-nucleon potential. Comparison shows that the calculated energy levels and electromagnetic transition rates are in very good agreement with the experimental data. (APS

Recent measurements of the spherical and deformed isomers using the Lohengrin fission-fragment spectrometer

International audienc

Shape coexistence in the very neutron-rich odd-odd 96Rb^{96}Rb

Microsecond isomers of neutron-rich nuclei in the masses A=96 and 98 were reinvestigated at the ILL reactor (Grenoble). These nuclei were produced by thermal-neutron induced fission of $^{241}$Pu. The detection is based on time correlation between fission fragments selected by the Lohengrin mass spectrometer, and the $\gamma$ rays and conversion electrons from the isomers. A new level scheme of $^{96}$Rb is proposed. We have found that the ground state and low-lying levels of this nucleus are rather spherical, while a rotational band develops at 461 keV energy. This bans has properties consistent with a $\pi[431 3/2] x \nu[541 3/2]K = 3^-$ Nilsson assignment and a deformation $\beta_2 > 0.28$. It is fed by a $10^-$ microsecond isomer consistent with a $\pi(g_{9/2})\nu(h_{11/2})$ sperical configuration. It is interesting to note that the same unique-parity states $\pi(g_{9/2})$ and $\nu(h_{11/2})$ are present in the same nucleus in a deformed and in a spherical configuration. The neighbouring odd-odd nucleus $^{98}$Y presents a strong analogy with $^{96}$Rb and is also discussed

New Spins for ground states and isomers in 115^{115}Pd and 117^{117}Pd

Levels in 115Pd and 117Pd nuclei, populated in the spontaneous fission of 248Cm were studied by means of prompt gamma spectroscopy using the EUROGAM2 array of Anti-Compton spectrometers. Negative-parity, I = 9/2 excitations were identified, which are associated with the long-lived isomers in these nuclei, reported previously as 11/2- excitations. The new data indicate spin and parity 3/2 + for ground states in 115Pd and 117Pd instead of 5/2 + proposed in previous works. This result implicates changes of spin assignments to other levels in both nuclei

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasi-free production mechanism a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.Comment: 12 pages, 3 figures, submitted to Phys. Rev. Let

Anatomy of the Soft-Photon Approximation in Hadron-Hadron Bremsstrahlung

A modified Low procedure for constructing soft-photon amplitudes has been used to derive two general soft-photon amplitudes, a two-s-two-t special amplitude $M^{TsTts}_{\mu}$ and a two-u-two-t special amplitude $M^{TuTts}_{\mu}$, where s, t and u are the Mandelstam variables. $M^{TsTts}_{\mu}$ depends only on the elastic T-matrix evaluated at four sets of (s,t) fixed by the requirement that the amplitude be free of derivatives ($\partial$T/$\partial$s and /or $\partial$T/$\partial t$). Likewise $M^{TuTts}_{\mu}$ depends only on the elastic T-matrix evaluated at four sets of (u,t). In deriving these amplitudes, we impose the condition that $M^{TsTts}_{\mu}$ and $M^{TuTts}_{\mu}$ reduce to $\bar{M}^{TsTts}_{\mu}$ and $\bar{M}^{TuTts}_{\mu}$, respectively, their tree level approximations. The amplitude $\bar{M}^{TsTts}_{\mu}$ represents photon emission from a sum of one-particle t-channel exchange diagrams and one-particle s-channel exchange diagrams, while the amplitude $\bar{M}^{TuTts} _{\mu}$ represents photon emission from a sum of one-particle t-channel exchange diagrams and one-particle u-channel exchange diagrams. The precise expressions for $\bar{M}^{TsTts}_{\mu}$ and $\bar{M}^{TuTts}_{\mu}$ are determined by using the radiation decomposition identities of Brodsky and Brown. We point out that it is theoretically impossible to describe all bremsstrahlung processes by using only a single class of soft-photon amplitudes. At least two different classes are required: the amplitudes which depend on s and t or the amplitudes which depend on u and t. When resonance effects are important, the amplitude $M^{TsTts}_{\mu}$, not $M^{Low(st)}_{\mu}$, should be used. For processes with strong u-channel exchange effects, the amplitude $M^{TuTts}_{\mu}$ should be the first choice.Comment: 49 pages report # LA-UR-92-270