New low-energy measurement of the 18F(p,α)15O reaction

The ¹⁸F, produced during nova explosions, is the main responsible for the 511 keV γ-ray emitted during the outburst that could be detected with satellite missions or future γ-ray telescopes. In spite of many experimental efforts, the amount of ¹⁸F synthesized still suffers from large uncertainties concerning mainly the ¹⁸F(p,α)¹⁵O reaction. We report here on a new direct measurement of the ¹⁸F(p,α)¹⁵O cross section recently carried out at the RIB facility at Louvain-la-Neuve. Cross section data down to a center-of-mass energy of 400 keV have been obtained using an isobarically pure ¹⁸F beam (averaged intensity of the order of 10⁶ pps) and a thin polyethylene target. The reaction products were detected using the multi-strip detector array LEDA. The cross section data are analysed in the framework of the R-matrix model. Our main aim is to determine the interference sign between three 3/2⁺ resonances above threshold that can significantly affect the extrapolation of the ¹⁸F(p,α)¹⁵O cross section to the energies relevant for novae, and thus, the modeling of nova explosions.     

The effect of size on the strength of FCC metals at elevated temperatures: annealed copper

As the length scale of sample dimensions is reduced to the micron and sub-micron scales, the strength of various materials has been observed to increase with decreasing size, a fact commonly referred to as the ‘sample size effect’. In this work, the influence of temperature on the sample size effect in copper is investigated using in situ microcompression testing at 25, 200 and 400 °C in the SEM on vacuum-annealed copper structures, and the resulting deformed structures were analysed using X-ray μLaue diffraction and scanning electron microscopy. For pillars with sizes between 0.4 and 4 μm, the size effect was measured to be constant with temperature, within the measurement precision, up to half of the melting point of copper. It is expected that the size effect will remain constant with temperature until diffusion-controlled dislocation motion becomes significant at higher temperatures and/or lower strain rates. Furthermore, the annealing treatment of the copper micropillars produced structures which yielded at stresses three times greater than their un-annealed, FIB-machined counterparts.     

Coulomb excitation of 72 30Zn42

The reduced transition probability B(E2: 0₁ ⁺→ 2⁺) of ⁷²Zn has been measured for the first time by Coulomb excitation at intermediate energy. The result B(E2: 0₁ ⁺→ 2⁺) = 1740±210 e²fm⁴, corresponds to the deformation parameter β₂ of 0.23, in close agreement with expectations derived from the neighboring nucleus ⁷³Zn. A discussion of the evolution of the N = 40 sub-shell closure as a function of Z is presented. Received: 19 December 2001 / Accepted: 14 March 2002     

Intramolecular motion in uranium β-diketonates in solution

A ¹H NMR study of the chelate tetrakis (dipivaloylmethanato) uranium IVin CS₂ solution is presented. An intramolecular rearrangement is shown to take place between conformers of D₂ symmetry.The kinetics of this motion is also determined.     

Inclusive measurements of the break-up of 156 MeV6Li-ions at extreme forward angles

Inclusive alpha particle and deuteron spectra from collisions of 156 MeV⁶Li-ions with¹²C and²⁰⁸Pb were measured at extreme forward emission angles including zero degree. The measurements were performed with the Karlsruhe magnetic spectrograph ‘Little John’ and required an efficient reduction of the background from small-angle scattering. The observed double differential cross sections and angular distributions have been analysed on the basis of Serber's spectator break-up model. When going to angles smaller than grazing, where Coulomb effects are expected to be dominating, transitional features may appear. Corresponding effects probably associated with Coulomb break-up are observed with the²⁰⁸Pb-target and require a slight extension of the Serber approach. In the case of the¹²C-target the break-up cross sections in forward direction seem to reflect the shape of the internal momentum distribution of the alpha particle and deuteron cluster in the⁶Li-projectile and are in agreement with a 2S-type wave function. However, at larger angles the shape appears to be distorted, possibly by final state interactions.     

Extremalität von Ellipsoiden und die Faltungsungleichung von Sobolev

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