Mark Huyse

The SPEDE spectrometer: combined in-beam

The SPEDE spectrometer [1] aims to combine a silicon detector, for the detection of electrons, with the MINIBALL γ-ray detection array for in-beam studies employing radioactive ion beams at the HIE-ISOLDE facility at CERN. The setup will be primarily used for octupole collectivity [2] and shape coexistence studies [3, 4] in Coulomb excitation experiments. In the shape coexistence cases the transitions between states of the same spin and parity have enhanced E0 strength [5]. Additionally the 0→0 transitions, typically present in nuclei exhibiting shape coexistence [6], can only occur via E0 transitions, i.e. via internal conversion electron emission.

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The radioactive beam facility of Louvain-la-Neuve, based on a two-cyclotron scheme, currently produces post-accelerated beams of {sup 6}He, {sup 13}N, {sup 18}Ne and {sup 19}Ne. In the past two years the main effort has been concentrated on measurements dealing with the {sup 13}N(p,{gamma}){sup 14}O cross section, which opens the hot CNO cycle as an alternative to the standard cold CNO cycle. First of all, the authors measured the {sup 13}N(p,{gamma}){sup 14}O cross section, which at stellar energies, is dominated by a resonance corresponding to the first excited state of {sup 14}O (E{sub exc} = 5.17 MeV), which subsequently decays by {gamma} radiation to the ground state with a probability of about 10{sup {minus}4}. The p + {sup 13}N elastic scattering was measured to better determine the resonance energy and width. Finally the measurement of the {sup 13}N(d,n) {sup 14}O reaction provided some knowledge on the nonresonant {sup 13}N(p,{gamma}){sup 14}O cross section. The same set of measurements will soon be performed on the {sup 19}Ne(p,{gamma}){sup 20}Na reaction, responsible for the breakout from the hot CNO cycle. The excited states of {sup 20}Na which are the most important for astrophysics are not yet known, due to the uncertainty on the spin, parity, morexa0» and partial widths of the levels between threshold (2.2 MeV) and 3.1 MeV excitation energy in {sup 20}Na. Two different techniques will be used to measure the {sup 19}Ne(p,{gamma}){sup 20}Na cross section: detection of the positrons from the decay of {sup 20}Na; or of the delayed {alpha} particles from the decay of some {sup 20}Ne* levels fed by the decay of {sup 20}Na. The p+{sup 19}Ne elastic scattering will be studied with a large silicon strip detector covering a solid angle of 2{pi} in the centre of mass frame. «xa0less

-ray and conversion electron spectroscopy with radioactive ion beams

By studying the β +/EC and α decay of mass—separated neutron—deficient Hg to Fr nuclei, the spectroscopy group at the Leuven Isotope Separator On Line (LISOL) project collected in recent years extensive systematics on shell—model intruder states in the Pb region 1–10. These shell—model intruder states, leading to shape coexistence, have been identified in odd—proton At, Bi and Tl nuclei 3–4, odd—neutron Pb nuclei 8 9, even — even Pb nuclei 1 2 5 7 and odd—odd Tl nuclei 6 10. A remarkable systematic behavior of the excitation energy of the intruder—based states as a function of neutron number is now evident and several theoretical descriptions have been developed to reproduce this specific trend (see the references 11 to 15 and the contributions to this conference by K. Heyde, R. Bengtsson and N. Tajima).