Experiments

Photodissociation experiments of astrophysical interest - Mo isotopes as a test case (S295)

Astrophysical background

About half of the elemental abundances beyond the Fe-Ni-peak have been produced in explosive events. Such scenarios imply time scales of a few seconds and temperatures of 2 – 3 GK. In principle, these conditions allow to overcome the Coulomb barrier for the production of proton-rich nuclides by proton capture, but on the other hand also trigger photo-dissociation processes in the thermal photon bath. Proton- (and neutron-) captures as well as photo-dissociation processes of type (γ,n), (γ,p), and (γ,α) occurring on r and s process seed nuclei are important for the origin of the so-called p nuclei, stable proton-rich isotopes between Se and Hg, which cannot be made by the s and r neutron-capture processes [1].

Whereas proton capture is thought to dominate for light p nuclei, photo-dissociation of heavier s process seed nuclei (the so-called γ-process) most likely prevails in the production of the heavier p nuclei [1]. The γ-process sets out as a series of (γ,n) reactions until the p/α separation energies are low enough that (γ,p) and (γ,α) processes start to compete. When the stellar temperature drops, β-decays and neutron captures lead to the final p nuclei observed in nature. A theoretical calculation of the isotopic p abundances requires a huge reaction network involving thousands of isotopes and correspondingly large numbers of nuclear reactions (e.g. Ref.[1,2]). It is clear that most of the reaction rates have to be calculated, e.g. by the Hauser-Feshbach (HF) statistical model. It is important, however, that as many rates as possible are measured accurately to provide pivot points for the HF calculations.

The present proposal [...] aims at verifying that accurate (γ,n) data close to the neutron threshold can be obtained for medium-mass nuclei using the CD method at LAND. An especially interesting region of the nuclide chart in this context is centered around 92Mo. This isotope has one of the highest cosmic abundances of all p nuclei. This enhancement, although attracting much attention among theoretical nuclear astrophysicists, has not been explained even by recent nucleosynthesis calculations in massive stars [1,2]: the isotopes 92,94Mo and 96,98Ru are persistently underproduced in practically all state-of-the-art network calculations.

We propose to perform (γ,n) reactions on 92Mo and 100Mo with both, real and virtual photons, the former provided by bremsstrahlung photon beams at ELBE [6] and S- DALINAC [11], the latter provided by accelerating (stable and unstable) Mo beams at 208Pb targets. The expected GSI/SIS to about 500 A MeV and shooting them on thick agreement of both results should establish the accuracy of the CD method. Once this goal is achieved, CD measurements on many critical (but unstable) nuclei for the γ-process can be envisaged. As a first example we propose to measure CD for 93Mo, an unstable nucleus that cannot be prepared as a target.

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