Astrophysics
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The nucleosynthesis of mid-heavy and heavy elements in the Universe is one of the eleven big enigmas of Science. Its understanding is not immediate because it happens far from stability in explosive environments, in supernovae or neutron stars, and it concerns mainly nuclei that are impossible to produce on Earth in laboratory. In order to be able to extrapolate towards these nuclei, we have to understand the nuclear structure in general.
For example, all the explosive astrophysical processes are very sensitive to the nuclear shell closures, especially for the neutron-rich nuclei. The abundance of elements in the Universe undergoes sharp variations that are correlated to the shell closure positions. In the explosive phenomena, the surviving nuclei are localised at these shell closures where the most strongly bound nuclei are found. There are accumulations of these nuclei, the genitors of the stable nuclei via beta decay, creating abundance peaks of elements. To study if a shell closure survives or crumbles allows us to understand these abundance peaks and, beyond, to fix the stellar conditions that leads to these peaks. It is kind of a symbiosis of two approaches: an hydrodynamic approach via the junctions of stellar explosion models and a nuclear structure approach that has a fundamental importance for the localisation of these peaks.
With the development of radioactive beams for about ten years, physicists have surmised that the magic numbers that were postulated by our ancestors disappear for nuclei far from stability, already with a moderately high neutron/proton ratio. However, these were so far strong indications, as unusual behaviours of the mass surface or the binding energy, rather than proofs. With my former student Laurent Gaudefroy, we have submitted an article to Physical Review Letters on a study of the spin-orbit splitting in the shell N=28, which is the first magic number created by the spin-orbit potential as invoked by M. Goeppert Mayer. To determine to what extent the N=28 shell closure disappears, we have used a SPIRAL beam of 46Ar and the MUST array to induce (d,p) reaction to localise the occupied and valence orbitals in 47Ar. By comparing the level scheme of 49Ca to that of 47Ar, we have deduced that in 47Ar the N=28 is weakened by about 350 keV and that the spin-orbit potentials for the neutron p and f orbitals are reduced by 900 keV and 800 keV, respectively. The probable reason for such a large unexpected variations while removing only two protons from 49Ca are likely to be due to proton-neutron tensor forces and to the dependence of the spin-orbit interaction with the nuclear matter density.
In a near future, we would like to better pin-down which internal forces are causing these phenomena. To achieve this goal we need to know precisely from which in which orbitals the two protons have been removed from 49Ca to that of 47Ar. So far, we presumed that they are equally removed from the s1/2 and d3/2 orbitals. A different ratio would modify the strength of the tensor and density-dependent forces.
With the SPIRAL2 facility and the MUST2 detectors that are currently developed, we plan a very ambitious program in order to apply all what we learned for N=28 to the N=82 shell closure, where the abundance peaks of the elements around A=132 is built. We plan to investigate which nuclear forces come into play for modifying the N=82 shell closure while 10 protons are removed from the g9/2 orbital from 132Sn to 122Zr.
More info:
Olivier Sorlin's e-mail
Laurent Gaudefroy's e-mail
Astrophysics session
Words collected by K. Turzó at the XVe Colloque GANIL, Giens, France, from May 29th to June 2nd, 2006.
For example, all the explosive astrophysical processes are very sensitive to the nuclear shell closures, especially for the neutron-rich nuclei. The abundance of elements in the Universe undergoes sharp variations that are correlated to the shell closure positions. In the explosive phenomena, the surviving nuclei are localised at these shell closures where the most strongly bound nuclei are found. There are accumulations of these nuclei, the genitors of the stable nuclei via beta decay, creating abundance peaks of elements. To study if a shell closure survives or crumbles allows us to understand these abundance peaks and, beyond, to fix the stellar conditions that leads to these peaks. It is kind of a symbiosis of two approaches: an hydrodynamic approach via the junctions of stellar explosion models and a nuclear structure approach that has a fundamental importance for the localisation of these peaks.
With the development of radioactive beams for about ten years, physicists have surmised that the magic numbers that were postulated by our ancestors disappear for nuclei far from stability, already with a moderately high neutron/proton ratio. However, these were so far strong indications, as unusual behaviours of the mass surface or the binding energy, rather than proofs. With my former student Laurent Gaudefroy, we have submitted an article to Physical Review Letters on a study of the spin-orbit splitting in the shell N=28, which is the first magic number created by the spin-orbit potential as invoked by M. Goeppert Mayer. To determine to what extent the N=28 shell closure disappears, we have used a SPIRAL beam of 46Ar and the MUST array to induce (d,p) reaction to localise the occupied and valence orbitals in 47Ar. By comparing the level scheme of 49Ca to that of 47Ar, we have deduced that in 47Ar the N=28 is weakened by about 350 keV and that the spin-orbit potentials for the neutron p and f orbitals are reduced by 900 keV and 800 keV, respectively. The probable reason for such a large unexpected variations while removing only two protons from 49Ca are likely to be due to proton-neutron tensor forces and to the dependence of the spin-orbit interaction with the nuclear matter density.
In a near future, we would like to better pin-down which internal forces are causing these phenomena. To achieve this goal we need to know precisely from which in which orbitals the two protons have been removed from 49Ca to that of 47Ar. So far, we presumed that they are equally removed from the s1/2 and d3/2 orbitals. A different ratio would modify the strength of the tensor and density-dependent forces.
With the SPIRAL2 facility and the MUST2 detectors that are currently developed, we plan a very ambitious program in order to apply all what we learned for N=28 to the N=82 shell closure, where the abundance peaks of the elements around A=132 is built. We plan to investigate which nuclear forces come into play for modifying the N=82 shell closure while 10 protons are removed from the g9/2 orbital from 132Sn to 122Zr.
More info:
Olivier Sorlin's e-mail
Laurent Gaudefroy's e-mail
Astrophysics session
Words collected by K. Turzó at the XVe Colloque GANIL, Giens, France, from May 29th to June 2nd, 2006.
