The origin of low-abundance neutron-deficient stable isotopes and the relevant cross section measurements
The synthesis of the elements beyond iron has been known to be dominated by the rapid neutron capture(r-process)and slow neutron capture(s-process).Nevertheless,approximately 30 stable,neuron-deficient isotopes between 74Se and 196Hg,known as the p-nuclei,cannot be synthesized in that way.This is because they are shielded by the valley of β stability,which cannot be reached by the s-process and r-process flows.The γ process,also often referred to as the p-process,was proposed as one of the most promising candidates for producing the bulk of p-isotopes.The origin of these p-nuclei is explained by the burning of pre-existing more neutron-rich isotopes in stellar environments of high enough temperature(T9≡ T/(109 K)=[2,3]),where photodisintegrations of such nuclei can occur.Such temperature conditions are fulfilled in the oxygen-/neon-rich layers of Type Ⅱ supernovae(SNII)or in Type Ⅰa supernovae(SNIa).The γ-process starts with sequences of(γ,n)reactions.Several mass units away from stability,the(γ,n)reactions will compete with(γ,p)and(γ,α)reactions as well as β-decays,leading to deflections in the γ-process path.Nowadays,the contribution of different stellar sources to the observed distribution of p-nuclei in the solar system is still under debate.The reliable modeling of the γ-process flows typically necessitates the consideration of an extensive network of approximately two thousand nuclei and thousands of reactions.The largest part of the γ-process reaction network lies in the region of neuron-deficient unstable nuclei,most of the reaction rates are not yet accessible by experiments.The scarcity of the relevant information makes it mandatory to rely heavily on rate predictions.Such predictions are exclusively based on the Hauser-Feshbach(HF)statistical model and the various nuclear ingredients required in such a framework.These nuclear ingredients include the optical model potential(OMP),gamma-ray strength function(GSF),and nuclear level density(NLD).It is important to test the reliability of these nuclear physics input in the HF model based on the rare experimental data.Experimentally,the γ-process nucleosynthesis can be studied by the γ-induced and proton capture reactions.Under the stellar conditions relevant to p-nuclei synthesis,all constituents of the stellar plasma,including nuclei,are in thermal equilibrium.This implies that a fraction of the nuclei will be present in an excited state.Most of the rate measurements involve the target ground state only,so that a correction factor,called the stellar enhancement factor,has to be applied to the laboratory data in order to account for the possible contribution of the target excited states to the stellar capture rates.Owing to the huge effect of the stellar enhancement factor in the case of γ-induced reactions,it is preferable to study the inverse capture reactions on the basis of the reciprocity theorem.There have been various experimental investigations of charged particle capture reactions.These measurements have been performed with charged particle beams impinging onto stable or long-lived targets and the cross-sections determined by activation and in-beam γ-ray detection techniques.While successful,these techniques have been limited to long-lived isotopes whose chemical properties allow for a target,typically isotopically enriched,to be made.To overcome this limitation and expand the experimental scope for capture reaction measurements,reactions can be performed in inverse kinematics with heavy beams impinging on p or α targets.In this paper,we review the physical mechanisms of the γ-process and the astrophysics of Hauser-Feshbach models.We introduce the experiment methods and the progress in p-nuclei research,including our recent work on the Dy isotopes.
nucleiorigin of elementsnucleosynthesisγ-processreaction cross section
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维普
