Objectives and Challenges

According to what we know today about stellar nucleosynthesis, heavy elements up to the Fe-Ni region are produced in cosmos primarily by charged-particle thermo-nuclear reactions either in sequential order (chain reactions) or by forming cycles. These reactions may be accompanied by β decays. Under certain stellar temperature conditions, they compete with their reverse photodisintegration reactions. The mass domains, where these chain reactions or cycles of charged-particle reactions operate, driving nucleosynthesis to higher masses, are shown in panel (a) of Fig 1.

When reaching Fe, the Coulomb barrier becomes the highest, making any attempt to proceed to heavier masses with charged-particle induced reactions almost impossible. To overcome this problem, nature employs neutrons to be captured by already formed nuclides, which then decay by emitting electrons (β− decays) and nucleosynthesis proceeds to heavier mass regions with two main processes, the s process [5] and r process [6].

Although the vast majority of the nuclides heavier than iron are synthesized by these two processes, there exist 35 nuclei that cannot be synthesized by either the s- or the r process. These nuclei, known as p nuclei, lie “north-west” of the stability valley between 74Se and 196Hg, as indicated with black squares in panel (b) of figure 1. Their origin in the cosmos is still a complicated puzzle yet to be solved by any model of heavy-element nucleosynthesis. Nevertheless, to date, p nuclei have been observed only in the solar system and p-process nucleosynthesis models are able to reproduce most of their abundances within a factor of 3 but fail completely in the case of the light p nuclei, i.e., p nuclei with atomic number Z≤ 120.

Intense scientific efforts from the astrophysics side are devoted to the understanding of the synthesis of the p nuclei on the basis of astrophysical processes occurring outside the solar system, like exploding supernovae of type II (SNII) or on Helium-accreting white dwarves with mass below the Chandrasekhar limit. However, as is all too often stressed by astronomers and astrophysicists, in order to explain the solar-system p-nuclei abundances and on top of any astrophysical model improvements, it is imperative that the nuclear physics uncertainties entering abundance calculations are reduced or set under control at least. Given this challenge, the main goal of the present project is to shed light on nuclear physics aspects of the p-process puzzle that still remain diffuse.

Indeed, in addition to the uncertainties of pure astrophysics nature, nuclear physics uncertainties also need to be considered, since astrophysical abundance calculations require knowledge of a huge number of reaction cross sections and β−decay rates exceeding 20000. This number is, in fact, the number of nuclear reactions entering a reaction network involving almost 2000 stable or unstable isotopes heavier than iron. Given these numbers, it becomes obvious that the measurement of the necessary cross sections of every single reaction of the network is a quite unrealistic task. To overcome this problem, abundance calculations make an extensive use of the Hauser-Feshbach (HF) theory [7] that provides the cross-section σ of any nuclear reaction and, hence, the corresponding reaction rate using Eq. 1. For the HF calculations certain nuclear properties, i.e., the nucleon-nucleus and α-particle–nucleus Optical Model Potentials (OMPs), the Nuclear Level Densities (NLDs) and the γ-ray strength functions (γSFs) are required as input (see, e.g., in [8] or [9] for these properties) Under these conditions, the nuclear physics uncertainties affecting the abundance calculations concern uncertainties in the OMPs, NLDs and γSFs, which are described either by microscopic or phenomenological models. Depending on the model used, the resulting HF reaction cross-sections and reaction rates may deviate significantly. Hence, for a sensitive reliability check of the HF calculations is imperative to assess the nuclear physics uncertainties involved in p-nuclei abundance calculations. This check can be performed by comparing HF calculations with experimental cross sections. For this reason, the proposed project combines experimental activities with theoretical calculations and the specific research objectives of the proposal are:

1. To establish a reliable database of proton and α-particle induced capture-reaction cross sections in medium-mass nuclei. To achieve this, systematic cross section measurements need to be performed together with a compilation and a thorough evaluation of existing data.
2. To improve existing models entering HF calculations and, especially, develop a microscopic α-particle–nucleus Optical Model Potential (αOMP), as this is still poorly known. Consequently, the (α,γ) cross sections obtained from HF calculations can be highly uncertain and abundance calculations may strongly be affected.

Both objectives are strongly interconnected: Testing the credibility of models requires to compare calculations with reliable experimental cross sections, i.e., data with as small as possible uncertainties. On the other hand, in order to conduct measurements capable to provide data for model testing, one needs to perform first HF calculations to pinpoint the energies, where the predictions of different models differ at the most and, therefore, measurements only in these energies could provide conclusive results.