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Up to date the situation concerning the ability of the existing p-process nucleosynthesis models to reproduce the p-nuclei solar-system abundances within a factor of 3 is improved, but they still fail to do so in many cases, with the most striking one being that of 92Mo, which is taken to be the performance indicator for all p-process models. It was often claimed by certain p-process scenarios that the previously observed strong discrepancies, partly up to two orders of magnitude, between the solar-system abundance of 92Mo and the corresponding predictions have been drastically reduced. In all such cases, however, this reduction required unrealistic parameters in the description of the astrophysical conditions. In conclusion, strong discrepancies are still on table indicating that serious deficiencies still exist in the pure astrophysical modeling of the p process and uncertainties in the nuclear physics parameters entering abundance calculations are still not under control.

Under these conditions, the majority of the experimental efforts to set the aforementioned nuclear physics uncertainties under control agree in the following:

• In most cases of (p,γ) cross-section investigations, the uncertainties affecting nuclear input (OMP, NLD, γSF) give rise to at most 50% uncertainties in the reaction rates that are used in abundance calculations. The Hauser–Feshbach predictions are more sensitive to the proton-nucleus Optical Model Potential (pOMP) at energies below ≈2.5 MeV rather than to the Nuclear Level Densities (NLDs) and the γ-ray strength functions (γSFs). However, the latter two quantities, and especially the γSFs, have a strong contribution to HF calculations above this energy. This implies that the existing models for all three nuclear parameters need to be properly investigated. Moreover, no global predictions are possible at this stage using a given OMP-NLD-γSF combination in the HF calculations, although the pOMP seems to be well described by the phenomenological OMP model proposed by Koning and Delaroche [10].
• Principally, (p,γ) cross-section measurements alone cannot provide clear conclusions on the best NLDs and γSFs. They could, however, distinguish between different pOMPs, if conducted at properly selected proton energies. The rule hereby is to measure (p,γ) cross sections at the lowest possible energies This rule pre-assumes, however, that at these energies, the competing proton-induced reactions, i.e., the (p,p’), (p,n) and (p,α) channels, are closed or negligible.
• In recent investigations of our group, an attempt to improve the semi-microscopic pOMP of Bauge et al. [11] at energies relevant to the p process was successfully made. For this purpose, the normalization factors for the real and imaginary components of this pOMP were optimized through comparison with our experimental data. Furthermore, using the optimized normalization factors, we were able to reproduce some of our data with a combination of purely semi-microscopic models of OMPs, NLDs and γSFs. The achieved agreement compares equally good with a combination of the most advanced purely phenomeno-logical models. Further improvements have been identified and will be implemented within the course of the present project.
• The investigation of the so-called primary γ-transitions, that are discussed in section 2.1 below, can provide important information on the shape of the γSF as well as conclusions on the predictive power of existing γSF models. The research groups from the institutions collaborating with us in the framework of this project (iThemba LABS, Cape Town and Inst. of Nuclear Physics, Univ. of Cologne) have a long-term expertise in this type of investigations. It is expected that through this collaboration, we will also be able to check the predictions of existing γSF models through joint measurements.
• The conclusions based on (α,γ) cross-section investigations are different: The α-nucleus OMP (αOMP) is still poorly known; Consequently, the astrophysical (α,γ) reaction rates obtained from HF calculations can be highly uncertain and abundance calculations may strongly be affected. So far, the semi-microscopic αOMP of Demetriou et al., [12] reproduces new experimental data. However, more data are needed in the mass range A≈90-100 and higher, to further constrain potential parameters. This is a key task in the workplan of our proposal and a further development of the αOMP of Demetriou et al., is among its goals.
• Some recent developments in the description of the αOMP by others, have been successfully tested in HF calculations. However, they all refer to phenomenological models and as such they have no global character, which is most wanted in our efforts to calculate reaction rates across the table of isotopes from iron up to bismuth.

In terms of research methodology, many new approaches will be applied for the first time here. Hence, for our measurements, we will employ:

• an array of brand new hyper-pure germanium detectors (HPGe) that will be surrounded by BGO scintillating detectors acting as Anti-Compton shields to eliminate the Compton background,
• a brand new large-volume NaI(Tl) calorimeter (NEOPTOLEMOS) with unprecedented absolute efficiency for γ rays, and
• a “sphere” of 80 BGO scintillator detectors (GASPAR) donated by the Italian national nuclear physics laboratory of Legnaro (LNL).

These three instruments are installed at the TANDEM accelerator laboratory of NCSR “Demokritos”. Their value together with their electronic units and supporting structures, exceeds one million Euros. In addition, the first two detection systems are installed at the same beamline at a distance of ≈2 meters. This allows for translating in vacuum, with a proper mechanical arm, the target from the HPGe location to that of the NaI(Tl) calorimeter, combining this way almost at the same time two different experimental methods. Moreover, with the GASPAR BGO sphere, it will be possible to determine the so-called “γ-multiplicity” of the reaction under investigation. Knowledge of this quantity is crucial for extracting the absolute efficiency of the NaI(Tl) calorimeter NEOPTOLEMOS and, hence, to conduct reliable measurements.

Additional measurements with state-of-the art detectors and other supporting facilities are proposed to be conducted at the 3MV TANDETRON accelerator of the South African National Laboratories for Accelerator-Based Sciences (iThemba LABS) in Cape Town as well as at the 10MV TANDEM accelerator of the Institute of Nuclear Physics of the University of Cologne, Germany, using the HORUS multi-detector array [13]. HORUS can deliver almost background free spectra, whereas experiments at the iThemba’s 3MV Tandetron accelerator will allow for cross section measurements at very low proton beam energies, even below 1.5 MeV, due to the high-current proton beams (up to 1 mA) that can be delivered by the most advanced Multi-Cusp-type ion sources installed at this laboratory.

The use of three independent methods and complementary experimental setups will allow for an independent triple check of the cross-section results, the optimization of the experimental effort, a thorough investigation of systematic errors and a unique training of the young researchers who will be hired in state-of-the art detector technologies and nuclear instrumentation, in general. All instruments that will be available for the needs of the ARENA project are displayed in figure 2.