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Comprehensive study of γ-ray strength function: microscopic model calculation, experimental determination, and astrophysical application
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Acronym: COMgSF
Contracting Authority: UEFISCDI
Number / Date of the contract: 103 PCE / 2025-08-01
Program:
Project Manager: Yi Xu
Partners:
Starting date / finishing date: 2025-08-01 / 2028-07-31
Project value: 1 137 368 RON

Abstract: Study of γ-ray strength function (γSF) attracts intensive attention in nuclear physics community. In this project, three activities related to γSF are proposed:
(I) We will perform systematic study of the magnetic dipole (M1) and electric quadrupole (E2) γSF using microscopic Hartree-Fock-Bogoliubov plus Quasiparticle Random Phase Approximation theory for about 10000 nuclei with Z in [8,124] lying between proton and neutron drip-lines. Additional parameterization on such microscopic calculation will be implemented considering experimental constraints. For the first time, the M1 and E2 γSF for all these nuclei will be investigated within the same theory.
(II) Currently the experimental (γ,p) cross section for most target is not available at lower energies. Therefore, we propose to measure the (γ,p) cross section, in order to experimentally determine the photo-absorption cross section including the complete channels of (γ,γ’), (γ,n) and (γ,p). Such photo-absorption cross section is crucial to verify and determine the γSF. In particular, we will perform GEANT4 simulation of the (γ,p) measurement for interesting targets, develop the needed charged-particles detector, and conduct the scheduled (γ,p) measurement.
(III) Using our previous E1 γSF and the M1+E2 γSF to be obtained from (I), we will compute the cross sections and astrophysical reaction rates of capture reactions. It is expected that the reaction rates derived from the new γSF could be used for nucleosynthesis study.

Motivation:
The reliability of the photon strength predictions can be greatly improved by the use of microscopic model, simultaneously taking into account the constraints described by phenomenological corrections. Efforts can be found in Refs. [1-6] where the electric dipole (E1) photon strengths were derived from quasiparticle random-phase approximation (QRPA) calculations. Furthermore, parameters determined from available experimental data are taken into account when folding the QRPA γ-ray strength with a Lorentzian-type expression. Note that for practical applications, such phenomenological corrections with experimental data on mean-field plus QRPA calculations are needed.
The experimental photo-absorption cross section is crucial to determine the γ-ray strength function. In principle, the photo-absorption cross section consists of three contributions, namely the cross sections of photo scattering (γ,γ’), photon neutron emission (γ,n) and photon proton emission (γ,p). The neutron emission channel (γ,n) is generally the predominant channel. However, the (γ,p) channel can be important for some light and medium nuclei and neutron-deficient nuclei. The competition between (γ,n) and (γ,p) cross sections may impact various nuclear and nuclear astrophysics scenarios, for example the nuclear collective properties, isospin asymmetry, and various application of photonuclear physics including astrophysical reaction path in nucleosynthesis and medical radioisotope productions [7,8]. Radiative capture cross sections play a key role in nucleosynthesis studies. Despite a huge effort to measure such capture reaction cross sections, theoretical predictions are required to fill the gaps, both for the unstable targets of which measurements are not feasible at the present time, and for the energies that cannot be reached in the laboratory. In particular, applications of nuclear astrophysics also require the determination of radiative capture cross sections and astrophysical reaction rates for a large number of exotic neutron-rich nuclei [9]. In this case, large-scale calculations need to be performed on the basis of sound and accurate nuclear reaction and structure models, to ensure a reliable extrapolation far away from the experimentally known region.

Objectives:
Nuclear excitation and decay via γ-ray absorption and emission have been attracting a lot of attention from nuclear physics community. Valuable theoretical and experimental investigation of the γ-ray strength function for the whole nuclear chart is of great interest, especially in nuclear astrophysics. A comprehensive study of γ-ray strength function is proposed in this project, which includes three activities.
(Activity I) We will present the systematic study of the magnetic dipole (M1) and electric quadrupole (E2) γ-ray strength functions using microscopic Hartree-Fock-Bogoliubov plus Quasiparticle Random Phase Approximation (HFB+QRPA) theory for about 10000 nuclei with 8≤Z≤124 lying between the proton and the neutron drip-lines on nuclear chart.
(Activity II) We propose to measure the (γ,p) cross section on interesting targets, to obtain the complete photo-absorption cross section including the present (γ,p) cross sections. Such photoabsorption cross section can be used to experimentally verify and determine the photon strength function. In particular, we will perform feasibility study of the (γ,p) measurements focusing on the simulation and development for charged-particles detector. (Activity III) We will conduct large-scale calculation of the cross sections and astrophysical reaction rates for neutron and proton capture reactions, by employing the complete γ-ray strength functions including the contributions from E1, E2 and M1 γ-ray strength functions. It is expected that the promising results can be used for further astrophysical simulation of nucleosynthesis.

Implementation plan:
Year 1:
(1) Update and develop the computer program for the calculations of M1 and E2 γ-ray strength functions in the framework of HFB+QRPA theory.
(2) Feasibility study of (γ,p) reactions, GEANT4 simulation of expected (γ,p) measurements.
Year 2:
(1) Calculate the M1 and E2 γ-ray strength functions for about 10000 nuclei with 8≤Z≤124 lying between the proton and the neutron drip-lines.
(2) Article preparation and results dissemination for the calculation of γ-ray strength function.
(3) Develop the new charged-particles detector and perform the 116Sn(γ,p) measurement.
(4) Calculate the cross sections and reaction rates for neutron and proton capture reactions using the γ-ray strength functions obtained from (1). Year 3:
(1) Conduct data analysis of 116Sn(γ,p) to obtain the cross section, and perform theoretical investigation on γ-ray strength function considering the new (γ,p) cross section.
(2) Article preparation and results dissemination for detector development and (γ,p) measurement results.
(3) Article preparation and results dissemination for the calculation of cross sections and astrophysical reaction rates.

References:
[1] S. Goriely, E. Khan, and M. Samyn, Nucl. Phys. A, 739, 331 (2004)
[2] Y. Xu, S. Goriely, and E. Khan, Phys. Rev. C, 104, 044301 (2021)
[3] M. Martini, S. Peru, S. Hilaire, S. Goriely, and F. Lechaftois, Phys. Rev. C, 94, 014304 (2016)
[4] G. Colo, and P.F. Bortignon, Nucl. Phys. A, 696, 427 (2001)
[5] A. A. Raduta, R. Budaca, and Al. H. Raduta, Phys. Rev. A, 79, 023202 (2009)
[6] V. Baran, D. I. Palade, M. Colonna, M. Di Toro, A. Croitoru, and A. I. Nicolin, Phys. Rev. C, 91, 054303 (2015)
[7] A. Zilges, D.L. Balabanski, J. Isaak, and N. Pietralla, Progress in Particle and Nuclear Physics, 122, 103903 (2022)
[8] D. Savran, T. Aumann, and A. Zilges, Progress in Particle and Nuclear Physics, 70, 210 (2013)
[9] M. Arnould, and S. Goriely, Progress in Particle and Nuclear Physics, 112, 103766 (2020)

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