Feasibility of studying astrophysically important charged-particle emission with the variable energy gamma-ray system at the Extreme Light Infrastructure-Nuclear Physics facility

In the environment of a hot plasma, as achieved in stellar explosions, capture and photodisintegration reactions
proceeding on excited states in the nucleus can considerably contribute to the astrophysical reaction rate. Usually,
such reaction rates including the excited-state contribution are obtained from theoretical calculations as the
direct experimental determination of these astrophysical rates is currently unfeasible. Future experiments could
provide constraining information on the current reaction models which would improve the predictive power
of the theoretical reaction rates. In the present study, experiments of photodisintegration with charged-particle
emission leading to specific excited states in the residual nucleus are proposed. The expected experimental results
can be used to determine the particle-transmission coefficients in the model calculations of photodisintegration
and capture reactions. With such constrained transmission coefficients, the astrophysical reaction rates especially
involving the excited-state contributions can be better predicted and implemented in astrophysical simulations.
In particular, (γ , p) and (γ,α) reactions in the mass and energy range relevant to the astrophysical p process are
considered and the feasibility of measuring them with the ELISSA detector system at the future Variable Energy
γ -ray (VEGA) facility at Extreme Light Infrastructure–Nuclear Physics is investigated. To this end, in a first step
17 reactions with proton emission and 17 reactions with α emission are selected and the dependence of calculated
partial cross sections on the variation of nuclear property input is tested. The simulation results reveal that, for the
(γ , p) reaction on 12 targets of 29Si, 56Fe, 74Se, 84Sr, 91Zr, 96,98Ru, 102Pd, 106Cd, and 115,117,119Sn, and the (γ,α)
reaction on five targets of 50V, 87Sr, 123,125Te, and 149Sm, the yields of the reaction channels with the transitions to
the excited states in the residual nucleus, namely (γ , Xi) with i = 0, are relevant and even dominant. Therefore,
these 17 reactions are considered in the further feasibility study. For each of the 17 photon-induced reactions, in
order to attain the detectable limit of 100 counts per day for the total proton or α-particle yields, the minimum
required γ -beam energies Elow for the measurements are estimated. It is further found that for each considered
reaction, the total yields of the charged-particle X may be dominantly contributed from one, two, or three (γ , Xi)
channels within a specific, narrow energy range of the incident γ beam. If the actual measurements of these
photon-induced reactions are performed in this energy range, the sum of the yields of the dominant (γ , Xi)
channels can be approximated by the measured yields of the charged particle X within acceptable uncertainty.
This allows to experimentally obtain the yields of the (γ , Xi) channels which dominantly contribute to the
total yields of X. Using the simulated yields, these energy ranges for each of the 17 photon-induced reactions
are derived. Furthermore, the energy spectra of the (γ , Xi) channels with 0 i 10 are simulated for each
considered reaction, with the incident γ -beam energies in the respective energy range as derived before. Based
on the energy spectra, the identification of the individual dominant (γ , Xi) channels is discussed. It becomes
evident that measurements of the photon-induced reactions with charged-particle emissions considered in this
work are feasible with the VEGA+ELISSA system and will provide knowledge useful for nuclear astrophysics.