Feasibility of studying astrophysically important charged-particle emission with the variable energy γ-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, (gamma, p) and (gamma, alpha) 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 gamma-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 alpha 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 (gamma, p) reaction on 12 targets of Si-29, Fe-56, Se-74, Sr-84, Zr-91, Ru-96,Ru-98, Pd-102, Cd-106, and Sn-115,Sn-117,Sn-119, and the (gamma, alpha) reaction on five targets of V-50, Sr-87, Te-123,Te-125, and Sm-149, the yields of the reaction channels with the transitions to the excited states in the residual nucleus, namely (gamma, X-i) with i not equal 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 alpha-particle yields, the minimum required gamma-beam energies E-low 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 (gamma, X-i) channels within a specific, narrow energy range of the incident gamma beam. If the actual measurements of these photon-induced reactions are performed in this energy range, the sum of the yields of the dominant (gamma, X-i) 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 (gamma, X-i) 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 (gamma, Xi) channels with 0 <= i <= 10 are simulated for each considered reaction, with the incident gamma-beam energies in the respective energy range as derived before. Based on the energy spectra, the identification of the individual dominant (gamma, 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.

Automated summary

This study investigates the contribution of capture and photodisintegration reactions on excited states in the nucleus to the astrophysical reaction rate in hot plasma environments like stellar explosions. Experimental measurements are proposed to improve the predictive power of theoretical reaction rates and constrain reaction models. The feasibility of measuring (gamma, p) and (gamma, alpha) reactions using the ELISSA detector system at the VEGA facility is investigated. The results show that these measurements are feasible and provide valuable knowledge for nuclear astrophysics.