In this review, we cover radiative capture reactions in various astrophysical environments but specifically exclude big bang nucleosynthesis and the neutron captures of the s and r processes. Inference of thermonuclear reaction rates from the analysis of related transfer and breakup reactions also relies on models of direct nuclear reactions. Extrapolation of measured radiative capture reaction cross sections downward from accessible to astrophysical energies can be done reliably only with the aid of nuclear theory. Theoretical insight is required at nearly every step. The difficulties of direct measurements at astrophysical energies necessitate the use of indirect experimental techniques involving intermediate energy transfer and breakup reactions that have much higher yields. When one of the reactants is radioactive, arranging for sufficient beam intensity or target thickness to enable a measurement at low energy is very challenging.
Measuring the cross sections of radiative capture reactions at the relevant energies is the most reliable way to evaluate their rates, but due to the vanishingly small reaction probability this is not possible for every interesting case. As such, they often control the reaction flow and rate of nucleosynthesis in a process. Second, radiative capture reactions proceed slowly compared with strong interactions, rendering them rate-limiting steps in a number of reaction pathways and cycles. First, for many nuclei, radiative captures are the only proton- or α-induced reactions possible with positive Q values, making knowledge of their rates essential for determining reaction pathways and energy release. Two features make them particularly influential in stellar astrophysics. In addition, radiative capture reactions are prominent in the explosive conditions found in novae, X-ray bursts, and supernovae. Their involvement in powering the stars via the pp chain and CN cycle was first described by Bethe & Critchfield ( 1) and Bethe ( 2), respectively, in 1938. Radiative capture reactions, namely those in which an atomic nucleus fuses with one or more nucleons or nuclei with the emission of electromagnetic radiation, play an important role in astrophysics.
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