Photonuclear Reactions in Astrophysics

Abstract

Photodisintegration in stellar environmentsNucleosynthesis in stars and stellar explosions proceeds via nuclear reactions in thermalized plasmas. Nuclear reactions not only transmutate elements and their isotopes, and thus create all known elements from primordial hydrogen and helium, they also release energy to keep stars in hydrostatic equilibrium over astronomical timescales. A stellar plasma has to be hot enough to provide sufficient kinetic energy to the plasma components to overcome Coulomb barriers and to allow interactions between them. Plasma components in thermal equilibrium are bare atomic nuclei, free electrons, and photons (radiation). Typical temperatures of plasmas experiencing nuclear burning range from 10 7 K for hydrostatic hydrogen burning (mainly interactions among protons and He isotopes) to 10 10 K or more in explosive events, such as supernovae or neutron star mergers. This still translates into low interaction energies by nuclear physics standards, as the most probable energy E between reaction partners in terms of temperature is derived from Maxwell-Boltzmann statistics and yields = 9 11.6045 ⁄ MeV, where T9 is the plasma temperature in GK.Photodisintegration reactions only significantly contribute when the plasma temperature is sufficiently high to have an appreciable number of photons (given by a Planck radiation distribution) at energies exceeding the energy required to separate neutrons, protons, and/or  particles from a nucleus. Forward and reverse reactions are always competing in a stellar plasma and thus photodisintegrations have to be at the same level or faster than capture reactions in order to affect nucleosynthesis. Since the number of captures per second and volume (the capture rate) not only scales with temperature but also with plasma density [1], the threshold temperature at which photodisintegrations cannot be neglected is higher for denser plasmas. On the other hand, photons require less energy when the particle separation energies are small. This is the case when approaching the neutron-or proton-dripline or for (,) reactions in the region of spontaneous -emitters. Based on the reciprocity relation for nuclear reactions, the principle of detailed balance can be derived, which relates the reactivity of the forward reaction (capture) rf * to the reaction rate of the reverse reaction r * (photodisintegration) [1]. Apart from factors containing spin weights and reduced masses, r * is proportional to 9 3 2 ⁄ exp(−11.6045 / 9 ) * , where Q is the reaction Q-value given in MeV, describing the energy release of the forward reaction. This means that mainly the Q-value sets the temperature at which the reverse reaction becomes fast enough to compete with the forward reaction and affect the amount of a given nuclide in the stellar plasma. The Q-value of a capture reaction is just the separation energy of the projectile in the final nucleus.Due to the above relations, photodisintegrations are found to be important in roughly three contexts: 1. (Almost) complete photodisinteg...

Horizons: nuclear astrophysics in the 2020s and beyond

“…Although the p-process consists of photodisintegration reactions starting on stable seed nuclei and moving off stability to neutron-deficient nuclei, these reactions are more often better probed with the inverse charged-particle reaction measurements [262]. Nonetheless, γ-beam facilities such as HIγS can provide unique insights for special cases [263].…”

Horizons: Nuclear Astrophysics in the 2020s and Beyond

“…Although the p-process consists of photodisintegration reactions starting on stable seed nuclei and moving off stability to neutron-deficient nuclei, these reactions are more often better probed with the inverse charged-particle reaction measurements [259]. Nonetheless, γ-beam facilities such as HIγS can provide unique insights for special cases [260].…”

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

“…Additionally, it has been suggested that photodisintegration experiments can also probe particle emission to excited states in the final nucleus and thus provide information on the transitions needed to calculate astrophysical capture rates [38][39][40]. This allows to better constrain the reaction models by being able to test calculations, e.g., by using different optical potentials in the computation of the relevant transitions and comparing them to the data [41].…”