Core photo effect

Nuclear photoeffect (term in radiation physics ) or photo disintegration (term in astrophysics ) are nuclear reactions triggered by the impact of a photon , in which one or a few components are "knocked out" of the target core , e.g. B. one or two neutrons , a proton or even an alpha particle (i.e. a helium- 4 atomic nucleus ). The name was chosen because of the conceptual similarity with photoionization in the atomic shell ; The latter is usually simply called "photo effect" in the technical language of nuclear physics.

The short notation usual for nuclear reactions is therefore -, -, - or -reactions. ${\displaystyle ~(\gamma ,\mathrm {n} )}$ ${\displaystyle ~(\gamma ,\mathrm {2n} )}$ ${\displaystyle ~(\gamma ,\mathrm {p} )}$ ${\displaystyle ~(\gamma ,\alpha )}$

The energy of the photon must at least correspond to the binding energy of the weakest bound nucleon in the nucleus for the effect to take place. For example, the energy required for a reaction with deuterium is 2.225 MeV . ${\displaystyle ~(\gamma ,n)}$ ${\displaystyle (^{2}\mathrm {H} )}$

Medical radiation protection

The core photo effect occurs in the energy range above 2.18 MeV and plays an important role in radiation protection in medicine. In classical photon radiation therapy , energies up to 18 MeV are used. Between the radiation source and the patient there is room air that becomes radioactive due to the nuclear photo effect . These are short-lived radionuclides . In order to protect the medical and technical staff from this radiation, air suction devices are used, which are to be monitored from the outside. Since the half-life of the radionuclides is short, this radiation exposure no longer affects people after leaving the radiation protection bunker.

astrophysics

Shortly after the Big Bang, the photo-disintegration caused the destruction of the deuterium nuclei that had just formed (see nucleosynthesis ). But it also continuously plays a role in stars with more than eight solar masses that have reached the neon phase :

${\displaystyle ^{20}\mathrm {Ne} +\gamma \rightarrow \,^{16}\mathrm {O} +\,^{4}\mathrm {He} }$

When burning silicon , the last burning phase of a star, the following photo-disintegrations are possible:

${\displaystyle ^{28}\mathrm {Si} +\gamma \rightarrow \,^{27}\mathrm {Al} +\,\mathrm {p} }$

${\displaystyle ^{28}\mathrm {Si} +\gamma \rightarrow \,^{24}\mathrm {Mg} +\,^{4}\mathrm {He} }$

Individual evidence

1. ↑ Hanno Krieger: Fundamentals of radiation physics and radiation protection. 4th edition, ISBN 3834818151
2. ↑ W. Rapp, J. Görres, M. Wiescher, H. Schatz, F. Käppeler: Sensitivity of p-Process Nucleosynthesis to Nuclear Reaction Rates in a 25 M ☉ Supernova Model . In: The Astrophysical Journal . tape 653 , 2006, p. 474-489 , doi : 10.1086 / 508402 .