Photonuclear reaction

Photonuclear reactions ( born photodisintegration , phototransmutation ) are nuclear reactions that occur when gamma quanta are absorbed by atomic nuclei ^[1] . The phenomenon of nucleon emission from nucleons during this reaction is called a nuclear photoelectric effect . This phenomenon was discovered by Chadwick and Goldhaber in 1934 ^[2] and was further investigated by Bothe and Wolfgang Gentner ^[3] , and later by Niels Bohr ^[4] ^[5] .

When a gamma-quantum is absorbed, the nucleus receives an excess of energy without changing its nucleon composition, and the nucleus with an excess of energy is a composite nucleus . Like other nuclear reactions, the absorption of a gamma-quantum by the nucleus is possible only with the fulfillment of the necessary energy and spin ratios. If the energy transferred to the nucleus exceeds the binding energy of the nucleon in the nucleus, then the decay of the resulting composite nucleus occurs most often with the emission of nucleons, mainly neutrons . Such decay leads to nuclear reactions. $(\gamma ,n)$ ${\ displaystyle (\ gamma, n)}$ ${\ displaystyle (\ gamma, n)}$ and $(\gamma ,p)$ ${\ displaystyle (\ gamma, p)}$ ${\ displaystyle (\ gamma, p)}$ which are called photonuclear , and the phenomenon of nucleon emission in these reactions is the nuclear photoelectric effect . Legend:

$\gamma$ ${\ displaystyle \ gamma}$ $\ gamma$ - particle of gamma radiation or gamma quantum ( photon with high energy);
$n$ ${\ displaystyle n}$ $n$ - neutron ;
$p$ ${\ displaystyle p}$ $p$ - proton .

In the theory of photonuclear reactions, the statistical model of the compound nucleus and the model of the resonant direct photoeffect are used ^[6] .

Photonuclear reactions proceed with the formation of a compound nucleus, but when the reactions are excited $(\gamma ,p)$ ${\ displaystyle (\ gamma, p)}$ ${\ displaystyle (\ gamma, p)}$ on mass number cores $A>100$ ${\ displaystyle A> 100}$ ${\ displaystyle A> 100}$ too large a yield was experimentally found compared to the yield predicted by this mechanism. In addition, the angular distribution of protons with the highest energy turned out to be non-isotropic. These facts point to an additional mechanism of direct interaction, which is essential only in $(\gamma ,p)$ ${\ displaystyle (\ gamma, p)}$ ${\ displaystyle (\ gamma, p)}$ -reactions on heavy and medium nuclei. Reaction same $(\gamma ,n)$ ${\ displaystyle (\ gamma, n)}$ ${\ displaystyle (\ gamma, n)}$ always comes with the formation of a compound nucleus.

The first observed photonuclear reaction was photo-splitting of the deuteron :

\gamma +{}^{2}{\textrm {H}}\rightarrow p+n

{\ displaystyle \ gamma + {} ^ {2} {\ textrm {H}} \ rightarrow p + n}

\ gamma + {} ^ {{2}} {\ textrm {H}} \ rightarrow p + n

It goes without the formation of a compound nucleus, since the deuterium nucleus has no excited states, and can be caused by gamma quanta of relatively low energy (above 2.23 MeV ^[7] ).

However, there are only a few nuclides with a low binding energy of nucleons, and photons with an energy of at least 8 MeV are necessary to initiate photonuclear reactions with other nuclei. Photons with such energy arise in some nuclear reactions or are obtained when decelerated in a substance of very fast electrons . During radioactive decay , as a rule, such gamma-quanta are not formed, therefore, gamma-quanta of β-decay cannot initiate photonuclear reactions and cause the appearance of new induced radioactivity in other substances.

If beryllium or heavy water serves as a moderator in a nuclear reactor , then due to the unusually low neutron binding energy of ⁹ Be and ² H under the action of gamma rays of radioactive decay on the nuclei of these nuclides, photonuclear reactions take place. $(\gamma ,n)$ ${\ displaystyle (\ gamma, n)}$ ${\ displaystyle (\ gamma, n)}$ . At that, radioactive fission products of uranium are especially abundant in gamma-quanta, but gamma-quanta in a nuclear reactor emit other substances activated by neutrons. Thus, an additional neutron source is present in the heavy water and beryllium nuclear reactors, due to the photonuclear reaction ^[1] .

Notes

↑ ¹ ² Klimov A.N. Nuclear Physics and Nuclear Reactors. - Moscow: Energoatomizdat, 1985. - p. 352.
↑ J. Chadwick , M. Goldhaber . Nuclear Photo Effect (Deuteron Decomposition by γ-Rays) // UFN . - 1934 . - V. 14 , № 8 .
↑ W. Bothe und W. Gentner. Atomumwandlungen durch γ-Strahlen // Zeitschrift für Physik A Hadrons and Nuclei. - 1937 . - T. 106 , № 3-4 .
↑ N.Bohr . Nuclear Photo-effects // Nature . - 1938 . - № 141 .
↑ N. Bor . Nuclear Photo Effect // UFN . - 1938 . - № 7 .
↑ J.Levinger. Photonuclear reactions. - Moscow: IL, 1962. - p. 258.
↑ NCRP Report No. 79. Neutron Contamination from Medical Electron Accelerators. - National Council on Radiation Protection and Measurements, 1984. - P. 19. - ISBN 0-913392-70-7 . ISSN 0083-209X

Links

Photonuclear reactions - an article from the Great Soviet Encyclopedia .
Photonuclear reactions , Physical Encyclopedia .

[К-1] ¹ ² Klimov A.N. Nuclear Physics and Nuclear Reactors. - Moscow: Energoatomizdat, 1985. - p. 352.

[2] J. Chadwick , M. Goldhaber . Nuclear Photo Effect (Deuteron Decomposition by γ-Rays) // UFN . - 1934 . - V. 14 , № 8 .

[3] W. Bothe und W. Gentner. Atomumwandlungen durch γ-Strahlen // Zeitschrift für Physik A Hadrons and Nuclei. - 1937 . - T. 106 , № 3-4 .

[4] N.Bohr . Nuclear Photo-effects // Nature . - 1938 . - № 141 .

[5] N. Bor . Nuclear Photo Effect // UFN . - 1938 . - № 7 .

[6] J.Levinger. Photonuclear reactions. - Moscow: IL, 1962. - p. 258.

[7] NCRP Report No. 79. Neutron Contamination from Medical Electron Accelerators. - National Council on Radiation Protection and Measurements, 1984. - P. 19. - ISBN 0-913392-70-7 . ISSN 0083-209X

Photonuclear reaction

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