Photonuclear Reaction

(also nuclear photoelectric effect), the absorption of gamma-ray photons by atomic nuclei and the accompanying ejection of protons p, neutrons n, or heavier particles from the nuclei. The (γ, p) and (γ, n) photonuclear reactions have been studied the most; other reactions, such as (γ, d), (γ, pn), and (γ, t) are also known. For a proton or a neutron, which are called nucleons, to be ejected from an atomic nucleus, the gamma-ray photon energy ℰ_γ must exceed the binding energy of the nucleon in the nucleus. The total cross section for all possible photonuclear reactions is called the gamma-ray absorption cross section for a nucleus.

For all except very light nuclei, the cross section σ_γ is small at high and low gamma-ray energies but has a high broad maximum, which is called a giant resonance (Figure 1), at some intermediate energy. As the mass number A of the nucleus increases, the position of the giant resonance decreases monotonically from 20–25 million electron volts (MeV) in light nuclei to 13 MeV in

Figure 1. A giant resonance

heavy nuclei. The relation between the energy ℰ_m that corresponds to the resonance peak and A is described by the equation ℰ_m = 34A^–1/6. The width Γ of the resonance is about 4–8 MeV; it is minimum for magic number nuclei—for example, Γ(²⁰⁸Pb) = 3.9 MeV—and is maximum for deformed nuclei—for example, Γ(^l65Ho) = 7 MeV. In the region of a giant resonance, the absorption curve is not monotonic but has a definite structure. For deformed nuclei, the absorption curve has a double peak (Figure 2,a). For light and medium nuclei, as well as for certain heavy nuclei, the absorption curve has several maxima with widths of hundreds of thousands of electron volts (Figure 2,b). The energy distribution of photoneutrons in a resonance region is nearly Maxwellian (seeMAXWELLIAN DISTRIBUTION). At the same time, there are some deviations; for example, the number of neutrons in the high-energy region of the spectrum is larger than is expected for a Maxwellian distribution. In most cases, the distribution of photoprotons is not Maxwellian.

A giant resonance is associated with the excitation by gammaray photons of natural oscillations of protons relative to neutrons; such oscillations are known as dipole oscillations. Nucleons may escape directly from a nucleus during dipole oscillations but may be ejected only after the oscillations are damped. The ordered oscillations of nucleons gradually evolve into a somewhat complicated thermal motion. As a result, an excited compound nucleus is formed, from which protons or neutrons are “evaporated.” The width Γ of a giant resonance is determined by the lifetime of the dipole oscillations. As a rule, when the gamma-ray energy exceeds the energy of a giant resonance, nucleons that absorb the gamma-ray photons escape rapidly from the nucleus. In this case, no dipole oscillations occur since the nucleus has not built them up, and the photonuclear reaction mechanism is direct (seeDIRECT NUCLEAR REACTION). For example, at ε_γ ≥ 70 MeV, the gamma-ray absorption mechanism becomes a two-nucleon mechanism. In addition to dipole oscillations in a nucleus, quadrupole, octupole, and other types of oscillations may be excited, but the role of such higher-order oscillations in photonuclear reactions is not substantial.

Figure 2. The fine structure of a giant resonance: (a) for deformed nuclei, (b) for spherical nuclei

The term “photonuclear reactions” is sometimes applied to processes in which the absorption of high-energy gamma rays—that is, photons with energies of ~ 1.5 × 10⁸ eV—by nuclei or individual nucleons results in the production of pions or other elementary particles. Examples of such reactions include γ + p → n + π^– and γ + p → p + π⁰.