Photonuclear reactions

Kinsey Nuclear Reactions, Levels, and Spectra of Heavy Nuclei Sect. 47. 

The most noticeable feature of photonuclear reactions is that the total cross section is greatest at relatively low energies. For example, the cross section of the (y, n) reaction, whose energy variation was first studied for C and Cu by Baldwin and Klaiber, rises rapidly to a maximum near 20 Mev and then falls off for higher energies. Thus, most of the photonuclear reaction is confined to energies below 25 Mev or so, and only those reactions are of importance for which the thresholds lie below this rather low figure. In heavy elements the most important reactions are the primary reactions (y, n) and (y, p), and the secondary reactions (y, (y, 2n), etc. Sometimes y,d) reactions are also important  

The giant resonance. It was pointed out in the previous section that the total photonuclear reaction cross section rises to a maximum near 20 Mev. This effect is to be seen in the cross sections for both the (y, n) and (y, p) reactions these cross sections rise to a peak near 20 Mev then fall off rather rapidly. This peak is called the giant resonance. In general appearance the variation of 

Beyond the peak the cross section falls to some low value. The yield of neutrons at higher energies has been measured by Jones and Terwilliger2 from about I3 Mev to 320 Mev. The slow rise in the neutron yield found by these authors can be attributed, in part, to an increase in the neutron multiplicity as successively more reactions become energetically possible. 

This bump is called the giant dipole resonance (GDR). Goldhaber and Teller (1948) provided a model for this reaction in which the giant dipole resonance is 

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A giant dipolar resonance (GDR) exists in the majority of photoabsorption and photonuclear reactions. This resonance energy corresponds to the fundamental frequency for absorption of electric dipole radiation by the nucleus acting as a whole. It can be envisioned as an oscillation of neutrons against the protons in a nucleus. The GDR occurs at energies of 20-24 MeV in light material and of 13-15 MeV in heavy nuclei. A compendium of the GDR parameters is found in Ref [3]. 

A. V. Varlamov, V. V. Varlamov, D. S. Rudenko, M. E. Stepanov, Adas of Photonuclear Reactions Table of GDR Parameters and Graphs of Cross Sections, International Atomic Energy Agency, Report INDC(NDS)-394, January 1999. 

PHOTONUCLEAR REACTION. A nuclear reaction induced by a photon. In some cases the reaction probably takes place via a compound nucleus formed by absorption of the photon followed by distribution of its energy among the nuclear constituents. One or more nuclear particles then "evaporate from the nuclear surface, or occasionally the nucleus undergoes pliotofissioii. In other cases the photon apparently interacts directly with a single nucleon, which is ejected as a photoneutron or photoproton without appreciable excitation of the rest of the nucleus. 

Radiation beams induce radioactivity primarily by photonuclear reactions. In these reactions, the absorption of energy from the incident electron, x-ray, or 7-ray will produce an excited nucleus that will then emit a neutron, proton, triton, 7-ray, or other secondary radiation. The chart of the nuclides from carbon to sodium in Figure 1 demonstrates the type of nuclide resulting from the emission of a secondary radiation from a given parent nuclide (10). The threshold energies necessary for the incoming radiation to produce a secondary radiation are given for a few of the reactions most relevant to this report. 

Additionally, a number of the radionuclides that can be produced by photonuclear reactions do not appear in the available MPC tables (16). E. G. Fuller, National Bureau of Standards, has prepared a complete table of radionuclides that could be produced in radiation processing. K. Z. 

E. G. Fuller, E. Hayward (Eds.), Photonuclear Reactions, Dowden, Hutchinson and Ross, New York, 1976... 

The potential value of high-energy electron-producing machines such as the linear accelerator for activation analysis must not be overlooked. Here photonuclear reactions y,n) can be used, either to produce a high neutron flux (as most charged-particle machines can by choice of a suitable reaction), or directly on samples. This may well be valuable, particularly for some light element determinations. 

Note that the above considerations concern only thermonuclear modes of p-nuclide synthesis. Some non-thermonuclear scenarios have in fact been proposed, like production by spallation reactions in the interstellar medium [71,72], or photonuclear reactions triggered by non-thermalized photons [73]. These models suffer from either too low efficiencies, or from constraints that limit their astro-physical plausibility. They are not discussed further here. 

Photon activation complements neutron and charged-particle activation. Photons are better than neutrons in certain cases. For example, photons are preferred if the product of the neutron activation is an isotope that has a very short half-life or emits only low-energy betas or low-energy X-rays. The cross sections for photonuclear reactions are generally smaller than those for neutrons and charged particles. Table 15.3 gives several photonuclear reactions that have been used in activation analysis. 

Thus it is clear that some form of direct interaction must occur in the photo-nuclear process and it is very probable, especially in heavy elements, that compound nuclei are also formed. What is not so clear is the mechanism of the direct interaction, the extent to which direct interaction and compound nucleus formation occur, and the relationship between them. It is quite reasonable to suppose that direct interaction is an essential preliminary to all photonuclear reactions, in the same sense as direct interaction for heavy particles is the preliminary to compound nucleus formation, because there is a high probability that the nucleon acted upon is absorbed in the process of ejection. In the heavy elements, where proton emission is hindered by the Coulomb barrier, it is clear that direct interaction on the proton must nearly always lead to compound nucleus formation, for the retention of the proton must produce a general excitation of the nucleus. 

The Coulomb barrier. The energy spectra of the protons and a-particles emitted in photonuclear reactions are usually found to be peaked at energies... 

A complete list of references of both theoretical and experimental papers will be found in a bibhography of all photonuclear reactions compiled by Toms. ... 

M. E. Toms Bibliography of Photonuclear Reaction . Naval Research Laboratory, June 1955 (545 references). 

Fig. 32. A schematic representation of the relative yield of various nuclides from photonuclear reactions with 320 Mev X-rays. The yield is indicated by the height above the proton-neutron plane. The dark line in the proton-neutron plane indicates the center of the valley of stability. [Figure from Halpern etal.i Phys. Rev. 97, 1327 (1955).]...

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