Nuclear constitution and the interpretation of fission phenomena

Energy Levels of Light Nuclei. V

Energy Levels of Light Nuclei. IV

Reactions produced by the nuclear capture of slow negative π-mesons have been studied by the photographic method. The disintegrations of the light and heavy elements of the emulsion have been distinguished by methods which are described in detail. Because of the large number of “stars” observed—more than 2500—it has been possible to make a statistical approach to the study of the disintegrations. The general features of the “stars” produced by the disintegration of heavy nuclei are in satisfactory agreement with the predictions of the evaporation theory. In addition, the emission of fast, singly-charged particles has been observed, most of which must be attributed to an alternative process to that of evaporation. Preliminary results have been obtained for the frequency of emission and the energy “spectrum” of these particles. The conclusion is reached that in the initial process of capture—the so-called “primary” process—the energy corresponding to the rest-mass of the φ− meson is shared among a ...

LIV. Nuclear reactions produced by slow negative π-mesons

Recent years have seen the accumulation of a considerable amount of new information, both experimental and theoretical, pertaining to the location and character of the excited states in the light nuclei. We have attempted in the following sections to summarize the pertinent facts appearing in the literature and to present this material in a systematic way. To a large extend the organization of the present article follows the pattern of the two previous papers on this subject.(1,2) The present article contains a summary of the most important information appearing in the earlier papers and is intended to supersede them entirely.

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Energy levels of light nuclei. III

The cross-section for the photodisintegration 12 C( γ , 3 α ) has been determined for γ -ray energies up to about 60 MeV from a study of 2500 stars in nuclear emulsions. The methods used in selecting and identifying the stars are described, and full details are given of the corrections (for escape, observer efficiency, etc.) required for converting the experimental data into cross-section values. The cross-section exhibits at least five resonances, situated at γ -ray energies ( E y ) of 17.3, 18.3, 21.9, 24.3 and 29.4 MeV, and a strong minimum at E y ~20.5 MeV. This behaviour suggests that a well-defined compound nucleus is formed, the minimum near 20.5 MeV resulting from ( γ , n ) and ( γ , p ) competition. Furthermore, the finer details of the cross-section data are consistent with current knowledge of the 12 C level structure. The integrated cross-section is 1.21 ± 0.16 MeV mb for E y E y E γ -27 cm 2 ). As a subsidiary result of the main work, the existence of the reaction 13 C( γ , n ) 3 α has been established.

https://www.jstor.org/stable/pdfplus/99190.pdf

Cross-Section for the Photodisintegration of Carbon into Three α -Particles

Multiple resonances in photodisintegration cross sections

The Photo-disintegration of Carbon into Three Alpha-particles

The mechanism of the 12 C(γ, 3α) reaction, for γ-ray energies, E γ , up to about 40 MeV, has been determined from a study of over 2500 stars in nuclear emulsions. The study includes investigation of the angular distributions and correlations of the α-particles. The reaction is initiated mainly by electric-dipole and electric-quadrupole γ-ray interaction, the former being unexpectedly strong when E γ E γ 8 Be (spin J = 0), and to 2⋅95 ± 0⋅10 MeV ( J = 2) and 4⋅0 ± 0⋅1 MeV ( J = 2 or 4) levels of 8 Be. Transitions to levels near 6, 10 and 15 MeV (all J = 0, 2 or 4) become predominant when 25 MeV ≤ E γ E γ ≥ 26 MeV, most transitions lead to 16⋅8 ± 0⋅2 MeV ( J = 2) and 17⋅6 ± 0⋅2 MeV ( J = 2, possibly 0) levels, and possibly to a further 16⋅4 ± 0⋅2 MeV ( J = 0 or 2) level, levels which have not been detected in other reactions. The reaction mechanism is interpreted in terms of competing modes of decay of a compound nucleus, demonstrating the strong influence of the isotopic spins ( T ) of the levels of 12 C and 8 Be involved. For example, the 2 + levels of 12 C involved when 16 MeV ≤ E γ T = 1, and the 16⋅8 and 17⋅6 MeV levels of 8 Be are also found to have T = 1. The relationship of the 12 C (γ, 3α) reaction to other 12 C photodisintegration reactions (including some new reactions established during the present experiments) is discussed.

The 12C(γ, 3α) reaction energy levels of 8Be and 12C

IN the study of the slow-neutron ternary fission of uranium-235, there is evidence that short-range particles of greater mass than α-particles are emitted. Dewan and Allen1 have reported ionization chamber experiments indicating the emission of particles of mass 13 ± 4 having ranges less than 1 cm. of air, and Titterton2 has recorded events in photographic plates which suggest the emission of nuclei heavier than α-particles having ranges up to 3 cm. air equivalent. An example of such an event is given in Fig. 1, from which it will be seen that, owing to the short range of the third fragment, certain interpretation cannot be made.

J J Wilkins

Papers

Cross-Section for the Photodisintegration of Carbon into Three α -Particles

Multiple resonances in photodisintegration cross sections

The Photo-disintegration of Carbon into Three Alpha-particles

The 12C(γ, 3α) reaction energy levels of 8Be and 12C

Photo-fission of uranium with possible emission of a beryllium nucleus.