The discovery of neutron-induced fission of uranium nuclei in the experiments performed by Hahn and Strassmann (Ha 39, Me 39) has signified an observation of nuclear transmutations of a radically new type—the processes of large-scale rearrangement of finite amounts of nuclear matter. As a matter of fact, the fission is a result of an irreversible deformation, that grows in time, in the course of which an original, nearly spherical heavy nucleus undergoes drastic changes in its topology and is transformed into two fragments of comparable, yet generally unequal, mass. The time evolution of a fissioning system is accompanied by multiple redistribution of available energy between various degrees of freedom, and culminates in the release of a vast amount of energy in the form of kinetic energy of the fission fragments and energy (mass) of accompanying radiation, i.e., prompt neutrons and γ rays. To initiate such radical rearrangement, an extremely weak external perturbation proved to suffice—the capture of a slow neutron causing only tiny changes in the total number of nucleons as well as in the total energy of the original nucleus.

Heavy Ions and Nuclear Fission

Isomeric yield ratios of 30 fission products in 24 MeV proton-induced fission of238U were measured by the use of the ion-guide isotope separator on-line. The obtained isomeric yield ratios were converted to the angular momenta of primary fission fragments based on the statistical model. The deduced angular momenta were examined from various aspects. It is found that in general the angular momentum continuously increases with the fragment mass number including the region of symmetric mass division. However, there are some exceptions. For Sn isotopes the deduced angular momenta are quite small due to the spherical shape of the nuclear shell configuration. It is also concluded from the consideration of the charge distribution that the angular momentum of fission product scatters considerably within the narrow range of mass division. The dependence of the angular momentum on the available energy of fragments at scission point indicates that the individual fragment possesses a characteristic deformation at scission and/or the deduced angular momentum is seriously affected by the particle excitation after scission.

Isomeric yield ratios of fission products in the system of 24 MeV proton-induced fission of238U

Isomer ratios have been measured in Sb and Te isotopes prepared in the 115In( alpha ,n), 115In( alpha ,3n), 118,120,122,124Sn( alpha ,pn), and 120Sn( alpha ,3n) reactions, using a range of bombarding energies between 16.6 and 38.4 MeV. In most of the product nuclei the isomer ratios are smaller than the predictions of the sharp cut-off model, showing that the isomer is less than 100% efficient in trapping high-spin products. The variation of isomer ratio with bombarding energy is described by a simple expression for the trapping efficiency.

https://iopscience.iop.org/article/10.1088/0305-4616/5/12/013/pdf

Isomer ratios in antimony and tellurium isotopes

As a fissioning nucleus passes from its ground state to the scission point-the point where it breaks into two fragments-the sequence of shapes it follows depends principally on the potential energy surface of the deformed nucleus The theoretical techniques used to predict the potential energy are described and the predictions are tested against theory Other properties of nuclei on the path to scission are also discussed: the structure of individual low-lying states, level densities at high excitation, and the magnitude of interstitial parameters may all be important in determining the nature of the fission process Finally, the current state of knowledge about specifically dynamical effects in fission is summarized

http://iopscience.iop.org/article/10.1088/0034-4885/39/10/002/pdf

The fission process

We report the first experimental determination of independent isomeric yield ratios using direct ion counting with a Penning trap, which offered such a high resolution in mass that isomeric states ...

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First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments

Isomer ratios in antimony and bromine fission fragments have been measured in the (12C+209Bi), ( alpha +204Pb) and (12C+196Pt) reactions, The values of the isomer ratios and their variation with energy are in accordance with the fragment angular momenta predicted by the Fermi gas model for the fissioning nucleus.

Isomer ratios in fission fragments at high excitation

Radiochemical studies of charge distribution and dispersion in the 12C-induced fission of 209Bi

Fast separation methods were devised for short-lived fission products embedded in Mg catcher foils; these are based on solvent extraction techniques.34Cl is a major contamination problem because it is formed directly by spallation in the catcher foil. The methods for Ag, Br and I had a decontamination factor of >1000: 1 for this isotope.

J. E. Freeman

Papers

Isomer ratios in fission fragments at high excitation

Radiochemical studies of charge distribution and dispersion in the 12C-induced fission of 209Bi

Fast separations of yttrium, silver, bromine and iodine in heavy-ion-induced fission