M. Tanikawa

Angular momentum effects on mass division in actinide fission

Angular momentum (/) effects on mass yield curves were studied for the fission from the same compound nucleus (CN) Np of the same excitation energy but with different angular momenta, produced by the Li+Th and p+U reactions. The effects were examined only for the complete fusion-fission events after careful estimation of the contribution of incomplete fusion-fission. It is found that the width of the whole mass yield curve is larger while the peak-to-valley ratio is smaller for the Li+Th fission at the same excitation energy (Ex) of the CN, the former value increases while the latter ratio decreases with Ex. Statistical model calculations incorporating angular momentum dependent fission barriers were made to reproduce the observed cross section data of evaporation residues and fission. The observations can well be explained if the angular momentum dependence of fission barriers is taken into account in the competition process of fission and neutron evaporation in the decay of a compound nucleus except the observed trend of the width of the mass yield curve which could not be explained straightforwardly.

Two deformation paths in fission of light actinides

The correlation among the fission threshold energy, the scission configuration and the mass yield distribution has been studied in proton-induced fission of light actinides. It was found that there exist at least two fission paths from the threshold region to the scission. The elongated scission configuration is related with the fission process that goes over a higher threshold energy and results in a symmetric mass division mode, while the compact scission configuration with the process that experiences a lower threshold and ends up with an asymmetric mass division mode.

Experimental verification of two deformation paths in the mass division process of actinides

Abstract Double TOF measurements were performed to obtain accurate kinetic energies of fission fragments produced in the proton-induced fission of 244 Pu, 238 U and 232 Th. The presence of two families of the distances between the two complementary fragments at scission, D sym and D asym , is confirmed and their systematic variation from the pre-actinide to the medium actinide region is examined. Variations of the mean total kinetic energies of symmetric and asymmetric mass division products are also studied for a wide range of fissioning nuclides.

Primary fragment mass-yield distributions for asymmetric fission path of heavy nuclei

The primary fragment mass-yield distribution for the asymmetric fission path in heavy nuclei, 233Pa, 239Np, 245Am and 249Bk at the excitation energy of ~20 MeV are experimentally constructed based on the intensities of total kinetic energies for individual mass splits. The results revealed an interesting phenomenon: in all the studied fissioning systems, the inner wings of the mass-yield distributions in the asymmetric fission path appear along the same mass-wall of A = 130 fragment mass. The asymmetric mass-yield distribution indicates the strong effect of structural shells in fragments on the final mass division process of the asymmetric fission path.

Characteristics of two fission modes

Characteristics of nuclear deformation properties at scission of two fission modes, symmetric and asymmetric, are reviewed. Existence of two kinds of scission configurations associated with the symmetric and asymmetric fission modes is pointed out: elongated and compact configurations. Each symmetric and asymmetric scission property is discussed in terms of shape elongation evaluated from fragment total kinetic energy (TKE). Fragment deformation at scission is also discussed based on neutron multiplicity data. From the systematic study of the scission properties in a wide range of actinide fission, the bimodal fission observed in spontaneous fission (SF) of the heavy actinides is interpreted as the result of the presence of the two fission paths of the ordinary asymmetric mode and a strongly shell-influenced symmetric one. The correlation between the fragment shell structures and the asymmetric mass-yield curves is described.

Nuclear‐Charge Polarization at Scission in Proton‐Induced Fission of Light Actinides

Fragment mass yields and the average neutron multiplicity in the proton‐induced fission of 232Th and 233U were measured by a double time‐of‐flight method. The most probable charges of secondary fragments were evaluated from the fragment mass yields measured and the fractional cumulative yields reported. The nuclear charge polarization of primary fragments at scission was obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The results show that the nuclear‐charge polarization at scission is associated with the liquid‐drop properties of nuclei and the proton shell effect with Z = 50 of heavy fragments and that it is practically insensitive to mass and excitation energy of the fissioning nucleus in the region of light actinides.

Shapes of Fragment Mass-Yield Distributions and Shapes of Scissioning Nuclei in Actinides

Three fimdamental types (named A-, B- and O-type) in the fragment mass-yield distribution and three independent types of the scission deformation (β~1.65, 1.53 and 1.33) for actinide nuclei in fission process are experimentally observed. The correlations between the final shapes of fissioning nuclei and the shapes of the fragment mass-yield distributions are hence revealed and presented. A-type: deformation path with β~1.65 resulting in a typical FWHM of >20 u; B-type: deformation path with β~1.53 leading to a typical FWHM of ~15 u; O-type: deformation path with β~1.33 producing a FWHM of ~9 u.

Symmetric and asymmetric scission properties

Existence of two kinds of scission configurations associated with the symmetric and asymmetric fission modes is pointed out in the fission of actinides: elongated and compact configurations. Each symmetric and asymmetric scission property is discussed in terms of shape elongation evaluated from fragment total kinetic energy (TKE). It is found that the shape elongation associated with the asymmetric fission mode is nearly constant for a wide range of fissioning nuclei, while that with the symmetric one is also constant except for low-energy induced fission and spontaneous fission (SF) of heavy nuclei. Prom the systematic study of the scission properties, the bimodal fission observed in SF of the heavy actinides is interpreted as the presence of the two fission paths of the ordinary asymmetric mode and a strongly shell-influenced symmetric one.

Characteristcs of binary scission configurations in proton-induced fission of actinides

Two kinds of scission configurations for certain mass division have been verified by the accurate measurements of fragment velocities in protoninduced fission of actinides: compact and elongated scission configurations. A correlation between the binary scission configurations and mass yield distributions reveals that elongated scission configurations are associated with the symmetric mass distribution and compact scission configurations with the asymmetric mass distribution. The elongation properties of nuclei at scission in a wide range of actinides fissions are studied. The results suggest that the compact and the elongated scission configurations in light actinides fission smoothly change to the scission properties of the symmetric and the asymmetic modes in heavy actinides fission.

HIGHLY ASYMMETRIC MASS DIVISION IN LOW-ENERGY PROTON-INDUCED FISSION OF 232TH AND 244PU

Excitation functions and mass yield distributions of rare earth products in the proton-induced fission of Th and Pu have been measured by means of an automatic rapid ion-exchange separation system for incident energies from 9 to 16 MeV. Excitation functions of highly asymmetric products with mass numbers A > 150 showed different energy dependence compared with those for typical asymmetric mass division products with mass number around A = 140 in the 2 3 2 Th (p , f ) reaction while no clear different energy dependence between the products with A > 150 and A ~140 was observed in the PU(/J,/) reaction. The gross feature of the highly asymmetric mass division would phenomenologically be explained by statistical considerations when they are compared in terms of the asymmetry parameters. However, the different energy dependence of highly asymmetric products between the two fission reactions might be caused by an additional fission mode or a slight shell effect of Ν = 50 in the light complementary fragments.