Shape isomerism in uranium nuclei

Abstract

In the present experiment, the cross-section for isomeric fission resulting from the 14.5 MeV neutron bombardment of ²³⁸U has been measured using annular plastic (makrofol) track detectors in a recoil geometry. The neutrons were obtained through the ³H(d,n)⁴ He reaction. The plastic track detector was kept in the same'plane as the target so that it would not register the prompt fission fragments. A catcher foil 1.5 cms from the target stopped the recoiling uranium nuclei which, if excited into the shape-isomeric state, would fission in flight or after reaching the catcher foil. The makrofol detector foil detected fission fragments from the isomeric fissions and was later processed and the tracks counted.

Seven runs were done with ²³⁸U targets and two with ²³⁵U targets. Each time the target was exposed to an average of 10¹²to 10¹³ neutrons. Each run on Uranium was followed by a run on ²³²Th. The yield of fission tracks per 10¹² neutrons was (.75 ±.14)for ²³⁸U targets and (.30 ± .08) for thorium targets. As there are no known shape isomers in thorium, this latter number represents the background. Subtracting this background from the uranium target yield gives a delayed fission yield of (.45 ±.16) per 10¹² neutrons for the ²³⁸U targets. This amounts to a delayed fission cross-section of (43 ± 14) μbarns for the ²³⁸U(n,n')²³⁸ mUreaction. In the case of the ²³⁵U targets the data are not statistically significant as only two experimental runs could be made, yielding a delayed fission cross-section of 50 ± 100 μbarns.

The isomer to prompt fission ratio obtained from these measurements as well as from other available data has been compared with a statistical model calculation, employing standard methods to evaluate the neutron, fission and gamma decay widths. The calculations used both a recent semi-empirical level density formula given by Ramamurthy et al and the conventional Gilbert and Cameron level density expression. The double-hump barrier 'parameters, were taken from Back et.al, obtained from charged particle induced fission experiments. The sensitivity of the calculations to variations in some of these parameters has been tested. The calculation bears out the general trend in the experimental data on excitation of shape isomer through neutron bombardment but raises some doubts regarding the reaction mechanism assumptions made in the case of experiments where deuterons are used to excite the shape isomers.