Spontaneously Fissioning Isomers

Since the discovery of the first spontaneously fissioning isomer, a number of other examples have been found. The positions of these nuclei in the chart of nuclides are 

---

Nuclei can be trapped in the secondary minimum of the fission barrier. Such trapped nuclei will experience a significant hindrance of their y-ray decay back to the ground state (because of the large shape change involved) and an enhancement of their decay by spontaneous fission (due to the thinner barrier they would have to penetrate.) Such nuclei are called spontaneously fissioning isomers, and they were first observed in 1962 and are discussed below. They are members of a general class of nuclei, called superdeformed nuclei, that have shapes with axes ratios of 2 1. These nuclei are all trapped in a pocket in the potential energy surface due to a shell effect at this deformation. 

Figure 11.6 Position of the known spontaneously fissioning isomers in the nuclide chart. (Figure also appears in color figure section.)...

Figure 11.6 Position of the known spontaneously fissioning isomers in the nuclide chart.

Selected nuclear properties of the principal isotopes of berkelium are listed in Table I (6). In addition to these isotopes, ranging from mass numbers 240 to 251, there are spontaneously fissioning isomers known for berkelium mass numbers 242, 243, 244, and 245, all with half-lives of less than 1 /usee. Only 249Bk is available in bulk quantities for chemical studies, as a result of prolonged neutron irradiation of Pu, Am, or Cm (7). About 0.66 g of this isotope has been isolated from... 

One should remember in this connection that many cases of spontaneous fission of excited (isomer) nuclear states with lifetimes about 10 2 sec (quite close to the time of addition of a new link to the growing polyformaldehyde chain near absolute zero of temperature) were observed and successfully studied after the pioneering works of Dubna scientists. 

The double-humped fission barrier in Fig. 5.19 also makes it possible to explain the very short half-lives, of the order of nano- to microseconds, observed for some spontaneously fissioning nuclear isomers (e.g. "Am). By meas-... 

It has long been recognized that the liquid-drop model semi-empirical mass equation cannot calculate the correct masses in the vicinity of neutron and proton magic numbers. More recently it was realized that it is less successful also for very deformed nuclei midway between closed nucleon shells. Introduction of magic numbers and deformations in the liquid drop model improved its predictions for deformed nuclei and of fission barrier heights. However, an additional complication with the liquid-drop model arose when isomers were discovered which decayed by spontaneous fission. Between uranium and... 

This is indeed the case, and such a short-lived state is called a fission isomer. Actually, it was the discovery of a spontaneously fissioning nucleus with an abnormally short period that led Polikanov et al. to postulate the existence of shape isomers and brought Strutinsky to formulate his theory (Strutinsky 1967). Polikanov s experimental setup is shown in Fig. 4.30. 

A beam of Ne or was directed on a target of By the impact, the fission isomers formed were projected onto a rotating collector wheel and transported in front of two ionization chambers. Any fission fragments formed from a spontaneously fissioning nuclide on the wheel were to be detected by the ionization chambers. The beam of Ne or O ions was stopped in a Ta-collector connected to a current meter for monitoring the beam intensity. The apparatus was equipped to allow a calibration of the chambers using fission fragments from the reaction U(nth,f). 

The ratio of count rates in the two ionization chambers at a known rotational speed of the wheel (800-1,400 rpm) allowed to calculate the half-life of the spontaneously fissioning nuclides. The value obtained was 0.02 s - about lo times shorter than expected for a normal spontaneously fissioning nuclide of this fissility parameter (see O Fig. 4.4). (The shortest-lived normal spontaneously fissioning nuclide that could be considered is pm formed in the reaction ( 0,2n) Fm. Its fissility parameter would be Z /A = 39.7. The half-life extrapolated from the systematics (O Fig. 4.4) would be of the order of 10 -10 s. The species observed here was later identified to be a shape isomer of Am (fissility parameter of 37.3). The ground state of Am decays by P decay and electron capture with a half-life of 16 h.)... 

Half-lives for spontaneous fission from the second minimum of the potential energy curve as shown schematically in O Fig. 4.8 (fission isomers) full points) as a function of the fissility parameter compared to the corresponding half-lives from the ground states open points). The latter points are identical to those in O Fig. 4.4. All data are from the compilation of (Wagemans 1991a)... 

The notations for various decay modes used in this book are a for alpha decay, for / decay, P for positron decay, EC for electron capture, IT for isomeric transition, and SF for spontaneous fission. The letter m after a mass number denotes an isomer. Isomers with a half-life of less than 1 s and fission isomers are omitted from the tables. Energies are given only for the most abundant a groups and y rays for P particles the maximum energies p , are tabulated. In the last column, only the convenient methods for the production of nuclides are given nature denotes that the nuclide occurs in nature and multiple neutron capture means that this nuclide is produced by long irradiation in a high-flux reactor. 

The detection of light involves the initial conversion of 11-ds-retinal to its all-tmns isomer. This is the only obvious role of light in this process. The high energy of a quantum of visible light promotes the fission of the tt bond between carbons 11 and 12. When the tt bond breaks, free rotation about the a bond in the resulting radical is possible. When the ir bond re-forms after such rotation, all-frans-retinal results. All-irans-retinal is more stable than 11-czs-retinal, which is the reason the isomerization proceeds spontaneously in the direction shown in the following equation. 

---