Fission isomers

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. 

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... 

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.

It gives reasonable fission barriers, fission isomer energies, and fission isomer lifetimes [KUM86]. Agreement with the fission lifetimes is also quite reasonable (see Fig. 7). This did require the introduction of two additional parameters (the strength and the A A -dependence) for the nuclear part of the fragment-fragment interaction. 

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... 

Strutinsky developed an extension of the liquid drop model which satisfactorily explains the fission isomers and asymmetric fission. For such short half-lives the barrier must be only 2-3 MeV. Noting the manner in which the shell model levels vary with deformation ( 11.5, the "Nilsson levels"), Strutinsky added shell corrections to the basic liquid-drop model and obtained the "double-well" potential energy curve in Figure 14.14b. In the first well the nucleus is a spheroid with the major axis about 25 % larger than the minor. In the second well, the deformation is much larger, the axis ratio being about 1.8. A nucleus in the second well is metastable (i.e. in isomeric state) as it is unstable to y-decay to the first well or to fission. Fission from the second well is hindered by a 2 - 3 MeV barrier, while from the first well the barrier is 5 - 6 MeV, accounting for the difference in half-lives. 

Abstract This chapter first gives a survey on the history of the discovery of nuclear fission. It briefly presents the liquid-drop and shell models and their application to the fission process. The most important quantities accessible to experimental determination such as mass yields, nuclear charge distribution, prompt neutron emission, kinetic energy distribution, ternary fragment yields, angular distributions, and properties of fission isomers are presented as well as the instrumentation and techniques used for their measurement. The contribution concentrates on the fundamental aspects of nuclear fission. The practical aspects of nuclear fission are discussed in O Chap. 57 of Vol. 6. 

Phenomena, like the existence of fission isomers, to be discussed later. 

The result of his approach for the potential energy as a function of deformation is shown in O Fig. 4.8. These calculations result in a two-humped, double fission barrier (O Fig. 4.8), which is confirmed by multiple experimental evidence, such as the asymmetry of fission, the existence of elements with Z > 100 and of fission isomers as will be discussed further below. 

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). 

Soon after this discovery, fission isomers were observed in a multitude of other fission reactions (Michaudon 1973 Wagemans 1991a) (O Fig. 4.31). Most of these fission isomers are... 

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)... 

It was a lucky circumstance for Polikanov et al. that they chose a reaction leading to the longest-lived of the fission isomers their experimental setup would not have been able to detect any other fission isomer. 

A particularly interesting variant of the measurements described above is the charge plunger experiment. It allows one to obtain some direct experimental information on the spectroscopic transitions in the second minimum and from this on the degree of deformation of the fission isomers. The experimental setup is shown in O Fig. 4.32. 

Besides the information on the deformation of the fission isomers based on rotational excitations described above, some information can also be obtained on the level density of vibrational and single-particle excitations in fission isomers. 

When the fission isomeric states were discovered (Polikhanov et al. 1962), it became clear that the fission potential should have a second minimum as a function of the deformation and the observed fission isomers are shape isomers in such a second minimum. Strutinsky s proposal (see O Chap. 2 in this Volume) was to combine the virtues of both the LDM and shell models to describe such a second minimum of the fission potential. 

The value of n = (2.25 0.20) MeV - extracted for the excitation energy of the ground state in the second minimum of Pu - is in good agreement with the fission isomer energy obtained from the well-known method of extrapolated excitation functions of various experiments (see, e.g., Wagemans 1991 and references therein). Thus having proven the reliability of this method, an excellent tool has been obtained to address the question of the depth of the (hyperdeformed) third minimum of the potential surface. This tool will be discussed in O Sect. 5.5. 

It is interesting to note that when either 2 or Nis odd, the half-life tends to be much longer (see Fig. 4.5 in Chap. 4). On the other hand, the half-lives of the fission isomers are many orders of magnitude below the general trend (see Fig. 4.31 in Chap. A) ... 

Spectroscopic studies of superdefonned fission isomers have been reviewed (Vandenbosch 1977 Metag et al. 1980 Bjornholm and Lynn 1980), while hyperdeformed nuclear shapes in the third minimum of the potential energy surface was discussed by M5ller et al. (1972) and recently by Cwiok et al. (1994). Krasznahorkay et al. (1998) succeeded to observe hyperdeformed rotational bands in actinide nucleL... 

Shape Isomers can be found in the second minimum of the fission barriers of actinides. If the nucleus is prepared in the lowest state in the second minimum it is much more deformed than in any of the states in the first minimum. Thus any transition out of the second minimum will require the rearranging of all nucleons, which leads to the observed very small matrix elements and thus the formation of an isomeric state. These isomers are also known as fission isomers. 

Most isomers occur because of the large difference between the spins of the excited state and the ground state. However, there is a class of isomers that decay by fission, and these are caused not by the difference in the angular momenta of the states but by difference in the shapes. These are called fission isomers or shape isomers , and have half-lives in the range ofl0 stol0 s. These isomers have deformation almost twice that of the ground-state nucleus. More than 50 fission isomers have been discovered [10]. 

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. 

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