Isobar chain

Unfortunately, precise knowledge of the distribution of direct yield among several competing isobars is generally not available furthermore, the radioactive half-lives involved are frequently completely unknown since the fission process gives rise directly to between 300 and 400 radioactive species, and the separation of such a complex mixture usually involves a time which is quite long compared with the lifetimes of interest. We do know that each isobaric chain is formed directly as a number of different isobars and that the width of the isobaric yield distribution is such that to account for 90% or more of a chain one must consider at least three or perhaps four chemical elements. 

Some of the isobar chains from fission contain isotopes with very high n,y cross sections. One of the most important is y4 = 135 and it s effect on reactor kinetics is used as an example. The genetic relations in the y4 135 chain is ... 

As an example, is not formed to any appreciable extent in fission because it is shielded by the stable Xe. Hence, no Cs is normally observed in the remains after a nuclear explosion in the atmosphere. However, primary fission products in the A 133 isobar chain have time to decay to stable Cs during reactor operation and Cs is produced by the reaction Cs (n,y) Cs. Giv the cooling time, the ratio between the decay rates of Cs and Cs can be used to estimate the bumup of fuel from a given reactor, see Figure 21.6. 

In this model the fractional independent yields in an isobaric chain (mass chain) are described by a Gaussian curve with a maximum (most probable charge, Z ) and a width... 

In general, these parameters depend on the mass number of the isobaric chain considered. In the region of normal asymmetric fission (>99% of the fission yields), e.g., AZ can be given as a linear function of the mass of the heavier fi-agment ... 

In general, the independent yield is highest for the first members of an isobaric chain and decreases significantly in the development of the chain the final mem-... 

However, there are deviations from this isobaric chain behavior, the most important ones will be shortly mentioned. 

Another secondary effect is the production of radionuclides which themselves are not fission products but which are generated by neutron capture in long-lived or stable fission product nuclides. Examples of the products of such reactions are Cs and Cs, which are separated from the true members of the isobaric chains by the stable nuclides Xe and Xe, and which are formed by neutron capture in the fission products Cs and Cs, respectively. Because of the two-fold neutron-induced nuclear reaction which is necessary for their production, their concentration in the irradiated fuel depends approximately on the square of the local neutron fluence. 

A large fraction of possible contenders have short half-lives, and these you may well not need to consider. On the other hand, all fission products are members of isobaric chains and decay will usually result in other activities. 

This URL gives access to diagrams showing simplified decay schemes arranged in isobaric chains, as described in Chapter 1. Diagrams can be downloaded in either Postscript or. pdf format. 

DELAYED NEUTRON A neutron emitted by a fission product in an isobaric chain the delay caused by the time taken for beta decay in the chain. 

The yield of each member of the isobaric series integrates, by virtue of the intervening (3 decay, the yields of its precursors. Such yields are referred to as cumulative yields. For example, the cumulative yield of the mass 140 chain in the thermal neutron-induced fission of 235U is 6.25%. 

Fig. 5. Specific volume temperature curves at different pressures of the l.c. side chain polymer (isobaric measurements)37,3S)...

If one could reproduce the neutron end beta efficiencies from one mass to the next, one could use a known Pn value to determine the ratio of beta efficiency to neutron efficiency and then calculate all other Pn values using Eq. 1. With our present apparatus, the neutron efficiency is insensitive to changes from one mass number to the next, but the beta efficiency depends more critically on how the beam is tuned. We thus use Eq. 2 to calculate Pn because the beta efficiency does not appear in the expression. In some mass chains with several precursors (either isomers or isobars) it is possible to determine the beta efficiency for one of the prominent precursors from the ratio of the coincidence counting rate to the neutron counting rate. We then calculate Pn values for the other precursors in that mass chain by use of Eq. 1. In all cases we use the neutron counting efficiency determined for 9 Rb by use of Eq. 2 ond the Pn value of 13.6 0.9 X. 

Many decay chains, each with about 5 isobars (isotopes with the same mass number), have been observed. 

Ultratraces of Po are distributed worldwide as nuclear contamination in the environment, bnt °Po also occurs as a natural radioactive decay product in the decay chain of and can be analyzed by a-spectrometry. In spite of the short Po half life (tj j = 138days), traces can also be detected after careful separation (due to isobaric interference with Pb in nuclear samples and environmental materials) using advanced mass spectrometric techniques such as CE-ICP-MS or AMS as demonstrated in references" " ... 

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