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Chapter: 4 Atomic and nuclear physics
    Section: 4.6 Radioactive elements
        SubSection: 4.6.2 The radioactive series and their precursors

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4.6.2 The radioactive series and their precursors

The three naturally occurring radioactive series have great historical importance. Nowadays they, the 4n + 1 (Neptunium) series and all their precursors amongst the man-made nuclides are of considerable technological importance in the development of nuclear power. Precursors which feed the heads of the main chains are shown in Figs. A and B. Those which lead laterally (i.e. by β- decay) into the main α-chains have been excluded. Lifetimes are given to two significant figures only. Isomers that decay by α, β+, β or electron capture (EC) are included, but not those that decay by isomeric transition (IT) alone. Where two lifetimes are listed without distinction, the identity of the ground-state is unclear. Lack of space precludes separate entries for the decay branching ratios of ground and isomeric states; further information can be found in the Table of nuclides (4.6.1).

Where α, β branching occurs, the intensity of the weaker branch is usually listed (see, for example 214Bi). Where there is apparently no competitor to α-decay, e.g. 252Cf, and a branch intensity is given, or where the labelled branch intensities do not add up to 100%, the competing decay process is spontaneous fission. All known branches have been included irrespective of their intensity. Prior to the discovery of fission only three α, β branches were known in the natural series. Now there are 11 cases in addition to those in the Neptunium series. Only the Thorium series has no new branches.

The classical names of the members of the natural radioactive series, though rarely used nowadays, are included because of their historical interest; they indicate the state of historical knowledge before 1939. The name, where applicable, is shown outside the bottom right-hand corner of the square representing the particular nuclide.

Certain nuclides warrant specific mention. The isomer of 234Pa (in the 4n + 2 series) is formed in 100% of transitions from its parent 234Th. In 99.87% of cases the isomer decays by β emission with a half-life of 1.2 minutes. In the remainder of cases γ-emission to the ground state of 234Pa takes place prior to β decay with a half-life of 6.8 hours. The isomeric state known to exist in 210Bi is not excited in the natural radioactive decay chain and is not listed.

210Tl (in the 4n + 2 series) is a weak delayed neutron emitter (in 0.007 + 0.007%− 0.004% of cases). It is the only example of a transition from one chain to another. The neutron emission is from an excited state of 210Pb, leading to 209Pb, a member of the Neptunium series (4n + 1), which β decays to state 209Bi.

The longest lived member and titular parent of the Neptunium series, 237Np, is formed principally by α-decay of reactor produced 241Am. The next member of the series, 233U, is also abundantly produced in the sequence

232Th(n, γ)233Th 233Pa 233U

D.J.Parker


Fig. A

(click the Images to view Larger Images)

ThumbNail of 4.6.2 Fig A



Fig. B

(click the Images to view Larger Images)

ThumbNail 4.6.2 Fig B



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