Page 110 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 99

half-life through a-decay is 1.41 x 1010 y. The terminal nuclide in this decay series is the
stable species ~~ (also known as ThD). The transformation from the original parent to
the final product requires 6 a- and 4 B-decays. The longest-lived intermediate is 5.76 y
228Ra"
The uranium decay series consist of a group of nuclides that, when their mass number is
divided by 4, have a remainder of 2 (the 4n + 2 series). The parent of this series is 238U
with a natural abundance of 99.3 %; it undergoes a-decay with a half-life of 4.46 • 109 y.
The stable end product of the uranium series is 2~ which is reached after 8 a- and 6
B-decay steps.
The specific activity of 238U is 12.44 MBq/kg 238U. However, because natural uranium
consists of 3 isotopes, 238 U, 235 U and 234 U, whose isotopic abundances are 99.2745 %,
0.7200% and 0.0055%, respectively, the specific activity of natural uranium is 25.4
MBq/kg.
The uranium decay series provides the most important isotopes of dements radium, radon,
and polonium, which can be isolated in the processing of uranium minerals. Each ton of
uranium is associated with 0.340 g of 226Ra. Freshly isolated 226Ra reaches radioactive
equilibrium with its decay products to 21~ in about two weeks (see Fig. 1.2). Many of
these products emit energetic 7-rays, which resulted in the use of Ra as a ),-source in
medical treatment of cancer (radiation therapy). However, the medical importance of
radium has diminished greatly since the introduction of other radiation sources, and
presently the largest use of radium is as small neutron sources (see Table 12.2).
Although the chemistry of radium is relatively simple (like barium), the fact that it
produces a radioactive gas (radon) complicates its handling. The decay of radon produces
"airborne" radioactive atoms of At, Po, Bi, and Pb. Since uranium is a common element
in rocks (see w it is also a common constituent of building materials. Such material
emits Rn, as discussed further in w Work with radium compounds should be carried out
within enclosures to avoid exposure to Rn and its daughters.
The actinium decay series consists of a group of nuclides whose mass number divided by
4 leaves a remainder of 3 (the 4n + 3 series). This series begins with the uranium isotope
235U, which has a half-life of 7.04 • l08 y and a specific activity of 8 x 10 4 MBq/kg. The
stable end product of the series is 2~ which is formed after 7 a- and 4 fl-decays. The
actinium series includes the most important isotopes of the elements protactinium, actinium,
francium, and astatine. Inasmuch as 235 U is a component of natural uranium, these elements
can be isolated in the processing of uranium mjnerals. The longest-lived protactinium
isotope, 231pa (t~A 3.28 x l04 y) has been isolated on the 100 g scale, and is the main
isotope for the study of protactinium chemistry. 227Ac (t~6 21.8 y) is the longest-lived
actinium isotope.

5.3. Transuranic elements in nature and the Np decay series

A fourth long radioactive decay series, the neptunium series (Fig. 1.2), is composed of
nuclides having mass numbers which divided by 4 have a remainder of 1 (the 4n + 1
series). The name comes from the longest lived A = 4n + 1 nuclide heavier than Bi,
237Np, which is considered as the parent species; it has a half-life of 2.14 x 10 6 y.
Inasmuch as this half-life is considerably shorter than the age of the earth, primordial 237Np
no longer exists on earth, and, therefore, the neptunium series is not found as a natural