Page 156 - Soil and water contamination, 2nd edition

8

Radionuclides

8.1 INTRODUCTION

Radionuclides are elements that have unstable nuclei which disintegrate or change
spontaneously with a loss of energy in the form of ionising radiation (that is any radiation
which displaces orbital electrons from atoms, so producing ions). The instability of an
atomic nucleus is caused by an imbalance of the number of protons (Z) and neutrons (N)
in the nuclei. Stable nuclei have neutron and proton numbers which are closely related.
This is illustrated in Figure 8.1, which shows that stable nuclei occur only within a narrow
band of increasing neutron and proton numbers. Unstable nuclei break down, ultimately
forming stable nuclei. The most unstable nuclei disintegrate rapidly and do not now exist
in measurable quantities in the environment. Other unstable nuclides, however, have a slow
decay rate and still exist in significant amounts.
Atoms that have the same number of protons but different numbers of neutrons are

known as isotopes of an element. Isotopes are usually denoted by their mass number A (i.e.
1
the sum of the number of protons and neutrons in a nucleus: A = Z + N), e.g. H or H-1
206
207
204
for hydrogen. For example, lead has four stable isotopes Pb (1.4 %), Pb (24.1 %), Pb
(22.1 %), and 208 Pb (52.3 %), and eight instable isotopes 202 Pb, 203 Pb, 205 Pb, 209 Pb, 210 Pb,
214
211 Pb, Pb and Pb. The percentages in brackets refer to the average relative abundance of
212
each isotope in naturally occurring lead. The unstable lead isotopes are found in only trace
amounts. Note that the relative abundance of the isotopes varies for the different ores, giving
each region its own lead isotope signature.
Radioactive decay is a first-order kinetic process, which implies that the number of nuclei
that disintegrate per second (dN/dt) is proportional to the number of nuclei N at any time t:
dN
N (8.1)
dt
where λ is the disintegration or decay constant. Radioactive decay is a stochastic process and
the decay rate constant is an invariable property of each radioisotope and is independent
of factors such as pressure, temperature, chemical form, and time. The number of
disintegrations per second is measured in Becquerel (Bq), named after the discoverer of
radioactivity in 1896. One Bq is equal to one disintegration per second. It replaces the older
10
unit Curie (1 Ci = 3.7·10 Bq) named after Marie and Pierre Curie, the discoverers of the
radioactive element radium.
If N is the number of nuclei present at time t = 0, then by integration of Equation (8.1)
0
we obtain:
N N 0 e t (8.2)

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