RADIATION PROTECTION - ILEA

6 | Beta Particles 25
lion eV, and serve to trace the presence of the nuclei from which they are
emitted.
6.1 Properties of Some Common Beta-Emitting Radionuclides
Let us examine some radionuclides that emit only beta particles. Properties
of beta emitters that are most commonly used in research at universities
are given in Table 2.1. It is not surprising that the radioactive isotopes
that trace carbon (carbon-14), sulfur (sulfur-35), calcium (calcium-45),
phosphorous (phosphorous-32), and hydrogen (tritium or hydrogen-3) are
most popular, as these elements have a basic role in chemical and biological
processes. Strontium-90 is included because its decay product, yttrium-90,
which is always present with strontium-90, gives off the most energetic
beta particle found among the common radioactive nuclides. Hence, this
radionuclide is often used as a source of penetrating beta radiation. Let
us examine Table 2.1 in detail, discussing the quantities listed and their
values.
6.1.1 Half-Lives
Each beta particle given off by a radioactive source results from the
transformation or decay of an atom of that source to an atom of another
element whose atomic number is greater by one. The rate at which the atoms
undergo transformations—and, consequently, the rate of emission of
beta particles—is proportional to the number of radioactive atoms present.
Thus, as the number of radioactive atoms in the source decreases owing to
the radioactive transformations, the rate of emission of beta particles decreases.
When half the atoms in a sample have decayed, the rate of emission
of beta particles is also cut in half. The time in which half the atoms of
a radionuclide are transformed through radioactive decay is known as the
half-life of the particular radionuclide.
A radionuclide that is frequently used to demonstrate the nature of radioactive
decay is indium-116 ( 116 In). This is produced from indium-115,
which is a naturally occurring, nonradioactive metal, by the absorption of
neutrons; suitable neutron sources are usually available at research or educational
institutions. Figure 2.3 shows the counting rate of a sample of
116
In as a function of time, as measured with a Geiger-Mueller counter.
The data, when plotted on semilogarithmic graph paper, fall on a straight
line. No matter when the measurement (count) is made, the counting rate
decreases with a half-life of 54 minutes. Each radionuclide has a unique
half-life.
From Table 2.1 we see that the half-lives of commonly used radio-