RADIATION PROTECTION - ILEA

6 | Beta Particles 27
2.3 Decrease of counting rate of an indium-
116 source after an arbitrary starting time.
Measurements were made with a G-M counter.
Each count was taken for one minute, starting
30 seconds before the time recorded for the
count. When plotted on semilogarithmic paper,
the results of the measurements fall close to a
straight line drawn as a best fit, with a half-life
of 54 minutes. They do not fall exactly on the
line because of statistical variations in the
number of disintegrations in a fixed counting
interval (see Part Four, section 6).
Counts in 1 minute
10,000
9000
8000
7000
6000
5000
4000
3000
2000
1000
20 40 60 80 100
Decoy time (min)
mately 79 percent of the 238 U and 99 percent of the 40 K have been lost by
decay since creation because of their radioactivity.
6.1.2 Maximum and Average Energies of Beta Particles
The energies of the beta particles given off in radioactive decay of the
various radionuclides listed in Table 2.1 are expressed in terms of both a
maximum energy and an average energy. The maximum energies range
from 0.018 million electron volts (MeV) for tritium to 2.24 MeV for
strontium-90 (from the yttrium-90 decay product). The tritium betas are
very weak, the maximum energy of 0.018 MeV, or 18,000 eV, being less
than the energies of electrons hitting the screens of most TV tubes. Average
energies are given for the beta sources because, as stated earlier, the nature
of beta decay is such that any individual beta particle can have any energy
up to the maximum (E max ). However, only a very small fraction of the
emitted beta particles have energies near the maximum. The frequency
with which energies below the maximum are carried by the beta particles is
given by energy spectrum curves, which have characteristic shapes. Energy
spectra for tritium and carbon-14, low-energy beta emitters, and for phosphorous-32,
a high-energy beta emitter, are shown in Figure 2.4. Energy