Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

R ADIATION SOURCES 91
Figure 4-4. Phosphorous-32 beta
spectrum.
radioisotopes is about 30–40% of the maximum energy. Unless otherwise specified,
whenever the energy of a beta emitter is given, it implies the maximum energy.
The fact that beta radiation is emitted with a continuous energy distribution up to
a definite maximum seems to violate the established energy–mass conservation laws.
This is explained by the simultaneous emission of a second type of particle, called a
neutrino, 1 whose energy is equal to the difference between the kinetic energy of the
accompanying beta particle and the maximum energy of the spectral distribution.
The neutrino, as postulated, has no electrical charge and a vanishingly small mass.
Although these two characteristics make detection of the neutrino difficult, neutrinos
have been measured and the neutrino hypothesis has been experimentally
verified. Equation (4.6) should therefore be modified to
1
0n → 1 0
1H + −1e + ν, (4.8)
where ν represents the neutrino.
Phosphorus-32, like several other beta emitters—including 3 H, 14 C, 90 Sr, and
90 Y—emits no gamma rays. These isotopes are known as pure beta emitters. The
opposite of a pure beta emitter is a beta–gamma emitter. In this case, the beta
particle is followed instantaneously (in most cases) by a gamma ray. For those radionuclides
where the gamma-ray emission is delayed, as in the case of 99m Tc and
137 Cs, the gamma-ray emission is called an isomeric transition. In an isomeric transition,
the atomic number and the atomic mass number of the radionuclide is
not changed. The explanation for the gamma ray here is the same as that in the
case of the alpha. The daughter nucleus, after the emission of a beta, is left in
an excited condition and rids itself of the energy of excitation by the emission
of a gamma ray. Mercury-203 may be given as an example. It emits a 0.21-MeV
beta and a 0.279-MeV gamma, as seen in the transformation scheme shown in
Figure 4-5.
Both illustrations given above ( 32 P and 203 Hg) are for beta emitters with simple
spectra, that is, for emitters with only one group of beta particles. Complex beta emitters
are those radionuclides whose beta spectra contain more than one distinct group
1 Technically this is an antineutrino, but in common parlance, unless there is a need to be
more specific, this particle is referred to as a neutrino.