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

116 CHAPTER 4
A third common characteristic among the three natural radioactive series is that
the end product in each case is lead. In the case of the uranium series (Table 4-3),
the final member is stable 206 Pb; in the actinium series, it is 207 Pb; and in the thorium
series, it is 208 Pb. The artificial neptunium series differs in this characteristic too from
the natural series; the terminal member is stable bismuth, 209 Bi.
These four radioactive decay series, the three naturally occurring ones and the
artificially produced neptunium series, are often designated as the 4n, 4n + 1,
4n + 2, and 4n + 3 series. These identification numbers refer to the divisibility
of the mass numbers of each of the series by 4. The atomic mass number of 232 Th,
the first member of the thorium series, is exactly divisible by 4. Since all disintegrations
in the series are accomplished by the emission of either an alpha particle
of 4 atomic mass units or a beta particle of 0 atomic mass units, it follows that
the mass numbers of all members of the thorium series are exactly divisible by 4.
The uranium series, whose first member is 238 U, consists of radionuclides whose
mass numbers are divisible by 4 and leave a remainder of 2 (238 ÷ 4 = 59 + 2/4).
This series, therefore, is called the 4n + 2 series. The actinium series, whose first
member is 235 U (actinouranium), is the 4n + 3 series. The “missing” series, 4n + 1,
is the artificially produced neptunium series, which begins with 241 Pu.
Primordial radionuclides found in nature are not restricted to the thorium,
uranium, and actinium series. Several of the elements among the lower-atomicnumbered
members of the periodic table also have radioactive isotopes. The most
important of these low-atomic-numbered natural emitters are listed in Table 4-5.
Of these naturally radioactive isotopes, 40 K—by virtue of the widespread distribution
of potassium in the environment (the average concentration of potassium in
crustal rocks is about 27 g/kg and in the ocean is about 380 mg/L) and in plants
and animals, including humans (the average concentration of potassium in humans
is about 1.7 g/kg)—is the most important from the health physics point of view.
Estimates of body burden of many radioactive materials, from which the degree
of exposure to environmental contaminants may be inferred, are made from radiochemical
analysis of urine from persons suspected of overexposure. Potassium,
whose concentration in urine is about 1.5 g/L, may interfere with the determination
of the suspected contaminant unless special care is taken to remove the potassium
from the urine or unless allowance is made for the 40 K activity. That this interfering
activity must be considered is clearly shown by a comparison of the 40 K activity
in urine with that of certain isotopic concentrations thought to be indicative of a
significant body burden.
TABLE 4-5. Some Low-Atomic-Numbered Naturally Occurring Radioisotopes
PRINCIPAL RADIATIONS
ISOTOPIC HALF-LIFE
NUCLIDE ABUNDANCE (%) (YRS) PARTICLES (MeV) GAMMA (MeV)
40 K 0.0119 1.3 × 10 9 1.35 1.46
87 Rb 27.85 5 × 10 10 0.275 None
138 La 0.089 1.1 × 10 11 1.0 0.80,1.43
147 Sm 15.07 1.3 × 10 11 2.18 None
176 Lu 2.6 3 × 10 10 0.43 0.20,0.31
187 Re 62.93 5 × 10 10 0.043 None