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

112 CHAPTER 4
TABLE 4-1. Thorium Series (4n)
ENERGY (MeV)
GAMMA (PHOTONS/
NUCLIDE HALF-LIFE ALPHA a BETA TRANS.) b
232
90Th 1.39 1010 yrs 3.98
228
88Ra (MsTh1) 6.7 yrs 0.01
228
89Ac (MsTh2) 6.13 h Complex decay scheme 1.59 (n.v.) 0.966 (0.2)
Most intense beta
group is 1.11 MeV
0.908 (0.25)
228
90Th (RdTh) 1.91 yrs 5.421 0.084 (0.016)
224
88Ra (ThX) 3.64 d 5.681 0.241 (0.038)
220
86Rn (Tn) 52 s 6.278 0.542 (0.0002)
216
82Po (ThA) 0.158 s 6.774
212
82Pb (ThB) 10.64 h 0.35, 0.59 0.239 (0.40)
212
83Bi (ThC) 60.5 min 6.086 (33.7%)c 2.25 (66.3%) c 0.04 (0.034 branch)
212
84Po (ThC′ ) 3.04 × 10−7 s 8.776
208
81Tl (ThC′′ ) 3.1 min 1.80, 1.29, 1.52 2.615 (0.997)
208
82Pb (ThD) Stable
aOnly the highest-energy alpha is given. Complete information on alpha energies may be obtained from Sullivan’s Trilinear Chart of
Nuclides, Government Printing Office, Washington, DC, 1957.
bOnly the most prominent gamma rays are listed. For the complete gamma-ray information, consult T. P. Kohman: Natural
radioactivity, in H. Blatz (ed.): Radiation Hygiene Handbook. McGraw-Hill, New York, 1959, pp. 6–13.
cIndicates branching. The percentage enclosed in the parentheses gives the proportional decay by the indicated mode.
All of the isotopes that are members of a radioactive series are found in the upper
portion of the periodic table; the lowest atomic number in these groups is 81, while
the lowest mass number is 207.
All the radioactive series have several common characteristics. The first member of
each series is very long-lived, with a half-life that may be measured in geological time
units. That the first member of each must be very long-lived is obvious because, if the
time since the creation of the world is considered, relatively short-lived radioactive
material would have decayed away during the 4.5 billion years that the earth is
believed to have been in existence. This point is well illustrated by considering the
artificially produced neptunium series. In this case, the first member is the transuranic
element 241 Pu, which is produced in the laboratory by neutron irradiation of reactorproduced
239 Pu. The half-life of 241 Pu, however, is only 13 years. Because of this short
half-life, even a period of a century is long enough to permit most of the 241 Pu to
decay away. Even the half-life of the longest-lived member of this series, 237 Np, which
is 2.2 × 10 6 years, is sufficiently short for this element to have essentially disappeared
if it had been created at the same time as all the other elements of the earth.
A second characteristic common to all three naturally occurring series is that each
has a gaseous member and, furthermore, that the radioactive gas in each case is a
different isotope of the element radon. In the case of the uranium series, the gas
(Rn) is called radon; in the thorium series, the gas is called thoron; and in the actinium
series (Table 4-4), it is called actinon. It should be noted that the artificial neptunium
series (Table 4-2) has no gaseous member. The existence of the radioactive gases in