Nuclear Spectroscopy

E X P E R I M E N T 13
Uranium 238 Series
INTRODUCTION
Like thorium, uranium is found is most building
materials and soils on the earth. There are three
isotopes of uranium still found abundantly in the
earth’s crust. Mass 238 is the most abundant (99.28%),
and the other isotopes are much less abundant, 235 U
(0.72%), 234 U (0.0055%).
The vast difference in the relative abundances of 238 U
and 235 U is supposedly due to the factor of 6 difference
in their half-lives (see Tables 13.1 and 13.2), and
the long time from the creation of nearly equal
quantities of the two isotopes. This was about 6 x 10 9
years ago, the postulated time for the supernova
whose remnants eventually became our solar system.
The presence of 234 U after this great length of time is
due only to its being created in the decay chain
(shown in Figure 13.1 and 13.2) starting with 238 U.
235
U has its own decay chain, ending with the stable
isotope, 207 Pb. For most environmental measurements,
gammas from isotopes in the 235 U decay series
are too weak to observe with small NaI(Tl) detectors.
Table 13.1
U-238 4.47 x 10 9 yr 1.41 x 10 17 sec
Th-234 24.1 day 2.08 x 10 6 sec
Pa-234 1.17 min 68.4 sec
U-234 2.46 x 10 5 yr 7.76 x 10 12 sec
Th-230 7.54 x 10 4 yr 2.38 x 10 12 sec
Ra-226 1.6 x 10 3 yr 5.05 x 10 10 sec
Rn-222 3.8235 day 3.30 x 10 5 sec
Po-218 3.10 min 186 sec
Pb-214 26.8 min 1.61 x 10 3 sec
Bi-214 19.9 min 1.19 x 10 3 sec
**Po-214 163.7 µsec 1.64 x 10 -4 sec
**Tl-210 1.3 min 78 sec
Pb-210 22.3 yr 7.04 x 10 8 sec
Bi-210 5.01 day 4.33 x 10 5 sec
Po-210 138.38 day 1.20 x 10 7 sec
Pb-206 stable
226
Ra
88
63.3 (3.8%)
92.6 (5.4%)
234 γ Th
90
1001.6 keV (0.6%)
γ 67.7 keV (23.4%)
230
Th
90
234
Pa
91
Figure 13.1 The uranium decay series to radium.
238
U
92
The 238 U decay chain contains radium ( 226 Ra) and
radon ( 222 Rn) isotopes discovered by Marie and Pierre
Curie at the beginning of the 20th century. This radon
isotope is the one that seems to worry some health
physicists and environmentalists the most, as far as its
concentration in houses, mines, and buildings. You
will handle sources with radon and radium in some lab
exercises below. Your exposure to these isotopes will
not be any more health-threatening to you than exposure
to other isotopes suggested for use in this book of
experiments. As always, proper handling of radioisotopes
should always be followed, whether they pose a
significant or insignificant health risk. Whether exposure
to radon is a health risk or a benefit has been
called into question (see Cohen).
Radon will only appear as a significant isotope in
naturally occurring uranium ores. Uranium compounds
from the chemistry lab have not had enough
time to populate 226 Ra significantly because of the long
uranium half-lives. The resulting activity of 222 Rn is the
same as 226 Ra, very small.
γ
U
234
92
γ 53.2 keV (28%)
40