Nuclear Spectroscopy

E X P E R I M E N T 14
Environmental Sampling
INTRODUCTION
There are many more radioactive isotopes in our
environment than just 40 K, and the members of the
uranium and thorium decay series. Most of the
isotopes you have purchased are decay products from
nuclear fission, or have been created in accelerators
or reactors. Many of these are now in our environment
from atmospheric nuclear weapons tests or
discharges from reactors and their wastes. Obtaining
such environmental sources can be difficult because
they are not sold commercially. However there are
many other naturally-occurring sources, some of which
are gamma emitters. A list of the best (for your
purposes) is given in Table 14.1 at the end of this
chapter.
Observation of these isotopes is interesting by itself,
but they also provide information on the dynamics of
their production and accumulation. 7 Be is produced
in the upper atmosphere by the cosmic-ray proton
bombardment of 16 O and 14 N. Measurements of the
gamma emissions from 7 Be provides the means to
monitor the cosmic ray flux that produces the 7 Be in
the upper atmosphere or to monitor the variations in
the dynamics of the atmosphere that bring 7 Be near
the earth’s surface.
Lanthanum and lutetium are rare-earth elements
easily (though somewhat expensive) obtained from
chemical supply houses. There are many commercial
uses, especially for lanthanum oxide, a phosphor in
newer fluorescent tubes. Both elements have naturally
occurring, radioactive isotopes that emit gammas.
The specific activity (decay rate per gram of the
element) of lutetium is much larger than that of
lanthanum, and its gammas are at a lower energy than
the gamma from lanthanum, resulting in more efficient
detection of lutetium’s gammas.
Use of radionuclides in medicine, for imaging and
therapy, results in walking human sources that are
orders of magnitude more radioactive (for a few days)
than any exempt-quantity source you may purchase.
For people receiving doses from an isotope given
internally, a substantial fraction of that isotope will be
eliminated in urine, ending in the sewage system.
Your local hospital may be helpful in supplying small
quantities of these short-lived isotopes.
Not all important radioisotopes emit gammas when
they decay, especially tritium ( 3 H), carbon ( 14 C), and
rubidium ( 87 Rb). These isotopes are observed with
liquid scintillation or direct beta detection, but this
book is focused on the subject of gamma detection
with NaI(Tl) detectors.
Collection of environmental samples requires experimental
skills from all branches of science, so only the
simplest will be suggested in the following exercises.
If your sample-gathering skills are better than those
demanded in this book, then use them.
Exercise 14.1
Gamma Spectra from Air Samples
The Russian Chernobyl reactor accident was not
publicly announced until scientists in Sweden observed
gammas from short-lived fission products in
air samples, and deduced that an accident had occurred.
Atmospheric weapon’s tests always leave their
fingerprint of gamma-emitting, radioactive isotopes
in the air.
OBJECTIVE
Observe gammas and identify the isotopic sources
obtained by sampling the air, both indoors and outdoors.
Gammas from isotopes of radon and radon
daughters, as well as 7 Be, could possibly be part of
your collection.
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