Page 257 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 241

example, since strontium and yttrium are not chemically identical, a gross B-count of
strontium samples including 9~ may include an unknown fraction of 90y activity present
from 9~ decay because or the relationship

9~ t,,~ 28.5 y)9~ t,/~ 2.671 d)9~

Beta-absorption, and/3- or -y-scintillation techniques which use energy discrimination, are
frequently useful in such parent-daughter cases. If equilibrium is rapidly established
between the parent and daughter activities it is usually simpler to count the samples after
sufficient time for this to occur, the contribution to the observed count rate by the daughter
is then proportional to the amount of mother in the sample. In the case of 9~176
radioactive equilibrium is established in about 25 d. If 137Cs (/3-'y, t,/~ 30.0 y) is being used
to study cesium chemistry it is necessary to wait only 15 - 20 min after sampling until
counting as the daughter 137tuBa (IT, tl,~ 2.55 rain) reaches an equilibrium level within that
time. Since the ratio of the 137tuBa and the 137Cs activity is the same in all samples at
equilibrium, the total count rate before and after a chemical step is then a true measure of
the behavior of cesium alone. If radioactive equilibrium is not re-established in a convenient
time, it may be necessary to either discriminate against the activity not involved in the
chemical system, to take into account its contributions to the net count rate, or to remove
it immediately before counting.
It may be necessary or expedient to use a radioactive nuclide which can undergo
significant decay during the chemical investigation. In these cases, in order to compare
results at different points in the process, it is necessary to correct all counts to the same
time (usually starting time of the experiment).

9.2. Chemistry of trace concentrations
Consider a sample containing a pure radionuclide with a disintegration rate of 107 dpm.
For a t,a of 1 h the number of atoms is (w 8.7• for a t,~ of 1 y it is 7.6•
If such a sample is dissolved in one liter of solution, the respective concentrations would
be 1.4 x 10-15 M and 1.3 x 10-11 M. At such concentrations the chemical behavior may be
quite different than it is at higher concentrations. Addition of macroscopic amounts (e.g.
at the gram level) of non-radioactive (isotopic) atoms of the element results in
concentrations of 10 -3 to 10 -1 M. The non-radioactive component is called a carrier as it
"carries" the radioactive and ensures normal chemical behavior. Many applications of
radiotracers involve mixing the tracer atoms with a much larger amount of nonradioactive
isotopic atoms prior to use.
If a radionuclide is to follow the chemical properties of an isotopic carrier it is necessary
that the radionuclide and the carrier undergo isotopic exchange. If it is not known a priori
that such exchange takes place between two compounds with a common element this must
be determined by experimentation before it can be assumed that the tracer and the carrier
would act similarly in a chemical system. This consideration must be particularly borne in
mind if the radioactive tracer and the inert carrier are in different oxidation states when
mixed.