Page 269 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 253

Figure 9.6 shows a typical NAA spectrum obtained with a multichannel analyzer equipped
with scintillation (upper curve) or semiconductor (lower curve) detectors. Each peak can
be ascribed to a certain -t-energy, which in most cases identifies the nuclide. A number of
nuclides can be identified simultaneously with semiconductor detectors, but with NaI(TI)
scintillation detectors the poor resolution limits simultaneous multi-element analysis.
The area under the peak (shaded area in Fig. 8.5(b)) is proportional to the amount of the
radioactive nuclide. If all other factors in (9.9) are known, the number of target nuclide
atoms N can be calculated.
When complex mixtures are irradiated, such as geological or biological samples, there
may be some difficulties in peak assignment. The energy spectrum is then scanned at
repeated time intervals and from the decrease of the peak area with time the half-life of the
peak may be established. This is a valuable additional aid in the assignment of the peak to
a certain nuclide.
While in principle it is possible to calculate the amount of the desired element through the
use of the proper values for the cross-section, flux, irradiation time, and half-life in (9.9),
a simpler approach has been developed that avoids errors implicit in the uncertainties of
each of these values. The unknown and a known standard of similar composition are
irradiated and counted in an identical fashion. A direct comparison can be made according
to the following relationship:

Weight of element in unknown Activity of element in unknown

Weight of element in standard Activity of element in standard

or

w u/w 0 = R u/R 0 (9.10)
Sometimes such a large number of competing radioactivities are produced in the
bombardment process that it may be necessary to conduct some chemical purification
(RNAA, radiochemical NAA). This is particularly true if simple counting of/3-activity or
-y-ray spectrometry using NaI(TI) counting is used. However, with the development of
semiconductor detectors and INAA the increasexl resolution in the spectrum allows
simultaneous determination of as many as 15 or 20 competing radioactivities, usually
without the necessity of chemical purification. Analysis of trace constituents in air and
water, in soil and geological samples, in marine and in biological systems are some of the
interesting applications of the NAA technique. Examples of on-line NAA include sorting
ore minerals and oil well logging. In forensic science, by using NAA to measure the
composition of the material adhering to a hand which has held a gun during firing, it is
possible to determine the type of ammunition and even the number of shots fired. Trace
metal analysis of plants can be used to determine the location in which that plant has been
grown (used, for example, for identification of marijuana growers). The trace constituents
of archeological and art objects play an important role in ascertaining their authenticity and
the identification of place of origin; the use of nondestructive NAA has been extremely
valuable in this field. Activation analysis of the mineral content of pigments has enabled
scientists to determine the authenticity of paintings attributed to certain artists since, in
times past, each artist prepared his own paints by distinctive and individual formulae. A