Page 569 - Instrumentation Reference Book 3E

Materials analysis 551

23.2! Materials analysis radiation emitted as the excited atoms of the elem-
ent of interest decay with emission of gamma rays
Nuclear methods of analysis, particularly neutron or beta particles, while not responding to other
activation analysis, offer the most sensitive types of radiation or to different energies from
methods available for most elements in nature. other elements which may be present in the sample.
However, no one method is suitable for all elements, The decay half-life of the radiation is also used to
and it is necessary to select the techniques to be used identify the element of interest, while the actual
with due regard to the various factors involved, magnitude of the response at the particular energy
some of which may be listed as follows: involved is a direct measure of the amount of the
element of interest in the sample. Quantitatively the
(1) Element to be detected: basic relation between A, the induced activity pres-
(2) Quantities involved; ent at the end of the irradiation in Becquerels, iie.,
(3) Accuracy of the quantitative analysis disintegrations per second, is given by
required;
(4) Costs of various methods available;
(5) A.vailability of equipment to carry out the
a:nalysis to the statistical limits required;
(6) Time required; where N is the number of target atoms present, 0
(7) Matrix in which the element to be measured is the cross-section (cm’), @ the irradiation flux
located; (neutrons cm-’ s-l), ti the irradiation time and
(8) Feasibility of changing the matrix mentioned Ti the half-life of product nuclide.
irn (7) to a more amenable substance. -From this we may calculate N, the number of
For example, the environmental material sample atoms of the element of interest present, after
may be analyzed by many of the methods appropriate correction factors have been evaluated.
described, but the choice of method must be a Beside the activation of the element and measure-
compromise, depending on all the factors ment of its subsequent decay products one may
involved. count directly the excitation products produced
whilst the sample is being bombarded. This is called
“prompt gamma-ray analysis,” and it has been
23.2.11 Activation analysis used; for example, to analyze the surface of the
When a material is irradiated by neutrons, photons, moon.
alpha or beta particles, protons, etc. a reaction may Most elements do not produce radioactivity
take ]place in the material depending on a number when bombarded with electrons, beta particles.
of factors. The most important of these are: or gamma rays. However, most will emit charac-
teristic X-rays when bombarded and the emitted
(1) The type of particle or photon used for the X-rays are characteristic of each element present.
irradiation: This is the basis of X-ray fluorescence analysis.
(2) The energy of the irradiation; discussed below.
(3) The flux in the material; Electrons and protons have also been used to
(4) The time of irradiation. excite elements to emit characteristic X-rays or
The nost useful type of particle has been found particles, but this technique requires operation in
to be neutrons, since their neutral charge allows a very high vacuum chamber,
them to penetrate the high field barriers surround- High-energy gamma rays have the property of
ing most atoms; and relatively low energies are exciting a few elements to emit neutrons - this
required. In fact, one of the most useful means of occurs with beryllium when irradiated with
irradiation is the extreimely low energy neutrons of gamma rays of energy greater than 1.67MeV
thermal energy (0.025 eV) which are produced and deuterium with gamma rays of energy greater
abundantly in nuclear reactors. The interactions than 2.23 MeV. This forms the basis of a portable
which occur cause some of the various elements monitor for beryllium prospecting in the field,
present to become radioactive, and in their subse- using an ‘”Sb source to excite the beryllium.
quent decay into neutral atoms again, to emit par- Higher-energy gamma rays from accelerators,
ticles and radiation which are indicative of the etc., have the property of exciting many elements
elements present. Neutron activation analysis, as but require extremely expensive equipment.
this is called, has become one of the most useful
and s,ensitive methods of identifying certain elem- 23.2.2 X-ray fluorescence analysis
ents in minute amounts without permanently 23.2.2.1 Dispersive X-ray fluorescence analysis
damaging the specimen, as would be the case in
most chemical methods of analysis. A detector sys- In dispersive X-ray fluorescence analysis the
tem can be selected which responds uniquely to the energy spectrum of the characteristic X-ray