Page 209 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 193

ship crossed very intense showers of high energy cosmic rays. People who have been
involved in criticality accidents experiencing high intensities of n and "y have noted a
fluorescence in their eyes and felt a heat shock in their body.
However, we are not physiologically aware of the normal radiation fields of our
environment. In such low fields we must entirely rely on instruments.
The ionization and/or excitation of atoms and molecules when the energies of nuclear
particles are absorbed in matter is the basis for the detection of individual particles.
Macroscopic collective effects, such as chemical changes and heat evolution, can also be
used. The most important of the latter have been described before because of their
importance for dose measurements (e.g. the blackening of photographic films and other
chemical reactions, excitation of crystals (thermoluminescence), and heat evolved in
calorimeters; Ch. 7).
In this chapter we consider only the common techniques used for detection and
quantitative measurement of individual nuclear particles. We also discuss the problem of
proper preparation of the sample to be measured as well as consideration of the statistics
of the counting of nuclear particles necessary to ensure proper precision (i.e. how well a
value is determined) and accuracy (i.e. agreement between measured and true value).

8.1. Track measurements

The most striking evidence for the existence of atoms comes from the observation of
tracks formed by nuclear particles in cloud chambers, in solids and in photographic
emulsions. The tracks reveal individual nuclear reactions and radioactive decay processes.
From a detailed study of such tracks, the mass, charge and energy of the particle can be
determined.
The tracks formed can be directly observed by the naked eye in cloud and bubble
chambers, but the tracks remain only for a short time before they fade. For a permanent
record we must use photography. On the other hand, in solid state nuclear track detectors
(SSNTD), of which the photographic emulsion is the most common variant, the tracks have
a much longer lifetime during which they can be made permanent and visible by a suitable
chemical treatment. Because of the much higher density of the absorber, the tracks are also
much shorter and often therefore not visible for the naked eye. Thus the microscope is an
essential tool for studying tracks in solids.

8.1.1. Cloud and bubble chambers

The principle of a cloud chamber is shown in Figure 8.1. A volume of saturated vapor
contained in a vessel is made supersaturated through a sudden adiabatic expansion. When
ionizing radiation passes through such a supersaturated vapor the ionization produced m the
vapor serves as condensation nuclei. As a result small droplets of liquid can be observed
along the path of the radiation. These condensation tracks have a lifetime of less than a
second and can be photographed through the chamber window. The density of the
condensation depends on the ionization power of the projectile as well as on the nature of
the vapor, which is often an alcohol or water. Cloud chamber photographs are shown in