Radiation Sources
The radiations of primary concern in this
text originate in atomic or nuclear processes. They are conveniently
categorized into four general types as follows:
Charged particulate radiation {Fast
electrons, Heavy charged particles
Uncharged radiation {Electromagnetic
radiation, Neutrons
Fast
electrons include beta particles (positive or negative) emitted in nuclear decay,
as well as energetic electrons produced by any other process. Heavy charged particles denote a
category that encompasses all energetic ions with a mass of one atomic mass unit
or greater, such as alpha particles, protons, fission products, or the products
of many nuclear reactions. The electromagnetic
radiation of interest includes X-rays emitted in the rearrangement of
electron shells of atoms and gamma rays that originate from transitions within
the nucleus itself. Neutrons
generated in various nuclear processes constitute the final major category,
which is often further divided into a slow neutron and fast neutron subcategories.
The energy range of interest spans over six
decades, ranging from about 10 eV to 20 MeV. (Slow neutrons are technically an
exception but are included because of their technological importance.) The
lower energy bound is set by the minimum energy required to produce ionization
in typical materials by the radiation or the secondary products of its
interaction. Radiations with energy greater than this minimum are classified as
ionizing radiations. The upper bound is chosen to limit the topics in this
coverage to those of primary concern in nuclear science and technology.
The main emphasis in this chapter will be the
laboratory-scale sources of these radiations, which are likely to be of
interest either in the calibration and testing of radiation detectors described
in the following chapters or as objects of the measurements themselves. Natural
background radiation is an essential additional source.
The radiations of interest differ in their
"hardness" or ability to penetrate the thicknesses of material. It is
also of considerable concern to determine the physical form of radiation
sources. Soft radiations, such as alpha particles or low-energy X-rays,
penetrate only small material thicknesses. Radioisotope sources must
therefore be deposited in very thin layers if a large fraction of these
radiations is to escape from the source itself. Sources that are physically
thicker are subject to "self-absorption," which is likely to affect
both the number and the energy spectrum of the radiations that emerge from their
surface. Typical thicknesses for such sources are therefore measured in micrometres.
Beta particles are generally more penetrating, and sources up to a few tenths
of a millimetre in thickness can usually be tolerated. Harder radiations, such
as gamma rays or neutrons, are much less affected by self-absorption, and
sources can be millimetres or centimetres in dimension without seriously
affecting the radiation properties.