TYPES OF RADIATION

a.  General.  Four major forms of radiation are commonly found emanating from radioactive matter:  alpha, beta, gamma, and X radiation.  The marked differences in the characteristics of these radiations strongly influence their difficulty in detection and consequently, the detection methods used.  

b.  Alpha.  An alpha particle is the heaviest and most highly charged of the common nuclear radiations.  As a result, alpha particles very quickly give up their energy to any medium through which they pass, rapidly coming to equilibrium with, and disappearing in, the medium.  Since nearly all common alpha radioactive contaminants emit particles of about the same energy, 5 MeV, some general statements may be made about the penetration length of alpha radiation.  Generally speaking, a sheet of paper, a thin layer (a few hundredths of a millimeter) of dust, any coating of water or less than 4 cm of air are sufficient to stop alpha radiation.  As a result, alpha radiation is the most difficult to detect.  Moreover, since even traces of such materials are sufficient to stop some of the alpha particles and thus change detector readings, quantitative measurement of alpha radiation is impossible outside of a laboratory environment where special care may be given to sample preparation and detector efficiency.  

c.  Beta.  Beta particles are energetic electrons emitted from the nuclei of many natural and manmade materials.  Being much lighter than alpha particles, beta particles are much more penetrating.  For example, a 500-keV beta particle has a range in air that is orders of magnitude longer than that of the alpha particle from plutonium, even though the latter has 10 times more energy; however, many beta-active elements emit particles with very low energies.  For example, tritium emits a (maximum energy) 18.6-keV beta particle.  At this low an energy, beta particles are less penetrating than common alpha particles, requiring very special techniques for detection.  

d.  Gamma and X Radiation.  Gamma rays are a form of electromagnetic radiation and, as such, are the most penetrating of the four radiations and easiest to detect.  Once emitted, gamma rays differ from X-rays only in their energies, with X-rays usually lying below a few hundred keV.  As a result, X-rays are less penetrating and harder to detect; however, even a 60- keV gamma ray has a typical range of a 100 meters in air and might penetrate a centimeter of aluminum.  In situations in which several kinds of radiations are present, these penetration properties make X-ray and/or gamma-ray detection the technique of choice.  

e.  Radiations from the Common Contaminants.  Table 1., below, lists some of the commonly considered radioactive contaminants and their primary associated radiations.
 

Table 1.  Commonly Considered Radioactive Contaminants and Their Primary Associated Radioactive Emissions