318 ENVlRONMEN'IALENGINEERINGTable 16-2. Properties of Ionizing Radiation
Particle or photon (Wave) Mass (amu) Electric chargeAP~ me4)^4 +2
Beta (electron) 5.5 10-~ -1
Gamma (X-ray) -0 0
Positron (positive electron) 5.5 x 10-~ +1Neutron 1 0and the charge balance is6= -1+7A typical reaction for a! decay, the first step in the U-238 decay chain, is
When a radionuclide emits a B, the mass number remains unchanged and the atomic
number increases by 1 (B decay is thought to be the decay of a neutron in the nucleus
to a proton and a B, with subsequent emission of the B). When a nuclide emits an a!,
the atomic mass decreases by 4 and the atomic number decreases by 2. y emission
does not result in a change of either atomic mass or atomic number.
Nuclear reactions may also be written for bombardment of nuclei with subatomic
particles. For example, tritium (H-3) is produced by bombarding a lithium target with
neutronsThese reactions tell us nothing about the energy with which ionizing radiation is emit-
ted, or the relative biological damage that can result from transfer of this energy in
collisions. These effects are discussed briefly later in this chapter.Units for Measuring Ionizing RadiationDamage to living organisms is directly related to the amounts of energy transferred to
tissue by collisions with a! and /3 particles, neutrons, and y radiation. This energy, in
the form of ionization and excitation of molecules, results in heat damage to the tissue.
Many of the units discussed in this section are thus related to energy transfer.
The International System of Units (SI units) for measuring ionizing radiation,
based on the meter-kilogram-second (mks) system, was defined by the General
Conference on Weights and Measures in 1960, and adoption of these units has been
recommended by the International Atomic Energy Agency. SI units replaced the units
that had been in use since about 1930. SI units are used in this chapter, and their
relationship to the historical units is also discussed.