Environmental Engineering FOURTH EDITION

316 ENVIRONMENTALENGINEERING

N = (5.47 x 1022)e-47*52 = (5.47 x 1022)(2.30 x = 126 atoms.

(116 atoms)(l% @mole)

6.02 x lG3 atomdmole

= 2.52 x lo-’’ g

Useful figures of merit for radioactive decay are:

0 After 10 half-lives,

0 After 20 half-lives,

(or 0.1%) of the original quantity of radioactive material

of the original quantity of radioactive material is left.

is left.

Alpha, Beta, and Gamma Radiation

Emissions from radioactive nuclei are called, collectively, ionizing radiation because

collision between these emissions and an atom or molecule ionizes that atom or

molecule. Ionizing radiation may be characterized further as alpha, beta, or gamma

radiation by its behavior in a magnetic field. Apparatus for such characterization is

shown in Fig. 16-2. A beam of radioactively disintegrating atoms is aimed with a

lead barrel at a fluorescent screen that is designed to glow when hit by the radiation.

Alternately charged probes direct the a and /3 radiation accordingly. The y radiation is

seen to be “invisible light,” a stream of neutral particles that passes undeflected through

the electromagnetic field. a and B emissions have some mass, and are considered

particles, while y emissions are photons of electromagnetic radiation.

Alpha radiation has been identified as helium nuclei that have been stripped of

their planetary electrons, and each consists of two protons and two neutrons. a particles

thus have a mass of about 4 amu (6.642 x

They are emitted with kinetic energies between 4 and 10 MeV from the nuclei

of relatively heavy elements. As these charged particles travel at approximately

10,OOO miles/s, they collide with other atoms. Each collision results in a transfer

of energy to the electrons of these other atoms, and thus in production of an ion pair: a

negatively charged electron and an associated positively charged ion. An alpha parti-

cle produces between 30,000 and 100,OOO ion pairs per centimeter of air traveled. Its

kinetic energy is depleted rapidly and it has a range of between 1 and 8 cm in air and

much less in denser media like solid objects or human skin cells. External radiation

by (Y particles presents no direct health hazard, because even the most energetic are

stopped by the epidermal layer of skin and rarely reach more sensitive layers. A health

hazard occurs when material contaminated with a-emitting radionuclides is eaten or

g) each, and a positive charge of 2.l

‘This electric charge is expressed in units relative to an electronic charge of - 1.

‘Mass is designated as energy in the equation E = mc2. One megaelectron volt (MeV) = lo6 electron

volts (eV); 1 eV = 1.603 x J, and is the energy of an electron accelerated through a field of a million

volts.