Environmental Engineering FOURTH EDITION

320 ENVIRONMENTAL ENGINEERING

Table 16-3. Sample Quality Factors for Internal Radiation

Type of radiation Quality factor

Internal alpha 10

Plutonium alpha in bone 20

Neutrons (atom bomb survivor dose)

Fission spectrum neutrons 2-100

Ema > 0.03MeV 1 .o

20

Beta and Gamma

Emax c 0.03MeV 1.7

a Emax refers to the maximum energy of emissions from the /3 or y source.

ionizing radiation. Table 16-3 gives sample QFs for the internal dose fromradionuclides

incorporated into human tissue.

Dose equivalents are often expressed in terms of population dose equivalent,

which is measured in persondu. The population dose equivalent is the product of

the number of people affected and the average dose equivalent in sieverts. That is, if

a population of 100,000 persons receives an average dose equivalent of 0.05 Sv, the

population dose equivalent will be 5000 person-Sv. The utility of this concept will

become evident in the discussion on health effects.

Table 16-4 gives 1996 estimated average radiation dose equivalents in the United

States. The dose equivalent is product of the absorbed dose and the QF. The efective

dose equivalent (EDE) is the risk-weighted sum of the dose equivalents to the individ-

ually irradiated tissues or organs. The committed eflectiue dose equivalent (CEDE) is

the total effective dose equivalent to the individual over a 50-year period. The EDE

and CEDE are usually cited by the U.S. Environmental Protection Agency (EPA) and

other regulatory agencies. Note that 55% of the “background” effective dose equiv-

alent is from radon exposure and 82% is from natural (nonanthropogenic) sources.

The effective dose equivalent is the number usually cited as average annual back-

ground exposure for an individual living in the United States. Different radionuclides

exhibit different EDEs and CEDES. These values may be found for each isotope in the

Handbook of Health Physics and Radiological Health (SchIeien et al. 1998).

Measuring Ionizing Radiation

The particle counter, the ionization chamber, photographic film, and the thermolumi-

nescent detector axe four methods widely used to measure radiation dose, dose rate,

and the quantity of radioactive material present.

Particle counters are designed to detect the movement of single particles through

a defined volume. Gas-filled counters collect the ionization produced by the radiation

as it passes through the gas and amplify it to produce an audible pulse or other signal.

Counters are used to determine radioactivity by measuring the number of particles

emitted by radioactive material in a given time.