PLUTONIUM
D-9
APPENDIX D
inflammation of the
basement membrane, designated as the histohematic barrier (HHB), which may
progress to fibrosis. In slow renewal and nonrenewal systems, the radiation may have little effect on the
parenchymal cells, but ultimate parenchymal atrophy and death over several months result from HHB
fibrosis and occlusion of the microcirculation.
D.4.3 Low Level Radiation Effects
Cancer is the major latent harmful effect produced by ionizing radiation and the one that most people
exposed to radiation are concerned about. The ability of alpha, beta, and gamma radiation to produce
cancer in virtually every tissue and organ in laboratory animals has been well-demonstrated. The
development of cancer is not an immediate effect. Radiation-induced leukemia has the shortest latent
period at 2 years, while other radiation induced cancers have latent periods >20 years. The
mechanism by
which cancer is induced in living cells is complex and is a topic of intense study. Exposure to ionizing
radiation can produce cancer at any site within the body; however, some sites appear to be more common
than others, such as the breast, lung, stomach, and thyroid.
DNA is a major target molecule during exposure to ionizing radiation. Other macromolecules, such as
lipids and
proteins, are also at risk of damage when exposed to ionizing radiation. The genotoxicity of
ionizing radiation is an area of intense study, as damage to the DNA is ultimately responsible for many of
the adverse toxicological effects ascribed to ionizing radiation, including cancer. Damage to genetic
material is basic to developmental or teratogenic effects, as well. However, for effects other than cancer,
there is little evidence of human effects at low levels of exposure.
D.5 UNITS IN RADIATION PROTECTION AND REGULATION
D.5.1 Dose Equivalent (or equivalent dose)
Dose equivalent (as measured in rem or Sievert) is a special radiation protection quantity that is used, for
administrative and radiation safety purposes only, to express the absorbed dose in a manner which
considers the difference in biological effectiveness of various kinds of ionizing radiation. The dose
equivalent concept is applicable only to doses that are not great enough to produce biomedical effects.
The NRC defines the dose equivalent, H, as the product of the absorbed dose, D, and the quality factor, Q,
at the point of interest in biological tissue. This relationship is expressed as H = D x Q. The NCRP
defines equivalent dose, H, as the product of the absorbed dose, D, and the radiation
weighting factor, w
r
.
This relationship is expressed as H = Dw
r
.
The quality factor or radiation weighting factor is a dimensionless quantity that depends in part on the
stopping power for charged
particles, and it accounts for the differences in biological effectiveness found
among the types of radiation. Originally relative biological effectiveness (RBE) was used rather than Q
to define the quantity, rem, which was of use in risk assessment. The generally accepted values for
quality factors and radiation weighting factors for various radiation types are provided in Table D-3. The
dose equivalent rate is the time rate of change of the dose equivalent to organs
and tissues and is
expressed as rem/unit time or sievert/unit time.
PLUTONIUM
D-10
APPENDIX D
Table D-3. Quality Factors (Q) and Radiation Weighting Factors
Radiation Weighting Factor (w
r
)
Type of radiation
Quality factor (Q)
X, gamma, or beta radiation
1
1
Alpha particles, multiple-charged
20
20
particles, fission fragments and
heavy particles of unknown charge
Neutrons (100 keV to 2 MeV),
7.5-11 depending on
20
protons, alpha particles, charged
energy
particles of unknown energy
Neutrons of unknown energy
10
Thermal neutrons
2
5
High-energy protons
10
5
Source:
USNRC. 2004. Standards for the protection against radiation, tables 1004(b).1 and 1004(b).2. 10 CFR 20.1004. U.S.
Nuclear Regulatory Commission, Washington, D.C.
NCRP 1993
D.5.2 Relative Biological Effectiveness
RBE is used to denote the experimentally determined ratio of the absorbed dose from one radiation type
to the absorbed dose of a reference radiation required to produce an identical biologic effect under the
same conditions. Gamma rays from cobalt-60 and 200–250 keV x-rays
have been used as reference
standards. The term RBE has been widely used in experimental radiobiology, and the term quality factor
(or radiation weighting factor) used in calculations of dose equivalent for radiation safety purposes (ICRP
1977; NCRP 1971; UNSCEAR 1982). RBE applies only to a specific biological end point, in a specific
exposure, under specific conditions to a specific species. There are no generally accepted values of RBE.
D.5.3 Effective Dose Equivalent (or effective dose)
The absorbed dose is usually defined as the mean absorbed dose within an organ or tissue. This
represents a simplification of the actual problem. Normally when an individual ingests or inhales a
radionuclide or is exposed to external radiation that enters the body (gamma), the dose is not uniform
throughout the whole body. The simplifying assumption is that the detriment will be the same whether
the body is uniformly or non-uniformly irradiated. In an attempt to compare detriment from absorbed
dose of a limited portion of the body with the detriment from total body dose, the ICRP (1977) derived a
concept of effective dose equivalent. ICRP (1990) changes
this term to effective dose, but it has not yet
been adopted by the NRC or DOE.
The effective dose equivalent, H
E
, is
H
E
= ∑ W
t
H
t
where H
t
is the dose equivalent (or equivalent dose) in the tissue t, W
t
is the tissue weighting factor in that
tissue, which represents the estimated proportion of the stochastic risk resulting from tissue, t, to the