Appendix B to Part 191 - Calculation of Annual Committed Effective Dose
40:27.0.1.1.2.5.1.1.2 : Appendix B
Appendix B to Part 191 - Calculation of Annual Committed Effective
Dose I. Equivalent Dose
The calculation of the committed effective dose (CED) begins
with the determination of the equivalent dose, HT, to a tissue or
organ, T, listed in Table B.2 below by using the equation:
where DT, R is the absorbed dose in rads (one
gray, an SI unit, equals 100 rads) averaged over the tissue or
organ, T, due to radiation type, R, and wR is the radiation
weighting factor which is given in Table B.1 below. The unit of
equivalent dose is the rem (sievert, in SI units).
Table B.1 - Radiation Weighting Factors, wR
1
Radiation type and energy
range 2
wR value
Photons, all
energies
1
Electrons and
muons, all energies
1
Neutrons, energy
<10 keV
5
10 keV to 100
keV
10
>100 keV to
2 MeV
20
>2 MeV to 20
MeV
10
>20 MeV
5
Protons, other
than recoil protons, >2 MeV
5
Alpha particles,
fission fragments, heavy nuclei
20
1 All values relate to the
radiation incident on the body or, for internal sources, emitted
from the source.
2 See paragraph A14 in ICRP
Publication 60 for the choice of values for other radiation types
and energies not in the table.
II. Effective Dose
The next step is the calculation of the effective dose, E. The
probability of occurrence of a stochastic effect in a tissue or
organ is assumed to be proportional to the equivalent dose in the
tissue or organ. The constant of proportionality differs for the
various tissues of the body, but in assessing health detriment the
total risk is required. This is taken into account using the tissue
weighting factors, wT in Table B.2, which represent the proportion
of the stochastic risk resulting from irradiation of the tissue or
organ to the total risk when the whole body is irradiated uniformly
and HT is the equivalent dose in the tissue or organ, T, in the
equation:
Table B.2 - Tissue Weighting Factors, wT
1
Tissue or organ
wT value
Gonads
0.25
Breast
0.15
Red bone
marrow
0.12
Lung
0.12
Thyroid
0.03
Bone surfaces
0.03
Remainder
2 0.30
1 The values are considered to be
appropriate for protection for individuals of both sexes and all
ages.
2 For purposes of calculation,
the remainder is comprised of the five tissues or organs not
specifically listed in Table B.2 that receive the highest dose
equivalents; a weighting factor of 0.06 is applied to each of them,
including the various sections of the gastrointestinal tract which
are treated as separate organs. This covers all tissues and organs
except the hands and forearms, the feet and ankles, the skin and
the lens of the eye. The excepted tissues and organs should be
excluded from the computation of HE.
III. Annual Committed Tissue or Organ Equivalent Dose
For internal irradiation from incorporated radionuclides, the
total absorbed dose will be spread out in time, being gradually
delivered as the radionuclide decays. The time distribution of the
absorbed dose rate will vary with the radionuclide, its form, the
mode of intake and the tissue within which it is incorporated. To
take account of this distribution the quantity committed equivalent
dose, HΤ(τ) where is the integration time in years following an
intake over any particular year, is used and is the integral over
time of the equivalent dose rate in a particular tissue or organ
that will be received by an individual following an intake of
radioactive material into the body. The time period, τ, is taken as
50 years as an average time of exposure following intake:
for a single intake of activity at time t0
where HT(t) is the relevant equivalent-dose rate in a tissue or
organ at time t. For the purposes of this part, the previously
mentioned single intake may be considered to be an annual intake.
IV. Annual Committed Effective Dose
If the committed equivalent doses to the individual tissues or
organs resulting from an annual intake are multiplied by the
appropriate weighting factors, wT, and then summed, the result will
be the annual committed effective dose, E(τ):