Biomedical Engineering Reference
In-Depth Information
Committed Equivalent Dose
When a radionuclide is taken into the body, it can become distributed in various
tissues and organs and irradiate them for some time. For the single intake of a
radionuclide at time
t
0
, the committed equivalent dose over a subrequent time
τ
in
an organ or tissue T is defined as
t
0
+
τ
H
T
d
t
,
H
T
(
τ
) =
(14.6)
t
0
where
H
T
is the equivalent-dose rate in T at time
t
. Unless otherwise indicated, an
integration time
τ
=
50 y after intake is implied for occupational use and 70 y for
members of the public.
2)
Committed Effective Dose
By extension, the committed effective dose
E
(
)
following the intake of a radionu-
clide is the weighted sum of the committed equivalent doses in the various tis-
sues T:
τ
(14.7)
E
(
τ
)
=
w
T
H
T
(
τ
).
T
As we shall see in more detail in Chapter 16, the effective half-life of a radionuclide
in a tissue is determined by its radiological half-life and its metabolic turnover
rate. For radionuclides with effective half-lives of no more than a few months, the
committed quantities, Eqs. (14.6) and (14.7), are practically realized within one year
after intake. If a radionuclide is retained in the body for a long time, then the
annual equivalent and effective doses it delivers will be considerably less than the
committed quantities.
The committed effective dose replaces the earlier committed effective dose equiv-
alent. The latter is defined like Eq. (14.7), with
H
T
representing the committed dose
equivalent in the organ or tissue T.
Collective Quantities
The quantities just defined relate to the exposure of an individual person. The
ICRP has defined other dosimetric quantities that apply to the exposure of groups
or populations to radiation. The
collective equivalent dose
and the
collective effective
dose
are obtained by multiplying the average value of these quantities in a pop-
ulation or group by the number of persons therein. The collective quantities are
then expressed in the unit, “person-sievert,” and can be associated with the total
consequences of a given exposure of the population or group. The Commission ad-
2 According to NCRP Report No. 116. ICRP
Publication 60 (p. 9) specifies an implied
“50 y for adults and from intake to age 70 y
for children.”
