Radiation-Protection Criteria and Exposure Limits - Atoms, Radiation, and-Radiation Protection - page 446

ues to be regulated under the dose-equivalent system, the newer ICRP/NCRP dose
quantities are largely employed elsewhere throughout the world today. In practical
terms, both systems work in maintaining exposures not only well below acceptable
limits, but at low levels in keeping with the ALARA principle. We shall describe
both systems in turn (Sections 14.6 and 14.7).
At the time of this writing (2007), the ICRP has before it a major new draft state-
ment, the “2007 ICRP Recommendations.” While the numerical limits in Publi-
cation 60 continue to be indorsed as providing an appropriate level of protection
for normal operations, fundamental changes are proposed in certain concepts and
approaches to radiation protection. The 2007 Recommendations will be considered
in Section 14.8.

14.4
NCRP/ICRP Dosimetric Quantities

Equivalent Dose

The equivalent dose, H T,R , in a tissue or organ T due to radiation R, is defined as
the product of the average absorbed dose, D T,R , in T from R and a dimensionless
radiation weighting factor, w R , for each radiation:

H T,R = w R D T,R .

(14.1)

The values of w R specified by the NCRP are shown in Table 14.1. (The values rec-
ommended by the ICRP are the same, except that w R = 5 for protons in the next-

to-last entry.) When the radiation consists of components with different w R ,then
the equivalent dose in T is given by summing all contributions:

H T = w R D T,R .

(14.2)

R

With D T,R expressed in Gy (1 Gy = 1Jkg -1 ), H T,R and H T are in Sv (1 Sv = 1Jkg -1 ).
The equivalent dose replaces the dose equivalent for a tissue or organ, defined
in Section 12.2. The two are conceptually different. Whereas dose equivalent in an
organ is defined as a point function in terms of the absorbed dose weighted by
a quality factor everywhere, equivalent dose in the organ is given simply by the
average absorbed dose weighted by the factor w R .
For radiation types and energies not included in Table 14.1, the ICRP and NCRP
give a pr escription for calculating an approximate value of w R as an average quality
factor, Q . For this purpose, the quality factor Q is defined in terms of the linear
energy transfer L by means of Table 12.2, given earlier in the text. One computes
the dose-average value of Q at a depth of 10 mm in the standard tissue sphere of