Biomedical Engineering Reference
In-Depth Information
Table 12.1
Dependence of Quality Factor
Q
on LET of Radiation
as Formerly Recommended by ICRP, NCRP, and ICRU
LET (keV
µ
m
-1
in Water)
Q
3.5 or less
1
3.5-7.0
1-2
7.0-23
2-5
23-53
5-10
53-175
10-20
Gamma rays, X rays, electrons,
positrons of any LET
1
Table 12.2
Dependence of Quality Factor
Q
on LET as Currently
Recommended by ICRP, NCRP, and ICRU
LET,
L
(keV
µ
m
-1
in Water)
Q
<10
1
10-100
0.32
L
-
2
.2
300/
√
L
>100
ICRP, NCRP, and ICRU have defined
Q
in accordance with Table 12.2. In the con-
text of quality factor, LET is the unrestricted stopping power,
L
∞
, as discussed in
Section 7.3. For incident charged particles, it is the LET of the radiation in water,
expressed in keV per
µ
m of travel. For neutrons, photons, and other uncharged
radiation, LET refers to that which the secondary charged particles they generate
would have in water. Like absorbed dose, dose equivalent is a point function. When
dose is expressed in Gy, the (SI) unit of dose equivalent is the sievert (Sv). With the
dose in rad, the older unit of dose equivalent is the rem (“roentgen-equivalent-
man”). Since 1 Gy = 100 rad, 1 Sv = 100 rem.
Dose equivalent has been used extensively in protection programs as the quantity
in terms of which radiation limits are specified for the exposure of individuals.
Dose equivalents from different types of radiation are simply additive.
Example
A worker receives a whole-body dose of 0.10 mGy from 2-MeV neutrons. Estimate the
dose equivalent, based on Table 12.1.
Solution
Most of the absorbed dose is due to the elastic scattering of the neutrons by the
hydrogen in tissue (cf. Table 12.6). To make a rough estimate of the quality fac-
tor, we first find
Q
for a 1-MeV proton—the average recoil energy for 2-MeV neu-
trons. From Table 5.3 we see that the stopping power for a 1-MeV proton in water
is 270 MeV cm
-1
m
-1
. Under the current recommendations of the ICRP,
NCRP, and ICRU,
Q
is defined according to Table 12.2. However, the older recom-
=
27 keV
µ
