Biomedical Engineering Reference
In-Depth Information
one can express the equivalent-dose rate in a small volume of tissue in the body by
writing
Cskg
E
ρ
A
H
E
=
Svh
-1
,
(16.64)
where
g
E
is a geometrical factor that allows for shielding by intervening tissues.
For alpha particles and low-energy beta particles, such as those of tritium,
g
E
=
0
.
These radiations cannot penetrate to the lens of the eye (at a depth of 3 mm) or to
the basal layer of the epidermis (at a depth of 70
µ
m). For most other beta emitters
and for low-energy photons,
g
E
=
0.5
near the body surfaces and approaches zero
with increasing depth. For high-energy photons,
g
E
=
1
throughout the body.
For irradiation from gas absorbed in the body, the ICRP considers a prolonged
exposure to the cloud, which results in equilibrium concentrations of the gas in the
air and in tissue. The concentration
C
T
of gas in the tissue is then given by
C
T
=
δ
C
ρ
T
Bqg
-1
,
(16.65)
where
ρ
T
is the density of tissue (∼
10
6
gm
-3
)and
δ
is the solubility of the gas
in tissue, expressed as the volume of gas in equilibrium with a unit volume of
tissue at atmospheric pressure. The solubility increases with the atomic weight of
the gas, varying in water at body temperature from ∼
0.02 for hydrogen to ∼
0.1
for xenon. For adipose tissue the values may be larger by a factor of 3-20. For the
equivalent-dose rate in tissue from absorbed gas, the ICRP writes
s
δ
Cg
A
ρ
T
Svh
-1
,
H
A
=
(16.66)
where
g
A
is another geometric factor, depending on the size of a person and the
range of the radiation. For alpha and beta particles and low-energy photons,
g
A
=
1
for tissues deep inside the body and
g
A
=
0.5
for tissues at the surface. For energetic
photons
g
A
1
.
The equivalent-dose rate in the lung from the gas it contains can be written
sCV
L
g
L
M
L
H
L
=
Svh
-1
,
(16.67)
10
-3
m
3
),
M
L
is the mass
where
V
L
is the average volume of air in the lungs (
∼
3
×
of the lungs (1000 g), and
g
L
is a geometrical factor (=
1
for alpha and beta particles
and low-energy photons and decreasing with increasing photon energy).
The three rates (16.64), (16.66), and (16.67) can be applied in the following way.
For tritium,
H
E
= 0
for all relevant tissues of the body, because this nuclide emits
only low-energy beta particles. The ratio of the equivalent-dose rate in any tissue
from absorbed gas to the rate in the lung from the gas it contains is
H
A
H
L
=
g
A
M
L
V
L
g
L
ρ
T
.
δ
(16.68)
