Biomedical Engineering Reference
In-Depth Information
no dose and 0.2% will have a dose of
∼
500 mGy.
3)
The difference arises from the
fact that the neutron dose is deposited by recoil nuclei, which have a short range.
A proton having an energy of 500 keV has a range of
8 × 10
-4
cm
=
8
µ
min
soft tissue (Table 5.3), compared with a range of 0.174 cm for a 500-keV electron
(Table 6.1). Both particles deposit the same energy. The proton range is comparable
to the cell-nucleus diameter; the electron travels the equivalent of
∼1740/5 = 350
nuclear diameters.
Specific Energy
When a particle or photon of radiation interacts in a small volume of tissue, one
refers to the interaction as an energy-deposition event. The energy deposited by
the incident particle and all of the secondary electrons in the volume is called the
energy imparted,
. Because of the statistical nature of radiation interaction, the
energy imparted is a stochastic quantity. The specific energy (imparted) in a volume
of mass
m
is defined as
m
.
z
=
(12.32)
It has the dimensions of absorbed dose. When the volume is irradiated, it experi-
ences a number of energy-deposition events, which are characterized by the single-
event distribution in the values of
z
that occur. The average absorbed dose in the
volume from a number of events is the mean value of
z
. Studies of the distributions
in
z
from different radiations in different-size small volumes of tissue are made in
microdosimetry.
Similarly, one can regard an ensemble of identical small volumes throughout
an irradiated body and the distribution of specific energy in the volumes due to
any number of events. Thus, the example cited from the BEIR-III Report in the
next-to-last paragraph can be conveniently described in terms of the distribution of
specific energy
z
in the cell nuclei. For the low-LET radiation,
2
3
of the nuclei have
0;
in the other
3
,
z
varies widely with a mean value of
∼
3 mGy. For the fission
neutrons, 99.8% of the nuclei have
z
z
=
0;
in the other 0.2%,
z
varies by many orders
of magnitude with a mean value of
∼
500 mGy.
The stochastic specific energy plays an important role as the microdosimetric
analogue of the conventional absorbed dose, which is a nonstochastic quantity.
=
Lineal Energy
The lineal energy
y
is defined as the ratio of the energy imparted
from a single
event in a small volume and the mean length
¯
x
of isotropic chords through the
volume:
y
=
¯
x
.
(12.33)
3
The Effects on Populations of Exposure to Low
Levels of Ionizing Radiation: 1980
, BEIR-III
Report, p. 14, National Academy of Sciences,
Washington, D.C. (1980).
