Biomedical Engineering Reference
In-Depth Information
Fig. 12.8
Measurement of dose or dose rate as a function of
depth in water exposed to a beam of charged particles.
from measurements made with a proportional counter.
2)
A spherically shaped
counter (usually tissue equivalent) is used and a pulse-height spectrum measured
in the radiation field. If energy-loss straggling is ignored and the counter gas pres-
sure is low, so that a charged particle from the wall does not lose a large fraction of
its energy in traversing the gas, then the pulse size is equal to the product of the
LET and the chord length. The distribution of isotropic chord lengths
x
in a sphere
of radius
R
is given by the simple linear expression
x
2
R
2
d
x
.
P
(
x
)d
x
=
(12.20)
Thus, the probability that a given chord has a length between
x
and
x
+d
x
is
P
(
x
)d
x
,
this function giving unity when integrated from
x
=
0
to
2
R
. Using analytic tech-
niques, one can, in principle, unfold the LET spectrum from the measured pulse-
height spectrum and the distribution
P
(
x
)
of track lengths through the gas. How-
ever, energy-loss straggling and other factors complicate the practical application
of this method.
Precise LET determination presents a difficult technical problem. Usually, practi-
cal needs are satisfied by using estimates of the quality factor or radiation weighting
factor based on conservative assumptions.
2C .
Microdosimetry
, ICRU Report 36,
International Commission on Radiation
Units and Measurements, Bethesda, MD
(1983).
