Biomedical Engineering Reference
In-Depth Information
Tab l e 7 . 2
Restricted Collisional Mass Stopping Power of Water,
(-d
E
/
ρ
d
x
)
in MeV cm
2
g
-1
, for Electrons
-
d
E
ρ
d
x
100 eV
-
d
E
ρ
d
x
1keV
-
d
E
ρ
d
x
10 keV
-
d
E
ρ
d
x
Energy (MeV)
∞
0.0002
298.
298.
298.
298.
0.0005
183.
194.
194.
194.
0.001
109.
126.
126.
126.
0.003
40.6
54.4
60.1
60.1
0.005
24.9
34.0
42.6
42.6
0.01
15.1
20.2
23.2
23.2
0.05
4.12
5.26
6.35
6.75
0.10
2.52
3.15
3.78
4.20
1.00
1.05
1.28
1.48
1.89
power is due to collisions that transfer more than 10 keV. Corresponding data for
the restricted collisional mass stopping power for electrons are presented in Ta-
ble 7.2. Here, the restricted stopping powers are different at much lower energies
than in Table 7.1.
7.3
Linear Energy Transfer (LET)
The concept of linear energy transfer, or LET, was introduced in the early 1950s to
characterize the rate of energy transfer per unit distance along a charged-particle
track. As such, LET and stopping power were synonymous. In studying radiation
effects in terms of LET, the distinction was made between the energy transferred
from a charged particle in a target and the energy actually absorbed there. In 1962
the International Commission on Radiation Units and Measurements (ICRU) de-
fined LET as the quotient
-d
E
L
/d
x
, where
d
E
L
is the “average energy locally im-
parted” to a medium by a charged particle in traversing a distance
d
x
. The words
“locally imparted,” however, were not precisely specified, and LET was not always
used with exactly the same meaning. In 1980, the ICRU defined LET
as the re-
stricted stopping power for energy losses not exceeding
:
-
d
E
d
x
LET
=
,
(7.3)
with the symbol LET
∞
denoting the usual (unrestricted) stopping power.
LET is often found in the literature with no subscript or other clarification. It can
generally be assumed then that the unrestricted stopping power is implied.
Example
Use Table 7.1 to determine LET
1keV
and LET
5keV
for 1-MeV protons in water.
