Biomedical Engineering Reference
In-Depth Information
Table 12.6
Analysis of First-Collision Dose for Neutrons in Soft Tissue
Neutron
First-Collision Dose per Unit Neutron Fluence for Collisions with Various
Elements (10
-11
Gy cm
2
)
Energy
(MeV)
H
O
C
N
Total
0.01
0.091
0.002
0.001
0.000
0.094
0.02
0.172
0.004
0.001
0.001
0.178
0.03
0.244
0.005
0.002
0.001
0.252
0.05
0.369
0.008
0.003
0.001
0.381
0.07
0.472
0.012
0.004
0.001
0.489
0.10
0.603
0.017
0.006
0.002
0.628
0.20
0.914
0.034
0.012
0.003
0.963
0.30
1.14
0.052
0.016
0.003
1.21
0.50
1.47
0.122
0.023
0.004
1.62
0.70
1.73
0.089
0.029
0.005
1.85
1.0
2.06
0.390
0.036
0.007
2.49
2.0
2.78
0.156
0.047
0.012
3.00
3.0
3.26
0.205
0.045
0.018
3.53
5.0
3.88
0.244
0.079
0.024
4.23
7.0
4.22
0.485
0.094
0.032
4.83
10.0
4.48
0.595
0.157
0.046
5.28
14.0
4.62
1.10
0.259
0.077
6.06
Source
: From “Measurement of Absorbed Dose of Neutrons
and Mixtures of Neutrons and Gamma Rays,”
National Bureau
of Standards Handbook 75
, Washington, D.C. (1961).
distribution of events that neutrons have in nature. Such Monte Carlo calculations
can be made under general conditions of target composition and geometry as well
as incident neutron spectra and directions of incidence. Compilations of the re-
sults for a large number of neutrons then provide dose and LET distributions as
functions of position, as well as any other desired information, to within the statis-
tical fluctuations of the compilations. Using a larger number of neutron histories
reduces the variance in the quantities calculated, but increases computer time.
Figure 12.10 shows the results of Monte Carlo calculations carried out for 5-MeV
neutrons incident normally on a 30-cm soft-tissue slab, approximating the thick-
ness of the body. (The geometry is identical to that shown for the charged particles
in Fig. 12.9.) The curve labeled
E
T
is the total dose,
E
p
is the dose due to H recoil nu-
clei (protons),
E
γ
is the dose from gamma rays from the
1
H(n,γ
)
2
H slow-neutron
capture reaction, and
E
H
is the dose from the heavy (O, C, N) recoil nuclei. The
total dose builds up somewhat in the first few cm of depth and then decreases as
the beam becomes degraded in energy and neutrons are absorbed. The proton and
heavy-recoil curves,
E
p
and
E
H
, show a similar pattern. As the neutrons penetrate,
they are moderated and approach thermal energies. This is reflected in the rise
of the gamma-dose curve,
E
γ
, which has a broad maximum over the region from
