Biomedical Engineering Reference
In-Depth Information
the specific gamma-ray constant, but includes the exposure rate from
all
photons
emitted with energies greater than a specified value
δ
. In the case of
125
I, the spe-
cific gamma-ray constant is 0.0042 Rm
2
Ci
-1
h
-1
and the exposure-rate constant is
0.13 Rm
2
Ci
-1
h
-1
for photons with energies greater than about 10 keV.
Neutrons
As discussed in Chapter 9, fast neutrons lose energy primarily by elastic scattering
while slow and thermal neutrons have a high probability of being captured. The two
principal capture reactions in tissue are
1
H
(n
,
γ
)
2
Hand
14
N
(n
,
p)
14
C. Slow neutrons
are quickly thermalized by the body. The first capture reaction releases a 2.22-MeV
gamma ray, which could deposit a fraction of its energy in escaping the body. In
contrast, the nitrogen-capture reaction releases an energy of 0.626 MeV, which is
deposited by the proton and recoil carbon nucleus in the immediate vicinity of
the capture site. The resulting dose from exposure to thermal neutrons can be
calculated, as the next example illustrates.
Example
Calculate the dose in a 150-g sample of soft tissue exposed to a fluence of 10
7
thermal
neutrons cm
-2
.
Solution
From Table 12.3, the density of nitrogen atoms in soft tissue is
N
10
21
cm
-3
,
14
N being over 99.6% abundant. The thermal-neutron capture cross section is
=
1.29
×
σ
=
10
-24
cm
2
(Section 9.7). Each capture event by nitrogen results in the depo-
sition of energy
E
=
1.70
×
0.626 MeV, which will be absorbed in the unit-density sample
1gcm
-3
). The number of interactions per unit fluence per unit volume of the
tissue is
Nσ
(
ρ
=
10
7
cm
-2
is therefore
. The dose from the fluence
ϕ
=
ϕ
N
σ
E
D
=
ρ
10
7
cm
-2
10
21
cm
-3
10
-24
cm
2
×
1.29
×
×
1.70
×
×
0.626 MeV
=
1gcm
-3
1.6
×
10
-13
J
MeV
1
10
-3
kg g
-1
=
10
-6
Gy.
×
×
2.20
×
(12.30)
Some additional dose would be deposited by the gamma rays produced by the
1
H(n,γ)
2
H reaction, for which the cross section is 3.3 × 10
-25
cm
2
. However, in a
tissue sample as small as 150 g, the contribution of this gamma-ray dose is negligi-
ble. It is not negligible in a large target, such as the whole body.
The absorbed dose from fast neutrons is due almost entirely to the energy trans-
ferred to the atomic nuclei in tissue by elastic scattering. As discussed in Sec-
tion 9.6, a fast neutron loses an average of one-half its energy in a single colli-
sion with hydrogen. For the other nuclei in soft tissue, the average energy loss is
