Biomedical Engineering Reference
In-Depth Information
43.
A parallel beam of monoenergetic photons emerged from a
source when the shielding was removed for a short time. The
photon energy
h
ν
and the total fluence
ϕ
of photons are known.
(a)
Write a formula from which one can calculate the absorbed
dose in air in rad from
h
ν
, expressed in MeV, and
ϕ
,
expressed in cm
-2
.
(b)
Write a formula for calculating the exposure in R.
The thermal-neutron capture cross section for the
14
N(n,p)
14
C
reaction is 1.70 barns. Calculate
(a)
44.
the
Q
value for the reaction
(b)
the resulting dose in soft tissue per unit fluence of thermal
neutrons.
45.
A 100-cm
3
sample of water is exposed to 1500 thermal
neutrons cm
-2
s
-1
. How many photons are emitted per second
as a result of neutron capture by hydrogen? The cross section
for the
1
H(n,γ
)
2
H reaction is 3.3
10
-25
cm
2
.
×
46.
A uniform target with a volume of 5 L is exposed to 100
thermal neutrons cm
-2
s
-1
. It contains an unknown number of
hydrogen atoms. While exposed to the thermal neutrons, it
emits
1.11
10
4
photons s
-1
as the result of thermal-neutron
capture by hydrogen (cross section
=
0.33 barn). No other
radiation is emitted. What is the density of H atoms in the
target? Neglect attenuation of the neutrons and photons as they
penetrate the target.
47. (a)
×
Calculate the average recoil energies of a hydrogen nucleus
and a carbon nucleus elastically scattered by 4-MeV
neutrons.
(b)
What can one say about the relative contributions that
these two processes make to absorbed dose and dose
equivalent in soft tissue?
48.
Using Table 12.6, plot the percentage of the first-collision tissue
dose that is due to elastic scattering from hydrogen for
neutrons with energies between 0.01 MeV and 14.0 MeV.
49.
Calculate the first-collision dose per unit fluence for 14-MeV
neutrons based on their interactions with tissue hydrogen
alone. Compare the result with Table 12.6.
50.
From Table 12.6, the total first-collision dose per unit fluence
for 14-MeV neutrons in soft tissue is 6.06
10
-11
Gy cm
2
. From
Table 12.5, the average quality factor for 14-MeV neutrons
is 7.5. Use these two values to estimate the constant fluence
rate of 14-MeV neutrons that gives a first-collision dose
equivalent of 1 mSv in 40 h. How do you account for the lower
value, 12 neutrons cm
-2
s
-1
, given in the last column of
Table 12.5?
×
