Biomedical Engineering Reference
In-Depth Information
When the two chambers are used together to monitor an
unknown mixed gamma-neutron field, the TE chamber reads
0.27 mGy h
-1
and the C CO
2
chamber reads 0.21 mGy h
-1
.
What are the individual gamma and neutron dose rates?
(Assume that the neutrons have an energy of 0.5 MeV.)
24.
AC CO
2
chamber like that in Fig. 12.4 is calibrated to read
directly in mGy h
-1
for 1-MeV photons. The chamber satisfies
the Bragg-Gray principle for neutrons as well as gamma rays.
(a)
The chamber is exposed to 1-MeV neutrons and gives a
reading of 22 mGy h
-1
. What is the neutron dose rate for
soft tissue?
(b)
A 1-MeV gamma source is added, and the reading is
increased to 84 mGy h
-1
. What is the gamma dose rate to
tissue?
(c)
What would a tissue-equivalent chamber read in part (b)
when exposed to both the neutrons and gamma rays
together?
25.
In a proportional counter such as the one shown in Fig. 12.6,
why, on the average, would pulses produced by gamma photons
be much smaller than pulses produced by fast neutrons?
40
eV ip
-1
for a
1-MeV carbon recoil nucleus in a proportional-counter gas,
what is the ratio of the pulse heights produced by these two
particles if they stop completely in the gas?
30
eV ip
-1
for a 2-MeV proton and
W
26.
If
W
=
=
27.
A beam of fast neutrons is directed toward a proportional
counter that operates with methane (CH
4
)gas.Ifthe
maximum pulse height registered is 4.2 MeV, what is the
maximum energy of the neutrons?
An air ion chamber having a volume of 2.5 cm
3
(STP) is placed
at a certain depth in a water tank, as shown in Fig. 12.8. An
electron beam incident on the tank produces a current of
0.004
µ
A in the chamber. What is the dose rate at that depth?
28.
29.
What is the average whole-body dose rate in a 22-g mouse that
contains
1.85
10
5
Bq of
14
C distributed in its body?
×
30.
A patient receives an injection of 1.11
10
8
Bq of
131
I, 30% of
which goes to the thyroid, having a mass of 20 g. What is the
average dose rate in the organ?
×
31.
Tritium often gets into body water following an exposure and
quickly becomes distributed uniformly throughout the body.
What uniform concentration of
3
H, in Bq g
-1
,wouldgivea
dose-equivalent rate of 1 mSv wk
-1
?
A 36-g mouse is to be injected with
32
P (half-life
=
14.3 d;
average beta energy
=
0.70 MeV; no gamma [Appendix D]).
Assume that the
32
P distributes itself almost instantaneously
throughout the body following injection and that none is lost
32.
