Biomedical Engineering Reference
In-Depth Information
(
b
)Fe
(
c
)Au.
A small 1.85-GBq
198
Au source (maximum beta-particle energy
0.961 MeV, average 0.315 MeV) is enclosed in a lead shield just
thick enough to absorb all the beta particles.
(
13.
a
) Estimate the energy fluence rate due to bremsstrahlung at
a point 50 cm away.
(
b
) What is the estimated bremsstrahlung photon fluence rate
at that point for the purpose of assessing the potential
radiation hazard?
14.
A 12-mA electron beam is accelerated through a potential
difference of
2
10
5
V in an X-ray tube with a tungsten target.
X rays are generated as bremsstrahlung from the electrons
stopping in the target. Neglect absorption in the tube.
(
×
a
) Estimate the fraction of the beam power that is emitted as
radiation.
(
b
) How much power is radiated as bremsstrahlung?
15.
Estimate the range in cm of the maximum-energy beta ray
(2.28 MeV) from
90
Y in bone of density 1.9 g cm
-3
.
16.
Use Eq. (6.15) to estimate the range of a 400-keV beta particle
in water. How does the answer compare with Table 6.1?
17.
Derive Eq. (6.16) from Eq. (6.15).
18.
Use Table 6.1 to estimate the range in cm in air at STP for
electrons of energy
(
a
) 50 keV
(
b
) 830 keV
(
c
) 100 MeV.
19.
A positron emerges normally from a 4-mm-thick plastic slab
(density 1.14 g cm
-3
) with an energy of 1.62 MeV. What was its
energy when it entered the slab?
20.
Use the range curve in Fig. 5.7 and make up a formula or
formulas like Eqs. (6.15) and (6.17), giving the range of
electrons in lead as a function of energy.
21.
A cell culture (Fig. 6.9) is covered with a 1-cm sheet of Lucite
(density
1.19 gcm
-3
). What thickness of lead (in cm) is
needed on top of the Lucite to prevent 10-MeV beta rays from
reaching the culture? Use the approximate empirical formulas
relating range
R
in g cm
-2
to electron kinetic energy
T
in MeV:
Lucite:
R
=
0.334
T
1.48
,
=
0
≤
T
≤
4
R
= 0.426
T
1.14
,
lead:
0.1 ≤
T
≤ 10
.
22.
To protect the cell culture in the last problem from radiation,
what advantage would be gained if the positions of the Lucite
and the lead were swapped, so that the Lucite was on top?
