Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

EXTERNAL RADIATION SAFETY 575
for a pure high-energy beta emitter, the bremsstrahlung produced in the shield must
be considered in the shielding design.
Neutron shielding is based on slowing down fast neutrons to thermal energies
and then absorbing the thermal neutrons. Low-atomic-numbered materials, such
as water, and hydrogenous compounds, such as paraffin, are effective slowing-down
materials. The thermalized neutrons are most readily absorbed by materials that have
a high absorption cross section, such as boron or cadmium. In designing shielding
against neutrons, it must be remembered that absorption of neutrons can lead to
capture gammas and to induced radioactivity and the consequent production of
gamma radiation.
Although radiation protection measures can be designed to reduce radiation dose
to levels approaching that of the natural background, expenditure of resources to
do this may be wasteful since the benefits that accrue from such low doses are less
than the benefits that might result from expenditure of these resources in other
avenues. Accordingly, we optimize the degree of radiation protection at the level at
which the benefits are equal to the cost of protection.
m Problems
10.1. A Po–Be neutron source emits 107 neutrons/s of average energy 4 MeV. The
source is to be stored in a paraffin shield of sufficient thickness to reduce the
fast flux at the surface to 10 neutrons/cm2 /s. Consider paraffin to be essentially
CH2 (for the purpose of this problem) and to have a density of 0.89 g/cm3 .
(a) What is the minimum thickness of the paraffin shield?
(b) If the slowing-down length is 6 cm, the thermal diffusion length is 3 cm,
and the diffusion coefficient is 0.381 cm, what will be the thermal-neutron
leakage flux at the surface of the shield?
(c) What is the gamma-ray dose rate, due to the hydrogen-capture gammas, at
the surface of the paraffin shield?
10.2. An X-ray therapy machine operates at 250 kVp and 20 mA. At a target to skin
distance of 100 cm, the dose rate is 0.2 Gy/min. The workload is 100 Gy/wk. The
X-ray tube is constrained to point vertically downward. At a distance of 4 m from
the target is an uncontrolled waiting room. Calculate the thickness of lead to be
added to the wall if the total thickness of the wall (which is made of hollow tile
and plaster, density 2.35 g/cm3 ) is 2 in. (5 cm).
10.3. A7.4 × 1013 Bq (2000 Ci) 60Co teletherapy unit is to be installed in an existing
concrete room in the basement of a hospital so that the source is 4 m from the
north and west walls—which are 30 in. thick. Beyond the north wall is a fully
occupied controlled room. Beyond the west wall is a public parking lot. The
useful beam is to be directed toward the north wall for a maximum of 5 hours
per week during radiation therapy. The beam will be directed at the west wall
1 hour per week. Considering only the radiation from the primary beam, how
much additional shielding, if any, is required for each of the walls?