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

564 CHAPTER 10
TABLE 10-9. Tenth Value Layers (TVLs) of Concrete for Leakage Radiation
ACCELERATOR VOLTAGE (MV) TVL1(cm) TVLe(cm)
4 33 28
6 34 29
10 35 31
15 36 33
18 36 34
20 36 34
25 37 35
30 37 36
60 Co 21 21
Reproduced with permission of the National Council on Radiation Protection and Measurements from Structural
Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities. Bethesda, MD: National
Council on Radiation Protection & Measurement; 2005. NCRP Report 151.
As shown in the example above, even the secondary barrier is very thick, usually
on the order of 1–1.25 m (3–4 ft.) concrete. To shield the entrance doorway into the
treatment room would require a massive amount of lead, the exact thickness would
depend on the distance of the doorway from the accelerator head. To reduce the
shield thickness at the door, megavoltage therapy rooms are designed in the form
of a maze (Fig. 10-20). The maze wall is a barrier that reduces the patient-scattered
radiation and the leakage radiation at the door to a manageable level. To reach the
door, the patient-scattered radiation must again be scattered through a large angle,
thereby greatly decreasing the energy of the scattered photons and making it easier
to shield the radiation at the door. Details for the design of a maze may be found in
NCRP 151.
Neutron Production
Except for the 9 Be(γ , n) 8 Be reaction whose gamma threshold energy is 1.666 MeV,
all other thresholds for photoneutron production are about 8.5 MeV, and production
becomes significant at quantum energies greater than 10 MeV. The neutron
energy spectrum is almost independent of the photon energy at these energies, and
the average neutron energy is about 1–2 MeV, depending on the exact photodisintegration
reaction. The energy spectrum of the neutrons is important to the health
physicist because the response of the neutron survey meter may depend on energy
of the neutrons. Most neutron-surveying instruments are calibrated with one of the
following three sources:
Pu–Be, whose average neutron energy = 4.2 MeV,
Am–Be, whose average neutron energy = 4.5 MeV, or
252 Cf, whose fission neutron spectrum is similar to the spectral distribution of
the photoneutrons, and whose average neutron energy, 2.2 MeV, is not far from
that of the photoneutrons.
Photoneutron production is important in shielding design for three reasons:
Neutrons add to the X-ray dose, and must be accounted for in the shield design.
The neutrons may be absorbed by material in the environment and make the
absorbing atoms radioactive, thus leaving radioactivity and a radiation field after
the machine is turned off.