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

EXTERNAL RADIATION SAFETY 565
Gamma rays (capture gammas) are almost always produced in the neutron absorption
reactions. The average energy of capture gammas in concrete is 3.6
MeV.
Shielding design of high-energy radiotherapy facilities is based on well-known
principles of bremsstrahlung production, interaction of photons with matter, neutron
production, and neutron interactions, including absorption and activation.
However, the application of these principles to the design of a particular facility is
a very complex undertaking. Detailed application of these principles may be found
in several applied publications, such as NCRP 144, NCRP 147, and NCRP 151. The
complexity of shielding design for high-energy radiation therapy facilities makes
shielding design an ideal candidate for computerization, and numerous computer
codes may be found in the literature.
Airborne Contaminant Production
Interaction of energy by the atmosphere from environmental radiation fields can
lead to radiolytic dissociation of oxygen and nitrogen molecules in the air. The
resulting disrupted molecules can recombine to form ozone, nitrogen oxide, and
nitrogen dioxide. Ozone, whose OSHA permissible exposure level (PEL) is 0.1 parts
per million (ppm), and nitrogen dioxide, whose PEL is 5 ppm, are of concern because
of their relatively high toxicity. Neutrons can interact with the 40 Ar constituent
in ordinary air to produce 110-minute half-lived 41 Ar, and with atmospheric dusts
to produce airborne radioactive particles. According to NCRP 147, ventilation of a
normal clinical treatment room at a rate of three air changes per hour is sufficient
for health protection.
Beta Shielding
Two factors must be considered in designing a shield against high-intensity
radiation—namely, the beta particles and the bremsstrahlung that are generated
due to absorption in the source itself and in the shield. Because of these factors,
the beta shield consists of a low-atomic-numbered substance (to minimize the production
of bremsstrahlung) sufficiently thick to stop all the betas, followed by a
high-atomic-numbered material thick enough to attenuate the bremsstrahlung intensity
to an acceptable level.
TABLE 10-10. Tenth Value Layers (TVLs) of Concrete for Patient-Scattered Radiation at 90 ◦
Scattering Angle
ACCELERATOR VOLTAGE (MV) TVL (cm)
4 17
6 17
10 18
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.