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

The required lead-equivalent leaded glass window is calculated from
B = I
I0
t = lnB
−μ
= e −μt
ln 0.193
= = 0.86 cm = 8.6mm=11
−1
−1.92 cm 32 in.
EXTERNAL RADIATION SAFETY 557
This simplified calculational model was used to illustrate some of the considerations
in the design of shielding for a PET facility without either a cyclotron or a CT scanner.
A computerized shielding model using a realistic body phantom as the source term,
calculated a window thickness of 9.9-mm lead equivalent for exposure conditions
similar to those in this simplified illustration. 3
Radiotherapy Machines 4
Radiation from radiotherapy machines span a very wide range of energies, from
15 keV Grenz ray X-rays for superficial therapy to 25 MeV accelerators for deep
therapy. The basic principles of shielding radiotherapy devices is the same as for
diagnostic devices, as is the basic calculational methodology. In all cases, we
1. must know the maximum radiation level to be produced by the device,
2. calculate the unshielded radiation level at the dose point that we wish to protect,
3. calculate the required degree of attenuation, or the maximum transmission, of
the shielding barrier, taking into account the use of protected area, and
4. calculate the thickness of the barrier that will give the required degree of attenuation
of the radiation.
Although the basic principles are the same for all shielding-design calculations, there
are significant difference in the details. Some of these differences include
1. Photoneutrons are produced when high-energy X-rays, E > 10 MeV, interact
with matter, such as collimaters, shielding, etc. Thus, when discussing shielding
of high-energy machines, we have two categories:
a. ≤ 10MeV, where we do not have neutrons because the threshold energy for
photoneutron production is about 8.5 MeV for most materials, and the cross
section remains very small until the quantum energy exceeds 10 MeV.
b. >10 MeV, where we have neutrons that contribute significantly to the radiation
dose, and which, therefore, must be shielded.
2. Because of the presence of photoneutrons, safety criteria are expressed in Sv
rather than Gy.
3. Specification of the workload—For diagnostic X-ray machines, the workload is
specified in milliamp-minutes per week at a given kVp. For therapeutic facilities,
3 Methe, Brian M. Oper Rad Saf, S83-S88, May, 2003.
4 This introduction to radiotherapy machine shielding design is based on NCRP Report
151, Structural Shielding Design and Evaluation for Megavoltage X- and Gamma Ray Radio Therapy
Facilities. Complete detailed information can be found in NCRP 151.