Shielding Flashcards
What is the typical thickness of shielding for an HDR suite?
~30-60 cm of concrete
~5 cm on lead if you use lead (not common)
What is the primary shielding reference?
NCRP151
How do you calculate your barrier transmission (B)?
B = Pd^2/WUT
How do you get the number of TVLs for a shielding calculation?
n = -log(B)
What is your design goal for a controlled area?
0.1 mSv/week
What is your design goal for an uncontrolled area?
0.02 mSv/week
What material is most often used in doors for high-energy linac vaults?
BPE - borated polyethylene
Why is boron included in doors for high-energy linac vaults?
Boron has a large cross section for thermal neutrons
How much boron is typically included in high-energy linac vault doors?
5% by weight
What does neutron capture by boron produce?
an alpha and a ~0.5 MeV photon
What is a typical thickness of a primary barrier for a high-energy linac?
2-3 m
How does the TVL of steel compare to concrete?
~1/4
How does the TVL of lead compare to concrete?
~1/8
What is the typical thickness of shielding for a PET/CT suite?
~1 cm of Pb or 13 cm of concrete
What is the typical thickness of shielding for a CT suite?
1/16th inch of lead (embedded in drywall)
What is the typical shielding thickness of a secondary barrier?
1-2 m
What is the typical thickness of materials in a high-energy linac door?
With a maze: 2-3 cm of lead plus 2-4 cm of BPE
No maze: 10 cm lead and 15-25 cm BPE
What are some assumptions in NCRP-151
all conservative:
- neglects patient attenuation
- beam always takes the shortest path through the barrier
- head leakage is the maximum allowable magnitude
- occupancy factors are conservatively high
- two-source rule
What is the two-source rule in NCRP-151?
Sources of radiation (like types of secondary radiation) are considered to be independent. If the TVLs required from 2 are about the same (within 1 TVL), then you need to take the higher of the 2 and add 1 HVL. If they differ by more than 1 TVL, then you just need to use the bigger of the 2.
How do you calculate transmission for laminated (multi-material) barriers?
You multiply the transmission from the various components: Btot = Bconcrete * Blead * Bsteel
What is a typical primary barrier width?
10-12 ft
By how much does neutron fluence decrease with each turn in a maze?
1/3
