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ABR Part III > Shielding > Flashcards

Shielding Flashcards

1
Q

What is the typical thickness of shielding for an HDR suite?

A

~30-60 cm of concrete
~5 cm on lead if you use lead (not common)

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2
Q

What is the primary shielding reference?

A

NCRP151

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3
Q

How do you calculate your barrier transmission (B)?

A

B = Pd^2/WUT

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4
Q

How do you get the number of TVLs for a shielding calculation?

A

n = -log(B)

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5
Q

What is your design goal for a controlled area?

A

0.1 mSv/week

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6
Q

What is your design goal for an uncontrolled area?

A

0.02 mSv/week

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7
Q

What material is most often used in doors for high-energy linac vaults?

A

BPE - borated polyethylene

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8
Q

Why is boron included in doors for high-energy linac vaults?

A

Boron has a large cross section for thermal neutrons

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9
Q

How much boron is typically included in high-energy linac vault doors?

A

5% by weight

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10
Q

What does neutron capture by boron produce?

A

an alpha and a ~0.5 MeV photon

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11
Q

What is a typical thickness of a primary barrier for a high-energy linac?

A

2-3 m

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12
Q

How does the TVL of steel compare to concrete?

A

~1/4

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13
Q

How does the TVL of lead compare to concrete?

A

~1/8

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14
Q

What is the typical thickness of shielding for a PET/CT suite?

A

~1 cm of Pb or 13 cm of concrete

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15
Q

What is the typical thickness of shielding for a CT suite?

A

1/16th inch of lead (embedded in drywall)

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16
Q

What is the typical shielding thickness of a secondary barrier?

A

1-2 m

17
Q

What is the typical thickness of materials in a high-energy linac door?

A

With a maze: 2-3 cm of lead plus 2-4 cm of BPE
No maze: 10 cm lead and 15-25 cm BPE

18
Q

What are some assumptions in NCRP-151

A

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

19
Q

What is the two-source rule in NCRP-151?

A

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.

20
Q

How do you calculate transmission for laminated (multi-material) barriers?

A

You multiply the transmission from the various components: Btot = Bconcrete * Blead * Bsteel

21
Q

What is a typical primary barrier width?

A

10-12 ft

22
Q

By how much does neutron fluence decrease with each turn in a maze?

A

1/3

ABR Part III (13 decks)

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