Radiation Safety and Protection Flashcards

In linear accelerator vault design, which barrier is struck directly by the useful beam?

Answer: Primary barrier

The primary barrier intercepts the useful beam and is therefore the thickest shielding in the vault.

What use factor (U) is assigned to a secondary barrier when calculating shielding?

Answer: 1

Leakage and scatter reach secondary barriers whenever the beam is on, so U = 1.

According to NCRP Report 151, what is the weekly shielding design goal for an uncontrolled area?

Answer: 0.02 mSv/week

NCRP 151 uses 0.02 mSv per week for uncontrolled areas and 0.1 mSv per week for controlled areas.

If one tenth-value layer (TVL) of concrete is added to a barrier, the transmitted radiation intensity becomes what fraction of the original?

Answer: 1/10

A TVL reduces beam intensity to one-tenth of its incident value.

Why is borated polyethylene often added to the maze door of an 18 MV linac vault?

Answer: To moderate and capture neutrons

Photon beams above about 10 MV produce photoneutrons, which hydrogen moderates and boron captures.

What occupancy factor (T) is typically assigned to a fully occupied area such as an office or control console?

Answer: 1

NCRP 151 assigns T = 1 to areas occupied full time, such as offices and control areas.

A therapist doubles their distance from a point source from 1 m to 2 m. By what factor does the exposure rate change?

Answer: It drops to one-quarter

Per the inverse square law, doubling the distance reduces intensity to 1/4.