Rad Safety Part 3

"What is the definition and characteristics of a Secondary Radiation Barrier's Scattered Radiation?"

"Radiation is scattered from the patient in all directions. The amount depends on beam intensity, quality, area of the beam at scatter, and scattering angle."

"What is the maximum energy for a 90-degree scattered photon, and what is the reason for this limit?",

"0.5 MeV, due to Compton scattering."

"What is the Absorbed Dose Rate Limit for leakage radiation (for radiation > 0.5 MeV) within a 2m radius?",

"Absorbed dose rate due to leakage within a 2m radius shall not exceed 0.2% of the useful beam."

"What is the quality of leakage radiation compared to the primary beam?",

"Equal to the primary beam."

"For MV beams, which is more penetrating: leakage radiation or scattered radiation?",

"Leakage radiation is more penetrating than scattered radiation."

"What effect does IMRT have on leakage radiation compared to 3D treatments?",

"IMRT increases leakage radiation compared to 3D."

"What rule determines the thickness of the secondary barrier if the thickness needed for leakage and scatter differ by three or more HVLs (Half Value Layers)?",

"If the thicker of the two (leakage or scatter thickness) differ by >= 3 HVLs, the thicker of the two will be used."

"What rule determines the thickness of the secondary barrier if the thickness needed for leakage and scatter differ by less than three HVLs?",

"If the thicker of the two (leakage or scatter thickness) differ by < 3 HVLs, the thicker of the two + 1 HVL will be used."

"What shielding requirement must a door satisfy relative to the wall around it?","

"Must provide shielding equivalent to the wall around it.

"What is the source of neutron contamination in linear accelerators operating with X-rays greater than 10 MV?", "

High energy photons and electrons incident on the target, flattening filter, collimators, and other shielding materials."

Radiation scattered from patient depends on what factors?

Beam intensity, beam quality, beam area at scatter, and scattering angle

Maximum energy of a 90-degree scattered photon?

0.5 MeV

Leakage radiation limit within 2 m for >0.5 MeV beams?

≤0.2% of useful beam

Is leakage radiation always present when the machine is on?

Yes, leakage always exists (U = 1)

Neutron contribution within useful beam vs X-ray dose?

<1% of X-ray dose

What to do if leakage barrier thickness differs from scatter by ≥3 HVLs?

Use the thicker barrier

What to do if leakage barrier differs by

Use thicker barrier + 1 HVL

Purpose of a maze in shielding design?

Prevent direct radiation incidence on the door

Best materials for slowing neutrons?

Hydrogen-rich materials like polyethylene, water, concrete

Recommended maze length for neutron protection?

Greater than 5 meters

Typical neutron-protective door structure?

Lead (inside) → concrete → lead (outside)

Examples of brachytherapy source shielding?

Lead-lined safes, lead-filled drawers, filtered sinks, hot boxes

Three principles of radiation protection?

Time, distance, shielding

Most effective method to reduce external exposure?

Increase distance (inverse square law)

Shielding material for gamma radiation?

Lead or concrete

Shielding material for beta radiation?

Plastic or aluminum

Shielding material for alpha radiation?

Paper; air; stopped by skin but dangerous internally

Definition of Half Value Layer (HVL)?

Thickness of material that reduces beam intensity to one-half

Definition of Tenth Value Layer (TVL)?

Thickness of material reducing beam intensity to one-tenth

Inverse square law effect when distance reduces from 3 m with 315mR/h to what at 1 m?

Exposure increases by factor of 9

Definition of a Radiation Area?

>5 mrem/hr at 30 cm

Definition of a High Radiation Area?

>100 mrem/hr at 30 cm

Definition of a Very High Radiation Area?

>500 rad (5 Gy)/hr at 1 m

Purpose of an ionization chamber?

Measure radiation accurately

Purpose of a Geiger-Müller counter?

Detect radiation (not for dose measurement)

Why can’t GM counters be used for accelerators?

Slow recovery time

How are thermal neutrons detected?

BF₃-filled neutron detectors

How often should source leak testing be done?

Every 6 months

Purpose of pocket dosimeters?

Instant monitoring of exposure

Film badge features?

Energy dependent; cumulative dose monitoring; worn on chest/abdomen

TLD characteristics?

Reusable, slow readout, energy independent at MV energies

OSLD characteristics?

Reusable, fast readout, more sensitive; replaced film badges

Three radionuclide intake pathways?

Inhalation, absorption, ingestion

Hematopoietic syndrome threshold and symptoms?

800–1000 rem; bone marrow failure; infections; bleeding

GI syndrome threshold and symptoms?

800–5000 rem; mucosal destruction; diarrhea; dehydration

CNS syndrome threshold and symptoms?

>5000 rem; rapid neurovascular collapse; seizures; coma

Meaning of ALARA?

As Low As Reasonably Achievable

Which radiation is ionizing?

Gamma rays

Annual occupational effective dose limit?

50 mSv

Unit that measures absorbed dose?

Gray (Gy)

Primary source of natural background radiation?

Radon gas

Lifetime dose limit for workers?

10 mSv × age (years)

Fetal dose limit during pregnancy?

5 mSv total

Agency regulating reactor-produced materials?

NRC

Dose equivalent of 1 Sv in rem?

100 rem

Most radiosensitive time in pregnancy?

2–15 weeks

Effect on dose when distance doubles?

Dose reduces to one-fourth

Quality factor for gamma radiation?

1

Dosimeter providing fastest readout?

OSLD