CHP Radiation Shielding Design and Analysis 2 — Questions and Answers
Question 1: NCRP Report No. 151 provides shielding guidance for medical X-ray facilities. The concept of 'workload' (W) is defined as:
- The total number of patients imaged per week
- The weekly time-integrated tube current expressed in mA·min/week (Correct answer)
- The peak kilovoltage setting of the X-ray tube
- The occupancy factor of the area to be shielded
Correct answer: The weekly time-integrated tube current expressed in mA·min/week
Workload (W) in NCRP 151 is the time-integrated tube current per week (mA·min/week), representing the total use of the X-ray machine and the radiation output generated.
Question 2: In NCRP 151 shielding calculations for diagnostic X-ray rooms, the 'use factor' (U) accounts for:
- The fraction of time the room is occupied by non-radiation workers
- The fraction of the workload during which the primary beam is directed at a given wall (Correct answer)
- The efficiency of the shielding material at the relevant photon energy
- The fraction of total patients requiring repeat exposures
Correct answer: The fraction of the workload during which the primary beam is directed at a given wall
The use factor (U) is the fraction of the total workload for which the primary beam is directed toward a particular barrier, reflecting that not all exposures target every wall.
Question 3: For a point isotropic gamma source in air, if the distance from the source doubles, the dose rate changes by a factor of:
- Decreases by factor of 2
- Increases by factor of 4
- Decreases by factor of 4 (Correct answer)
- Decreases by factor of 8
Correct answer: Decreases by factor of 4
By the inverse square law, dose rate is proportional to 1/r²; doubling the distance (r×2) reduces dose rate by a factor of 4.
Question 4: Concrete is commonly used for gamma and neutron shielding in nuclear facilities primarily because:
- It has a high Z number comparable to lead, maximizing photoelectric attenuation
- It provides both gamma attenuation (high density) and neutron moderation (hydrogen content) in a structural material (Correct answer)
- It eliminates secondary gamma production from neutron capture reactions
- It has the lowest cost per HVL among all shielding materials
Correct answer: It provides both gamma attenuation (high density) and neutron moderation (hydrogen content) in a structural material
Concrete combines structural integrity, gamma attenuation from its density, and neutron moderation from its hydrogen content (water of hydration), making it versatile for mixed-field shielding.
Question 5: The linear attenuation coefficient (μ) for photons in a material has units of:
- cm²/g
- cm⁻¹ (Correct answer)
- g/cm²
- MeV⁻¹
Correct answer: cm⁻¹
The linear attenuation coefficient μ has units of inverse length (cm⁻¹), as it describes the fraction of photons removed per unit thickness of material.
Question 6: When designing shielding for a PET facility, which photon energy must the shielding primarily address?
- 140 keV (from Tc-99m)
- 511 keV (annihilation photons from positron emission) (Correct answer)
- 1.274 MeV (from Na-22 gamma decay)
- 364 keV (from I-131 gamma emission)
Correct answer: 511 keV (annihilation photons from positron emission)
PET imaging uses positron-emitting radionuclides; each positron annihilates with an electron to produce two 511 keV photons, which is the primary shielding design energy.
Question 7: In reactor shielding design, 'skyshine' refers to:
- Neutron activation of atmospheric nitrogen above the reactor
- Radiation that travels upward, scatters in the atmosphere, and returns to ground level at a distance from the facility (Correct answer)
- Reflected gamma radiation from the reactor building roof
- Direct streaming through ventilation ducts in the reactor building
Correct answer: Radiation that travels upward, scatters in the atmosphere, and returns to ground level at a distance from the facility
Skyshine is radiation (typically gamma) that scatters off the atmosphere and deposits dose at distances well beyond the facility, requiring analysis for dose assessment to the surrounding public.
CHP Radiation Shielding Design and Analysis 2
NCRP Report No. 151 provides shielding guidance for medical X-ray facilities.
The concept of 'workload' (W) is defined as: