PIC Nuclear Density Gauge Operation 3 — Questions and Answers
Question 1: What information must be displayed on the exterior of a nuclear gauge transport case per NRC/DOT requirements?
- Radioactive material warning label, UN number, and proper shipping name (Correct answer)
- Only the operator's name and license number
- Gauge serial number and purchase date only
- Manufacturer's name and model number only
Correct answer: Radioactive material warning label, UN number, and proper shipping name
DOT regulations require radioactive material labels (Yellow-II or Yellow-III), the UN number (UN2910 or UN3332), and proper shipping name on transport cases.
Question 2: An inspector uses a nuclear gauge on a granular base course and obtains a percent compaction of 96%. The specification requires 95% minimum. The gauge's moisture reading is suspect. What should the inspector do?
- Accept the material since density meets spec regardless of moisture accuracy
- Take a nuclear density reading and verify moisture independently with an oven-dry or speedy moisture test (Correct answer)
- Reject the material immediately without further testing
- Double the number of gauge readings and average them
Correct answer: Take a nuclear density reading and verify moisture independently with an oven-dry or speedy moisture test
When moisture accuracy is in question, verifying with a reference method (oven-dry or calcium carbide test) ensures the density calculation and compaction percentage are correct.
Question 3: What is the significance of the 'gauge factor' or 'density calibration offset' entered into a nuclear gauge?
- It adjusts readings to correct for the specific material composition of the material being tested (Correct answer)
- It sets the maximum depth of source rod insertion
- It controls the gamma source decay rate
- It programs the gauge's GPS location
Correct answer: It adjusts readings to correct for the specific material composition of the material being tested
Nuclear gauges are calibrated to standard reference materials; a gauge factor corrects for differences in chemical composition (element density and atomic number) of the actual material being tested.
Question 4: A nuclear gauge operator must maintain a distance of at least how many feet from the gauge when it is in use, according to typical NRC license conditions?
- 1 foot
- 2 feet (Correct answer)
- 6 feet
- 10 feet
Correct answer: 2 feet
Most NRC license conditions require operators to remain at least 2 feet from the gauge while a reading is being taken to limit radiation exposure.
Question 5: Which type of radiation emitted by the nuclear density gauge is primarily used to measure moisture content?
- Gamma radiation from the Cesium-137 source
- Fast neutrons from the Americium-241/Beryllium source (Correct answer)
- Alpha particles from the source rod tip
- X-rays generated by the detector
Correct answer: Fast neutrons from the Americium-241/Beryllium source
Fast neutrons from the Am-241/Be source are thermalized (slowed) by hydrogen atoms in water molecules; the count of slow neutrons is proportional to moisture content.
Question 6: What action must be taken if a nuclear gauge is lost or stolen?
- File a police report only and continue operations
- Notify the NRC (or Agreement State) within 4 hours of discovery and file a written report within 30 days (Correct answer)
- Replace the gauge and report the loss at annual license renewal
- Notify the NRC only if the gauge is found outside the state
Correct answer: Notify the NRC (or Agreement State) within 4 hours of discovery and file a written report within 30 days
NRC regulations (10 CFR 20.2201) require immediate notification (within 4 hours) by telephone and a written report within 30 days for lost or stolen licensed radioactive material.
Question 7: When performing nuclear gauge testing on a thin HMA lift (less than 2 inches), which mode is preferred and why?
- Direct transmission at maximum depth, to get the most accurate reading
- Backscatter mode, because the source rod cannot penetrate the thin lift and underlying material would be included in the direct transmission reading (Correct answer)
- Either mode produces identical results on thin lifts
- Direct transmission at 2-inch depth, which isolates the lift perfectly
Correct answer: Backscatter mode, because the source rod cannot penetrate the thin lift and underlying material would be included in the direct transmission reading
For thin lifts, direct transmission would include the underlying base in the measurement zone; backscatter mode keeps the measurement near the surface, better isolating the thin layer.
PIC Nuclear Density Gauge Operation 3
What information must be displayed on the exterior of a nuclear gauge transport case per NRC/DOT requirements?