NRRPT Detection and Measurement 3 — Questions and Answers
Question 1: An OSL (optically stimulated luminescence) dosimeter differs from a TLD primarily in that it:
- Cannot measure gamma radiation
- Uses light (laser) rather than heat to read out stored dose information (Correct answer)
- Requires liquid nitrogen cooling during readout
- Can only be read once before the signal is erased
Correct answer: Uses light (laser) rather than heat to read out stored dose information
OSL dosimeters use laser light stimulation to release stored energy as luminescence, allowing non-destructive readout and the ability to re-read the same dosimeter multiple times.
Question 2: The Compton edge in a gamma-ray pulse-height spectrum represents:
- The photopeak of the highest-energy gamma emitter present
- The maximum energy transferred to a recoil electron in Compton scattering (Correct answer)
- The minimum detectable energy for the scintillation detector
- The energy at which pair production begins to dominate
Correct answer: The maximum energy transferred to a recoil electron in Compton scattering
The Compton edge is the maximum kinetic energy a recoil electron can receive from Compton scattering, which occurs when a photon backscatters at 180 degrees.
Question 3: What is the purpose of a moderator in a neutron rem meter (such as the Andersson-Braun rem meter)?
- To amplify the neutron signal electronically
- To slow fast neutrons to thermal energies where detection efficiency is higher (Correct answer)
- To shield against gamma radiation background
- To convert neutrons into alpha particles for detection
Correct answer: To slow fast neutrons to thermal energies where detection efficiency is higher
Moderating materials (typically polyethylene) slow fast neutrons through elastic scattering, converting them to thermal neutrons that are efficiently detected by the BF3 or He-3 tube.
Question 4: A survey meter with a pancake probe is particularly useful for:
- Measuring high-energy neutron fields
- Detecting low-energy beta and alpha contamination on surfaces (Correct answer)
- Performing gamma spectroscopy in the field
- Measuring dose rates inside reactor vessels
Correct answer: Detecting low-energy beta and alpha contamination on surfaces
The thin window on a pancake GM probe allows low-energy beta particles and alpha particles to enter the detector, making it ideal for surface contamination surveys.
Question 5: The energy resolution of a germanium (HPGe) detector compared to NaI(Tl) is:
- Significantly poorer, making spectroscopy impossible with HPGe
- Significantly better, allowing discrimination of closely spaced gamma energies (Correct answer)
- Essentially identical for practical radiation protection purposes
- Better only for low-energy X-rays below 100 keV
Correct answer: Significantly better, allowing discrimination of closely spaced gamma energies
HPGe detectors have energy resolution of ~0.1-0.2% FWHM compared to ~7% for NaI(Tl), enabling identification of radionuclides with very similar gamma-ray energies.
Question 6: When performing a minimum detectable activity (MDA) calculation, the critical level (Lc) represents:
- The maximum activity that can be measured without detector saturation
- The net count above background at which a decision is made that a signal is present (Correct answer)
- The legal action level for radioactive contamination
- The lowest calibrated point on the instrument's response curve
Correct answer: The net count above background at which a decision is made that a signal is present
The critical level (Lc) is a statistical threshold above which a measured count rate is considered to indicate the presence of radioactivity with defined probability of a false positive.
Question 7: A portable alpha spectrometry system used for plutonium air sampling requires the sample to be:
- In liquid form for accurate energy measurement
- Thin and dry to minimize alpha self-absorption within the sample (Correct answer)
- Contained in a sealed, pressurized chamber
- Collected on a lead-backed filter to enhance counting efficiency
Correct answer: Thin and dry to minimize alpha self-absorption within the sample
Alpha particles have very short ranges in matter, so thick or wet samples cause energy loss and self-absorption, degrading spectral resolution and reducing detected count rate.
NRRPT Detection and Measurement 3
An OSL (optically stimulated luminescence) dosimeter differs from a TLD primarily in that it: