CHP Radiation Measurement and Instrumentation 3 — Questions and Answers
Question 1: The W-value for air (approximately 34 eV/ion pair) is used to convert:
- Fluence to kerma
- Energy deposited to number of ion pairs created (Correct answer)
- Activity to dose rate
- Exposure to absorbed dose
Correct answer: Energy deposited to number of ion pairs created
The W-value (mean energy per ion pair) converts total energy deposited in a gas into the number of ion pairs produced, which is proportional to the measured charge.
Question 2: A Fano factor less than 1 in semiconductor detectors indicates:
- The detector is operating in Geiger mode
- Statistical fluctuations are sub-Poissonian, giving better resolution than expected (Correct answer)
- Charge collection efficiency is below 50%
- The depletion region has not fully formed
Correct answer: Statistical fluctuations are sub-Poissonian, giving better resolution than expected
A Fano factor F < 1 means the variance in ion-pair creation is smaller than Poisson statistics predict, which is why HPGe resolution exceeds the Poisson limit.
Question 3: Compton suppression in a gamma-ray spectrometer system is achieved by:
- Using a thicker NaI crystal to stop all Compton electrons
- Surrounding the primary detector with an annulus detector in anticoincidence (Correct answer)
- Increasing the bias voltage to improve charge collection
- Cooling the detector with liquid helium to reduce noise
Correct answer: Surrounding the primary detector with an annulus detector in anticoincidence
An anticoincidence annulus detector vetoes events where a Compton-scattered photon escapes the primary detector, reducing the Compton continuum.
Question 4: The quench gas in a Geiger-Müller tube serves to:
- Increase the ionization density for better detection efficiency
- Absorb UV photons and positive ions to terminate the discharge (Correct answer)
- Reduce the operating voltage below 300 V
- Improve neutron sensitivity through (n,p) reactions
Correct answer: Absorb UV photons and positive ions to terminate the discharge
The quench gas (e.g., halogen or organic vapor) absorbs UV photons and neutralizes positive ions, preventing spurious secondary discharges.
Question 5: An OSLD (optically stimulated luminescence dosimeter) differs from a TLD primarily in that readout is performed by:
- Heating the crystal to several hundred degrees Celsius
- Exposing the crystal to a specific wavelength of light (Correct answer)
- Passing an electric current through the dosimeter
- Counting delayed fluorescence pulses in darkness
Correct answer: Exposing the crystal to a specific wavelength of light
OSLDs are read by stimulating the crystal with laser light of a specific wavelength rather than by thermal heating as in TLDs.
Question 6: Plutonium-239 alpha particle emission is most accurately measured in the field using a:
- High-pressure xenon ionization chamber
- Thin-window proportional counter with low-energy response (Correct answer)
- Sodium iodide scintillation detector
- Geiger-Müller pancake probe
Correct answer: Thin-window proportional counter with low-energy response
Alpha particles have very short ranges in air and matter, requiring a thin-window (or windowless) proportional counter to detect them effectively.
Question 7: The moderation ratio used in Bonner sphere spectrometry refers to the ratio of:
- Detector response in fast versus thermal neutron fields
- Polyethylene sphere diameter to bare detector diameter (Correct answer)
- Neutron fluence at the surface to fluence at the center of the sphere
- Thermal neutron cross-section to fast neutron cross-section
Correct answer: Polyethylene sphere diameter to bare detector diameter
Bonner spheres of different polyethylene diameters produce response functions that peak at different neutron energies; the sphere diameter relative to the bare detector defines each channel's energy sensitivity.
CHP Radiation Measurement and Instrumentation 3
The W-value for air (approximately 34 eV/ion pair) is used to convert: