NMTCB Radiation Physics & Detection 5 — Questions and Answers
Question 1: The Bragg peak phenomenon associated with charged particle beams describes:
- A maximum in photon attenuation at intermediate energies
- The sharp increase in energy deposition near the end of a charged particle's range in matter (Correct answer)
- Resonance absorption of neutrons at specific energies
- Maximum scintillation efficiency at a specific gamma energy
Correct answer: The sharp increase in energy deposition near the end of a charged particle's range in matter
As a heavy charged particle slows near the end of its range, its interaction cross-section increases dramatically, causing a sharp peak in energy deposition (the Bragg peak).
Question 2: For a counting system, the minimum detectable activity (MDA) is primarily determined by:
- The maximum count rate the detector can handle without saturation
- Background count rate, detector efficiency, and counting time (Correct answer)
- The half-life of the radionuclide being measured
- The photomultiplier tube gain and dynode configuration
Correct answer: Background count rate, detector efficiency, and counting time
MDA depends on the background count rate (which creates statistical uncertainty), detector efficiency (converting decays to counts), and counting time (reducing statistical uncertainty).
Question 3: The backscatter peak observed in a pulse height spectrum is caused by:
- Beta particles backscattered from the detector housing
- Photons that undergo Compton scattering in shielding material and enter the detector at ~180° scatter angle (Correct answer)
- Characteristic X-rays from the NaI crystal iodine atoms
- Secondary electrons generated at the photocathode surface
Correct answer: Photons that undergo Compton scattering in shielding material and enter the detector at ~180° scatter angle
Photons Compton-scattered through approximately 180° in surrounding shielding lose maximum energy and enter the detector with ~200 keV (for 662 keV source), creating the backscatter peak.
Question 4: Transient equilibrium differs from secular equilibrium in that during transient equilibrium:
- Daughter activity equals parent activity at all times
- Parent and daughter have similar half-lives and daughter activity exceeds parent activity at equilibrium (Correct answer)
- No equilibrium is ever achieved between parent and daughter
- Both parent and daughter activities decrease at the physical decay rate of the daughter
Correct answer: Parent and daughter have similar half-lives and daughter activity exceeds parent activity at equilibrium
In transient equilibrium, the parent half-life is only moderately longer than the daughter's, so at equilibrium the daughter activity slightly exceeds the parent activity due to the branching factor.
Question 5: The isobar line on a Segré chart connects nuclides that have:
- The same number of neutrons but different proton numbers
- The same mass number (A) but different atomic numbers (Z) (Correct answer)
- The same atomic number but different mass numbers
- The same nuclear energy level configurations
Correct answer: The same mass number (A) but different atomic numbers (Z)
Isobars share the same mass number A = Z + N but have different numbers of protons and neutrons, appearing as diagonal lines on the Z vs N chart.
Question 6: Which parameter determines whether a proportional counter or ionization chamber region is being used?
- The type of fill gas used in the detector
- The applied voltage, which controls the degree of gas multiplication (Townsend avalanche) (Correct answer)
- The thickness of the detector wall material
- The energy of the radiation being detected
Correct answer: The applied voltage, which controls the degree of gas multiplication (Townsend avalanche)
Higher applied voltage causes the primary electrons to gain enough energy to ionize fill gas molecules, producing Townsend avalanche multiplication characteristic of proportional counters.
Question 7: The specific ionization of a charged particle in matter is defined as the number of:
- Photons produced per unit path length during scintillation
- Ion pairs formed per unit path length of travel in the absorbing medium (Correct answer)
- Electrons ejected from detector material per incident particle
- Secondary particles generated per primary particle interaction
Correct answer: Ion pairs formed per unit path length of travel in the absorbing medium
Specific ionization is the number of primary ion pairs created per unit length of particle track, which increases with particle charge and decreases with particle velocity.
NMTCB Radiation Physics & Detection 5
The Bragg peak phenomenon associated with charged particle beams describes: