Radiation Detection and Counting Systems
Core concepts
Detectors convert radiation into measurable signal. Gas-filled detectors operate in ionization, proportional, or Geiger–Müller regions depending on applied voltage; the Geiger plateau is the voltage range where count rate is nearly voltage-independent. Scintillation detectors (NaI(Tl), BGO, LSO, LYSO, GSO) absorb energy and emit light, which a photomultiplier tube (PMT) converts to electrons via the photocathode and multiplies through dynodes. Key detector properties: energy resolution (FWHM of the photopeak, expressed as %), efficiency (intrinsic vs absolute), and dead time. NaI(Tl) has excellent energy resolution (~6–7% at 140 keV) but is hygroscopic and has moderate stopping power; BGO has higher stopping power but poorer resolution; LSO/LYSO offer fast decay for PET.
Traps
- Intrinsic vs absolute efficiency: intrinsic ignores solid angle; absolute includes geometry.
- Energy resolution is reported as FWHM/peak energy × 100% - a smaller number is better.
- Confusing photopeak with Compton scatter and backscatter features; the Compton edge and backscatter peak are not the photopeak.
- Assuming higher voltage always increases counts - beyond the plateau, noise and spurious counts rise.
Rule of thumb
For a well-tuned NaI detector at 140 keV (Tc-99m), a photopeak FWHM of roughly 10% or less is acceptable; recheck energy calibration if it drifts.
Worked example: A Tc-99m photopeak at 140 keV has FWHM of 14 keV. Energy resolution = 14/140 × 100% = 10%.