Answer to Question #2299 Submitted to "Ask the Experts"

Q

In a laboratory setting, using a NaI scintillator (in conjunction with a PMT [photomultiplier tube] and pulse height analyzer) to detect gamma-ray emissions from a radioactive source, what would be the origin of the width of the photo peaks and the origin of the "Compton edge?" This is for a plot of MeV vs. counts.

A

The width (or resolution) of a photopeak at a given gamma energy is function of many statistical factors. These relate to the initial gamma interaction in the NaI crystal, that is, the gamma energy deposition, conversion to light photons, where in the detector volume the event occurred, the number of light photons reaching the photomultiplier tube (PMT) photocathode, photon conversion and initial yield of photoelectrons, and the subsequent electron multiplication down the dynode chain.

Because not all gamma rays undergo a complete energy deposition event in the NaI crystal through a photoelectric interaction with an electron, many interactions result in partial energy transfer to electrons via Compton scattering. These Compton scatter electrons cause a Compton continuum at lower energies below the photopeak, with a distinct edge.

See Radiation Detection and Measurement by Glenn F. Knoll (publisher, John Wiley & Sons) and NCRP Report No. 58, A Handbook of Radioactivity Measurements Procedures.

David J. Allard, CHP