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11 October 2008

Answer to Question #2898 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

With 4" x 5" NaI crystals, I have been periodically finding that peak widths are changing. Could this be caused by temperature changes of the crystals or maybe the preamplifiers on the photomultiplier tubes?

As Glenn Knoll points out in Chapter 10 of his text Radiation Detection and Measurement, the energy resolution of your NaI crystal detector will be impacted by several factors, such as the photomultiplier (PMT) charge collection statistics, electronic noise, variation in response over the active volume, and drift in operating parameters during a measurement. Also, with scintillation detectors, fluctuation in PMT gain can add to the measured resolution. My gut feeling is that temperature changes are affecting the PMT's photocathode, more than the crystal and the preamplifier. A good-quality "center well" NaI should have a 7-10 percent resolution, and a solid "right cylinder" NaI crystal should have a 6-7 percent resolution at a 0.662 ¹³⁷Cs gamma energy. Because drift can be a problem with NaI detector-based instruments, some manufacturers are placing a small ¹³⁷Cs source near the NaI crystal for regular automatic peak checks. If you can isolate the NaI/PMT assembly, and alter the temperature in a controlled manner with a fixed-source geometry, you'll be able to empirically evaluate the magnitude of your situation.

David J. Allard, CHP

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