Answer to Question #2829 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

How do I calculate (specify) instrument nonlinearity from a set of instruments' response data?

I assume you have a Geiger-Mueller counter, proportional counter, scintillation crystal, solid state detector, or ion chamber, where you've exposed the detector to an increasing radiation fluence, in a fixed geometry. I start with a plot of the resulting counts or dose rate on the X-axis and fluence on the Y-axis. If you plot this data, you'll note a linear portion of the curve, and without electronic correction (e.g., for dead time), the curve will become nonlinear at higher fluence rates. You could either visually determine the nonlinear region or, with a curve fitting routine, note where the correlation coefficient becomes unacceptable. However, I'd start with a plot of the data and simply state the upper limit of linearity.

David J. Allard, CHP

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