Answer to Question #3447 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

ANSI N323a-1997 Section 4.8 gives some guidance for performing source checks on instruments. It provides only for a tolerance of +/-20% (with an exception for high dose rate and neutron instruments). This criterion may be hard to meet, specifically for beta contamination survey instruments on the lower range due to variables such as background interference, needle fluctuation, source emission rates at low activity, etc. Is there additional guidance for this? Perhaps a -20%/+30% for this range or other allowances?

For most health physics instrumentation a tolerance of +/- 20% is the normal accuracy guidance; however, you should review the National Council on Radiation Protection and Measurements (NCRP) Report No. 112 "Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination." Section 5.5 of this report notes "An uncertainty of +/- 30% may be allowed for E_res < 0.3 MeV . . ." with calibration of beta dose instruments. Similarly, Section 4.5 notes the same for air kerma rates below 10 µGy per hour for photon measuring instruments used for external radiation field evaluation.

David J. Allard, CHP

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