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28 August 2008

Answer to Question #96 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

Where can I find a description of when, where, and how to use a pancake probe as opposed to the regular end/side-window survey meter probe?

GM Pancake probes are generally used for collecting surface contamination measurements, but may also be used for estimating low-level beta dose rates when proper conversion factors are used. The primary features of pancake detectors are thin windows (< 2 mg/cm^2) and relatively large areas (15.5 cm^2) which makes them well suited for these uses. Side-window and "end-window" GM detectors were once used routinely for surface contamination monitoring, but these uses are not typical today. GM side-window detectors are most commonly used for measuring exposure rates and usually employ an energy compensating sheathing. These detectors have good sensitivity and energy response characteristics--although they will over-respond (relative to 662 keV) at less than about 200 keV. A noncompensated or open window tube will display very poor linearity below about 200 keV. Robert L. Coleman Oak Ridge National Laboratory

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