The ratemeter or electronic component provides the interface for most contamination monitors. A ratemeter is an instrument that measures the counting rate of incoming electronic pulses. When connected to a radiation-detecting probe, it reports the rate at which radioactive particles are detected. An attached radiation detector provides a signal, which the ratemeter presents as a counting rate. The counting rate is generally given in either counts per minute (cpm) or counts per second (cps).
The knob on the ratemeter turns the meter on and off, and controls the scale of the readout. To find the correct cpm, multiply the reading on the gauge by the scale selected by the knob. For example, a reading of 2K on the x0.1 scale indicates a counting rate of 200 cpm. Every time you use the ratemeter, you should check the battery level to make sure it’s in the acceptable range. If the battery level falls below that range, the detector may still respond, but it won’t have enough power to respond reliably, so contamination may go undetected.
Some ratemeters, like the one shown above, display scales for both cpm and mR/hr. These scales are not interchangeable. To measure exposure rate (mR/hr), you would need a probe calibrated specifically for that purpose. Contamination surveys should be reported in cpm.
Each ratemeter should have a calibration sticker affixed to either the side or the bottom. This sticker lists the date of calibration as well as efficiencies for commonly used isotopes. An example of a calibration sticker is pictured below:
The efficiency is the fraction of emitted particles the detector picks up from a particular isotope. Using the above sticker as an example, this particular meter can detect 25% of the particles emitted by P-32, but only 6% of those emitted by C-14 and S-35. The efficiencies listed apply only to that specific probe used with that specific ratemeter. For meters with multiple probes, the numbers refer to the Geiger-Mueller detector unless otherwise specified.
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