Most portable instruments use a count-rate meter (ratemeter) to process the pulses from the geiger detector. A ratemeter displays the rate at which pulses or counts are received by transforming a series of randomly arriving pulses into a smoothed out direct current. This current is proportional to the rate at which the pulses arrive and is displayed on a meter calibrated in counts per minute.
The most important characteristic of a
ratemeter is its response time . This is
a measure of how long it takes the ratemeter
to average out
the pulse arrival
rate and convert it to a steady reading. It will not give an instantaneous
reading of the count rate. For example, Fig.
shows
the response of a ratemeter which had been receiving 100 counts per second
(cps) and was then suddenly exposed to 200 cps. The response time, about 6
seconds in this case, is taken as the time between the start of the new count
rate and the attainment of 90% of the final value.
Figure: Response Time of a Ratemeter
Some ratemeters are equipped with a control which permits different response times to be selected. Long response times give a steadier reading, but you have to wait longer for the reading to stabilize, whereas short response times may result in readings being erratic. When the radiation level is low a long response time is needed to smooth out the random rate at which the pulses arrive. For high levels a short response time may be used. Radiation instruments fitted with ratemeters have response times from several seconds to tens of seconds. You must give the instrument enough time to reach its final reading. This is especially important when you are monitoring yourself for low level contamination.