Radiological monitor MRS 110B-1 Manual
AMES d.o.o. Jamova 39, Ljubljana, Slovenia
7
1.2 DESCRIPTION OF THE MEASURING METHOD
Geiger Mueller tube detects photons that hit it and produces an electric pulse
for every detected photon. When we are measuring natural background, there
are normally 15 to 20 pulses per minute, which means in average one pulse
every 3 seconds. Time distribution of pulses is random. To achieve low
statistical uncertainty of the measured results, one should count pulses over
the longer time interval, since the relative statistical uncertainty is given with
the expression 1/
√
N (or, in percents, with 100/
√
N).
MRS 110B-1 uses the following measuring algorithm:
Instrument counts pulses in 2-second time intervals, storing data to the
internal memory. Each 2 seconds it computes the new sum of pulses by
adding the 2-seconds data from the memory, from the latest sample back
(running average), until one of the two conditions is fulfilled:
•
To sum 150 or more pulses
•
To sum the contents of 30 intervals, this means pulses for 1 minute
back.
When first or second condition is true, MRS 110B calculates new cpm value.
With the described algorithm, we achieve the following:
Under conditions, where there are only few cpm from the GM tube
(background measurements), summing time is limited to 1 minute. This also
defines the time response of the instrument. At 20 cpm, statistical uncertainty is
equal to 1/
√
20= 22.4%, with the response time of maximum 1 minute. When we
approach the radiation source, response time is, depending on the source activity,
significantly shorter (down to 2 seconds). In this case, count is limited by minimum
count of N=150 pulses, that gives the statistical uncertainty 1/
√
150= 8.2% or better.
Since MRS 110B-1 works with the running average that is updated every 2
seconds, measuring is smooth and without jumps in results, caused by the random
distribution of impulses.
