AMES MRS 110B-1 Посібник - Сторінка 7

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Radiological monitor MRS 110B-1

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.

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