1.
Dose Rate Measurement with Type G Counter Tube
The Type G end window counter tube is a pure contamination counter tube and not suitable for
dose rate measurement. Simply because of its very high sensitivity, this counter tube becomes
saturated relatively fast in cases of intense radiation which often arise in dose rate
measurements. Its longer dead time leads to coincidence losses at a higher impulse density.
Moreover, the all round radiation detection of the counter tube is not symmetrical due to the
relatively large end window, so di
ff
erent results would be obtained at di
ff
erent angles to a
specific radiation source. For dose rate measurement with ALPHAiX we recommend the Type A
or B counter tube. The fluctuations in background radiation can be calculated in millirem per
year (mrem/a) by multiplying the impulses by a factor of 4 and converting to 1 minute.
2.
Statistical Error of Measurement
120 mrem/a or 1.2 mSv/a is the usual background radiation (solar and earth radiation) which
can, however, show considerable local variation. The usual background radiation of a location
or test location can be determined if the radiation detection instrument is left running for 2
hours with no radioactive radiation source present in its vicinity. The recorded number of
impulses is converted to a value per minute. This value (Ipm) is then the so-called background
count. In measurement, only measured values above the background count indicate the
presence of radioactive exposure.
All measurements are subject to a statistical error of measurement. This is due to the fact that
radioactive radiation does not occur constantly in time and space but at varying intervals.
The error of measurement is calculated from the root of the counted impulses:
Error of measurement in % =
(N = total counted impulses)
This means the error of measurement declines as the number of impulses increases. In other
words, the longer the measurement, the more accurate the measurement. Thus a series of
measurements of, say, 100 impulses has an error of measurement of 10% but only 3.2% at
1,000 impulses and 1% at 10,000 impulses.
For food inspections a minimum measurement duration of 10 minutes is recommended.
Experience shows that the tolerance value for 10 minute measurements at background counts
of 30 Ipm is at 35 Ipm (30 + 5), i.e. only the impulses in excess of 35 per minute are the result
of additional radiation exposure. If a series of measurements is very close to the tolerance
value, the series must be repeated for a longer measuring period.
OPERATING INSTRUCTIONS
for Counter Tube Type G
JS Monday 10 January 2011 Page 2 of 9
