5.
KC1 Standard Compound
Experience shows that Geiger-Müller counter tubes have a service life of about 10 years. The
service life is reduced in the event of continuous use at intense radiation because the
quenching gas in the tube is used up faster.
It is expedient after many years of use to be able to check the operability of the radiation
detection tubes. We therefore o
ff
er a standard compound which emits 12 Bq ± 1 on one
surface. This is a KC1 tablet (5 gram) in which a natural radioactivity (K-40) of 85 Bq is
embedded - of which, however, only 12 Bq emerges at a surface because the bulk of the BETA
radiation remains in the tablet due to self-shielding.
The decay of potassium-40 releases up to 89.33% BETA radiation with a maximum energy of
1,312 keV and up to 10.67% GAMMA radiation of 1,461 keV.
Check measurement is carried out with opened covers both at the tablet and the counter tube
with the end window of the tube held directly on the tablet surface. In the counter tube A, the
metal bezel can be put on the tablet as the end window itself is sunk but in counter tube G a
distance of approx. 3 mm must be observed. In an operational counter tube the net impulse
value indicated (after deduction of the background counts) after 10 minutes must be for
Type G:
1054 impulses ± 41
The check values for
Type A counter tube:
205 impulses ± 24
Type B counter tube:
434 impulses ± 26
Type FSZ counter tube:
792 impulses ± 35
With a probability of 65% all measured impulse frequencies will be within the above bandwidth
around the check value. The pre-requisite is accurate enough determination of the background
counts to be deducted from the total number of impulses for calculating the net impulse
frequency. Experience shows that the background counts of counter tubes increase with age.
OPERATING INSTRUCTIONS
for Counter Tube Type G
JS Monday 10 January 2011 Page 5 of 9
