APPENDIX
Explanatory Notes on Using the Measuring Table
As can be seen, the relationship between the impulses of the counter tubes is proportionate to
the becquerel values - in other words, the higher impulse rates mean correspondingly higher
becquerel values. Conclusions for other measurements are thus possible.
If, for example, a specific object contaminated by caesium-137
***
must be examined, a 10
minute measurement at a distance of 30 mm from the sample should be carried out. The result
reduced to 1 minute must then be used for the table.
EXAMPLE:
A 10 minute measurement on a sample with caesium-137 using a Type G
counter tube shows after the measuring time a measured value of 500 impulses.
After reduction to 1 minute (500 : 10 = 50 Ipm) and after deduction of the
background counts (30 Ipm) a net impulse rate of 20 Ipm is left.
The column in the measuring table for a counter tube G shows under 100 Bq
Cs-137: 143 Ipm. Consequently, 20 Ipm corresponds to
100 Bq / 143 x 20 = 14 Bq
(14 Bq x 50 = 700 Bq/kg)
Experience shows that the pre-requisites often do not concur with those of the measuring
table. A shorter distance, usually 5 mm, is often chosen for surface measurements using the
type A or G end window counter tubes. The number of impulses at a distance of 5 mm is 5
times as high as that shown in the table, i.e. the corresponding value in the measuring table
must be multiplied by a factor of 5 before conversion.
Thus 715 Ipm would correspond to (143 x 5) 100 Bq.
Converted to the above 30 Ipm, that would be
(100 / 715 x 20) only 2.8 Bq Cs-137
The immersion counter tubes are not normally used for surface measurements. These are much
more e
ffi
cient as immersion probes. To obtain comparable results, the value in the measuring
table must be multiplied by the high factor of 10 in this case. This means that 100 Bq Cs-137
*** It must be assumed that the existing contamination in Europe due to the Chernobyl disaster
must be attributed almost exclusively to the nuclide caesium-137.
OPERATING INSTRUCTIONS
for Counter Tube Type G
JS Monday 10 January 2011 Page 9 of 9
If the sample weight is, for example, 20 gram this value must be extrapolated to 1 kg
would correspond to 1430 Ipm (143 x 10) for a counter tube type G.