To measure Beta radiation the institute is provided with a Risų GM-25-5
Beta-Multicounter, that is able to measure five solid samples with a maximum diameter
of 25mm and 8mm height. The counter detector array is surrounded by a lead shield of
10cm thickness and 500kg weight which attenuates the intruding natural background
radiation (Gamma radiation) to some 130cpm (counts per minute). Background radiation
entering the detectors despite of the shielding is compensated electronically.
Therefore a so-called guard counter, completely insulated from Alpha and Beta
radiation, is located above the sample detectors. When a count is registered in a
sample detector and the guard counter simultaneously it can only be due to Gamma
radiation and is classified as background radiation by an anticoincidence module. By
those means the beta-counter background can be reduced to 0,05cpm to allow measurements
for low-level samples.
Due to its construction and the partial self-absorption of Beta-Radiation within the
samples the measured activity is only a share of the real value. Ideally for each
sample the counting efficiency had to be estimated to convert the counts per minute to
the real sample activity, given in dpm (disintegrations per minute; 60dpm = 1Bq).
The beta-counter is primarily used to measure the activity of the naturally occuring
radionuclide 234-Thorium that is extracted from water samples to trace the dynamics and
fate of particulate matter in the oceans. Sample activities typically range between 0,1
and 5cpm which leads to real activities between 0,3 and 15dpm. For comparison: a human
being ingests about 5000dpm of natural radioactivity with every kilogram of food and
emits some 500000dpm at the body surface.
Working bench with beta-counter (front) and alpha-spectrometer (rear)