Simultaneous Alpha && Beta Determination

Technical data on detectors

The planchets of LB 790 (Ø 60 mm) are arranged in 2 rows on, 5 planchets each and with a tray made of copper while the proportional counter tube LB 761 has a diameter of 200 mm and a height of approx. 8 mm. The rear of the detectors is sealed by a thin copper plate (2 mm) (guard counter  behind it), the side facing the sample is covered by a very thin Hostafan foil (approx. 0.5 mg/cm2), with vaporized aluminum on one side, and connected to ground potential. The larger counter tube includes two counting wires,  the smaller one only one (tungsten wire,  diameter approx. 50 µm);  the counting wires of all detectors suspended separately via Teflon supports are supplied by one common positive bias voltage (RC filtered). Via a 1 nF high voltage capacitor the negative charge carriers created at the counting wire are transmitted to a chargesensitive pre-amplifier. The enclosed function diagram illustrates the method for Alpha/Beta separation.

The effective feedback capacity of the pre-amplifier is about 1.5 pF; together with the parallel connected load resistance of 681 kΩ one will thus get a time constant of approx. 1 µs. The rise of the positive output signal of the preamplifier is proportional to the drift time of the charge carrier in the detector; the amplitude is a measure for the total number of charge carriers created by gas amplification.