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.
The separation of Alpha and Beta pulses which differ significantly with regard to their amplitude is now performed in the following amplifier stages (Alpha channel, Beta channel). The input of the Alpha channel consists of a passive differentiation circuit (time constant approx. 0.12 µs), so that within the HV operation point the Beta pulses, due to their low amplitude, have only a slight chance to exceed the subsequent discriminator threshold in the Alpha channel, which is no problem for the differentiated large Alpha pulses.
In the Beta channel, the low Beta pulses are first amplified by a factor of 20, before they get to the discriminator stage. The Alpha pulses getting into the Beta channel will, of course, get the same amplification; these Alpha pulses will later have to be suppressed again by suitable provisions. The above mentioned discriminator stages include one integral threshold each. All pulses exceeding this threshold generate a standard pulse at the output, the width of that pulse being adjusted via a monostable flip-flop.
The Alpha channel includes 2 integral discriminators, one with a low threshold and one with a higher threshold. If one Alpha pulse, which gets into the Alpha as well as the Beta channel, exceeds the low integral threshold in the Alpha channel, a veto is generated in the Beta channel and no standard pulse appears at the output of the channel. A complete suppression of the Alpha pulses in the Beta channel, however, is not possible, because Alpha particles, due to their pre-absorption in the air and the detector foil, are subject to a pulse height distribution, and the lower amplitudes lie in the range of the Beta amplitudes. Selecting too low an integral threshold for veto generation would entail too high a loss of Beta pulses, since Beta particles with a greater pulse height will also exceed the integral threshold of this discriminator in the Alpha channel and would generate a veto signal. In any case, the Alpha spillover in the Beta channel, e.g. with 210Po is only a few percent (less than 3 %).
The second integral discriminator in the Alpha channel with a higher threshold serves for creating the Alpha count rate. The threshold is set such that as many Alpha events as possible will be detected, but no Beta particles (suppression of 10-5) are registered in the high voltage operation point.
The pulses supplied by monostable pulse shapers are applied to the amplifier output via drivers with an output resistance of approx. 50 Ω.
The suppression of cosmic radiation coming essentially from high energy muons or protons is done by means of the guard counter. Alpha or Beta radiation will not be registered by this guard counter, because it is surrounded on all sides by thick electrolyte copper walls.
Since cosmic radiation passing through the detection system will pass through the guard as well as the measuring counter in a large solid angle range triggering an ion avalanche in both counters, the cosmic share in the Beta channel can be eliminated to a substantial degree via an anti coincidence circuit. In the Alpha channel this share is negligible, since no sufficient pulse height will be created by this radiation.
The anticoincidence signal supplied by the guard counter amplifier is also fed to the Beta channel as an “or” with the veto signal. The suppression of the ambient Gamma radiation by means of this method does not work, because the probability that a Gamma will trigger an event in both detectors at the same time is extremely low. The only way to suppress this background is to surround the entire detector by a suitable lead shielding.
The 10-Channel Low-Level Counter LB 790 allows simultaneous and separate measurements of low activities for alpha and beta radiation emitting radionuclides with a detection limit of approx. 12 mBq for Alpha (Am-241) and approx. 22 mBq for Beta (Sr-90).
The measuring system LB 761 Alpha-Beta Low-Level Counter for 200 mm Planchets allows the simultaneous measurement of alpha- and Beta- activity of a sample in two separate measuring channels. The background count-rates of the system LB 761 are extraordinarily low, so that good detection limits can be achieved.