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Other electronic controls include : high voltage spectrum stabiliser, negative ion generator to remove static charges on counting vials, pulse shape analysis (PSA) capabilities to allow differentiation between alpha and ß emitters, and RF pickup.
Twin multichannel analysers (MCAs) record both coincident and anti-coincident events in both sets of tubes, with the data not only used for age calculation , but also for quality control purposes.
The 'total optimisation concept' (Polach, 1987; p.8) has been applied in the design of the Perkin Elmer 1220 Quantulus.
The counter components requiring optimisation are '...
shielding and electronics, data processing, data evaluation, and validation' (Polach, 1987; p.2).
The Quantulus achieves ultra-low background levels by both passive and active forms of shielding.
This reduces the possibility of pulses generated within either tube being recorded as genuine decay events, and results in a lowering of background levels.
Active shielding in the Quantulus is in the form of a liquid scintillation anticoincidence guard, which completely surrounds the sample phototubes, within the lead shield.
This active guard has an additional pair of 'guard' PMTs ('G' in Fig 1), which normally operate in anticoincidence with the sample PMTs, thus rejecting cosmic induced pulses.
Figure 1: Schematic diagram of Quantulus lead shield assembly and phototubes.
S = Sample PMTs; G = Guard PMTs The counting vial is placed into one of three trays located beneath the shield, and counting data (tray position number,1-60; sample identification number; counting requirements etc) entered into a laboratory computer.