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A layer of cadmium, lined with copper, surrounds the sample PMTs ('S' in Fig 1) to shield against neutrons (Kojola et al., 1984).
Consequently, each of the components of the LS counting system needs to be totally optimised for low-level counting, so as to maintain high counting efficiency whilst significantly reducing the background.
The Quantulus has been designed with state-of-the-art electronics that includes short coincidence resolution time, two-position coincidence bias, and pulse comparison analysis. All LS counters with horizontally-opposed PMTs record pulses initiated within each PMT only if they fall within a set coincidence resolution time interval.
This reduces the possibility of pulses generated within either tube being recorded as genuine decay events, and results in a lowering of background levels.
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 '...
For example, an event originating in one PMT may result in a very large pulse in one tube, and a coincident, small pulse in the other.