Gas proportional counting system (GPC)
The gas proportional counting system was developed for high precision radiocarbon dating and consists of nine electrolytic copper proportional counters of identical diameters with sensitive volumes of 0.35-0.73 dm3 and filled with CO2 at 1115 mbar and 2 bar.
The counter system is located in a lead shield (1250 mm x 950 mm x 800 mm) with a wall thickness of 100 mm and 200 mm on the top. A 200 mm boron loaded paraffin layer is used inside the lead shield for thermalization and absorption of neutrons produced by cosmic ray particles in the lead. The inner counters are surrounded by an anticoincidence shield consisting of five multiwire proportional flat counters filled with propane up to 1 bar. The pulses of the detectors are handled by integrated amplifiers, discriminators and anticoincidence units interfaced to a microprocessor-controlled data evaluation unit. The overall precision of the system for modern carbon samples is better than 4‰ after a counting period of seven days.