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Why do GM (sometimes) instruments fail at high dose rates? Pete Burgess

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Fail to danger at high dose rates GM units sometimes fail to danger at high dose rates. Causes include –Wrong counting threshold –Inappropriate detector –Amplifier paralysis –Insufficiently stiff HT supply

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Wrong threshold

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Problem? Some of the pulses at high count rates fall below the set counting threshold As the dose rate gets higher, mean pulse size decreases Eventually, at very high rates, all pulses are the same and an oscilloscope trace looks like RF. Good GMs have a pulse minimum about 20% of the low dose rate size. Se set below that, all are counted and no problem

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Inappropriate detector.

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Reliable detector

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Amplifier paralysis Maximum pulse rate = 1/dead time Can be as high as 220k per second (ZP1310 series as fitted to many GM dosemeters) Dead time = 4.5 microseconds If amplifier and pulse generator are slower than this then dead time can be extended Old fashioned cure = current trip

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HT stiffness At high count rates, current can be up to 130 microA If HT drops, pulse size drops and may fall below threshold.

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Acknowledgements Centronic Ltd David Allard Julian Ginniver

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