Ultra-low background alpha particle counter using pulse shape analysis

The need to measure alpha particle emissivities at levels below 0.005 /spl alpha//cm/sup 2/-hr is becoming increasingly important in fundamental physics experiments (e.g. neutrino and rare decay measurements), environmental monitoring, nuclear activities monitoring and semiconductor packaging materials. Present counters can barely reach this level, being limited both by cosmic ray events and by their own alpha emissions. Here we report a detector capable of measurements at 0.00005 /spl alpha//cm/sup 2/-hr using a large electrode ionization chamber with digital pulse shape analysis to locate the point of emission of each alpha particle. Filled with 1 atm N/sub 2/, the counter is essentially blind to both environmental gamma-rays and cosmic ray muon showers, so its background becomes limited by the ability of the pulse shape analysis to distinguish different points of alpha particle origin. The counter's geometry intentionally exaggerates differences between signals originating from its different surfaces, with an inter-electrode separation D over 3 times the alpha particle range L. Since signal risetimes equal charge drift times, anode events have 8-10 /spl mu/s risetimes, while sample event risetimes are 30-35 /spl mu/s and readily distinguished. Integrated charge also increases with drift length, producing a 2 to 1 difference between sample and anode events. Applying both risetime and amplitude cuts distinguishes between sample and anode emitted alpha particles at about 1 part in 1000. A guard electrode surrounding the anode allows alpha particles from the counter's sidewalls to be rejected at a similar ratio, so that essentially only alpha particles emanating from the sample are finally counted.