ABSTRACT
Samples for α counting must be thin enough that range straggling does not prevent α particles from entering the sensitive area of the detector or degrade the peak structure of the spectrum. This is accomplished by purifying α-emitting chemical fractions from all inert contaminants. In the case of long-lived αemitters, such as isotopes of Uand Th, care must be taken to balance the decay rate of the source against the quantity of analyte mass that accompanies it. Spreading an α-emitting sample over alarger area helps alleviate this problem until the dimensions of the source became comparable to the area subtended by the detector, atwhich point counting geometry causes aloss of counting ef–ciency. Final samples for αcounting are usually limited to lateral dimensions of 2.5 cm. Alpha samples for absolute emission measurements must be mounted on the same substrate as the calibration standard (usually Pt) so that the degree of backscatter (αparticles emitted into the sample backing that are subsequently re¢ected backward into the detector volume) is the same.
