K-40 decays to Ca-40 in 89 % of its decays by beta emission but 11% of the time it decays to Ar-40 via electron capture. A glance at the chart of the nuclides shows the decay modes and products while a reference such as Kocher's "Radioactive Decay Data Tables' gives the percent yeilds/intensities. Paul Frame Professional Training Programs ORISE framep@orau.gov