Actually most of the 15 Bq are electrons. Some are from 14C, others from 40K (89.21% β-). The remaining 10.72% of 40K are mostly electron captures, only 0.001% are β+ (but this branching ratio is from 1962). So roughly a positron every hour.
www.symmetrymagazine.org/2009/07/23/a...
The average banana produces a particle of antimatter roughly once every 75 minutes.
