Hi Sam:
I do not remember many things either. John D. was probably thinking
in terms of 100% efficient 4*Pi detector. But the efficiency is only a
constant multiplication factor, such as 0.01 (depends on what is being
counted, etc.). I would not agree, for example, that the probability of
a single Ra atom (T=1600 years) to decay at any given hour (or second)
is 1/2. It is much much smaller. The probability is 1/2 for decaying or
not during a much much longer time interval.
Regards, Ludwik