Bi207 -Reply

[FRAMEP@ORAU.GOV]

Denny:

Thanks for the opportunity. It was fun. 

It is worth noting that  6nCE is a rule of thumb. Its use can lead to
errors because it assumes that the exposure rate is directly
proportional to gamma energy. This is not true because the fraction
of the gamma ray energy absorbed by the air depends on the energy.

Better to multiply the exposure rate by the distance squared and then
divide by the specific gamma constant if you want to calculate
activity. There is still plenty of error here but it is a more
defensible approach.  In effect, this is what Ninni and I did.

Paul Frame