The betas are actually from protactinium(Pa)234 at Emax=2.3 MeV. The halflife of this branching (>99%) is 1.2min. The immediate parent is Th234 (1/2life=24d), so that, yes, after about 7 months, you have better than 99% ingrowth. I'd be very interested to know what sort of correlation you obtained between field and laboratory measurements, and what your y-intercept was, in pCi/gm. Chris Alston