Promethium-147

Promethium-147 decays by beta-minus decay, half-life 2.6234 years. Specific activity is 3.43e+13 Bq/g (928 Ci/g), so a gigabecquerel comes to 29.1 µg — weighable, but on an analytical balance. Over a year the activity falls to 76.78%, and over forty years to 2.6e-3%.

The air kerma rate constant is small — 5.19e-7 mGy·m²/(GBq·h), 7.7e+4× less than Cs-137 and 3.1e+5× less than Co-60, ranking 96 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 5.19e-7 mGy/h, and 1 Ci at the same distance 0.0000192 mGy/h. It takes 38.6 TBq at a metre to reach 20 µSv/h from the photons alone.

2 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 121.2 keV at 73.5% of the total — its emission probability is 0.00285%, which is also the highest.

Shielding barely arises: 0.119 mm of lead halves the air kerma rate and 0.513 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 1.80 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 4.3 times the half-value layer, not the 3.32 a single energy would give.

The beta endpoint is 0.225 MeV, mean 0.0618 MeV over 2 branches. That endpoint stops in 0.426 mm of acrylic or 0.201 mm of glass. Of the beta energy, 0.05% turns into X-rays in acrylic and 0.64% in lead.