Cobalt-60

Cobalt-60 decays by beta-minus decay, half-life 1925.28 days. Specific activity is 4.19e+13 Bq/g (1.13e+3 Ci/g), so a gigabecquerel comes to 23.9 µg — weighable, but on an analytical balance. Over a year the activity falls to 87.68%, and over forty years to 0.52%.

Among the strong external emitters here: Γ of 0.306 mGy·m²/(GBq·h) ranks 4 of 96, 4.0× Cs-137. 1 GBq at 1 m reads 0.306 mGy/h, and 1 Ci at the same distance 11.3 mGy/h — a metre-scale hazard at gigabecquerel activities, and 65.4 MBq is already 20 µSv/h at that distance.

5 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 1332.5 keV at 52.5% of the total — its emission probability is 99.9826%, which is also the highest.

This is a shield that has to be designed: 10.3 mm of lead for a factor of two and 34.4 mm for a factor of ten, or 16.5 mm of steel to halve it, at which point the mass of the shield is part of the problem. Reaching 20 µSv/h from 1 GBq at a metre takes 40.8 mm of lead.

The beta endpoint is 1.49 MeV, mean 0.0964 MeV over 2 branches; that endpoint comes from a branch of only 0.12%, while 99.88% of decays take the 0.318 MeV branch. Shielding is sized on the endpoint, dose on the mean. That endpoint stops in 5.70 mm of acrylic or 2.69 mm of glass. Of the beta energy, 0.31% turns into X-rays in acrylic and 4.28% in lead.