Answer

The detector specification is 33% (4π) for Pu-239 alpha.

Pu-239 decays entirely via alphas, having energies at or around 5.1 MeV and a total yield of 100%. There is no appreciable ingrowth of decay products, so that is it for radiations of significance from Pu-239.

The M43-1 specification does not provide a reference distance. Since only half the alphas are ever emitted towards the detector, the maximum 4π efficiency you can get is 50% (in contact with the source). I checked using my efficiency estimator and, for this detector, the value of 33% would be about right at 0.5 cm. At this distance, about 45% of the alphas emitted would be incident upon the detector face, so a 4π efficiency of 33% basically means that 33/45, or about ¾, of the alphas directed towards the detector make it through the window and result in a pulse, which is reasonable. A measured value of 12% is consistent with a measurement distance of 2.5 cm (1 inch).

The problem with extrapolating this to Ac-225 is that Ac-225 exists in equilibrium with 4 daughters: Fr-221, At-217, Bi-213, and Po-209. Bi-213 decays via B-, which I’m going to assume contributes nothing to the signal. The same goes for any and all gamma; this type of detector is designed not to respond to anything but alpha. However, the Ac-225 and the other three daughters all decay 100% via alpha decay, with the following approximate average energies:

• Ac-225 = 5.7 MeV
• Fr-221 = 6.3 MeV
• At-217 = 7.0 MeV
• Po-209 = 8.4 MeV

Note that the Canadian Nuclear Safety Commission (CNSC) table is limited to the highest three energies and yields. It actually omits the alphas from the Ac-225 itself, as they have the lowest energy. Also, the table does not include ALL the alphas from each isotope, only the most probable. Fr-221 has many possible alpha emissions, with the highest yield being 83.4% at 6.34 MeV, but there are many others in the 6 to 7 MeV range, with a total yield adding up to 100%.

Basically, there are four alphas emitted for every decay of the parent, Ac-225. The efficiency of detection for each of these should be similar to that for Pu-239—that is, about three quarters of those incident upon the detector will result in a “count.” It will actually be slightly more efficient due to the higher alpha energies.

So, in terms of the Ac-225 activity, I’d expect roughly four times the nominal detection efficiency (i.e., 133% at 0.5 cm or 48% at 2.5 cm).