Health Physics Corner, June 2026

by Jeff Sandeman · June 1, 2026

Estimating Ac‑225 Efficiency Using a Pu‑239 Surrogate

A few weeks ago, I received an email from a CRPA member posing an interesting question. Using the Ludlum M43-1 Scintillator, they were looking for help determining an extrapolated value for efficiency for Ac-225 using the surrogate source Pu-239. I thought it was a great question to share with you here.

Question

The specifications for the M43-1 mention an efficiency of 33% for Pu-239.

Following are datasheets for Ac-225 (Canadian Nuclear Safety Commission) and Pu-239 (Health Physics Society).

Canadian Nuclear Safety Commission. (2026, January). Radionuclide information booklet (Version 10.0). page 87.
https://www.cnsc-ccsn.gc.ca/eng/resources/radiation/radionuclide-information/

Argonne National Laboratory. (2001, October). Human health fact sheet: Plutonium.
https://hpschapters.org/northcarolina/NSDS/plutonium.pdf

If we choose the alpha energy of 6.34 MeV, assuming a measured efficiency of 12% for the Pu-239 surrogate source and using the branching ratio correction for the target nuclide, what is the extrapolated target efficiency for Ac-225?

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).

Jeff Sandeman

Jeff describes himself as “a grumpy old curmudgeon” who retired from Canadian Nuclear Safety Commission’s Accelerators and Class II Facilities Division in January 2021 after 17 years with the division. Prior to that, he spent 20 years at CancerCare Manitoba, the last few years of which he was the radiation safety officer for the radiation therapy facilities.

Throughout both careers, he was blessed with opportunities to explore a very broad range of radiation-safety-related topics, including non-ionizing, diagnostic X-ray, nuclear medicine, medical radiotherapy, cyclotron isotope production, and even high-energy research accelerators. For some strange reason, both institutions also allowed him to teach on topics such as radiation protection fundamentals and radiation shielding design, and to mentor (corrupt) young staff. The highlight of his professional life came in 2020, when CRPA awarded him Richard V. Osborne Founders’ Award.

Now that Jeff is retired, he spends most of his time attempting to golf (very badly), making sawdust in the basement (woodworking), or collecting dirty bits of paper (a.k.a. postage stamps). He loves single malt scotch whisky, of which he has far more bottles than his liver can possibly survive. He continues to dabble in radiation protection and has maintained his membership with CRPA (usually after 18 notices to renew, as he’s growing a bit forgetful in his old age).

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