Comparison of different age Am-241

[MartinM]

I acquired a rather unique piece of Am241-based smoke detector from 1960s. While current detectors use ~1 uCi Am241, that old source had ~80 uCi initially deposited, so it's rather hot. Open G-M detector capturing also alphas exceeds its upper threshold of 1-2 mSv/h, PDS-100 gives ~5000 cps.
Am241 decays by alpha with half-time of 432y to Np237 which has a sufficient long life (2 million y), so I was curious what difference will I see between a new and old source.
Here some first rather quick results done using the Mirion PDS-100.

Relatively new source (a few years old) with 0.9 uCi. As expected, the 59 keV dominant peak is clearly visible:

Am.png

Now the 60-year old source with 80 uCi:

Am+Np.png

The difference between these samples is clearly visible in the 80 - 160 keV region, which (I believe) can be attributed to Np237 buildup.
For Np237, the most dominant peaks are 29.3 keV (15%), 86.4 keV (12.4), 94.6 keV (0.6%), 143.3 keV (0.43%), 57.1 keV (0.39%), 151.4 keV (0.23%).

Interspec.png

More detailed tests shall follow...

[Geoff]

If you shield the source with a thin piece of cadmium you can filter out most of the 59 keV gamma rays and really bring out the neptunium spectrum.

[MartinM]

Thanks for the suggestion, I just need to find a Cd sheet, currently have only a large rod in my collection :) What would be the ideal thickness?

[Geoff]

I use neutron activation disks that are 0.5mm thick.