Gamma Spectrometry Forum

Here's a spectrum of 40ish year old reactor grade plutonium with significant 241Am and 237U buildup. 59keV peak attenuated with cadmium. spectrum has been running for 32 hours and counting.

241Am/237U from 241Pu at 59 and 208keV, 238Pu at 43 and 152keV, 239Pu at 129 and 203keV. calibration is only accurate to 650keV

RID-1 or 6M ?
What device have you used for spectrometry?
How thick is the Cd shield, have you also used some other material like Cu?

This is the source from a RID-6M. I'm using my modified GS1100A and an ASUS Xonar SE sound card. The detector is the model 7530 1" NaI(Tl) from Far West Technology Inc which is very sensitive between 30keV and 600keV. The attenuator is 1.1mm of pure cadmium in the form of 2 neutron activation foils. I didn't use any other attenuator between the sample and the detector, but my shield is made of copper followed by tin followed by tungsten and finally lead.

Thanks! I'm trying to acquire a similar spectra, but using a 2x2" NaI and since I don't have a Cd sheet, I'm using Sn+Cu to shield the sample (assuming Sn might also be a good choice as it's just Z+2 after Cd) .
However the spectra I get looks rather like pure Am-241. So not sure whether I'm doing something wrong (either large crystal for sufficient sensitivity in this energy range or the shielding is not suitable), or the material is not what I expected.
I also tried with the Mirion PDS-100 which has a 1" CsI crystal. This looks somewhat better but doesn't allow a more extensive measurement.

I think tin would work. I’m out of town right now but when I get home I can try attenuating with tin and tin+copper to see how well it works compared to cadmium.

Alright, here is three 10 minute runs for comparison. Unattenuated, 1mm cadmium plus 0.8mm copper, and 2mm tin plus 0.8mm copper. Both the cadmium and tin effectively attenuate the 59keV gamma ray from the 241americium. The tin being twice as thick only reduced the total count rate by ~0.3Bq. For comparison I also included a 4th run of a 0.8μCi 241americium source from a modern American smoke alarm, unattenuated.

Unattenuated
Cd-Cu
Sn-Cu
Modern source

Thank you for the results!
So it looks like my problem is the rather large crystal not sensitive enough to separate the different peaks in the 100-300 keV range.

Hi Geoff,

I think the 43 keV peak can rather be attributed to Am-241 and the other prominent ones of Am-241 are at 99 and 103 keV.
Pu-239 should have peaks at 129 keV, which is however hard to distinguish using such spectrometer from another Am-241 at 125 keV.
Probably the most prominent Pu-239 peak in an area that should not be affected by other daughters should be visible at 413 and 375 keV.

Regards,
Martin