Cobalt 60 with plain vanilla NaI:Tl: everybody needs a little pig :)

[ColoRad-o]

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Hello all,
I noticed Steven’s (newish?) Cobalt 60 spectrum with a CsI fancy detector, and took a look at Nephrondoctor’s spectrum with that phenomenal LBC detector. It was great to see TWO sets of Compton features.

I tried plain old NaI:Tl 2”x2” [GSB-2020, with GS-USB-PRO] at 700V bias and a and was not disappointed. I used a brand-new 1 microcurie check source. I imagine those with older sources have less of a problem with how ‘hot’ the sample is, given the short half-life. With two very high branching ratio peaks, there was so much pulse overlap I had to use the advanced pulse filtering to restrict the pulse width to 120 to 130 microseconds to clearly resolve things. I’m lucky enough to have a little lead pig which fits in the throat of the GS-STANDUP-20 enclosure I bought from Steven in 2019—holds about 95 lbs of lead shot (the enclosure, not the pig!). So I put the source in the pig and the pig in the enclosure, which attenuated the count rate to a nice manageable value.

A couple notes: (i) with so much Pb around, it’s not surprising I can see the 511 keV escape peak from the gammas produced when the positron from pair production in the shielding recombines with an electron, releasing two back-to-back gammas, one of which I detect. (ii) It looks to me like the narrow 1090 keV feature could be some hint of the second Compton plateau from the higher-energy main gamma; (iii) Steven assigned a bump around 2500 to the 2614 Tl208 background, but I could see it in my original ridiculously high rate count, and of course the better lower cps count, which would certainly have swamped background radiation. I believe this is the sum peak at 2506 keV from the two main gammas.

I was happy with how well PRA’s interpolation of 5 calibration points worked. As usual, InterSpec is fantastically useful. I enclose a linear and log PRA spectrum and the InterSpec window.

[Sesselmann]

Thanks for sharing.., nice clean Co60 spectrum with good analysis. It is entirely plausible that the bump at 2500 is a sum peak, especially since your source is new and active. I live in a brick house so I always have some counts from Ti208 upsetting my spectra.