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Radioactive Lens - Pentax Super-Takumar 50mm f1,4

Posted: 29 May 2019, 06:28
by Go-Figure
Hi guys,
I knew plenty of camera lenses produced in the past were radioactive and I was very curious to test one of them.
I tried to find one from this list and last week I finally found a Pentax Super-Takumar 50mm f1,4 lens.
As soon as it arrived I checked it with my Geiger counter and the back lens sent it to 26000 CPM. Not a bad start I thought, but lantern mantles gave me 22000 CPM so it didn't look much more active than that.
But when I tested it with the scintillator the difference was very visible. Lantern mantles gave me less than 1000 CPS, the lens went over 2000 CPS. So far it's the hottest sample I tested.

Radioactivity comes clearly (as well known) from Thorium 232, all the peaks are there.
When I calibrate with Cs137 the peaks are right when they should be below 1000-1200 keV and calibration starts to go off at higher energy. Here I knew I was going to find Thorium so I used a lantern mantle for calibration and for some reason peaks are 7-8 keV higher than they should be across the board.
I will repeat the test in order to see if it happens again.

I attach both counts per bin and energy per bin spectrum.

My dead time for this test was 160 ┬Ás, I think I can do better than that by improving my settings. This object was so active that maybe some pulses wasn't counted (as Steven recently pinted out to me), which is another reason to repeat the test. Anyway, I also attach a quantitative analysis based on the results of this test. I will check how they change with better settings.