Background


I found this knurled knob in my nuts and bolts junk bin, by chance, after I waved my Geiger counter past the bin and it went bezerk.   It'd been there for years and I had no idea that it was radioactive.   I'm not sure where it came from but from the looks of it, possibly a military radio?




Monday, 3rd December, 2012 01:20
Geiger Counter Testing of
Unidentified Radioactive Source
This is a video of testing the object with scintillators -  14 Mbytes
This is a video of testing the object with a CD V-700  -  7 Mbytes
The knurled knob is made from Aluminium with a brass shaft.   It has a dot of some material on the front face which although clear in the above photo, is almost indiscernible in natural light .
Placing it against the CD V-700 probe, it is showing around 37mR/hr with the Beta Shield open.   With the shield closed, the reading is zero.   Obviously it's a Beta emitter.
                                         The CD V-700  "x 100"  scale is visible at this angle.


The following two videos show some testing with different detectors.   The first shows a test with the Electro-Neutronics CD V -700.  

The second shows my Alpha / Beta scintillator responding to the source.   Note that the dial is switched to Alpha, but it's actually reading Beta because the internal threshold level sensor setting from the probe isn't set correctly.   The orange coloured scintillator detector should respond to Gamma emissions only.   It's showing 0.5 uSieverts/hr but it may possibly be responding to strong Beta emissions as it's much more sensitive than the CD V-700.    I'm not sure ...  I still need to validate that.

The scintillator crystal and photomultiplier casing is machined aluminium with a thin stainless steel disc at the front and the crystal is all surrounded by a soft removable plastic cover.  Normally I'd expect that to stop most Beta emissions, but without knowing the actual energy of the emitted Beta particles, nor the densities of the combined absorbers, I can't work out the maxiumum penetration range of the emissions, so it is possible some are getting through.

An amateur in VK2 emailed me in 2013 and sent me a photo of a similar item that he'd removed from an old WW2 military transmitter.   It appears that this is an adjustment knob and many of the knobs on his transmitter were coated with a Radium (Ra226) based photoluminescent paint which once glowed brightly at night.   Over time, the Radium's ionising alpha and gamma radiation has destroyed the phosphor in the paint mixture and it no longer glows, but the remaining Radium salt remains highly radioactive.   Given that Radium's half-life is 1,600 years, this item will remain radioactive for quite a few millennia.  It now sits in a sealed, lead lined, and labelled container!  

What is still unclear is why the Geiger Counter indicated that it was Beta radiation and not Gamma.   If this item is coated with a Radium 226 compound, then I would have assumed that its Gamma emissions would have penetrated the probe's shield and registered.   Possibly the Gamma emissions from Ra226 are not sufficiently energetic enough to penetrate this particular probe's Gamma shield.   I'll have to look into that further.   A Multi-Channel Analyser that can display the energy spectrum of the emissions would be handy!
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