It’s been two months since I last did a measurement with the GS 2’’x2’’ NaI(Tl), so I thought it was about time to do one.
I have several samples in line, I picked LYSO crystals for this one.
I think you all know them, Lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) are scintillator materials widely used in PET detectors for the detection of 511 keV annihilation photons. Natural lutetium contains 2.6% of Lu 176 which decays beta to excited states of Hf 176 producing both gamma rays and x-rays.
I got myself three of those crystals, they are 22 mm long and have a cross section of 4 mm x 4 mm.
Here they are:
As usual I started with the Geiger counter, which typically gives me a background around 55 CPM, the read I got on the Ranger was about three times as much (oscillating between 150 and 180 CPM), not much, but I’ve seen enough to know that this could be an highly misleading parameter as far as gammas are concerned.
Then it was time for the PDS to have his say, and there I got roughly my normal gamma background (50 CPS) + 60%.
That made me confident I was going to be able to extract a decent gamma spectrum and three hours later here it is, both in Linear and LOG view. Of course you can’t remove the background here so the two things are mixed up, but the two major peaks at 201.83 keV and 306.78 keV are very clear. The sum peak is there as well, although I would expected it to be at 508-509 keV while it’s shows at around 520 keV. A calibration problem? The 2’’x2’’ will tell.
In terms of dose it's my ordinary background + 20%.
By now I should already have improved my graded shield, alas I am having a slight delay with it and I don’t expect to receive the extra lead before another month, so for now I have to make do with what I have, namely 5 mm of plastic, 1 mm of copper , 4 mm of pewter and 13-15 mm of lead.
But the weakest part of my current shielding is what I use to close the chamber, a mere 2mm of lead that cannot even fully cover the height of the inner plastic, so basically a part of it is completely open. Not very good.
Anyway, since I have to assemble and disassemble it all the time my shielding doesn’t always perform the same way. This time it went marginally better than last time I used it, allowing through 66 CPS rather than 69, my normal background measured with the 2’’x2’’ GS NaI(Tl) is 255-256 CPS so the result is 75% less than the unshielded background.
The peak CPS reported in the picture very much depends on bin size, this one here is for a bin size of about 0.5 keV. If I was to use the same bin size I used last time it would have been 0.259 CPS versus 0.279 CPS of my last measurement.
The accumulation time was 14 hours.
The pewter again did an excellent job in reducing Lead fluorescence.
I fitted the three crystals in a plastic bag into a scotch tape roll so that the position of the LYSO crystals was pretty close to the scintillator’s axis. A very low tech solution but it worked.
And then I left it there for 20 hours.
Peaks emerged right from the start, after less than a minute they were all clearly there, so it was just a matter of letting it smooth up more and more. Here is the spectrum in counts-per-bin and energy-per-bin view.
You have the two major peak 201.83 keV and 306.78 keV as well as the smaller one at 88.34 keV.
Then you have the expected X-Rays peak. There are many contributors to it, the main one are at 54 and 55 keV (and the peak is actually centred at 55 keV) but I decided to mention the others as well in the spectrum (which maybe made the label a bit messy).
Then you have the sum peak, which again is around 520 keV instead of the 508-509 I expected so something is eluding me there and it’s not a calibration problem, calibration was really good for this measurement and checking the spectrum with the background not subtracted I can see it holds very well even at high energy (K40 peak is just 5 keV off).
And then you have the escape peak at around 25 keV which comes from the X-ray peak minus the escape of I (iodine) K electron shell x-rays. This is something I didn’t expect, I had to look it up to find out what it was. I still need to research about it.
I am happy with the result and as you can see the shield fluorescence has been neutralised enough not to be a factor here.
Last, the quantitative analysis. The exposure from the crystals measured at contact is 0.020 μSv/h, a little less than 1/5 of my normal background which is quite close to what I got from the PDS in terms of dose.
Well, that’s it.
Dealing with a new radionuclide was a good fun.
This is my last spectrum before the holiday season so Happy Christmas folks and….don’t fill up on sweets.
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