Wednesday, July 29, 2020

Radium-226 Spectrum

I was running my spectrometer thru its paces and figured that Ra-226 is a good isotope to test with. Radium thanks to Madame Curie is an "evergreen classic" after all :-) (well maybe not quite "evergreen" - half-life is 1600 years)

There is a good spread of peaks coming from Pb-214 and 

I calibrated using my usual process - K-40 -> LYSO crystal -> Cs-137 while doing some adjustments in the Theremino's MCA "Linearizer" feature. 
Running the Radium spectrum after calibration placed the peaks spot-on telling me that the linearity after the adjustments is rather excellent - from the low energy spectrum (Pb-210 @ 46.5 keV) where NaI(Tl) detector has a relatively poor linearity (for any energies less than 100 keV) all the way to above 2.2 MeV (the efficiency of the small crystal is really poor at these high energies so no peak there). I labeled the K-40 peak for reference as it is almost always present in spectrograms.
This spectrum was obtained during a 12 hr scan, preceded by about 9 hrs of Background radiation scan for subtraction, all with my normal lead shielding.

These Radium watch hands still glow in the dark. The sample activity at 5mm from LND7311 Geiger tube, thru the glass (no Alpha) is approx. 120-130 cpm. 
The watch hands were placed at 1 cm from the bottom of the detector crystal inside the lead shielding.

Here is another Radium spectrum. This is about 0.5 grams of Radium Sulfate in a sealed vial - scan duration is about 8.5 Hrs. The Radium Sulfate was precipitated from Uranium "Yellowcake" but also includes some of the natural Uranium as salts - evidence is the 143.76 keV peak from U-235, not found in the Radium paint spectrum (which obviously underwent a better purification).

I am quite happy with the resolution of the detector and the background noise suppression by my shielding. Obviously it is not a CeBr3 detector but NaI (Tl) detectors have lower background noise compared to CeBr3 or LaBr3 and when you don't try to resolve peaks that are too close to each other it works very well for such cost-efficiency. 
Both CeBe3 is LaBr3 crystals have impurities of Ac-227 but LaBr3 also has also intrinsic activity due to the presence of the naturally occurring La-138 (0.09%). Some very weak peaks will get swamped in the higher background counts from a LaBr3 detector. CeBr3 detectors on the other hand are wicked expensive and you still need to find a "cherry-picked" one with lower Ac-227 impurities. I think I'll stick to the good old NaI(Tl) for now or maybe venture and test the CsI (Tl).

No comments: