My shielding setup, and some questions

[Bob-O-Rama]

I'm trying to detect ( and maybe quantify ) radon adsorption by upholstery foams / materials - "as ones does" - using whatever I can afford to cobble together. Its a long story. The current setup is a GS-PRO-V5 + a 1.5 x 2.25 refurb Bicron probe from iRad. Without shielding I get about 37 cps over long periods.

I have about 600 pounds of bricks and settled on 2" thick walls and a 6 x 6 x 18" volume. That takes it from 37 cps to ~ 1.67 cps. ( Don't judge, the permanent home is behind the little door... which is inconveniently blocked by 600 pounds of lead. )



The lead-shine is the dominant feature. Hints of NORM also.



I tried various options for attenuation of the lead-shine. For whatever reason getting elementally pure tin is stupid expensive and seeing what others here have done tried pewter tankard + copper tubing. That worked, but left a pretty large lead signal - likely everything to was too thin and not fully enclosing the probe. I had a 4" diameter roll of plastic backed copper flashing - so think alternating layers of copper and poly-something-or-other about 1cm thick - its several pounds of copper. Its a common hardware store item used for protecting wood posts. This worked really well. The K-40 peak is the dominant feature. This is the low end...



and the high end, with K-40 and its straight out of compton continuum peak in the 1200 range - I think.



Once that was worked out, I was continuously getting the NORM spectrum at low levels. I figured it might be the lead itself - which would be bad. Or some radium containing thing in my collection was being seen. But no, it was radon IN THE ROOM plating out RDPs inside the shield. ( See first spectra, its there too. ) To fix this, I keep the lead at a slightly positive electrostatic potential using a cheap "ion generator" power supply. This repels the RDP and keeps it from plating out in / on the chamber. You can see they are now entirely absent in the second spectra. I got the idea from a RDP precipitator I had cobbled together - I figure just "run it in reverse."

The question: I am particularly interested in 46 keV - any suggestions on cleaning up that area specifically would be helpful.

-- Bob
( Mahar )

[Sesselmann]

Bob,

Interesting and novel approach to apply a static electric charge to the lead bricks, novel to me at least...

The gaussian correlation is pretty good at pulling out features in the noise, but detection below 100 keV is typically difficult, the housing of your detector might be attenuating 10 to 15% of the 46 keV energy and also NaI detectors have pretty poor resolution in this range.

If your detector is inside a 1.5 mm aluminium housing you might slightly improve efficiency by removing the housing and wrapping it in thin aluminium foil.

[Lposter]

I am tempted to suggest that the large ill defined hump around the 150-300 keV area is backscatter from the lead being in close proximity to the detector? As you seem to be primarily interested in energies fairly low down the scale, it might be worth trying to get more space between the detector and the shielding, even at the expense of overall shield thickness.

This may also serve to further reduce the lead x-ray signal somewhat. A thinner lead shield with a greater internal volume and the shield liners may reduce the problematic signals in the energy region you are interested in.

If your lead is not old lead (ie. 200 or 300 years old), there is also going to possibly be a 46 keV signal coming from that. The internal liners may take care of it (or not depending on the thickness of them) but its something that has to be thought about especially if your liners are not covering all the surfaces.

In general, to maximize your chances of being able to utilise the lower end of the spectrum, you are going to have to be fairly systematic and possibly have to spend some money.

Lars