One of the challenges of doing any kind of quantitative measurement is achieving the best possible signal to noise ratio. When performing gamma spectroscopy, especially with very low levels of radiation (signal), background radiation (noise) can easily mask/hide/swamp the desired signal. Of course there are many sophisticated noise-filtering algorithms, but following the KISS & GIGO principles, reducing background radiation (noise floor) is a choice first-step. Simply put, we wrap the detector and sample in a radiation shield to minimize external influence. This shielding is commonly referred to as a “castle“.
To build the lead castle, I bought 25 rough 99.9% lead ingots from Rotometals.com; these are available for a little over $2/lb, including shipping. The downside is a fair amount of non-uniformity in the ingots:
Quite a bit of re-forging (straightening/flattening), cutting, and filing was necessary to get them into useable shape where the sides overlap when placed next to each other with no gaps.
After the ingots were refashioned, I cleaned them with Acetone and spray-coated them with Rust-Oleum epoxy appliance paint. If you think you might want to eventually melt the lead down for other purposes, you may forgo the painting and simply wrap the ingots in tin-foil so they can be handled without gloves (lead safety).
If I had it to-do over again, I would go with well-formed, clean lead bricks. These cost a bit more, but I now consider the cost well worth-it 😉 Another alternative would be to use fine lead-shot (#12) poured into the void between concentric containers. Wall thickness would then be easy to experiment with since the shot can be poured in & out to give more or less shielding.
The results were averaged over a couple of hours, with a counts/minute (CPM) of 46.43. The logging rig was then fully enclosed:
Counts were then logged for another two hours, yielding an average of 22.57 CPM. The results showed the castle cuts down the background radiation by a little more than half. Admittedly, at my altitude (6500 feet), radiation fluctuations are significant over the course of a day (typically 10-15 CPM).
The Theremino software (running under wine in Ubuntu) showed an average of 71.1 counts/second (CPS), or 4266 CPM. As can be seen, the scintillation detector is much more sensitive than a Geiger-Mueller tube:
Enclosing the detector Yielded an average of 7.5 counts/second or 450 CPM; a reduction in background radiation (noise) by almost an order of magnitude. I’d call this experiment a success.
This will allow measurements to be taken over a much longer period of time, making low-level measurements more accurate.
Here are some notes for installing Theremino under Linux.