r/Radionuclides

DISCLAIMER: The method is just a rough approximation. It should not be used to judge the purity or safety of food and water samples. This is just for fun.

The aim is to determine the counting efficiency of the device for the given geometry and derive a conversion coefficient (Bq/cps) that would allow us to estimate the activity of the sample per unit mass, i.e. Bq/kg. Again, this is just a rough approximation, more of a fun experiment than a calibration, really.

To do that we need a standard source with a known specific gamma activity. Unfortunately, these are not commonly available to the average user. However, the specific gamma activity of many potassium salts (due to K-40) are known. In this case, analytical grade potassium chloride (KCl) was used. Its specific gamma (only gamma) activity is 600 Bq/kg. The procedure is to measure a known amount of KCl in a reproducible geometry, subtract the background (measured under same conditions) to arrive at some activity in cps above background. Knowing the mass of the sample and its specific gamma activity we can easily calculate our conversion coefficient C as:

C=specific sample activity (Bq/kg) x sample mass (kg)/measured activity above background (cps)

The result is our coefficient in Bq/cps, in other words “true” (note the quotation marks) activity per each cps above background registered by the device.

In this example I used 260 grams of analytical grade KCl inside a Radiacode Marinelli beaker, in a lead shield. Sample and background (empty shielded beaker) were recorded for 6.5 hours. You don't need ultra long acquisition time, but allow a few hours to get a good average activity. Using the above formula, after measuring that the sample activity is 2.5 cps above background we get:

C= 600x0.26/2.5=62.4 Bq/cps or a counting efficiency of 1.6% for RC 103G

I did this measurement twice with slightly different shielding configurations. My other measurement gave 55.5 Bq/cps. The two values differ by about 11 %, which is about what could be expected in terms of accuracy. Mind that this is around and above 1 MeV. At lower energies the efficiency will be higher and this coefficient will be lower, respectively.

PS: The image shows the103G and the Radiacode Marinelli beaker outside the shield, just to show the nice yellow color.

I recently measured 1 kg of Brazil nuts on my gamma spectrometer. I was looking for Ra‑226, but found none…

Instead, the spectrum was dominated by the characteristic Th‑232 chain:

• 2614 keV peak of Tl‑208 (the highest‑energy gamma in nature), but very weak.
• 583 keV (Tl‑208) and 239 keV (Pb‑212) lines
• No 609 keV Bi‑214 peak, confirming no measurable U‑238 / Ra‑226 (although the Tl208 peak at 583keV could be mistaken as the Bi214 peak)
• and I can also sense 911 & 969keV from Ac228, which is a strong indication for Th232 chain.

How does Th‑232 get into Brazil nuts when the tree’s roots can’t take it up?
This goes back to Otto Frindik (1989) in „Zeitschrift für Lebensmittel‑Untersuchung und -Forschung, 189:236‑240“. 

The explanation is soil resuspension: fine Th‑bearing soil particles become airborne and stick to the large, rough surface of Brazil nuts. The Th isn’t taken up by the plant – it’s a physical dust deposit on the shell. Frindik calculated that Thorium activity is enriched 740‑fold in Brazil nuts relative to the soil, far beyond what any root uptake could achieve.

So what I measured isn’t a failure to find Ra‑226 – it’s a demonstration of a completely different (and often overlooked) contamination pathway: atmospheric dust deposition, not root uptake.

So those Brazil nuts can not only contain Ra226, they can also be quite contaminated with Thorium via a completely different mechanism….

To be honest: I did not like those nuts from the very start, LOL, not my taste….

*Equipment: GS1515‑CsI(Tl) scintillator, GS‑MAX‑8000 digital MCA, custom lead/copper shield (>90 % background reduction)*

Just wanted to share my new setup and get some thoughts from the group.

I recently picked up a custom gamma scintillator probe for my Ludlum Model 3. It’s a Bicron BC412 polyvinyl toluene plastic scintillator with organic fluors, specifically for gamma detection. The crystal measures 3” diameter x 2.25” thick, polished on all sides with a reflective coating before coupling to the PMT.

The tube is a quality 3” photomultiplier, and the dynode chain is spec’d at >100 MΩ, so it plays nicely with battery-powered meters like the Ludlum Model 3 (no HV droop issues).

First numbers:

• Background: ~2400 CPM (40 CPS) on the Ludlum
• Radiacode 110 background: ~13 CPS

So the BC412 is seeing about 3x the gross count rate in background compared to the RC110. Makes sense given the massive volume difference, but it’s fun to see it quantified. Of course, the plastic scintillator has a different energy response than the CsI(Tl) in the Radiacode, so not a direct 1:1 sensitivity comparison, but still impressive.

The downside? Stealth mode is fully disabled. This thing is chunky and very obviously not a phone or a pager.

Testing on real sources:
The ebay-seller of the Ludlum was so kind to include a uranium glaze vase with the meter. The BC412 rushed up to 3k CPM almost instantly – very responsive. The Radiacode struggled to pick it up reliably by comparison.

Where I’m a bit disappointed: uranium glass. On weak pieces (thin or low-U content), the BC412 wasn’t dramatically better than the Radiacode. Maybe I expected too much given how hot the glaze source looked.

Overall, it’s a fun new toy. Looking forward to taking it to flea markets and watching people’s faces when the needle starts climbing. Absolute terror device. 😂

Anyone else running a large plastic scintillator on a Ludlum? Curious about your background counts and how it performs on low-activity stuff.

The remains of electrostatically charged balloon after collecting Rn-222 progeny for an hour. The device is Rdiacode 103G.