Low Level Gamma Radiation (2023 Update)

What does the X axis represent?

There's a long decay chain from naturally-occurring thallium 232 and uranium 238 down to stable lead, and a number of steps in the chain produce a gamma photon

Er, Thorium 232, not thallium

What do you know of the calibration of the unit itself? Does it have the equivalent of frequency response?

When I look at the decay chain, I don't see any gamma emissions. Is this in a more rare decay event?

For thorium the gory details are here:

For his device it looks like the plot is a histogram of absorbed energy, and can be set on the 1 MeV, 2 MeV and 3 MeV scales, see top of page 19:

If the widget is set on the 1 MeV scale and the left extreme of the X axis is about 100 keV and the right hand extreme is about 1 MeV, and the amplitude on a log plot it seems somewhat congruent with page 4 here:

Seems to be detecting background radiation if that's how it's set up, situation normal I think?

How about

I seem to remember that it is the most important gamma ray source in regular terrestrial environments.

As a general rule, X-rays excite fluorescences in lots of materials; unless you have only low-atomic-number elements around, some of those fluuorescences will be in the low X-ray region, and would presumably be a low-energy high-count source that penetrates the window of your sensor (whatever the sensor is). For some sources, secondary radiation is the easiest to detect (a detector can be transparent to high energy photons).

Thanks for your reply. As a beginner, it is easy to get confused. I'm confused.

To get more information, I decided to get the spectrum of Potassium-40 by extending the scale of the Radiacode to 3MV, and sitting it on 3 jars of Windsor Salt Free shown here:

Wikipedia gives the following information on Potassium-40 decay:

Potassium-40 is a rare example of a nuclide that undergoes both types of beta decay. In about 89.28% of events, it decays to calcium-40 (40Ca) with emission of a beta particle (an electron) with a maximum energy of 1.31 MeV and an antineutrino. In about 10.72% of events, it decays to argon-40 (40Ar) by electron capture (EC), with the emission of a neutrino and then a 1.460 MeV gamma ray.[1] The radioactive decay of this particular isotope explains the large abundance of argon (nearly 1%) in the Earth's atmosphere, as well as prevalence of 40Ar over other isotopes. Very rarely (0.001% of events), it decays to 40Ar by emitting a positron (?+) and a neutrino.[2]

The detector in the Radiacode is a 1 cm cube of Thallium Doped Caesium Iodide (CsI:TI). This is a very popular scintillation detector and has good performance when coupled to a avalanch diode.

The Potassium-40 spectrum is here:

You can see a slight hump at 1.31 MeV and a clearer hump at 1.46 MeV. This is very satisfying, but it's not clear how the hump at 1.31 MeV is produced. Is the Radiacode sensitive to beta decay?

Also notice the shelf extending back to zero energy. Where does this come from?

You mentioned above "X-rays excite fluorescences in lots of materials".

But the sources are presumably beta and gamma. Where is the fluorescence coming from?

Thanks for your help!

""In about 89.28% of events, it decays to calcium-40 (40Ca) with emission of a beta particle (an electron) with a maximum energy of 1.31 MeV and an antineutrino."

The neutrino was pretty much invented to explain why the electron came out with a range of energies - the neutrino carried away the rest of the energy.

Fluorescence can also be excited by energetic electrons - "beta rays". Gamma ray is just another name for an X-ray. It took a while for us to understand that they were both energetic photons.

And in the more modern physics it helps conserve spin and lepton number/flavor, they're on the lookout for something like muon -> electron + gamma where the energies are correct but lepton flavor conservation is violated. Haven't seen it yet AFAIK

I think in particle physics all photons that come from decay tend to be called "gamma" even if they overlap with the X-ray's domain below about

100 keV.

At the bottom of page 4:

"About 90% of the counts come from the low-energy part of the spectrum, which is degraded by Compton scattering." I think this means the area around 100 keV in the background radiation is very noisy with contributions from the scattered photons of lots of stuff.

Energy I presume. There will be a mix of elements contributing to the background and some will have characteristic lines. Potassium nitrate or instant coffee may have enough K40 in to allow some calibration.

>>

No they are common but they occur in conjunction (shortly after) either an alpha or beta decay due to the recoil and necessary rearrangement of the remaining components of the atomic nucleus.

Just after the alpha or beta particle escapes the nucleus is in an excited state with a hole in it where the emitted particle once sat. Gamma ray(s) get emitted as it rearranges back to its new ground state.

Only emissions that alter the atomic number and/or mass are normally shown on decay chain diagrams.

X-axis should be energy, the spectrum looks right for it to be that. Clearly with such a device you won't see much energy resolution, probably the 1461 keV line of 40K won't be visible even if it were within the energy range of the device (probably not, by the size of it the detector would be too small for that; buy a kilogram of bananas and measure it to see if that's the case). Here is what the 40K peak looks like (the marker, a red X, is on top of it):

If the banana pack yields higher counts per second and no visible peak (which is what I expect you will see) it will be due to Compton etc., others may be more familiar with the details, I just design the spectrometers and have learnt only as much as it takes to do the measurement and the evaluation of the spectra....

====================================================== Dimiter Popoff, TGI

In article snipped-for-privacy@googlegroups.com,

Correct. In fact, when building high-quality isolation chambers for measuring gamma specta (the gamma equivalent of a Faraday cage, in effect) it's necessary to use a layered approach. The outer layer is usually lead, but when gammas from outside hit the lead it will fluoresce in the X-ray spectrum. So, inside the lead, you have another layer which effectively absorbs those X-rays... and *it* may fluoresce at a lower-energy X-ray frequency, so you may need a third layer of yet another material.

I've used a home-made gamma spectrometer (NaI crystal, a PMT, and my own electronics) to look at some naturally-occurring radioactive materials. One interesting source is some monazite sand from a local beach - it has a significant amount of thorium in the mix and I get an appreciable count rate if I lower the sensor down onto a big box of the stuff.

The spectrum does show the expected gamma-ray peaks for thorium, but they're not as "clean" as for a purer thorium sample and there's a strong continuum of lower-energy gamma/X-rays just as the OP's photo showed. My understanding is that this is "degraded" gamma - in other words, gamma-induced fluorescence occurring within the sample itself. Reducing or eliminating this requires flattening out the sample (so that there's a better chance for a thorium-generated gamma to hit the NaI sensor before it hits an atom of the sand and causes fluorescence).

One spectrum I looked at was that of a "quantum energy pendant" that can be bought inexpensively on eBay and elsewhere. It's supposedly a natural negative-ion source with semi-mystical healing powers. What it actually seems to be is a pendant made from a natural ore rich in thorium. It emits "negative ions" in the form of beta-decay electrons, and reportedly its emission rates are high enough that if you wear it next to your skin every day you'd exceed certain government safety limits for ionizing-radiation exposure in that area (possible cancer risk or a localized radiation burn).

The green trace is the background radiation level in my work area. The purple trace is from sampling for the same amount of time, with the pendant in contact with the NaI sensor. The thorium-228 peak is clear, there's another from lead and radium decay daughters, and the actinium-228 peak is also visible.

With another sensor and voltage setting, the background signal from potassium-40 is visible... and bringing a bottle of Morton "lite salt" or a bag of water-softener potassium chloride around the sensor really makes it obvious!

This listing is also very interesting:

Basically 2000+ PMT+CsI(Tl) assemblies for sale at around $20 each. Wonder how they'd compare to the solid-state CsI(Tl) detector assembly in the Radiacode unit?

-- john, KE5FX

This is incredibly cheap indeed. I thought even Hamamatsu can't make PMT-s that small :). (4 years ago they sold us some of their R12421 at

450 euro each IIRC). But just the scintillation crystal would probably cost much more, let alone the PMT at Hamamtsu.... No idea how they manage that.

I tried to order one. Does not ship to Canada. Can you order one and mail it to me? I can pay you via Paypal. Thanks.

On a sunny day (Sat, 25 Jun 2022 18:16:39 -0000 (UTC)) it happened Mike Monett snipped-for-privacy@not.com wrote in <XnsAEC1913D785B5idtokenpost@144.76.35.252>:

I just found this on tomshardware.com:

spectra (github link)

Maybe. I'll have to surf through the Commerce and ITAR lists to see if there's a reason why the seller doesn't ship to Canada. Shifty folk, Canadians.

(It'll be a few days before I have time to deal with it, for various reasons.)

-- john, KE5FX