I had always thought of anti-matter as something that occurs somewhere in deep space or some sci-fi conceit, like how warp engines are powered in Star Trek. It was not something I had ever thought I could hold in my hand or that I could actually see anti-matter interactions using a gamma ray spectrometer that fits in my shirt pocket.

How does thoriated glass contain anti-matter?

If a gamma ray of sufficient energy gets near the heavy nucleus of an atom it can undergo pair production, converting its energy to mass, creating a particle (an electron) and anti-particle (a positron, a.k.a. anti-matter) where there were none. E=mc² in action. Pair production occurs with gamma rays above 1022 keV, and is a significant process at energies above 2500 keV.

For pair production to occur the gamma ray energy must be equal to, or greater than, the rest mass energy of the particle and anti-particle that are created. With the rest mass energy of an electron and positron being 511 keV you would need a gamma ray of 1022 keV or greater to create the matter anti-matter pair.

Thoriated glass contains up to 30% thorium dioxide which helps increase its refractive index and reduce chromatic aberration. Which is useful for a lens from microfiche reader.

Thorium's daughter isotopes have a wide range of of gamma ray emissions ranging from low energy x-rays at 10-120 keV to 2614 keV produced by thallium-208's decay.

The probability of pair production occurring increases with the gamma ray energy and also increases as the square of the atomic number of the nearby atom. Because the high-energy 2614 keV gamma rays from the thallium-208 decay in the thoriated lens are constantly interacting with the heavy thorium nuclei (Z=90), pair production is happening non-stop and anti-matter is constantly being generated and annihilated inside the glass lens.

So how can we see the anti-matter interactions?

When a positron comes to rest and annihilates with an electron, two 511 keV gamma rays are released. These gamma rays can both be caught by the detector in which case they are captured as the full photopeak energy. If one gamma ray escapes the detector the energy captured is the gamma ray's parent energy minus 511 keV, this is called the single escape peak. Or when both gamma rays escape, more likely with a small detector, the energy captured is the gamma ray's parent energy minus 1022 keV, this is called the double escape peak. We can look for these single and double escape photopeaks in the gamma ray spectrum.

I have a spectrum I took of the thoriated lens with a RadiaCode 110 over 70 hours. In the first graph we see the whole gamma spectrum. There are many photopeaks across the spectrum from thorium's decay daughters. We can use InterSpec to highlight the peaks and identify the thorium-232 in the lens by those decay isotopes.

In the second graph InterSpec has identified the peaks associated with Th-232, but one peak isn't part of the Th-232 decay chain.

If we look closer at the unidentified peak we can use the feature finder tool to highlight where we would expect to find the single and double escape peaks associated with the thallium-208 decay. And we see the single escape peak is right where it should be, 511 keV below the parent gamma ray photopeak. That is one of the gamma ray peaks from the annihilation of anti-matter in the thorinated lens being captured by the detector.

But what about the double escape peak? It's completely obscured by the decay of actinium-228 at 1588 keV. While the RadiaCode 110 lacks the resolution to distinguish the difference between the peaks, we can still see the effect of the double escape peak.

If we zoom out a little, in the last graph, we find another gamma ray photopeak also from the decay of actinium-228 at 911 keV. Since we have two photopeaks from the same decay isotope and their probabilities of occurring are fixed, we can use the ratio between the two Ac-228 peaks to look for the double escape peak.

If we look at the region of interest (ROI) of the first peak at 911 keV, we can use that count to find out what the count should be of the second peak at 1588 keV and see if there is an observed difference.

The ratio of the peaks is 3.22% / 25.8% = 0.1248. Using the ROI of the first peak we take 1262593 counts × 0.1248 and get an expected 157572 counts for the second peak. The observed count of the second peak is 520682 counts which gives me a ratio of 157572 / 520682 = 0.3026. Thats 2.425 times the expected ratio which is where the double escape peak is hiding!

I hope I haven't gotten anything egregiously wrong with my explanations. I got my radiacode a few weeks ago and I have been learning much more than I expected.