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I'm asking a similar question on the Photography SE, but here I'm more curious about a measurement setup.

In short, some lens manufacturers used thorium in some of the lens elements to increase the refractive index (similar to lead crystal glass). I have a student with such a lens and he asked me for a Geiger counter, so that he could measure the radiation given off from that (simply for academic curiosity, not due to health risks). I unfortunately don't have access to any such equipment. But I got an idea to maybe use photographic film. It's a photographic lens after all.

I'm wondering if any of you have any resources on using film in a home set-up for radiation dosimetry. My question is what is needed to get a quantitative measurement of the radioactivity of the lens in order to estimate the amount of thorium that's in the lens.

  • Is it possible to simply buy a film and load it into the camera and wait for a given time and count the number of exposed spots? Or is it perhaps better to put everyting in a light-tight box where distances and shielding can be more easily controlled?
  • What kind of (consumer-available) film is best suited for dosimetry? I'm guessing something with large film grain, so they become more easily spotted on the developed negative.
  • What kind of radiation should you expect? Thorium emits alphas, but I'm guessing that what makes it out of the lens should be beta or gamma from some decay product.

Sorry for blasting off several questions in one, but my main question is about the requirements for the measurement.

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I doubt there will be enough alphas and betas for you to detect - most of them will likely not make it out of the glass. But there might be some gamma radiation.

To detect the gamma you have to overcome two issues: (1) normal photographic film is not very sensitive to such a high energy radiation (you can x-ray it quite safely, as I was forced to learn at some airports). You need a scintillator to convert gamma to visible light. (2) Schwarzschild effect - if you try to detect very low level light with photographic film the sensitivity of the film reduces. The light has to form defects in crystals of silver bromide in the film emulsion (so called latent image). Each time a photon is absorbed in the crystal the defect grows. The twist is that below some critical size the defects are not stable - they can disappear over time. If you have a lot of light then the exposure happens in a fraction of a second and your latent image nicely matches the light intensity. But in low light situation some defects might disappear before you absorb another photon and you have to start again. Net result is that the sensitivity is reduced.

From a more practical point of view your best shot is to get hold of some x-ray sensitive film, ideally one intended for dosimetry (for example this Foma personal dosimetry film) or for x-ray imaging. It will have a scintillator layer and if it is meant to be a dosimeter then the Schwarzschild effect should be less prominent too.

EDIT: Maybe even more practical approach: If you can find some scintillator material then you can attach it to the lens and take a very long exposure photo of it (in the dark) with a digital camera. The scintillator should appear brighter than the rest of the image.

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I would expect any unexposed film would record a track if penetrated by a charged particle, but there are complications. Many alpha and beta rays would be absorbed within the lens. Only a small fraction of gammas would cause ionization within the film. You would need some way to estimate what fraction of the tracks were caused by background or cosmic radiation (perhaps long before you bought the film). You would need some systematic way of counting the tracks using a microscope.

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    $\begingroup$ I guess estimation of the background would be relatively easy: you need to reserve some of the frames from the same roll of film and develop them without exposure, and then compare with the exposed ones... $\endgroup$
    – Zeus
    Commented Sep 16, 2021 at 0:40
  • $\begingroup$ I was prompted to ask this question partly by this answer to the question linked in my question, which stated that the "exposure rate at a depth of 10 cm in the body of an individual carrying a camera containing 0.36 uCi of thorium would be approximately 0.01 mrem/hr", which is not insignificant compared to background radiation. Do you think that's mostly from gamma rays? They would penetrate the film without a trace, but with 10 cm of body tissue, the person would absorb said dose. $\endgroup$
    – Andréas Sundström
    Commented Sep 17, 2021 at 7:53
  • $\begingroup$ As I recall, alpha and beta do not penetrate he body beyond the skin. $\endgroup$
    – R.W. Bird
    Commented Sep 17, 2021 at 13:34
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An easy technology for dosimetry uses materials that develop the same kind of metastable electron states of photographic film, but aren't thin films chemically developed. Rather, they're thick slabs (so have more stopping power than the sensitive layer of photo film), and are read out by warming them in an oven and seeing the luminescence that results from relaxation of those excited electrons.

Thermoluminescent dosimetry only requires an IR-sensitive detector and a warm dark box. Calcium fluoride in a transparent plastic binder is a suitable sense element; you'd want at least two identical sensors, one as a control, and you'd start by 'erasing' them in a similar oven cycle to that which later will be used to read out the dose.

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