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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.

Andréas Sundström
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1 Answers1

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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.

R.W. Bird
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  • 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... – Zeus Sep 16 '21 at 00:40
  • 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. – Andréas Sundström Sep 17 '21 at 07:53
  • As I recall, alpha and beta do not penetrate he body beyond the skin. – R.W. Bird Sep 17 '21 at 13:34