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I'm also asking a similar question on Physics SE.

As we know, some old lenses used Thoriated glass which give off non-negligible amounts of radiation. Although the radiation isn't really harmful to the photographer, I'm wondering if it could be detected on film photographs. The radiation that hits the photographer would have had to travel through the film to get to him/her. (Nothing is, however, coming out of the eyepiece since the radioactive radiation doesn't usually reflect off the mirrors like light.)

For this SE, I'm wondering if anyone knows of an example photo where such radiation effects is visible (presumably as black specks on the negative). Perhaps someone has mistakenly wound past a frame without exposing it, and found specks on that unexposed frame? I'm guessing that it's most easily seen on some high-sensitivity and/or large-grain B&W film.

Maybe such effects can be picked up by a digital sensor, but I'm guessing that it could be hard to distinguish that from noise.

Andréas Sundström
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  • Side-note: The app "RadioactivityCounter" allows you to measure radioactivity levels using a smartphone-camera and a bit of black masking tape, by counting the pixels that light up due to background radiation. Sadly, the app has not been updated in a while (at least on iOS, latest supported Apple device is iPhone6s), but there has been a recent nature paper: https://www.nature.com/articles/s41598-021-92195-y – Martin J.H. Sep 16 '21 at 16:49

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I have a Super Takumar 50mm f/1.4, one of the more common lenses with a thoriated rear element. I've owned it for forty years, and a couple times left 400 speed film in the camera for several months with the lens mounted, and never seen any effect.

Why?

First, between the lens and the film is a mirror and shutter curtain (in an SLR -- in a Speed Graphic with an Aero Ektar, there's more distance, as well as a dark slide in almost all cases).

Second, because the primary radiation from decaying thorium is alpha particles (helium nuclei stripped of their electrons); they have very little penetrating power (in most cases, a few inches of air or a single sheet of paper will stop nearly all of them, never mind the metal reflective coating and glass of an SLR mirror). Virtually no alpha particles will penetrate metal parts of the camera body or lens body. Therefore the only radiation that could expose the film must pass through the reflex mirror, its mount plate (usually thin metal), and the shutter curtain (either opaque cloth as in my Spotmatic SP or metal blades as in my Ricoh Singlex II) -- and again, that isn't going to happen with alpha, or not enough of it to matter.

Now, most of these radioactive lenses are fifty-some years old (some as much as three decades older than that), so of course they have other decay products mixed with the thorium in the thoriated element(s), but most natural decay paths still produce primarily alpha radiation, with occasional beta (loose electrons potentially able to emit x-ray on impact with metals, but usually not energetic enough for that) and almost never direct gamma emission; beta penetrates more than alpha, but so little is produced that it won't expose the film in a reasonable time.

So, bottom line, even several months with the lens mounted on an SLR and 400 speed film loaded in the camera produces no noticeable fogging, at least in my experience since the early 1980s. The answer might be different if the camera shutter was locked open and mirror up (with lens capped, I presume), but that would be very, very unusual situation.

Zeiss Ikon
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    Re, "metal parts...reflex mirror...shutter curtain" Most of the alpha particles won't even escape from the lens element itself. Only those emitted from very close to the surface of the glass will be able to make it out. – Solomon Slow Sep 15 '21 at 17:04
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    In addition, only those particles that are stopped by the emulsion will alter it. Those that are penetrating enough to go through mirror+shutter will likely go through the emlsion as well. – xenoid Sep 15 '21 at 17:09
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    @xenoid While true, don't forget that film emulsion has a significant amount of silver in it, which like most other metals has a higher interaction profile (hence why better shielding per thickness for something like gamma) than organics, water, etc. – Zeiss Ikon Sep 15 '21 at 17:46
  • Minor point: regular beta decay releases electrons, not positrons. Beta+ is less common, and it looks like the decay chain of both thorium isotopes which occur naturally in appreciable amounts (230 and 232) only have regular beta decays in them – llama Sep 16 '21 at 17:00
  • @llama Thanks -- so even less likelihood of fogging, since the mirror glass and its metal support, plus a metal leaf shutter, will attenuate electrons without generating any gamma (and those aren't energetic enough to produce X-ray, IIRC). – Zeiss Ikon Sep 16 '21 at 17:46
  • @ZeissIkon hmm, they can definitely produce x-rays (see the wiki page on bremsstrahlung), I would guess the flux is just too low to be meaningful in this situation – llama Sep 16 '21 at 18:50
  • @llama They emit photons -- they're only X-ray photons if there's enough energy extracted to give short enough wavelength. Bremsstrahlung can be any wavelength from some upper limit (near UV?) to gamma -- but it appears that some of this radiation from beta decay does reach x-ray energies. Probably not much of that to begin with, though, and most SLRs have metal shutters, which make decent shielding for low energy radiation. – Zeiss Ikon Sep 16 '21 at 18:57
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    There is certainly SOME gamma radiation emitted - cheap geiger counters will respond decisively to radioactive lenses (not the kind with trace thorium in the glass really - but certainly those with INTENTIONALLY thoriated glass), and these are usually not equipped to measure alpha/beta radiation. – rackandboneman Sep 16 '21 at 19:53
  • @ZeissIkon My initial thought was along the lines of your answer. What prompted me to ask the question was that this answer to the linked question about radiation dose, mentions a non-negligible dose received by the photographer "carrying a camera containing 0.36 uCi of thorium". So my thinking was that any such radiation would have had to penetrate the camera (and film), but apparently at least you haven't observed any such effects. – Andréas Sundström Sep 17 '21 at 07:32
  • @ZeissIkon Also, while the radiation hasn't penetrated through the mirror and shutter when they have been closed. Do you have any long exposures, like a whole-night star trail, with unexpected white specks? (I know it would be nigh on impossible to distinguish radiation specks from real light specks in the scene.) – Andréas Sundström Sep 17 '21 at 07:43
  • @AndréasSundström I've never done star trail photography. I'd expect a single halide grain exposed by radiation to be easily distinguishable from hours-long star trails, however, as it would be a single dot instead of a long arc. Problem is, there are literally a dozen other sources of an exposed speck in a negative, so you'd never know where it came from -- and this level of fogging is unimportant. Radiation fogging would normally be overall density, demarcated by the frame mask (where the metal of the camera body shields the film). – Zeiss Ikon Sep 17 '21 at 11:07
  • @ZeissIkon Thanks for sharing your experiences! I was partly asking this question to see whether it would be feasible to use film as a way of measuring the radioactivity of a thoriated lens (my Physics SE question was for that). But It seems like, with what you have experienced, it would be quite hard to do that. It would, if possible, require that you leave the shutter open for long times, while blocking any light from coming in. – Andréas Sundström Sep 17 '21 at 11:44
  • @AndréasSundström You could try to recreated Becquerel's original accidental discovery, by leaving the lens sitting on a sheet of film or (for less money) photographic paper for a period, in the dark. The less distance, the more effect you'll see. – Zeiss Ikon Sep 17 '21 at 12:04
  • @ZeissIkon That's probably the best option, although leaving it on the camera with the shutter open, would remove the shielding from the cover of the film/photographic paper. I might try both, the inverse-square law can usually be a killer... (Not to mention that air also acts as shielding for alphas.) – Andréas Sundström Sep 17 '21 at 15:49
  • @rackandboneman A geiger counter counts ionization events inside the tube. Beta will do this more frequently than x-ray or gamma, because it's charged particles. I haven't handled a modern geiger counter, but the ones I saw in high school fifty years ago had a small mica window in a metal shielded tube to make detection somewhat directional, and that material was used because it was partially transparent to beta. – Zeiss Ikon Sep 17 '21 at 15:49
  • @ZeissIkon typical device you can currently buy inexpensively would be the GMC-300 series. These use new old stock tubes ... which are in most cases fully metal encased and described as suitable for "hard beta" and gamma, and housed in the plastic case of the device, no mica window. These will respond to thoriated lenses. – rackandboneman Sep 17 '21 at 16:35
  • @rackandboneman And if I were a nuclear engineer, I'd probably know which decay path step emits a high enough energy beta to register on that tube model. I still say it's more likely the beta than gamma in this case. – Zeiss Ikon Sep 17 '21 at 16:41
  • @ZeissIkon Here's a test of several lenses that gets a positive reading even through lead shielding, suggesting gamma radiation. – Steadybox Nov 27 '22 at 02:54
  • Yep, @Steadybox some thorium decay products are gamma emi4tters. That said, I've left film in the camera behind my 50 mm f/1.4 Super Takumar (which has a thorium glass element in the rear group) for multiple months on occasion (cloth shutter Spotmatic SP) and don't recall seeing frame fogging. Might have just not noticed, though. – Zeiss Ikon Nov 28 '22 at 12:10