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A Radioactive Lens (2018) (fourmilab.ch)
67 points by brudgers 75 days ago | hide | past | web | favorite | 26 comments

Back in the day, I was experimenting, a la Edison, to find an optimum anti-reflection coating for silicon photodiodes. One of the things I tried was thorium oxide. For some reason, coating the detectors ruined their dark current. Hm.

About an hour later, dawn broke over Marblehead.

What, why? What has Marblehead to do with anything?

I think he is referring to himself as "Marblehead", as someone whose head is made of marble or full of marbles, i.e. a dummy-head. In hindsight it seemed obvious that the radiation from the thorium would act as low level illumination for the photodiode and so ruin its dark current (the electricity that flows when there is no illumination).

PSA: If you have a yellowed lens that you'd like to use, great news: you can fix it:



This pops up with some regularity in the Pentax world. Turns out Pentax made a lot of radioactive lenses back in the day[0], and they sold them in considerable quantity.

I happen to own one (a Super-Multi-Coated Takumar 50mm f/1.4), and can confirm that sunlight works to clear the yellowing.

If you're trying to avoid radioactive lenses, I believe pretty much everything Pentax made after the SMC PENTAX-M is non-radioactive. I have a SMC PENTAX-M 50mm f/1.4 as well that shows no signs of yellowing.

[0] https://news.ycombinator.com/item?id=20612636 <- scroll down ginko's link, there are scads of them.

Quick reminder for people who have forgotten:

alpha particles are Helium nuclei. They are big, slow, and are easily absorbed by a couple of centimeters of air.

beta particles are high speed electrons (or positrons). They can be stopped by a couple of millimeters of aluminum, but they emit gamma radiation while being slowed.

gamma radiation is high energy photons, everything stronger than X-rays. You need a lot of dense material to stop them. Some elements, when absorbing gamma photons, will emit a free neutron.

Slow neutrons get absorbed by atoms; fast neutrons knock atoms apart. Either of these can produce unstable isotopes that may themselves emit radiation.

Another famous radioactive lens is the Kodak Aero Ekar 7"/2.5. The "aero" in the name refers to the design purpose of the lens: aerial photography.

The fact that the lens is radioactive is not particularly interesting. However, it is an incredibly fast lens for large format: on 4x5 film, f/4.5 produces a similar depth of field to f/1.2 on a 135* or equivalent digital camera. The 7" Aero Ektar produces a depth of field similar to f/.85 on 135.

Now, mounting one of these lenses to a rangefinder camera (typically a Graflex Pacemaker Speed Graphic press camera from the 1940s) for use in the field is a chore, and results in a very heavy and rather clumsy-to-use piece of kit.

David Burnett goes through the trouble, and the results [0] [1] are rather spectacular.

*135 is the normal "35mm" film format, the same size as "full frame" digital.



And thanks to Burnett, the prices these lenses go for has gone up quite a bit.

There's plenty of radioactive lenses out there. Many Takumars and Super Takumars for instance.


When I first heard the term "radioactive glass" I thought this was just a term used to refer to very sharp lenses. Boy was I wrong. :)

Please note that this particular geiger counter does not actually measure alpha radiation. Alpha radiation is the biggest concern with these lenses.

Is it though? Alpha radiation is nearly trivial to block, clothes or just a couple centimeters of air is enough to block most alpha radiation and it's not particularly damaging even if you are exposed to it. The camera body is going to block 99+% of the alpha emissions so even holding it up to your face isn't going to pose a big risk.

> Since the decay chain of thorium consists exclusively of alpha and beta particle emission, neither of which is very penetrating, the closed shutter protects the film from the radiation from the rear of the lens.

This isn't true. Thallium 208, for example, is on the decay chain. Yeah, it beta decays to lead, but it also emits a 2.6 MeV gamma ray in the process [1]. That's one of the highest energy gamma rays you typically see from radioactive decay, and it's highly penetrating. That's one example, and there's also lead 212 and actinium 228, which also peaks in the 100s of keVs.

[1] See the "gammas" part of http://nucleardata.nuclear.lu.se/toi/nuclide.asp?iZA=810208

Am I reading it wrong?

Tl208 to Pb is beta decay, there are no gamma particles involved. I would think the 2.6 MeV energy listed is for the electron emitted.

You're not reading it completely wrong. There is a beta decay.

Look at the last page of this table: http://www.nucleide.org/DDEP_WG/Nuclides/Tl-208_tables.pdf

The green arrows are possible beta decays to lead 208. But 208Tl never decays to the nuclear ground state of lead 208. And so after the beta decay, there are gamma rays (the black arrows) as the nucleus relaxes from whatever excited state it ended up in.

I wonder if there were any environmental or health issues associated with the manufacture and finishing/grinding of these lenses. The EU forced camera manufacturers to stop using leaded glass a while back because of worker's health.

The article mentions another interesting post on the site, which indicates that there is apparently a slight diurnal variation in background radiation levels: https://www.fourmilab.ch/documents/cosmic/

There is some good data, but no conclusion on why this would be the case. Anybody know of any followups that explain this effect?

Purely a guess on my part, but I'd wonder if it corresponds to whether your location on Earth is pointing towards or away the galactic core.

Does anyone have some example images taken with this kind of lens? I'm curious what the result would look like.

This blog post has some sample photos with a collapsible 5cm f2 summicron that looks like it's from the same era:


I have some lenses (Nikon 35mm and Pentax 55mm) that are identical other than having single-coating (thoriated glass) or multi-coating (non-thoriated glass).

The multi-coated versions all have a little bit more saturated colors. Other than that I have never noticed a difference.

I have a Nikkor 35mm f/1.4 with significant yellowing from the thoriated element. Apart from the yellow cast there’s no effect:


Why isn't there any mentioning of radiation exposure effects on film? Or are they so insignificant that it's not worth mentioning?

There is (in the paragraph after the last picture):

> First of all the most commonly used films in the early 1950s were slower (less sensitive) than modern emulsions, and consequently less prone to fogging due to radiation. Second, all Leica rangefinder cameras use a focal-plane shutter, which means the film behind the lens is shielded from the radiation it emits except for the instant the shutter is open when making an exposure, which would produce negligible fogging. Since the decay chain of thorium consists exclusively of alpha and beta particle emission, neither of which is very penetrating, the closed shutter protects the film from the radiation from the rear of the lens.

It's in the article. Search for film fogging. It's not significant.

The curtain alone would be enough to stop the alpha and beta particles, the mirror might cut down on some of the gamma, thorium dioxide is qouted as being 'no signficant source of gamma radiation' though so not a problem.

Its mentioned. According to the article, film was slower then too, so the effect was lesser then.

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