Lanthanum harmful to Image Sensor?

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mark-b

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Knowing well about lanthanum glass' radiation levels to be slightly double that of background radiation, I'm wondering if a particle hits the sensor, would it harm the sensor at all?
 
There is a metal shutter in front of the sensor so I think that the exposure time of the sensor is very short (if the radiation has an effect at all).
 
maddoc said:
There is a metal shutter in front of the sensor so I think that the exposure time of the sensor is very short (if the radiation has an effect at all).
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I don't think the shutter would do much to stop the radiation!

But I wouldn't worry about it...
 
gdi said:
But the glass contains thorium...
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I believe that's the general misconception; most of the "well-known" radioactive glasses used in lenses are actually doped with thorium, as gdi points out, not lanthanum, in order to tailor their indices of refraction.

One of the better (and more complete) explanations of this use of thoriated glass is given here, by a fellow who is also a big fan of Kodak's Aero-Ektar lenses, which also contain heavily thoriated glass. Early Pentax Takumars also have this "feature".

For me, the most striking characteristic of these types of lenses is that they will yellow (and then brown) with age, as the energy of radioactive decay products causes defects in the optical glass.

To be sure, lanthanum oxide is also used to increase index of refraction, as in the Industar I-61 L/D, but the natural occurrence of radionuclides in the most common isotope is very low, something like 800 ppm.

As for damage to the sensor, radioactive 138La decays via beta decay, producing a 1MeV electron. This isn't really ionizing radiation, and much of the energy of the mode is probably dissipated within the lens itself, so the electron probably won't reach the sensor.

The real challenge with beta decay is something called "bremsstrahlung", for which a poor translation gives braking radiation; this mode produces soft X-rays from the deceleration of energetic electrons, and traditionally has greater penetration depth. The effects of this mode are less easily definable, but I'm almost certain they also pose no danger.

Thorium, on the other hand, is very different (and always radioactive), and decays via alpha decay. The alpha particle (a helium nucleus) has much more kinetic energy and will cause other issues by knocking into other subatomic particles. I'm not as familiar with the alpha decay process, but the above linked article on Aero-Ektars has a bit of information on that.

Okay, this post is already too long. Sorry.


Cheers,
--joe.
 
Alpha radiation has an extremely short radiation length, it will be stopped by the shutter or any other piece of metal (or even a sheet of paper).
 
philipp.leser said:
Alpha radiation has an extremely short radiation length, it will be stopped by the shutter or any other piece of metal (or even a sheet of paper).
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The article PlanetJoe refers to above indicates, I believe, that isotopes produced by the decay of thorium do emit gamma rays. Though I can't vouch for the author's credentials here is what he says....


Under myths..

false: Thorium decays to Lead by alpha and beta emission and these radiations are easily shielded and so the radiation is not dangerous. The statement that Thorium and its daughters decay via alpha and beta emissions is correct, but neglects the fact that many of the daughter isotopes are created in excited states. These excited isotopes quickly transition to their ground states by the emission of gamma-rays. The atomic electrons also readjust their states by the emission of X-rays. Blocking the X-rays and especially the gamma-rays require much more shielding than blocking alpha and beta particles.
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Gamma goes easily through a metal shutter. It needs a thick concrete wall or something like that to be stopped.

I have an Aero Ektar 178/2.5 that is metered 17 microSieverts/hour which is about 100x the background radiation, but the metering is done directly off the rear lense. Radiation is realative to second power of distance, so it will be much less already just a little behind.

I have no idea how would radiation affect an image sensor, but thorium is radioactive and I think it produces gamma which is not stopped easily.
 
Interesting and revealing article, by Michael S. Briggs. Thanks for that, Joe. Looks to me there's more damage to the glass (visible in the yellowing or browning of the glass) than even by the Brehmstrallung effect that produces x-rays, thereby potentially damaging the sensor.
 
Svitantti said:
I have an Aero Ektar 178/2.5 that is metered 17 microSieverts/hour which is about 100x the background radiation, but the metering is done directly off the rear lense. Radiation is realative to second power of distance, so it will be much less already just a little behind.
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In December 2005 I ran an informal test on two Ektars with a friend to see whether the radioactivity would do anything. We put a sheet of Forte 400 B/W sheet film in a changing bag and put two Ektars on top of it for 30 hours, a 178/f2.5 Aero Ektar (rear element down), and a 113/f4 Printing Ektar. The Printing Ektar produced an almost indiscernible gray haze, the Aero Ektar didn't leave any trace.

I wouldn't be too worried about this. Don't keep your lens in trouser pocket.

Thorium-232, which makes up practically all of the thorium found in nature, is radioactive alpha decays into radium-228. As the name says, decay from thorium follows the thorium series. Look at a nuclide chart to see how this works. All decay processes in the thorium series down to lead-208, which is stable, produce alpha or beta radiation, which is stopped by shutters (or by a few mm's worth of glass). (EDIT: The wording in the original posting was a bit misleading. Actually there is gamma radiation involved in getting energy out of the various decay steps - gamma radiation in the thorium chain is what Lise Meitner was working on before she discovered nuclear fission with Otto Hahn. It's still only part of the output spectrum of the decays involved, and given the low amount of thorium present in the first place presents no real danger.) Given that thorium was used only in traces in optical glass anyway, and that thorium-232 with its half-life of 14 billion years is practically a stable element, there is nothing to worry about. You expose your camera to more radiation if you keep it in the basement, or if you wear an old-style wristwatch while changing lenses, or if you take the camera out in the rain, or if you take it on a plane.
 
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Smoking a cigarette will expose you to a dose of 0.07 mSv. A transatlantic flight will expose you to radiation in the same order of magnitude.

The allowed dose in the EU is 1 mSv per year, 20 mSv per year for people working with radiation.

The Ektar that Svitantti has is rated 17 µSv per hour, i.e. 0.02 mSv. So three hours of handling that lens is equivalent to smoking one cigarette.

I assume that other lenses of that era have similar ratings. This basically means that I wouldn't sleep with the lenses underneath my pillow, or lick them repeatedly, but the normal handling which happens when using them won't be harmful.

(I pulled the numbers from the German Wikipedia's "Strahlenbelastung" article. All numbers are equivalent doses, which already take into account that alpha radiation is more harmful than e.g. gamma rays).

Regards,
Philipp
 
Sensors are not affected by radiation the way film is. Film might be damaged by basic X-rays and radioactivity, sensors not. Cosmic radiation may in theory do some harm, but is of a much higher energy level than the radiation we are talking about here. The warning in the M8 manual about possible sensor damage on extended flights ( presumably transpolar too) refers to this, but is more legalistic than factual. There is not much one can do about it anyway, except using a camera case made of concrete a few meters thick, of course.
 
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I rest easier if that is the case :) Wouldn't hurt to shoot for a day with a lanthanum/thorium type lens, but I'll surely take it off when I'm done :D
 
jaapv said:
...Cosmic radiation may in theory do some harm, but is of a much higher energy level than the radiation we are talking about here.
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This is absolutely true, and important to point out. The limited amount of exposure (no pun intended) I've had to sensor design in space instruments has led me to believe that the practice of protecting charge-coupled devices - in our case, CMOS image sensors - from radiation is sort of a black art. In some cases exposure to ionizing radiation in the form of a cosmic ray might damage a single pixel permanently, but in most cases designers are really more worried about background noise and the resulting S/N ratio in the sensor.

In the latter case, shielding with aluminum is often indicated, but the weird nature of bremsstrahlung being what it is, thicker isn't always better, especially if the "cosmic ray" is a charged particle (in, say, the van Allen belts). Thin layers of tantalum and such are popular shielding, I think, for this application.

I'm a little fuzzy on the details, but I'm sure someone here knows a bunch more about this.


Cheers,
--joe.
 
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