Repro-Claron f/9 210 mm

[Ole Tjugen]

It is true that natural thorium, Th-238, is an alpha emitter, but Th-238 decays into a series of radioactive isotopes, which together emit a mixture of radiations, including gamma-rays. The Thallium-208 daughter emits a 2.6 MeV gamma-ray. The gamma-rays are much more penetrating the the alpha particles. One might as well minimize the radiation dose to people by keeping lenses with Thorium glass at a distance when not in use.

Natural Thorium is 232Th which decaus to Radium 228 by alpha emission. Since the half life of 232Th is 1.405×10^10 years and that of 228Ra is 6.7 years, we can safely assume there is no Radium in the glass. The next step is Actinium, 228Ac by beta emission. And so on and on down to lead...

But since the parent isotope, 232Th, is the most stable isotope in the chain, the vast majority of the radioactivity will be from that first step - and the radiaoctivity will decrease over time. Yes, Thallium 208 is formed in one of the two possible paths just before the end, but all that means is that you shouldn't sleep with the lens under your pillow - or crush it and inhale the dust.

To the best of my knowledge there is no such isotope as Thorium 238.

[Jiri Vasina]

I'll take it from the last:

Ole, there is a Thorium 238 isotope, but the only info I found about it is that it has a half-life of 9.4minutes. Not it's decay chain. (wikipedia).

Further calculations might be wrong (I might have a mistake somewhere), but I believe they are not fundamentally wrong:

The amount of radioactivity in thorium 232 is negligible - if it has a halflife on the order of 10^10 years, that means that in 1kg of pure Thorium 232 one radioactive decay every 1010 minutes - that is almost every 17hours between that. At that moment a Radium 228 is created, which is a beta emitter with half-life of 6.7 years - again, that means that every minute it has a 0.0000197% chance of decaying - a chance you would in other situations consider negligible.

And that is only speaking of time. If you also consider the aspect of space and all the possible directions the generated particle can travel in, the chance of the particle hitting your body is even less (by orders).

The long half-life of the mother isotope also means (and short half-life of the dangerous isotopes like all the beta emitters (there are no gama) 228Ra, 228Ac), that the equilibrium state achieved is one with very low amount of those dangerous particles. And the resulting radioactivity is very low.

I hope the above is at least a bit clear.

(I think that a chance of being asked for a date with the most-beautiful and most-intelligent lady you have met in the past year - and asked by her - is more likely than waiting for this single radioactive decay chain to hit you with any significant effect).

Tomorrow I'll take my 305mm Repro-Claron (S/N 8,xxx,xxx) to work and measure if any radiation is detectable (not-alpha. Have no means to detect that).

[Arne Croell]

A 355mm Repro-Claron I own (serial no. 85xxxxx) is good for about 9 µSv/h (micro-Sievert/hour) of just gamma radiation next to the front or back element; background radiation here is about 0.15µSv/h, so its about 60x higher than background.

[Jiri Vasina]

So I must have forgotten something important

That's yet more reason for me to take my 305mm Repro-Claron for a measurement tomorrow...

[Ole Tjugen]

It's possible that the Thorium in the glass is about as pure as that in a sample of Thorite I measured at one time - 0.8 millisievert/hour is quite a bit above "background". It's even far above the background where I found it!

[Greg Lockrey]

[Michael S. Briggs]

Natural Thorium is 232Th which decaus to Radium 228 by alpha emission. Since the half life of 232Th is 1.405×10^10 years and that of 228Ra is 6.7 years, we can safely assume there is no Radium in the glass. The next step is Actinium, 228Ac by beta emission. And so on and on down to lead...

But since the parent isotope, 232Th, is the most stable isotope in the chain, the vast majority of the radioactivity will be from that first step - and the radiaoctivity will decrease over time. Yes, Thallium 208 is formed in one of the two possible paths just before the end, but all that means is that you shouldn't sleep with the lens under your pillow - or crush it and inhale the dust.

To the best of my knowledge there is no such isotope as Thorium 238.

Yes, I mis-typed when I wrote 238 instead of 232.

But it isn't safe to assume that there is no radium in the glass or that the vast majority of the radioactivity comes from the first step. Because of the long half life of Th-232, the amount of Th-232 in the glass is almost constant and it is producing Ra-228 at an almost constant rate. Ra-228 has a half life of about 6 years, much shorter than the age of any lens with thorium glass. This means that there has been enough time for the Ra-228 content to reach its equilibrium value, which is not zero. In fact, all of the radioactive daughters in the decay chain have short half lives compared to the age of these lenses, so they have all reached their equilibrium concentrations, all the way down to the final radioactive daughter, Tl-208. Tl-208 is the isotope that emits the 2.6 MeV gamma-ray. If you don't believe this reasoning, take a gamma-ray spectrum of a lens with thorium glass, as I have -- you will observe the 2.6 MeV line.

[Michael S. Briggs]

I'll take it from the last:

Ole, there is a Thorium 238 isotope, but the only info I found about it is that it has a half-life of 9.4minutes. Not it's decay chain. (wikipedia).

Further calculations might be wrong (I might have a mistake somewhere), but I believe they are not fundamentally wrong:

The amount of radioactivity in thorium 232 is negligible - if it has a halflife on the order of 10^10 years, that means that in 1kg of pure Thorium 232 one radioactive decay every 1010 minutes - that is almost every 17hours between that. At that moment a Radium 228 is created, which is a beta emitter with half-life of 6.7 years - again, that means that every minute it has a 0.0000197% chance of decaying - a chance you would in other situations consider negligible.

And that is only speaking of time. If you also consider the aspect of space and all the possible directions the generated particle can travel in, the chance of the particle hitting your body is even less (by orders).

The long half-life of the mother isotope also means (and short half-life of the dangerous isotopes like all the beta emitters (there are no gama) 228Ra, 228Ac), that the equilibrium state achieved is one with very low amount of those dangerous particles. And the resulting radioactivity is very low.



Tomorrow I'll take my 305mm Repro-Claron (S/N 8,xxx,xxx) to work and measure if any radiation is detectable (not-alpha. Have no means to detect that).

I have measured many lenses with thorium glass. Over a 2 inch diameter Geiger detector the count rate is typically at least thousands per second, not approximately one per day.

Yes, the Wikipedia article lists only alpha and beta decay modes for the Thorium series. But that leaves some things out -- some of the resulting nuclei are created in excited states that emit gamma rays to reach their ground state. Also the electrons of the new atom will frequently reach their ground state by emitting x-rays. If you take a x-ray / gamma-ray spectrum, you will observe many lines.

[Jiri Vasina]

After sleeping relatively well, I also know that I have forgotten the gamma rays. I know that there are (almost) no natural pure alfa- or beta-only emitters, everyone of them also emits gamma. And I have forgotten that yesterday evening.

[Jiri Vasina]

I have just measured my 305mm Repro-Claron and at the front and rear of the glass (less than 1cm from the surface), the flow is at 9µSv/h (as measured by Arne).

But at 10cm from the glass, it has already fallen to 0.9-1µSv/h. And when measured on the sides of the barrel (where the barrel serves as shield) touching the lens barrel the flow is at 2-2.5µSv/h.

When I took the lens apart, the radioactive one is the frontmost glass in the front element, and the rear-most in the rear element (both the entrance and exit ones).

I don't have a spectrometer at hand, so I believe you with the spectral characteristics.