Repro-Claron f/9 210 mm

[Greg Lockrey]

OH NO!!! We're gonna die!

[Jiri Vasina]

No. I have just made further measurements.

It means that you should minimize your (tactile) contact with the lens - especially with the glass itself - to minimize irradiation of your hands.

When the lens is used, my 305 Claron is around 30cm from the ground glass, when composing, my eyes are around 20-30cm from the ground glass. My measured radioactive flow at 50cm from the lens (in clear air, without the intervening groundglass) is at the background rate 0.1-0.2µSv/h. So the dose to the eyes when used normally for normal composing/focusing time is negligible.

Also the flow at the side of the lens in 30cm distance is at the background rate (representing me standing next to the camera waiting for the "perfect light").

This means that in normal use the lens is not dangerous (because of radioactivity), but you should minimize your handling the lens, especially the glass itself.

[Greg Lockrey]

Whew... ! That was close.

[Arne Croell]

To keep things in perspective, the natural radiation at 33000ft is about 2-5µSv/h. So instead of flying from New York to LA you could fondle your Repro-Claron for about 1-2h instead to get the same dose. Also, since the extremities are the least susceptible parts of the body to radiation, normal handling is not really a problem. Just don't carry it in your pants pocket!
A more practical issue is that one should not leave these lenses close to film for a prolonged time, like in a backpack - that could result in fogging. I did some "autoradiographs" of Repro-Clarons on TMAX 100 a few years ago (I just placed the cells on a Readyload envelope), and 19 days gave considerable fog - you can see the result here: http://photo.net/photodb/photo.tcl?photo_id=1221738.

If you like your dosage a little higher, try a 300mm Apo-Lanthar, it comes in at 35µSv/h.

[Greg Lockrey]

Holy crap, Arne! I'll have to remember to keep it off of my lap.

[Steve Hamley]

Actually the radioactivity of radium daughter is equal to that of the parent in secular equilibrium, which takes about 7 half lives of the daughter, in the case of Ra-228, that's 6.7 x 7 or about 47 years. So if your Th-232 containing Apo-Lanthar (and not all of them did) is 47 years old, there's as much radium as thorium.

http://www.epa.gov/radiation/underst...uilibrium.html

if you're worried, I'll pay the freight to have all those Apo-Lanthars sent to me for safe storage.

Cheers,

Steve

[Michael S. Briggs]

Actually the radioactivity of radium daughter is equal to that of the parent in secular equilibrium, which takes about 7 half lives of the daughter, in the case of Ra-228, that's 6.7 x 7 or about 47 years. So if your Th-232 containing Apo-Lanthar (and not all of them did) is 47 years old, there's as much radium as thorium.

http://www.epa.gov/radiation/underst...uilibrium.html

This process of the daughters growing to their equilibrium concentration is what I was describing in my 04:23 message yesterday. The graph at the URL given by Steve makes it easier to understand -- the graph is the middle one -- "Radionuclide Has a Much Longer Half-Life" for Th-232 having a longer half-life than its first daughter Ra-228. The Table of Isotopes (http://ie.lbl.gov/toi/listnuc.asp?sq...28&A2=228&Z=88) gives the half life of Ra-228 as 5.75 years. It seems that the use of thorium glass in lenses ended circa 1980--1985. So the newest lens with thorium glass is 23 years / 4 half lives of Ra-228 old. From the graph on the EPA website, 7 half lives is required to reach full equilibrium, but 4 half lives is most of the way there. The other daughters have even shorter half lives (http://en.wikipedia.org/wiki/Decay_chain). So decay chains in the thorium glass of lenses have reached or nearly reached equilibrium all the way down to the final radioactive daughter.

[Greg Lockrey]

Actually the radioactivity of radium daughter is equal to that of the parent in secular equilibrium, which takes about 7 half lives of the daughter, in the case of Ra-228, that's 6.7 x 7 or about 47 years. So if your Th-232 containing Apo-Lanthar (and not all of them did) is 47 years old, there's as much radium as thorium.

http://www.epa.gov/radiation/underst...uilibrium.html

if you're worried, I'll pay the freight to have all those Apo-Lanthars sent to me for safe storage.

Cheers,

Steve

Right away, Steve.