Originally posted by mccririck It's well above background! Well above!
At what distance ? Less than 1 inch ? It's ok to hear all that popping at contact distance. At 2 inches it will drop dramatically, by the square of the distance, and from a few inches it will be at the background level.
Again, you have the distance of more than one inch from the lens back to you, and you have the metal camera back. You won't be harmed.
If this lens was so dangerous like people like to say, it would completely fog film ! I dare someone to show (and prove) that a film was fogged this way.
Lots of things in your house are also radioactive like concrete, cement, granite... You don't need to worry.
Digging here in my old papers I found some actual scientific studies about this:
Taylor et al. (1983) measured the absorbed dose rate at the back of a camera and the thorium content of the lens. The thorium in the lens was estimated to be 13 kBq (0.36 Ci). Using the 3–290 methodology described in Appendix A.4 for sources close to the body, the dose rate at 10 cm depth in the body was determined to be 1×10-4 mSv/h (0.01 mrem/h).
A serious outdoor photographer is assumed to spend 30 days/yr in the field (average photographers-10 days/yr) and to carry a camera next to the body for 6 hours per day during that time. This exposure time should be conservative for most photographers. Based on the assumed exposure time and the absorbed dose rate, the annual EDE would be 0.02 mSv (2 mrem). For an average photographer the EDE would be 0.007 mSv (0.7 mrem)
Note that 0.007 mSv is 0.2% of what you get annual from normal background radiation (3 mSv).
A more pertinent question might be what the dose rate to the eye is. Radiation exposure can lead to cataracts, and of course a camera lens is going to be very close to one's eye. From the same publication, they measured the dose rate at the surface of the camera lens to be 0.48 mrad/h, or about 5 micro-Sv/hr. The dose limit to the lens of the eye for members of the public is 15 mSv per year, so you would need to hold this lens against your eye for 3,000 hours to exceed that. With the lens attached to a camera, the dose rate dropped by a factor of 5 (due to blocking the electrons and alphas). At this level, one couldn't exceed the dose limit even if they continually held the camera to their eye for an entire year. Also note that dose limits to members of the public are already pretty conservative in terms of preventing effects.
So to summarize, there is almost no way to exceed the dose limits while using a camera of this type. Furthermore, the radiation you would receive is only a small fraction of the background radiation.
Here, another lecture from an old U.S. Army study (
http://www.irpa.net/irpa3/cdrom/VOL.3B/W3B_13.PDF)
I love this lecture. Read the introduction on page 883 (specially paragraph 4). Note the statement about the only potential problem is IF the ocular lens was highly thoriated.
Also take a look at the summary on the last page.
This page condensates some of the NRC conclusions about the same situation:
Thoriated Camera Lens (ca. 1970s)
According to the NRC the average background radiation dose in the US is about 360 mrem/yr, WAY MORE than you will ever receive from a thorium glass lens.
We live in a radioactive world, and radiation has always been all around us as a part of our natural environment. As explained above, the annual average dose per person from all sources is about 360 mrem, but it is not uncommon for any of us to receive more than that average dose in a given year (largely as a result of medical procedures). To find your personal annual radiation dose, use the interactive Personal Annual Radiation Dose Calculator or this printer friendly worksheet. (source: NRC)
There are plenty of studies with the same conclusions. If you like, read this enormous NUREG document:
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1717/nureg-1717.pdf (section 3.19)
Just for curiosity, there is a rule of thumb to estimate the beta range by energy and material density: For low density materials, it's about 2mm/MeV and for medium density materials, about 1mm/MeV. (for air, 3.5m/MeV ; glass 2.1mm ; aluminum 1.8mm ; copper 0.45mm, water 4.5mm)
A 5 MeV alpha is stopped by: 3.7 cm of air / 53 micrometers of paper / 45 micrometers of water and will not penetrate the skin.
The way
some people spread information is simlpy bad science, no scientific methodology and lots of misconceptions.
Please don't panic and enjoy your marvelous vintage lenses !