Is the FED-5 radioactive?

In the spring of 2001 when I was still in high school, my physics class had a radiation workshop. A guy from an Amsterdam university came by to help us do all kinds of junior physics tests about radioactive contamination, X-rays, detection, and other related subjects.

A couple of days before the workshop I remembered reading something on the Internet about the FED-5 and its lens being slightly radioactive (seems to be an eternal debate), so like a good komsomolets, I decided to take mine along to see if I could conduct some experiments to settle this once and for all. As the Russians say, doveryai no proveryai, trust but verify...

There are basically two reasons why the FED-5 and its lens, the Industar-61L/D, could be radioactive. One is that the Industar contains potentially radioactive lanthane glass. The second is that both were manufactured in the Ukraine, meaning that they could bear traces of the 1986 nuclear disaster in Chernobyl. (Admittedly a long shot, but you never know.) [This is totally bullshit, but what did I know. —Alfred, now studying at a Technical University, March 2006.]

The rare earth element lanthane, or La as it's known in the table of the elements, has atomic weight 57 and has two isotopes. One is non-radioactive and dominates 99,92 percent of the time. The other is the radioactive La-138 that comprises the leftover 0,08 percent. La-138 is a Beta-minus radiator, meaning that it emits electrons with a waylength (in air) of about four feet. Its level of radioactivity is very low, because its half-life time is in the oprder of 110 billion years.

At the workshop, I asked if I could do some experiments on my funny little potentially radioactive Ukranian camera. The guy in charge gave me the green light (in fact he was compelled to watch), so I got out my FED and went to work.

First I measured the background radiation, the ever-present radiation that comes either from radioactive materials around us or from cosmic particles. To measure its intensity I used a Geiger tube and an electronic counter. I found that the background radiation was in the order of eighteen to twenty ionisations per ten seconds. (The number of ionisations in the Geiger tube says something about the number of particles emitted on account of radioactivity. The unit of decay is the becquerel, which is defined as emitted particles per second.)

Then I placed the Geiger tube against the Industar-61L/D's glass surface and started the clock. The value I got was about twenty-five to twenty-eight ionisations per ten seconds. After subtracting the background radiation, that means the Industar-61L/D's radiation alone is responsible for around six to eight ionisations per ten seconds.

What does that mean? Most importantly, that the radiation coming from the Industar and the FED body is much less than the background radiation. That means that anybody who isn't affected by the three times stronger background radiation, won't be affected by that of the FED-5 and its lens. Also, because I measured the ionisations directly against the glass surface of the lens, I got a (much) higher reading than I would have gotten if I had measured on the back of the camera.

Concluding: as you'll sometimes read, the Industar-61L/D is indeed radioactive, but its radiation is already hard to make out against the background radiation, and comes nowhere near the levels you absorb when sunbathing or flying. So for all practical uses, the FED-5 with Industar-61L/D is perfectly safe.

Now there's a surprise...