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Physics/Radiation question

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Hello Mr. Nelson, I think you have answered a question for me in the past, I have a radiation question. The short question is, can scrap metal or other sources have enough radiation to be harmful?  This is probably more of an OCD problem than an actual physical threat, but I am compelled to ask, and it even sounds wildly outlandish to me as I type it. I had a guy a couple years ago come test a 1940s safe I bought for radiation ( it was built in the same plant that they were making uranium slugs for the atomic bomb), and he brought a Geiger counter and tested negative for excess radiation. Stay with me on this - we recently had a windstorm and a dead spot appeared in the grass in our backyard - my assumption (obviously OCD driven) was the when he came over (2-1/2 years ago) that he must have brought some radioactive material with him from previous tests and as he exited his car the material sat there in the street and then with the storm blew in our backyard and killed the spot (about 18" in diameter). I was afraid that he may have picked up some contaminated particles somewhere and that they were deposited at my house, although he was a very smart individual and seemed to have a very good handle on the whole radiation topic. I guess the final question is, and I'm sure I'm asking the wrong question, would a tiny radioactive source like a tiny shaving from a metal scrapyard be cause for concern? Like I said, I have no reason to believe he had any radioactive exposure or material in his possession, it's more of a fear of radiation to me. I know this all sounds crazy, but the fear of the unknown factor can weigh heavy on me- Thanks.

Answer
In general, absolutely not.  There have been rare exceptions with people ignoring radioactive material signs (or not knowing what they are) and scavenging used medical treatment machines, but that's definitely NOT what you're talking about here at all.  You're talking about dead grass.  If he somehow "tracked in" radioactive particles 2+ years ago, you'd have noticed the effect right then next to wherever they were dropped and if that's just what he tracked in he'd have been dead since before he got to your house.

You'd have to get a very specific piece of metal, and at those levels of radioactivity you'd see it glow and it would definitely give off heat.  That would be telltale, it would be warm or hot to the touch.  You're talking about extreme levels of radioactivity here for killing plants (which are surprisingly radiation resistant in general).  You should probably take a course on radiation safety so that you can gain a sense of scale for the subject, to get a scientific handle on it.

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Dr. Stephen O. Nelson

Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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