We've been having some pretty good success with fusors lately, and the question of are we getting hammered too badly by fast neutrons came up. I'm starting a stub here, and I am hoping that someone will come along on this (and other threads) here and fill in perhaps with some unit conversions -- numbers, rads, rems, sieverts, and all that kind of thing. I've read up on most of it, and the "biological relative effectiveness" of this and that, but it's daunting to someone who doesn't want to make a life work of it, and just wants to be reasonably safe while working this stuff.
At any rate, the book Fast Neutron Physics, Part 1 (1960) has a couple chapters on fast neutron biological effects I will share here, as most aren't likely to get the two volume set for $250 -- used.
It wouldn't surprise me if the dosage recommendations and limits had been tightened since that book was published, but I will have to go with data I have.
After studying data from actual injuries in accidents, the bombs in Japan, and also people who either had no troubles or non severe troubles, they came up with a number for 40 hr work weeks for 40 years with no observed troubles from exposure which was that 60 neutrons per sq centimeter per second was tolerable and didn't give eye cataracts over that time span, so it seems a usable number. They separated this data from data that also involved other rad exposure. They wind up recommending a lower number of 20 n/sec/sq cm for long term daily dosages, but note that no symptoms occur in their studies below 60 or more. Unfortunately, they flip units from rems to rads to....you name it throughout, and no conversions are given hat would make modern gear more useful in measureing. They are assuming RBE of 10 for fast neutrons (eg for the same energy deposited, they're ten times worse than gammas for us biological critters).
So, I worked that backwards to a fusor making 1 million neutrons per second -- how far away from the grid would you have to be to be safe in that situation if you were going to do it all day every day. The answer really surprised (and relieved) me. FWIW, elsewhere I worked out the total fusion energy from 1m n/s fusion rate, and it's about 1.1 uW -- not much. Most of the radiation from a fusor is X rays, mostly those of power supply volts (400 watt DC input vs 1.1 uW fusion output -- makes sense). The tank does get real hot, so a lot of that just goes to heat one way or another.
Ok, so assuming we have a point source and the neutrons are isotropic (have to begin somewhere) then what radius sphere do you have to have for the neutrons per second on the surface to be 60 n/s? The answer was a pleasent surprise. I worked this by dividing 1e6 by 60, which is 16,666.66....that's kind of the ratio we need to reduce by to get down to 60 n/s/sq-cm.
A sphere with 16,666 sq cm area is about 35-36 cm in radius, so I'd been far enough away the entire time, in fact perhaps never even closer than twice that during a run, as this is happening at the back end of a large tank I sit in front of. My face is perhaps 3 feet from the grid when I peer in through the lead glass window.
Now, during that same time, a geiger counter (not known for hyper sensitivity to X rays) that reads 42 cpm on my lab background, reads up to about 1000 cpm, or 10-20 times background, right between me and the lashup. Cut that ratio in half to compare to the background that existed when we were growing up from nuclear weapons tests, but still -- that's getting to a point where I worry a little bit. This is with my tank coated with 2-3 mm of lead all over, and 1/4" lead back near "the action". Without that the numbers would be much higher (they were). And at this point I suspect that a bunch of this is scatter from a couple of leaks near the "business end" where we put the neutron oven, and X rays and gammas that get out then scatter off the ceiling and so on to get into the geiger counter, so obviously I have more work to do there to be some sort of "safe". We plan to put a gamma spec head around this to see how much of that is the rare reaction that makes the 16 mev gammas, which would be one heck of a job to get stopped. My floor wouldn't support the lead needed to do that for the large tank, I'd have to go to a much smaller one to keep it down in the sub-ton range at the thickness required.
Now, if anyone knows how to change Sieverts into neutron/second counts, let me know so I can back check this with a BTI calibrated in Sieverts (actually micro Sieverts). The BTI's only respond to "fast" neutrons, which from my other readings are the most "biologically effective" and it seems to say that the 2.5 mev ones we make are about the worst there is -- perhaps 5-10 times worse than thermal ones. As the RBE goes down with neutron energy (so it seems) then perhaps a safety nut would want to use some moderator around the tank?
This isn't much concern right now, it seems, as it seems we are well under the doses that one worries about, or did in 1960, and we don't do this 8 hours a day. But as we do better (we're approaching 10 m neuts/second) it will become an issue it would seems, so it's a topic of some interest here.
Note, our Henny Penny scintillator survey meter goes absolutely nuts when the fusor is on, even at some distance. I think it must be seeing and counting some awful low energy stuff to do that -- it reads a couple k counts second off a feista ware plate. This is making BillF worry, even though it also does that inside a car simply driving past a shale outcropping...Maybe I should take it away from him? I'm kind of willing to trust a new 2" pancake geiger to tell me if anything is there that will hurt me. When we bought it from Geo, it came with a cal sheet for X rays so you could test in X ray facilities, so it has a known sensitivity to the ones of energies that can do bad things to you. Maybe I can find some old CRT TV's to let Bill put Henny in front of and watch it go nuts there too -- if that would kill you, we'd all be dead! I suspect, but haven't yet proven, that it responds all the way down to single digit kv X ray energies, and as a counter cannot tell the difference between those and the real zingers.