This topic has only been touched on before in the old BBS.
New data and info, albeit from an older source has come to light.
We all should, by now, know that the fast neutron is the most dangerous form of radiation known to man with an RBE (relative biological effectiveness) of 10 on a scale of 1 to 10. It turns out that neutrons in the 1-3mev range are the worst of all the fast neuts!! SO...... When we are doin' fusion we are kickin' out the badest of the bad. Again, somewhat old news. What would be nice to know is at what level of our fusion output we need to shield.
Nervous nellies, greenpeacers, and the current wisdom say that any exposure above background, no matter how insignificant, is bad.........very bad. People in th' biz, however, can't be so namby-pamby. They have to look at what might be termed a "safe dosage". Over the years, this has slid ever lower and lower for the sake of safety.
In the first volume of "Fast Neutron Physics", Pg 904, there is discussion involving the human carnage experienced with fast neutrons. It seems that some few poor bastards at Los Alamos in 1945 and then again in 1946 were actually handling fissile material when the masses went prompt critical! That's gotta' hurt. Anyway, the folks a mere 4-6 feet away not only didn't die but were only mildly ill. those at 10 feet or farther had no symptoms whatsoever. The neutron blast was on the order of 10e16 neuts emitted isotropically from the assembly before thermal expansion stoped the fast neutron process. This is fully 11 orders of magnitude greater than our best fusor run to date!
It seems that cataracts are one of the sad and rather immediate consequences of fast neutrons. A cataract safe continuous flux level of 60 fast nuet/sqcm/sec was established. That is continuous, day in and day out!!!
Let us figure what you might experience at the human head 1 meter from a 10e6 neut/sec isotropic fusor. With a point source assumed, no absorption in the air over the meter between you and the fusor, you might expect about 8 n/sqcm/s in the form of a fast flux. Even nervous nellies shouldn't worry about this. Besides, amateurs rarely operate for more than a minute or two near the limit of their capability. Again, virtually no exposure. Add to this the fact that no amateur has yet even hit the 10e6 neuts/sec in this example and you can see that no shielding is necessary for fast neutrons at the amateur level currently in progress.
Finally a bit of data about just how nasty these pesky little particles are.
1-2mev neuts have a LET of about 45kev/micron!!! Ouch!
There is so much absorption in tissue that half of the dose of any neutron radiation is delivered over the first 4 cm of tissue depth. At 8cm into your body, recoil protons and gamma make an equal contribution to your overall dose! Ouch again. Mercifully at 12cm and deeper almost all the dosage is due to gamma rays. So you see that fast neutrons are the gift that keeps on giving the deeper it bores into your watery massiveness. Like the energizer bunny, they keep on going and going and evolving and evolving.
Just remember that if you hit 10e7 neutrons/sec isotropic, it might be time to be making up sandbag baracades with parafin in the place of sand. Up to that point, just bask in the stellar glow of your smaller devices.
Richard Hull
Created on Thursday, March 08, 2001 4:41 PM EDT by Richard Hull