Science/Engineering/Technical forums

Seems like you could use a lead shutter over scint plastic to make sure it was only reporting hi-energy gamma, with about the same tradeoff as a big tube.

To an extent, you can. The problems are that it's not deterministic in re how much energy a given amount of lead will absorb from a given photon. There are a bunch of different energy loss mechanisms in play there. Another thing one learns quick around a big source of X and gammas is that the air itself messes things up -- they scatter, just like light in smoke, losing some unknown amount of energy each time. This means a leak in the shielding on the back shows up as radiation all around the room -- including right in front of the shielding in the front -- all smeared out in energy in the bargain. Found that one out the hard way.


To simplify his discussion, Haliday here assumes that scattered energy isn't an issue. But with a detector out in the world, say hand held, (not his good-geometry example) it's a lot more complex. Charged particles also have some "straggling" issues but no where near as bad. You can sort of use foils of known stopping power as shutters and get a coarse spectrum that way, assuming you have a zero energy threshold detector behind them (there is no such thing). One thing he omitted above in his #1 is that a knocked out electron leaves an ion in the shield, and when another electron takes its place, you emit another ?-energy photon and start the process again inside the shield - and since that's a high-Z material, it could itself be a pretty high energy X ray again.

The upshot is that it's impossible to do a perfect job on everything with existing stuff and knowledge, so all these sorts of things have to be traded off somewhat. Many-many books exist on this subject for designs to handle special cases, since there is no way to do the most general case.

For fusors, you can get rid of all the power-supply energy X rays with just a 1/8" piece of lead. But that's not the whole story by far. Some of the reactions make photons up in the high megavolts, as does neutron absorption in things -- 2.2 mev for absorption in hydrogen in the moderator (or your body). Now you're talking inches thick lead, not fun to carry (or design a shield for full coverage for with no leaks, either). To merely stop the secondaries from cosmics it takes about 4" thick lead and no cracks or leaks, but of course the primaries zing right through that without noticing it much. Which is why they do things down in salt mines sometimes - that's what it takes to eat those primaries -- 100's of meters of dirt and rock.

Most things with decent neutron capture cross section then emit gammas. http://www.nndc.bnl.gov/capgam/indexbyn.html
Shielding those is quite a chore in a home lab where you can't just have recourse to 6 feet of concrete. It's easy to make it worse, health wise.