Stopping X and Gamma rays

[Doug Coulter]

I've been getting some static about "why are you taking so long doing all this remotely when you could just use a buncha lead and be safe?".
Answer - because the amount of lead my floor will hold up is useless against high energy gammas, and of course, neutrons - remember atoms are mostly empty space, and it's like spraying neutrons between widely spaced bowling balls - little neutron energy is lost, they are just scattered a little - for that, low Z stuff works better.
Anyway, I gathered up a little data so I have something to point the static-givers at.

Note log scale - good for hospital energy X rays, horrible for all else.

So, what's that? 5 or so orders magnitude drop between low energy and 2 Mev or so? So it's not exactly linear (would be if that line were horizontal) with energy, right?
Here's a paper that goes into more detail why:

leadgamma.pdf

Paper explaining the above

(744.11 KiB) Downloaded 756 times

I don't have anything handy just now on neutron cross sections for lead and fast neutrons, but it's low for all speeds. Hydrogen slows them down the best, and various things like to "eat" the slow ones, most often giving off a capture gamma ray, and often that's up there in the half megavolt range which is getting hard to stop with lead.

Distance is my friend!

[Donovan Ready]

I wonder, (but not enough to research it myself), what the energy limit (or speed) of incoming neutrons on a boron absorber would break down. What other light, neutron-hungry metals are available at those energies?

I will do a little research, damn it. I can find tables of neutron cross sections in many places, I'm sure. Sorry to be lazy.

[Doug Coulter]

I should do a thread on that too - neutrons are their own ball of wax.
Almost everything that "eats" neutrons has a 1/V cross section - the slower, the better.
Almost everything gives off what's called a "capture gamma" or has some other subsequent decay (usually beta). It's that capture gamma that can be really problematic, as it tends to be high energy - in the region that lead (and for that matter everything else) really don't work well. A neutron decay (into hydrogen atom) and some other things are in the 2 megavolt range for that gamma.
Boron, He3, cadmium, lithium (particular isotopes of each) - less but still bad.

Light elements slow neutrons better - hydrogen is best (usually handled as water or plastic or oil). Remembering atoms are mostly empty - lead looks like a widely spaced array of bowling balls to BB sized neutrons which simply bounce off if they even hit...and retain most of their initial energy. When a neutron hits a proton, it can leave up to half its energy behind...

There are a couple of exceptions to the 1/V rule, but not many (u-238 is one) that are talked about much.

[johnf]

Surprisingly concrete is very good at absorbing x-ray / gammas and does a fine job of thermalising neutrons
Our Accelerator @ work has meter thick walls around it to contain the problem children of the accel beam tube, slits, bending magnet.
So all is good in our experimental area until you use deuterium when you have to be super careful not to bombard a light element like carbon with the beam
If you do you get a neutron fest in the lab -- so we normally limit our deuteron energy to 0.9 MeV to keep things simple and safe

mind you meter thick concrete walls would not cheap

[Doug Coulter]

Yep, all the big boys use concrete and there's a lot to be said for it - even with neutrons, as it's always wet so moderates them. As you say, not cheap, it's more like "build a building for this" or have one heck of one already that will hold that kind of thing up.

Which is why I'm using air - the price is right - over an admittedly much longer distance. Once you get to the thickness of 'crete you need to be good - you're forced into remote control anyway, and at that point the difference between a few feet (or meters) and a much longer air gap doesn't mean that much. The "just put it in a shed out back" technique works well here where land is cheap (and already bought).

Lucky for us with this high energy stuff we actually do better than 1/R2 - because X or gamma rays do lose energy interacting with whatever matter - be it air or concrete, as do neutrons and their resulting decay or capture gammas.

[APynckel]

There's something to be said about lining the exterior of a reactor with U-238 to use it's capture / enrichment cross section to your advantage.

[Doug Coulter]

Yup, if you want to breed Pu and suchlike. I hear 238 has a big cross section around 28 eV for neutrons, which is what requires the split up of fuel and moderator - it eats the neutrons needed to sustain a chain reaction if you can't get them slowed down below that before they encounter more fuel.
Our neutrons are just at the speed that if captured in 238 would give us fission (so the books say).
Not sure our friendly government would like that, or me like the resulting mess of hot byproducts...
I know many fusion project thought of driving fission this way more or less...I'm not yet at the point of assuming I'll fail without that, but...

That said, I hear that once they made counterweights for airplane control surfaces out of 238. Know where I can find some?