Saturday, April 26, 2008

Hello from Neutronworld.

So I’m at NIST, doing some scattering experiments at the National Center for Neutron Research this weekend. Please hold back your comments about what a party animal I am and how I live it up on the weekend.

**disclaimer: if you don't want to endure my lengthy description of Neutron scattering targeted to third grade teachers and musicians already familiar with x-ray scattering (perhaps that is a small demographic but whatever) please skip to the last paragraph.**

I actually think this Neutron stuff is pretty cool. Neutron scattering is similar to x-ray scattering, but the contrast mechanisms are different. When you shoot x-rays at a material, they scatter off of the atoms inside. The heavier the atom, the more the x-rays scatter (so P (atomic mass = 31) is a better scatterer than C (mass = 12), and H (mass = 1) is practically invisible). When you shoot neutrons at a material, they also scatter off the atoms inside. But the heavier atoms aren’t necessarily better than the light ones. For Neutrons, scattering is larger for atoms with a higher "scattering length" (I'll call it "b" for short), which doesn’t necessarily increase with atomic weight. For example, for C, b = 6.65, and for P, b = 5.13 .

There is a huge difference in scattering length between Hydrogen and Deuterium. Deuterium is Hydrogen with one extra neutron, so it's molar mass = 2. However, for D, b = 6.67, for H, b = -3.4. Big difference! Most molecules that have H in them can be made with D instead of H without really changing any other properties, so scientists can swap Ds for Hs in their molecules to get different scattering in the Neutron beam (so I don’t have to go into a whole shpiel about contrast matching, take my word for it that this is really really useful).

What all this boils down to is that you can do a very similar experiment with x-rays and neutrons and see totally different things. Below is an example of the difference between the contrast available from x-ray and neutron imaging, from UT Nuclear Engineering. "Neutron Imaging" is similar to "Neutron Scattering" in that you expose the material to neutrons, but the optics are different such that you get an image in real space. The contrast mechanisms are the same as with scattering, though, and you can see that each neutrons and x-rays allow you to see totally different things.

Here is an x-ray image of a camera.
The plastic parts of the camera are basically invisible (they're made up of light elements) and the metal parts are basically opaque (they're made of heavy elements).

Here is a neutron image of the same camera.
Now we can see the details in the plastic parts, including the film, because the metal parts are basically invisible. Isn't that cool?

They use neutron imaging a lot for fuel cell research. Inside a fuel cell there is water moving through channels in a plastic membrane, and the whole thing is enclosed in metal. Because of the metal casing, we can't use regular or x-ray imaging to see what's going on inside when the cell is running. But turns out Neutron imaging is perfect for this sort of thing, because the casing is basically invisible but the water shows up great. I would encourage you to read more just because I think it's such an elegant way to study fuel cells.

In case you are tired of my neutronbable, I will switch subjects entirely, to one less science-heavy. I think the picture on my NIST badge is one of the best taken of me in a really long time. Which is too bad because the badge is used for security purposes and belongs to the government, so I am quite sure it would be inappropriate for me to take a picture of it and post it here. Basically I am saying this great picture will be seen by no one, so it’s entirely useless to me. And you’ll just have to take it on faith that it actually exists.


  1. Ok, I feel like I'm missing something. Do I understand correctly that if the neutrons are scattered the image would appear darker ( or less invisable)on the resulting picture than if the neutrons are not scattered?

  2. So maybe I kind of brushed over the differences between scattering and imaging, but you're basically right. Where it probably got confusing is that you're not really "scattering" when taking an image - it's more like a regular camera but instead of light (photons) you're using neutrons.

    In both the scattering and imaging experiments, the contrast mechanism is the same. In the image, the material that interacts with the neutrons more will look darker. This material will also scatter more in a scattering experiment.