Mossbauer Spectroscopy

While waiting for my aged computer to boot-up on my return to work this morning, I was skimming through November’s Physics World magazine, and noted an obituary to Rudolf Mossbauer.

He is best known in the physics world for observing ‘resonance absorption’ of gamma rays, and then developing the technique of Mossbauer spectroscopy.

When a nucleus undergoes gamma decay, it emits a gamma ray, which is a high-energy electromagnetic wave. (Light, radio, microwaves are other examples of electromagnetic waves). What happens is that the nucleus moves from a high energy state to a lower energy state, and this difference in energy is carried away by the gamma ray.

Well, not quite. That’s because the nucleus will recoil when the gamma ray is emitted, like a gun will recoil when a bullet is fired. A small amount of the energy will be taken up by the recoil, so not quite all ends up in the gamma ray. This means that, if the gamma ray hits a similar nucleus in a low-energy state, it is not usual for the ray to excite the nucleus from the low-energy state back up to the high-energy state – ‘resonance absorption’.

What Mossbauer did was to eliminate the recoil by embedding the nucleus in a crystal. With no energy being taken away by recoiling nuclei, all is available to the gamma ray, and resonance absorption can occur.

If you’re not impressed by that clever bit of thinking, there’s more. One can alter the energy of the gamma ray with the Doppler effect by having a moving source of gamma radiation. If we move the source towards a sample of material, the energy of the gamma rays hitting the material will be increased slightly; conversely, if we move the source away they will be reduced slightly. Since the exact energy of photons that will be absorbed by a sample of material will depend upon the chemical environment of the nuclei in the sample, this provides a very clever spectroscopic way of probing the chemical environment of a sample.

We have a set-up in our lab – Unfortunately it hasn’t been used for a while since the cost of obtaining the Mossbauer sources (we use Cobalt-57 embedded into a crystal) is too great for it to be cost-effective for us. The source is mounted on a piston that can be driven at a very precise velocity towards a sample of material (that also contains Co-57 or Fe-57). By looking at the absorption at different source velocities we are effectively finding information out about the recoil of the Co/Fe-57 nuclei in the sample, which in turn tell us about the chemical environment of the Co/Fe-57. We don’t need to move the piston very quickly – just a few millimetres a second is sufficient to provide the range of photon energies we need.

A pretty clever technique, really, but one that isn’t greatly used today.

Rudolf Mossbauer received the Nobel Prize for Physics in 1961 for his work.

Leave a Reply