Question: How exactly does the technique Mossbauer spectroscopy work?

Rowena Fletcher-Wood answered on 9 Mar 2014:

This is a really hard question, but I have tried my best to explain everything. Feel free to ask me more questions about it.

Mössbauer spectroscopy works by probing nuclear energy levels. Just like electrons are in shells, there are vibrational energy levels in the nucleus of an atom. How close or spaced out these are depends on the element and what’s happening round the nucleus: its chemistry. I do Mössbauer spectroscopy of iron.

My gamma rays are produced when radioactive cobalt decays to iron. 57-Co –> 57-Fe + gamma radiation. The gamma ray is fired at the sample, and hits more 57-Fe nuclei, which absorb the gamma energy. But the gamma energy they absorb is very slightly different, because although it is the same KIND of nucleus it is a different actual one with different chemistry. So there is a tiny energy difference.

You already know Newton’s 3rd law: for every action there is an equal and opposite reaction. When the gamma rays hits the nucleus it is moving fast, so the nucleus should jump or recoil to obey this law. But when the gamma energy is less than the distance between energy levels, it can’t jump. This happens in solids. The solid crystal absorbs the energy instead and “shrugs”.

To record a Mössbauer spectrum, the 57-Co is moved forwards and backwards at constant acceleration. The crystal “shrugs” change the speed a tiny bit relative to the speed of gamma rays (3 x 10^11 mm/s). These differences are measured as a pattern that can be interpreted to tell us what the chemistry going on is.