Mossbauer SpectroscopyB. S. VINEETH (Roll# 120121007), B.Tech., IIT Guwahati

Introduction

• Versatile technique based on Mossbauer Effect that is widely used, even now, from the time it was discovered in 1957, in many areas:

• Physics, Chemistry, Biology, Archaeology,

Geology, Material Science, Metallurgy, …

• Rudolf L. Mossbauer

• NOBEL PRIZE Physics 1961 within 5 years

of its discovery!!

Nobel Prize, Physics (1961)

• Zeitschrift fur Physik 151, Issue 2 p. 124-143 (1958)

What is it good for?

• You name it!

• Very precise information on the: chemical, structural, magneticproperties of a material.

• Technique at the interface of Solid State Physics and Nuclear Physics.

Basic Principle

• Nuclear Energy Levels change (shift or split) because of interaction of the radioactive nucleus with the surrounding solidin which it is embedded.

Exclusively used by NASA• Mossbauer spectroscopy is used by NASA to study the composition, abundance, magnetic properties of iron-bearing materials in the outer space.

• MARS EXPLORATION ROVERS:

• It is ONE of FOUR facilities mounted

at the end of the ROVER ARM!

Measurements are taken by placing the Rover Arm head directly against a rock or soil sample.

Used Elements

• Most used: Fe-57

Every Iron containing material has approx. 98% of Fe-56 and 2% of Fe-57.

• I-129, Sn-119, Sb-121 also are used.

• Let us take the example of Fe-57 to demonstrate the concept

Isolated Fe-57 Nucleus

We expect the Gamma-ray emitted to be of 14.4 keV. (We are only interested till the

first excited state.)

Fe-57 in ferromagnetic surrounding

• Gamma rays of nearly 14.4 keV but

with differences in the order of

10-8 eV due to splitting+shifting.

Mossbauer spectroscopy exploits this

faint spectrum in the Gamma radiation

to assess the material environment.

Cubic Structure and Ferromagnetic

Fe 57 in stainless steel solid

• Cubic, No ferromagnetism here

• So, no splitting of levels

• But, due to the interaction of nucleus with the electron density (mainly s-electrons density), the energy levels only SHIFT in the order of 10-8 eV.

Fe-57 in Olivine solid (FeSiO4)

• Non-cubic crystal arrangement

• Non-ferromagnetic

• Ground level: As it is (no splitting)

• First excited state: mI = -3/2, +3/2 only

Construction

Source: Co-57 in Rhodium

Rh solid which is cubic and

non-ferromagnetic

So, only shifting of levels in

source.

Absorber: Is the sample to be

analyzed.

Setup Block Diagram

Working Theory and Physics

• As the Source is vibrating, the

absorber sees the Gamma rays

with a DOPPLER SHIFT.

So, if Es = E2 – E1, the absorber

sees it as:

E = Es (1 + v/c) (Doppler Shift)

(v lies from –vmax to +vmax)

Absorber: Simple Case

• Assume, the absorber is

cubic and non-ferromagnetic.

Then for Resonance, we need:

“Mossbauer Spectrum”

• So, at these particular velocities,

the detector takes a Dip as most

of the Gamma rays get absorbed

and don’t pass through.

Plot velocities versus detector count: “Spectrum”

If absorber has Olivine type surrounding

If absorber has ferromagnetic surr.

Spectrum if ferromagnetic absorber

A Serious Problem!!!

• Apart from the around 10-8 eV energy shift due to surrounding environment, there is an additional extremely significant (1,00,000 times) correction of around 1 milli-eV which is due to:

• NUCLEAR RECOIL EFFECT

Therefore, energy of emitted

Gamma ray becomes:

(E2 – E1) – ER

Nucleus recoils in the opposite

Direction while emission

…

• Similarly, when the absorber nucleus absorbs it, it recoils in the direction of Gamma-ray and therefore for resonance to take place (without Doppler effect) the energy required is:

• (E’2 – E’1) + ER

• Unfortunately, to cover this gap due to recoil, one needs vibrating the source with huge velocities. Then, how can we do Mossbauer spectroscopy??

Mossbauer Effect to our rescue

• It is a QM-cal phenomena that if you have a Nucleus with energy levels embedded in a Crystal Solid which itself has Crystal Energy levels, both of them couple and so:

• The Crystal has to Recoil whenever the Nucleus absorbs or emits the Gamma rays.

• But, if the Crystal Recoil Energy is less than the first-excited energy of the Nucleus, then the whole Recoil process will NOT occur, and the Gamma rays are emitted with exact energy level difference!!!

Interpreting Results:

Limitations

• The material to be analyzed must be composed of Iron or similar materials used in Mossbauer

• Vast majority of rock-forming minerals on Earth contain Fe 2+ in octahedral structure, and all of them have very similar Mossbauer Spectrum. (hard to distinguish)

References

• [1] R.L. Mossbauer, Zeitschrift fur Physik 151, Issue 2 p. 124-143

• [2] H.C. Verma, Concepts of Physics, Bharati Bhawan (1999)

• [3] E.N. Kaufmann, Characterization of Materials, Wiley (2003)

• [4] Obvious online sources (Wikipedia, etc.)

Thank You

Term paper submitted to Prof. Giri (Physics, IITG) by

B.S.VINEETH (Roll# 120121007).