What is Moessbauer Spectroscopy?

Mössbauer spectroscopy, a high precision technique that can be used to provide precise information about the magnetic properties, the structure of materials and chemical compositions, has been widely used in many areas of science. It is especially useful in measuring the hyperfine effects, the tiny shifts in the energy levels of the nuclei due to their interactions with the environment. It is accurate enough to detect the gravitational red shift predicted by Einstein. It is widely used through many fields including nuclear physics, Solid state physics, chemistry and biotechnologies. The sample being tested can be metal, insulator or semiconductor, the format of the sample can be foil, thin film, powder or low-temperature liquid. Therefore, it is worthwhile to study such a widely used technique.

What do we want to observe?

Threre are three types of hyperfine splits in the uncleus can be observed by Moessbuer Spectroscopy, namely: Zeeman Split, Quardrupole split and Isomer shift

How to observe?

The essence of this high precision technique is the Mössbauer effect, or the recoil-free resonant emission and absorption of gamma ray. The resonant emission and absorption takes place when the gamma ray emitted from an excited source nucleus as it makes transition to its ground state is absorbed by another ground-state absorber nucleus of the same kind so that the absorber nucleus makes a transition back to the same excited state as before. By embedding the emitting nuclei and the absorbing nuclei in a solid crystal lattice, the recoil momentum has to apply to the entire solid. Since the mass of the solid is far greater than that of the nuclei, the recoil velocity becomes infinitesimally small and the recoil kinetic energy is effectively zero.

To measure the hyperfine split, 57-Fe, a typical material, is chosen do be studied. The excited 57-Fe emits 14.4keV X-ray to transit to ground state. The mean life time of 57-Fe is 147ns, so the uncertainty of the energy of the photon emitted is 10-9eV according to the uncertainty principle, which is 1 part in 1013 relatively to 14.4keV. The excited 57-Fe will be provided from the beta decay of 57-Co. By changing the velocity of the radioactive source, the energy of the photon emitted is slightly changed to meet the energy drop of the sample. If the energy of the photon is exactly equal to the energy level of the sample, the sample will absorb the photon and reemit isotropically, otherwise the photon will go through the ample without interacting with any nuclei. Therefore, by putting an X-ray detector behind the sample, the count rate will drop as the absorption happens. The sample tested are Fe foil, α-phased Fe2O3 powder and Fe3O4 powder.

Equipment

Below is a diagram of experimental setup:

The 57-Co source is attached to the moving part of the linear motor which is driven by a feedback system provided by Mossbauer Driver S3. The feedback system adjusts the Drive signal to make sure that the velocity of the linear motor changes triangularly. 14Hz trig signal is fed into the Mossbauer Driver by a Function Generator HP33120A to guarantee that the drive frequency is 14Hz. The Silicon Detector is detects the X-ray received, and the Amplifier PX2T and the Ortec 550 Single Channel Analyzer (SCA) filter out any signal other than 14.4keV. Therefore, once the detector finds a 14.4keV photon emitted by 57-Fe, the SCA will give a trigger signal for the NI DAQ board to record the velocity signal processed by SR560 Pre-amplifier. The LabVIEW program is made to record the velocity and make a histogram. Therefore, the position of the absorption peak will be shown on the PC screen.

Results

Below are the histograms from our experiment:

The six absorption peaks are from six different transitions. The energy differs since the contributions of the three hyperfine splits are different to each peak:

by calculation, the energy split for each component are:

Here we have expressed the values and uncertainties of the energy shifts in both the unit of eV and mm/s, although it seems that the unit mm/s is more often used by most physicists doing similar experiments. Comparing to the Duarte and Campbel’s paper which is also about Moessbauer spectroscopy, our results are within one to two sigma from the accepted values.

Reference

1. Carrie Pfeifer, Moessbauer Effect Using Co57, Web. <https://wiki.umn.edu/MXP/M%f6ssbauerEffectLab>

2. The Moessbauer Effect, Havard University, Web. <https://wiki.umn.edu/pub/MXP/MossbauerLabReferencesAndources/Mossbauer_Harvard.pdf>

3. Moessbauer Spectroscopy, Javier M. G. Duarte & Sara L, Massachusetts Institute of Technology, web. <http://web.mit.edu/woodson/Public/8.14finalpapers/Duarte_mossbauer.pdf>