Mössbauer 57Fe

 

Principle

Resonant absorption experiments have long been performed in optics by exciting an atom from its ground state to an excited state by irradiating light. Before 1958, the γ-radiation emitted during nuclear transitions seemed not to follow this phenomenon: it is the recoil of the nucleus, in emission or absorption, and the Doppler broadening due to the thermal motion that is responsible for this anomaly. Rudolf L. Mössbauer in 1958 solved the problem by showing in the case of 191Ir that the resonant absorption increased at low temperature. He gave a theoretical explanation for this and was awarded the Nobel Prize in 1961. Thus, was born Mössbauer spectrometry, which is a nuclear spectroscopy with sufficient resolution to determine the hyperfine structure of the nucleus due to perturbations of the nuclear levels by its electronic environment. This resolution of 10-13 gives it extreme finesse and is ranked among the most accurate physical measurements. In the case of 57Fe, we are interested in the nuclear transition between the first excited state of this emitting isotope (from its radioactive parent 57Co) and its ground state generating a gamma photon emission E0 of 14.4 kev. After the gamma emission, the nucleus recedes and its energy will be E0 minus the recoil energy (Er = E02/2mc2 inversely proportional to its mass). This radiation is then absorbed resonantly by an iron target nucleus (absorber) if, within the Heisenberg width, it generates the opposite transition in the target of energy equal to the transition energy in the emitter plus the recoil energy of the nucleus. This energy mismatch follows from the conservation of momentum and total energy. Mössbauer demonstrated in 1958 that this resonance phenomenon can only be observed if the target nucleus is strongly bound to its neighbours in the case of a solid. Thus, it will completely absorb the recoil (mass m replaced by mass M of the solid) and thus the recoil energy of the crystal will become negligible without changing the vibrational state of the solid. Thus, the fraction f of nuclei giving rise to resonant absorption without recoil is called Lamb-Mössbauer f factor:

where

Er : recoil kinetic energy of the nucleus

  <x2> : mean square displacement of the probe nucleus in the solid along the direction of photon propagation γ which characterises the sample and is strongly temperature dependent.

For whom ?

Large values of Lamb-Mössbauer f factor are obtained for low energy transitions (via Er) for heavy nuclei with rigid chemical bonds and at the lowest temperatures. Although first discovered by physicists, the technique developed very quickly into a successful research method. Its applications, initially in condensed matter physics and chemistry, have proven to be valuable in other disciplines such as metallurgy, magnetism, coordination chemistry, catalysis, mineralogy and biology.

To do what ?

Like other nuclear probes, Mössbauer spectrometry gives local information about the nuclei it affects, in particular the local electron density and magnetic moment. This type of data provides information on the valence state of the corresponding atoms, the nature of the bonds they form with their neighbours and their position in a crystal lattice (local crystallographic, magnetic order and chemical environment). In the case of the 57Fe, metallurgy and mineralogy in earth sciences (or beyond with MIMOS Mars exploration) are prime areas for the use of the technique. In iron oxides, subtle phenomena such as the Morin transition (spin reversal) in hematite or the Vervew transition (Fe2+ to Fe3+ electron hopping) in magnetite are directly detectable in Mössbauer spectra which cannot be observed by diffraction or microscopy techniques. Moreover, the spin fluctuations giving rise to the dynamic effects occur at times comparable to the characteristic measurement times of Mössbauer spectrometry (lifetime of the first excited state of 57Fe). Thus, in the case of nanoparticles, blocking temperatures as well as superparamagnetic behaviours are also directly observable through the line shapes of Mössbauer spectra as a function of large temperature ranges.

How

Experimental implementation:

 

Isotopes: about fifty elements, including about forty metals, can be considered as active for Mössbauer spectrometry. Iron and tin, whose sources are 57Co and 119mSn, are the most studied elements, followed by the rare earths (Eu, Gd, Dy, …). As the sources of γ-rays are radioactive isotopes in their excited states with a short life span, as we are not in a nuclear experimental environment (accelerator, reactor), we need a radioactive parent (e.g., 57Co for 57Fe) which decays slowly and be long lived enough to allow time to setup the experiment while a conserving a significant activity.

Apparatus:

The spectrometer that measures the energy difference between the nuclear transitions of the emitter and absorber uses Döppler modulation of the energy of the emitted photon by moving the emitter and absorber relative to each other in a speed range of 1 to 300 mm.s-1. Almost exclusively the Mössbauer drive systems is the electromechanical driving transducer which drive the source, and a Mössbauer driving unit which feeds the velocity transducer with an electronically controlled voltage. The transducer and the driving unit together form a feedback system, which minimizes the deviation of the source motion from its correct value. There are two principal motions used, constant acceleration and sinusoidal. The velocity detection signal, solidary with the driving transducer, is proportional to the channel number of the analyser in which the pulses are counted. Experiments are generally performed by transmission, with the source mounted on the transducer. Detection is by means of a NaI or semiconductor scintillator connected after amplification to a single-channel discriminator (which selects the pulses from the only Mössbauer transition). The channels of a multichannel analyser are synchronised with a function generator (waveforms). Sample preparation is relatively simple. The sample to be analysed is powdered and spread across holder with a diameter equal to about fifteen mm. In cryogenic environments, cryostats supporting the samples and having optical windows made of Be or mylar (transparent to γ photons) are used.