Mössbauer effect

Under the Mössbauer effect ( after the discoverer Rudolf Mössbauer, erroneously written by back-translation from English and Mössbauer effect ) is the recoilless emission or absorption of a gamma quantum by a nucleus. To this end, the core must be in a crystal lattice, which can take the recoil, and his great mass of the gamma ray energy barely escapes (see elastic collision ). Combining emission and absorption is obtained with Mössbauer spectroscopy an extremely sensitive method for measuring the energy change of the quantum. Rudolf Mössbauer received the Nobel Prize in Physics for his discovery in 1961.

Properties of the gamma radiation

Since the beginning of the 20th century physicists know the gamma radiation as a component of radioactivity. As explorers apply Antoine Henri Becquerel and Paul Villard, the latter was able to prove in 1900 that it is extremely energy electromagnetic waves in the gamma radiation. Gamma radiation is produced, inter alia, when the nucleus is in an excited state as a result of an alpha - or beta decay.

The emission of gamma quanta does not change the core process, ie, In contrast to the α - or β -decay is no transformation takes another nuclide. Only stored in the nuclear excitation energy is emitted as gamma ray, as well as excited electrons release their energy in the form of photons. Nuclei can also absorb gamma quanta, which makes them into a - skip excited state - seen relative to the previous state.

From theoretical considerations we concluded early on that the radiation emitted by most cores gamma radiation is characterized by very sharp energy levels and thus must have a very small line width. One can also compare nuclei to a quartz oscillator, which can be excited with a certain frequency. In fact, the energy constant ( and thus the frequency accuracy ) of many gamma - ray transitions is comparable with the accuracy of atomic clocks.

The initial situation, in Mössbauer

The theoretically predicted spectral purity of the gamma radiation was virtually undetectable before the discovery made ​​by Mössbauer. Due to the high energy of the gamma - quantum, one can determine the frequency of which only very roughly by calorimetric methods. An electronic frequency counter no longer works in the frequency range of gamma radiation.

In addition, the core undergoes when emitting a gamma quantum a non-negligible recoil. This is due to the high energy of the quantum, but have as a photon Although no rest mass, a pulse. Acting on the core rebound causes a reduction in the energy of the gamma quantum: loses the core to the ground by the emission energy, so is the photon energy

Equivalent to this specification is a view from the perspective of the moving core are now taking account of the Doppler frequency shift. If now a core, the light emitted from another core gamma ray absorbing again, so this is really only possible if previously both cores are flown exactly with the double recoil velocity to each other ( twice, because even with the absorption by an equally strong rebound occurs ).

Mößbauers experiment

Mössbauer wanted to determine the probability of such an emission and subsequent absorption of a gamma quantum part of his dissertation. The condition that the two nuclei involved move towards one another at the correct speed should be met by the thermal motion of the atoms.

Here is the schematic experimental set-up of his experiment:

On the left side there is a radioactive source of gamma rays. Some of the rays hit right on an absorber which contains the same atoms as the source, but these are not inherently radioactive. If now a core taken in the absorber of a gamma photon, so, if above condition is satisfied, the gamma photon are scattered towards the detector. The direct path of the radiation to the detector is blocked by a shield made ​​of lead.

The temperature of solids, liquids and gases is correlated with the velocity of the particles (atoms, molecules ) in the same. The higher the temperature, the faster the particles move in the medium. However, the speed of all the particles is not the same, but a random distribution, as well as the directions of movement of the particles.

Mössbauer expected that with increasing temperature the probability of emission and subsequent absorption of a gamma quantum should increase, since statistically move more atoms with the proper speed. Conversely, should reduce the likelihood of this process to almost zero at very low temperatures, since the atoms are so slow in the middle, that the necessary difference in speed is hardly ever obtained.

The first surprising result

The first measurements near room temperature and above expectations Mößbauers seemed initially to confirm. However, when he started out of curiosity, cool source and absorber, he found, surprisingly, that the probabilities for the Gamma-Emission/Absorption at low temperatures suddenly rose steeply and indeed beyond the level that had been measured at higher temperatures.

Mössbauer conducted his experiments on solids. In these, the atoms vibrate around their equilibrium positions in the crystal lattice ( with increasing temperature with increasing amplitude). However, not all vibrational states are allowed due to the quantum mechanics, but only discrete energy states ( phonons). For this reason, the core can not transfer any strong impulse in the form of vibrational energy of the emission and absorption of a gamma quantum. Since the uptake and release of the vibration energy is quantized, there is a certain probability (given by the so-called Debye-Waller factor) such that the atomic vibration energy does not generate and transmit its recoil impulse to the entire crystal lattice. Since its mass substantially exceeds that of the core, carried gamma emission and absorption in this case almost recoilless.

Applications

Through the Mössbauer effect, completely novel methods result in the fields of solid state physics, materials science and chemistry. Furthermore, even predictions of general relativity can be examined with this effect. It was found in a Mössbauer experiment by Robert Pound and Glen Rebka 1960 that if the source and absorber vertically are apart from each other in some 20 meters distance, the gravitational potential of the Earth leads to a measurable change in energy of the quantum as it passes through the height difference (Pound - Rebka experiment).

The variety of applications is the Mossbauer effect today in chemistry. Since the expression of the electron cloud of a molecule reacts slightly on the energy levels of the excited states of its atomic nuclei, the Mössbauer effect has become an irreplaceable tool in chemical analysis (see: Mössbauer spectroscopy ).