Mössbauer effect

Mössbauer effect, nuclear process permitting the resonance absorption of gamma rays. It is made possible by fixing atomic nuclei in the lattice of solids so that energy is not lost in recoil during the emission and absorption of radiation. The process, discovered by the German-born physicist Rudolf L. Mössbauer in 1957, constitutes a useful tool for studying diverse scientific phenomena.

In order to understand the basis of the Mössbauer effect, it is necessary to understand several fundamental principles. The first of these is the Doppler shift. When a locomotive whistles, the frequency, or pitch, of the sound waves is increased while the whistle is approaching a listener and decreased as the whistle recedes. The Doppler formula expresses this change, or shift in frequency, of the waves as a linear function of the velocity of the locomotive. Similarly, when the nucleus of an atom radiates electromagnetic energy in the form of a wave packet known as a gamma-ray photon it is also subject to the Doppler shift. The frequency change, which is perceived as an energy change, depends on how fast the nucleus is moving with respect to the observer.

The second concept, that of nuclear recoil, may be illustrated by the behaviour of a rifle. If it is held loosely during firing, its recoil, or “kick,” will be violent. If it is firmly held against the marksman’s shoulder, the recoil will be greatly reduced. The difference in the two situations results from the fact that momentum (the product of mass and velocity) is conserved: the momentum of the system that fires a projectile must be opposite and equal to that of the projectile. By supporting the rifle firmly, the marksman includes his body, with its much greater mass, as part of the firing system and the backward velocity of the system is correspondingly reduced. An atomic nucleus is subject to the same law. When radiation is emitted in the form of a gamma ray, the atom with its nucleus experiences a recoil due to the momentum of the gamma ray. A similar recoil occurs during absorption of radiation by a nucleus.

Finally, it is necessary to understand the principles governing the absorption of gamma rays by nuclei. Nuclei can exist only in certain definite energy states. For a gamma ray to be absorbed its energy must be exactly equal to the difference between two of these states. Such an absorption is called resonance absorption. A gamma ray that is ejected from a nucleus in a free atom cannot be resonantly absorbed by a similar nucleus in another atom because its energy is less than the resonance energy by an amount equal to the kinetic energy given to the recoiling source nucleus.

Principles of application

By embedding the same kind of nuclei in two solids, those in one solid being in a radiating or excited state and those in the other being in an absorbing or ground state, it is possible to use gamma radiation from the first solid for resonant absorption in the second. The method is so sensitive that if one solid moves relative to the other with a velocity as small as a 10th of a centimetre per second the resonance is destroyed by the Doppler shift in the gamma-ray energy. In this fact lies the usefulness of the Mössbauer effect as a tool of experimental physics. If an energy state of the radiating or absorbing nucleus is disturbed by any external effect, resonance absorption will also be nullified. Then, by varying the relative velocity of the radiating and absorbing solids until resonant absorption is reestablished, the precise magnitude of the disturbing effect can be determined.

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Apparatus

The Mössbauer effect is usually observed by measuring the transmission of gamma rays from a radioactive source through an absorber containing the same isotope in the ground state. The device used for this experiment is termed a Mössbauer effect Doppler-velocity spectrometer because it utilizes the Doppler shift to scan a region of energy in the vicinity of the energy of the gamma ray itself. A spectrometer utilizing this concept is shown schematically in Figure 1. A radioactive source is mounted on an electromechanical transducer that moves with a well-defined velocity. Generally, velocities of a few centimetres per second are required. The energy of the emitted gamma rays is shifted, owing to the Doppler effect, by an amount proportional to the velocity. In this example, nuclei of the radioactive source and the resonant absorber are identical, not having been perturbed by an outside field. The gamma rays pass through an absorber containing the resonant isotope and are detected by a proportional counter. The gamma rays detected per second are plotted as a function of the Doppler velocity, resulting in a Mössbauer effect absorption spectrum like that shown in Figure 2. The drop in counting rate in the centre is due to resonant absorption—i.e., the Mössbauer effect. At high positive or negative velocity, the resonant absorption has been destroyed by the Doppler shift.

Isotope suitability

The Mössbauer effect has been observed in more than 35 isotopes. Suitable isotopes must have a stable or long-lived ground state and a low-lying excited state that decays to an appreciable extent by gamma-ray emission. The condition on the energy of the excited state arises from the need to achieve recoil-free gamma-ray emissions. Only if the free-atom recoil energy is small compared to the characteristic lattice vibration energy will there be a finite probability that momentum conservation is satisfied by the recoil of the crystal as a whole with negligible loss of energy. The resulting gamma ray has the full energy of the nuclear transition and is not broadened by thermal vibrations. It consequently has the exact energy for recoil-free resonant reabsorption by a nucleus in its ground state. The condition on the magnitude of the free-atom recoil energy generally restricts the Mössbauer effect to gamma rays with energy less than 150 KeV. The natural width of a resonance or nuclear level, Γ, is related to its lifetime, τ, by Γτ = h/2π, in which h is Planck’s constant. The lifetime of the excited state must be greater than 10⁻¹¹ second; otherwise the resonance level would be so broad that the distinction between ordinary and recoil-free emission is lost. The longest useful lifetime is about 10⁻⁵ sec because the corresponding width is comparable to that due to other mechanisms.

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