The Mossbauer effect: the discovery of the effect and its meaning

The article tells what the Mossbauer effect is. And also such concepts as a quantum, a level of energy in an atom and an atomic nucleus, a solid body and collective quasiparticles in it are revealed.

Mathematical fun

The breakthrough in physics, which occurred in the first decade of the twentieth century, required scientists to have serious knowledge in mathematics. Many discoveries were made, at the pen tip, at first: they were first calculated theoretically and only later found in practice.

For example, the presence of gravitational waves, predicted by Einstein in 1910, was able to confirm experimentally only in 2016. The fusion of two neutron stars gave rise to the trembling of space that terrestrial physicists caught and recorded, opening the era of gravitational measurements in the science of mankind. It is not for nothing that gravitation is mentioned here: it is for such studies that the Mössbauer effect is significant. But this is more an exception than the rule. Most often, theorists and experimenters advancing on each other's heels: one study generated the need for its mathematical description, and the side conclusions were the assumption of new, yet unavailable dependencies. The Mossbauer effect turned out to be one of such phenomena. This "side" phenomenon was the assumption of Max Planck, voiced at the end of 1900. It said that in the world of electrons and atomic nuclei all quantities can take only discrete values, that is, quantize. Moreover, according to his own conviction, this was just a mathematical trick, which made the calculations more convenient. Until the end of his life, he believed that a quantum, or the smallest possible portion, such as light, is only a suitable way of describing that does not carry a serious physical meaning.

Quantum world

However, other scientists interested in an adequate description of what is happening on the scale of the atom, considered the potential of such an inference and took for an axiom that everything is quantized. Electrons around nuclei can only be in certain orbits, the atomic nuclei themselves can have only specific energy levels. Jumping between them, the nuclei generate gamma quanta. The effect of Mossbauer argues that this action should generate a kind of return, but this does not happen. In general, all the quantities that describe the behavior of the nanoworld are subject to quantization - that is, discrete. However, do not forget that the impulse, which in the macrocosm is expressed as the product of the mass of the velocity, for an elementary particle is something fundamentally different, and hence it is also quantized. So in science the report, in which Max Planck derived his famous formula containing the value of h, or the minimum action, opened a new era. It was an era of quantum physics. The Mössbauer effect, the interpretation that was subsequently given to this phenomenon, became one of the most important milestones of twentieth-century science.

The discovery of the Mossbauer effect

As we noted above, the theoretical conclusions went hand in hand with experiment. Some practical conclusions were proved on installations assembled literally "on the knee" and from improvised materials. Scientists had to be able not only to derive formulas, but also to weld bulbs, sawing boards, working with metal and assembling plants. Of course, the heads of the laboratories only generalized the results of their wards. However, each experimenter was also an engineer, since the instruments were designed for specific purposes and directly in the process of investigation. The Mössbauer effect was no exception. Opening it would not have taken place if the stubborn doctoral student Rudolf Messbauer had not changed the way of measurement, cooling the unit, instead of heating it, as advised by the supervisor.

Solid

The theory, which we will tell the readers in this section, at first glance seems understandable. However, as you know, lightness is always achieved by incredible efforts. And so that we can now tell in simple words what the effect of Mössbauer is for the kettles literally, once the whole laboratories worked.

A solid is usually understood as a substance in the crystalline state. The nuclei of atoms in this case form a strict periodic lattice, while the electrons are more or less generalized. Of course, a very specific metallic bond is formed in the crystals of metals, owing to which the nuclei exist as it were, separately from the generalized electrons. The electronic cloud lives by its independent laws, not paying attention to the behavior of the crystal lattice. In crystals, where more traditional ionic and covalent bonds are present, electrons are more closely related to "their" nuclei. However, even there they move more freely between neighboring nodes than in gas or liquid.

The properties of a solid are determined not only by the chemical elements that they contain, but also by the symmetry of the arrangement of the atoms relative to each other. In the classic example of carbon, one structure gives rise to soft graphite, and the other is the hardest natural material, diamond. So the type of connection and the symmetry of the unit cell are many things for a solid. In the properties of the solid body, the disclosure of what the Mossbauer effect is is. Its nature is explained by the following: all atoms in a solid are connected.

Collective quasiparticles

Now imagine a fairly large three-dimensional lattice. For the model, salt is the most suitable: Na and Cl are located at the tops of the cubes, replacing each other. If somehow to capture one atom and pull it, to move from the usual place of balance, thanks to a sufficiently rigid connection, neighboring atoms will follow after it. Calculations show that a change in the position of one core has at least some significant effect on the neighbors of the third order. This means that if you "grab" sodium, neighboring chlorine atoms, followed by sodium atoms and another most distant layer of chlorine, will follow. The impact of this, obviously, will spread in all directions. It is usually said that the perturbations of the fourth-order neighbors are negligible. However, they are not equal to zero.

Therefore, if somehow "knock" the crystal harder (for example, send a laser or an electron beam to it), the crystal lattice will go "waves". Such collective motions, when many neighboring atoms of the crystal simultaneously experience a displacement, for example, up or down, are called phonons. In order to be able to describe what the Mossbauer effect is for dummies, we will not go into details and simply tell you that phonons, as it turned out, behave like elementary particles. For example, their energy is quantized, they have a wavelength, an impulse, and they are able to interact with each other. Thus, phonons are called collective quasiparticles. Their quantity and quality are given by the structure of the solid body in which they arise. You can calculate this by knowing the size, symmetry, and types of atoms in a unit cell. The appearance of phonons is also affected by the lengths and types of bonds between ions in the crystal lattice.

Zone theory

Since a rigid body generalizes all its electrons, then the orbitals (and hence their energies) must also be generalized. To begin with, we must remember that electrons belong to this class of particles, which are called fermions. Fermi, Dirac and Pauli jointly found out that in one state only one particle of this kind can be in the given system. If we return to the example of salt, then every crystal with which we sprinkle soup or meat contains an incredible amount of sodium and chlorine ions. And each of them has the same number of electrons that rotate in identical orbits. How to be? The solid leaves the position as follows: the energy of each electron orbiting the nucleus is slightly different from the energy of any other electron belonging to the same orbit of the other atom. Thus, it turns out: in the crystal there are incredibly many energy levels that differ from each other so little that they form a compressed zone. The perturbations introduced by phonons are small, since one atom does not oscillate very strongly. Value has only a collective movement as a whole. Therefore, the phonon energy "dissolves" in the energy of the band. This is the basis for the Mossbauer effect.

Electromagnetic scale

The motion of charged particles is accompanied by the appearance of an electromagnetic field. This fact raises, for example, the question of why one planet and its satellites have it, and others do not. Electromagnetic waves are usually divided into classes according to their frequency and, accordingly, energy. These two characteristics are interrelated, and also depend on the wavelength. What is the Mossbauer effect can be briefly told only if the reader understands where the gamma radiation is located on the electromagnetic scale. So, open the radio wave scale. Theoretically, the limit of their wavelength is the size of the universe. However, the energy of such emissions would be so small that it can not be registered. Slightly higher frequency in terahertz radiation. However, both it and radio waves are observed under very specific conditions: electron deceleration in a magnetic field, bending vibrations of polymers, motion of excitons in a solid. The following part of the electromagnetic spectrum is more understandable: infrared radiation. It transfers energy in the form of heat. The energy of visible radiation is even higher. The part of the spectrum perceived by the human eye is very small in comparison with the whole scale.

Red light carries the lowest energy, and purple - the largest. In connection with this, a paradox is known: colder water is denoted by a blue color, whose energy is higher than that of red radiation. The next ultraviolet portion of the electromagnetic scale already has a sufficiently high frequency to penetrate into the solid body. Despite the fact that people, like other living beings of our planet, do not perceive the ultraviolet, its significance for the normal functioning of biological organisms is enormous. The main source of ultraviolet research is the Sun. The higher energy and the ability to penetrate many substances has an x-ray. The source of such radiation is the deceleration of electrons in electromagnetic fields. In this case, the electrons can be either connected, that is, they belong to atoms, or free ones. In medical devices are devices on free electrons. Finally, gamma radiation is the hardest and shortest wavelength.

X-ray and gamma

The Mössbauer effect and its application in physics and technology require distinguishing gamma quanta and X-ray radiation. On the energy level and, accordingly, the wavelength, they overlap in a very wide spectrum. That is, there is both gamma and X-ray radiation with a wavelength of 5 picometers. There are different ways of obtaining them. As has already been explained above, x-ray emission occurs when electrons are decelerated. In addition, in some processes (including nuclear), an electron disappears from the inner shell of a sufficiently heavy atom, for example, uranium. In this case, other electrons tend to take its place. Such transitions become the source of X-ray radiation. Gamma quanta are the result of transitions of the nucleus itself from a more excited state. This radiation has a great penetrating power and ionizes the atoms with which it interacts. In this case, when the gamma quantum collides with the nucleus of an atom, there must be a so-called recoil. However, in practice it was found that the interaction of the gamma quantum with the nucleus of an atom belonging to a solid body, there is no recoil. This is explained by the fact that the additional energy is "spread out" over the electronic zones of the crystal, thus generating a phonon.

Isotopes

The Mossbauer effect and its application are closely related to one surprising fact: the phenomenon does not affect all the chemical elements of the periodic table. Moreover, it is essential only for certain isotopes of substances. If the reader suddenly forgot what isotopes are, let us recall. It is known that any single atom is electrically neutral. This means that in the nucleus of positive protons is the same as in the shell of electrons. However, the nucleus also contains neutrons, particles without charge. If we change their number in the nucleus, the electroneutrality is not violated, but the properties of such an atom will change slightly. In addition, it happens that the heavier isotope is radioactive and tends to decay, whereas the ordinary substance is completely stable. The list of elements and their isotopes, for which the Mossbauer effect is characteristic, is absolutely concrete. The detection of ⁵⁷ Fe, for example, is usually trusted precisely in this phenomenon.

The use of quantum effects

To make an experiment in which one or another hypothesis relating to the microworld is confirmed is often difficult. In addition, it is unclear what benefits can bring the same effect of Mossbauer? The application, however, is wide enough. Investigation of the properties of crystalline substances, amorphous bodies and finely crushed powders occurs, including with the help of this quantum phenomenon. Such data are required both in sections far enough from practice (theoretical physics) and in very close disciplines - for example, medicine. Thus, the Mössbauer effect and its application should be considered as an example of a theoretical discovery, which brings a lot of usefulness even in everyday life.