Recoilless emission of y rays

To gain an insight into the physical basis of the Mossbauer effect and the importance of recoilless emission of y-rays, we must consider the interplay of a variety of factors. These are best treated under five separate headings ... 

Looking back, physicists have been tempted to wonder why the recoilless emission of y-rays was not discovered earlier. The full theory had been worked out by Lamb in 1939 for the analogous case of absorption of neutrons [6]. Similarly the diffraction of X-rays depends on the same effect, namely the elastic scattering which leaves the wavelength (energy) of the X-ray beam unchanged. Indeed, the fact that in X-ray diffraction thermal motion of the... 

We have described the mechanism by which a y-ray can be emitted without recoil and the same arguments apply to resonant reabsorption. Since Mossbauer experiments usually utilise the recoilless emission of y-rays by a... 

Mossbauer effect - The recoilless emission of y-rays from nuclei bound in a crystal under conditions where the recoil energy associated with the y emission is taken up by the crystal as a whole. This results in a very narrow line width, which can be exploited in various types of precise measurements. 

The Mossbauer effect is the recoilless absorption and emission of y-rays by specific nuclei in a solid (Mossbauer 1958a, 1958b), and provides a means of studying the local atomic environment around the nuclei. It is a short-range probe, and is sensitive to (at most) the first two coordination shells, but has an extremely high energy resolution that enables the detection of small changes in the atomic environment. Mossbauer spectroscopy therefore provides information on phase transformations at the microscopic level. 

However, in contrast, the resonance effect increased by cooling both the source and the absorber. Mdssbauer not only observed this striking experimental effect that was not consistent with the prediction, but also presented an explanation that is based on zero-phonon processes associated with emission and absorption of y-rays in solids. Such events occur with a certain probability/, the recoil-free fraction of the nuclear transition (Sect. 2.4). Thus, the factor/is a measure of the recoilless nuclear absorption of y-radiation - the Mdssbauer effect. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

The y-rays from the source (which must be specially prepared to yield a high fraction of recoilless emission) are absorbed by the sample and re-emitted approximately isotropically. Compared with ordinary electronic absorption of y-rays by an equivalent material, detector (A) in the figure measures a decrease in transmitted y-ray intensity when... 

Since Mdssbauer s first publication (77) on recoilless emission or absorption of y-rays several extensive articles and books (12-24) have... 

The probability of recoilless emission of a y-ray from a nucleus is given by the recoil free fraction,/, where... 

Mossbauer effect is the recoilless (also called recoil-free) nuclear resonance emission/ absorption of y rays (see O Pig. 25.1). In the case of a nuclear transition, the de-excited nucleus is normally recoiled by the momentum of the y photon emitted, which makes its resonance absorption impossible by another ground-state nucleus of the same type. In solids, however, recoilless photons can be emitted (and reabsorbed by another ground-state nucleus) with some probability. 

Rare-earth ions inserted in the tetraborides have the 34- oxidation state, except for CeB4 and YbB4 (see Fig. 2). The abnormal volume contraction for the CeB4 unit cell can be explained by the presence of some Ce ions . Recoilless y-ray emission spectra and magnetic measurements indicate that ytterbium in YbB4 has an intermediate valence state as in YbAl3... 

The rare earths in their dodecaborides have the 3 + oxidation state except for Yb and Tm which have an intermediate valence state. A recoilless y-ray emission spectrum study of TmB,2 shows no magnetic ordering at 1.35 K the spectra of YbB,2 reveal no magnetic structure to 1.35 K. The compounds HoB,2, ErB,2 order antiferromagnetically, and ZrB,2 and LuB,2 become superconducting < 5.8 K and < 0.48 K, respectively. ... 

A recent development in physical techniques which may be of aid in evaluating the relative merits of theory is the Mossbauer effect. This effect is based upon recoilless y-ray emission (absorption) resulting from a nuclear transition in a particular atom with the resonance condition of zero-phonon processes. Since such nuclear transitions can be obtained with... 

In 1957, Rudolph Mossbauer, during his graduate studies, discovered an outstanding effect [49] that has generated an entire field in physics, that is, a very high-resolution spectroscopy in the y-ray region of the spectrum named Mossbauer spectroscopy [50-56], The effect consists of the fact that a y photon emitted by an excited nucleus can be resonantly absorbed by another nucleus [50-56], This means that a recoilless emission and absorption has occurred. 

Mossbauer experiments in the transmission mode (as shown in Fig. 1) usually use recoilless y-rays emitted from a radioactive source followed by resonant recoilless reabsorption by a non-radioactive absorber. If the probability of recoilless emission from a source is/ and the y-photon has width Fj at half-height then the number of transitions N(E) with energies between E -E) and (Ey-E+dE) is given by ... 

The probability of the recoilless emission or absorption is given by the recoilless fraction f, meaning the fraction of all y rays of the Mossbauer transition that are emitted (fs) or absorbed (A) without recoil-energy loss. This is also commonly referred to as the Mossbauer fraction, Debye-Waller factor, Mossbauer-Lamb factor or, simply, the /-factor. 

In order to measure a Mossbauer spectrum one has to detect recoilless absorption or emission of a selected y radiation as a function of the Doppler velocity of the sample and a reference material relative to each other. In the most common case of transmission geometry, absorption of the y rays is measured, and the reference material is a standard source, which is moved. Thus, the most important components of a Mossbauer spectrometer are a Doppler velocity drive system and an energy selective y detection chain with appropriate recording system. 

In 1956 and 1957, young physicist R. Mossbauer has performed the experiments concerning the scattering of the 129 keV y-ray of Ir by Ir and discovered an increase in scattering at low temperatures. Results obtained and his interpretations were published in 1958 [1-3], which is the beginning of the Mossbauer effects study and its development as the Mossbauer spectroscopy. The Nobel Prize for physics 1961 was awarded to him [4]. Mossbauer spectroscopy is the recoilless emission and the recoilless resonant absorption of the y-ray by the nucleus. After... 

In previous section we treat the Mossbauer spectroscopy using a radioactive y-ray source, which has an extremely narrow line width of the order of 10 eV in the case of Fe resonance. Mossbauer effect is a recoilless emission and a recoilless resonant absorption of low energy y photons in a combination with source and absorber nuclei. 

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