Chemistry Reference
In-Depth Information
levels and their decays and then discuss the radiation of c quanta. Nuclear levels
are influenced by the magnetic fields and the electronic charge through the
Coulomb interactions, which is so called hyperfine interactions of the nucleus, and
discussed in this chapter. Recent development of the synchrotron radiation facil-
ities made possible to use this modern X-ray source as a source for the nuclear
resonance instead of the radioisotope. Brief description for the nuclear scattering
with the synchrotron radiation is presented in the final section of this chapter.
1.1 What is the Mössbauer Effect?
Mössbauer effect is, as already mentioned, the recoilless emission and the
recoilless resonant absorption of c photons by the nucleus. Two nuclei, which are
in principal identical nucleus, are necessary to observe the Mössbauer effect.
Source is the nucleus, which emits c photon, and the absorber is the nucleus that
resonantly absorbs c photon. Since the source emits c photons, source nuclei have
unstable nuclear levels and prepared in principally by the nuclear reactions
induced for the stable nuclei by the bombardments of particles like deuteron,
proton and neutron so on. Excited nucleus is a parent nucleus having a moderate
lifetime and decays to the first excited state of Mössbauer source nucleus, which is
followed, by the decay to the stable ground state of the Mössbauer nucleus. Decay
to the ground state creates the emission of c-ray photons. The c-ray energy emitted
from nucleus is usually very high compared to the photons emitted by the atomic
process. During the emission and absorption of c-ray, the momentum and energy
conservation law needs usually the recoil of the nucleus. However, when the c-ray
has relatively low energy and the source nuclei embedded in solid, there is a large
probability to have zero recoil energy (recoilless) in the c-ray emission process. In
the same way for the absorption process, there is a large probability to have
recoilless resonant absorption. Figure
1.1
shows schematic illustration of the
source and absorber in a Mössbauer experiment. Creation of the source and the de-
excitation of the absorber after resonant absorption are also indicated. Doppler
velocity is used to modulate the c-ray energy by the first order Doppler effect, for
example, by the mechanical motion of the source. What is the Mössbauer effect?
The answer for this question is, by one word, as follows; that the Mössbauer effect
is recoilless emission and absorption of c photons by atomic nucleus.
1.1.1 Nuclear Levels
As shown in Fig.
1.1
, the parent nuclei for Mössbauer experiment should have a
moderate lifetime (for
57
Fe Mössbauer experiment,
57
Co has a half-life 270 days).
After the decay of the parent nucleus by a or b and so on to the excited state of
Mössbauer nucleus
57
Co decays by E. C. (electron capture) to
57
Fe excited states.
