196 5.5  Tools that Use Gamma Rays, Radioisotope Decays, and Neutrons

5.5.1  MÖSSBAUER SPECTROSCOPY

The Mössbauer effect consists of recoilless emission and absorption of gamma rays by/​from

an atomic nucleus in a solid or crystal lattice. When an excited nucleus emits a gamma ray,

it must recoil to conserve momentum since the gamma ray photon has momentum. This

implies that the emitted gamma ray photon has an energy, which is slightly too small to

excite an equivalent atomic nucleus transition due to absorption of another identical atomic

nucleus in the vicinity. However, if the gamma ray–​emitting atomic nuclei are located inside

a solid lattice, then, under sufficiently low temperatures, the atomic nucleus emitting the

gamma ray photon cannot recoil individually but instead the effective recoil is that of the

whole large lattice mass.

Under these conditions, the energy of a gamma ray photon may not be high enough to

excite phonon energy loss through the whole lattice and therefore these results in negligible

recoil energy loss of the emitted gamma ray photon. Thus, this photon can be absorbed by

another identical atomic nucleus to excite an atomic nuclear transition, with consequent

emission of a gamma ray photon, which therefore results in absorption resonance within the

sample. However, in a similar way to the fine structure of NMR resonance peaks discussed

previously in this chapter, the local chemical and physical environment can result in hyperfine

splitting of the atomic nucleus energy transition levels in atomic nuclear energy levels (due to

magnetic Zeeman splitting, quadrupole interactions, or isomer shifts, which are relevant to

nonidentical atomic radii between absorber and emitter), but which can shift the resonance

frequency by a much smaller amount than that observed in NMR, here by typically just one

part in ~10¹².

An important consequence of these small energy shifts, however, is that any relative motion

between the source and absorber of speed around a few millimeters per second can result

in comparable small shifts in the energy of the absorption lines; this can therefore result in

absorption resonance in a manner that depends on the relative velocity between the gamma

ray emission source and absorber. A typical Mössbauer spectrometer has a gamma ray source

mounted on a drive, which can move at different velocities up to several millimeters per

second, relative to a fixed absorber. A radiation Geiger counter is placed behind the absorber.

When the source moves and Doppler shifting of the radiated energy occurs, resonance

absorption in the fixed absorber decreases the measure transmission on the Geiger counter

since excited nuclei reradiate over a time scale of ~10⁻⁷ s but isotropically.

Several candidate atomic isotopes are suitable for Mössbauer spectroscopy; however, the

iron isotope ⁵⁷Fe is ideal in having both a relatively low-​energy gamma ray, which is a pre­

requisite for the Mössbauer effect, and relatively long-​lived excited state, thus manifesting as

a high-​resonance signal-​to-​noise ratio. The cobalt isotope ⁵⁷Co decays radioactively to ⁵⁷Fe

with emission of a 14.4 keV gamma ray photon and is thus typically used as the moving

gamma ray source for performing ⁵⁷Fe Mössbauer spectroscopy in the fixed absorber sample.

Iron is the most abundant transition metal in biological molecules and ⁵⁷Fe Mössbauer

spectroscopy has several biophysical applications, for example, biomolecules such as the

oxygen carrier hemoglobin inside red blood cells, various essential enzymes in bacteria and

plants, and also multicellular tissues that have high iron content, such as the liver and spleen.

In essence, the information obtained from such experiments are very sensitive estimates for

the number of distinct iron atom sites in the sample, along with their oxidation and spin

states. Importantly, a Mössbauer spectrum is still observed regardless of the actual oxidation

or spin state of the iron atoms, which differentiates from the EPR technique. These output

parameters then allow predictions of molecular structure and function in the vicinity of the

detected iron atoms to be made.

5.5.2  RADIOISOTOPE DECAY

An example of a radioactive isotope (or radioisotope) in ⁵⁷Co was discussed earlier in the con­

text of being a gamma ray emitter in decaying to the more stable ⁵⁷Fe isotope. But there are a

KEY BIOLOGICAL

APPLICATIONS: NMR

Determining atomic-​level precise

molecular structures without the

need for crystals; Identifying spe­

cific chemical bonds.