Velocity calibration, iron foil

N. E. Erickson Regarding iron for calibration, some workers run in a time mode but use a sinusoidal drive. This results in an extremely nonlinear velocity scale, and a two-line pattern, such as in sodium nitro-prusside, is just not reasonable for a standard. One needs more lines, with a wider separation. I would like a standard which gives six lines, and hence I favor iron. Perhaps the Bureau of Standards, and I know I have spoken with you (Spijkerman) about this and you argue against it, could calibrate iron foils, which would provide a known six line spectrum. 

The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32, 87, 88). In the latter technique, the spectrum of a compound with well-established Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. 

The sample was a 0.0127 mm thick iron foil, of 99.99% purity, cut into disks (2.5 cm diameter). Room temperature Mossbauer spectra were taken in the initial state, after treatment at 800°C for 30 minutes in 1 atm. argon saturated with water at room temperature, and finally after reaction in 1 atm. acetylene at 750°C for 0.5 minute. Velocities were calibrated using the known Mossbauer parameters of metallic iron at 25°C, and zero velocity is with respect to this standard absorber. 

To analyze the recorded spectra, the spectrometer needs to be calibrated. The three main calibration parameters are the velocity scale, the center point of the spectrum and the nonlinearity of the velocity/time profile of the oscillation compared to a standard reference. The calibration is performed using a spectrum recorded from an a-iron foil at room temperature using the well defined line positions of the sextet from a-iron, which occur at 5.312mms , 3.076mms , and 0.840mms The center of this a-iron spectrum at room temperature is taken as the reference point (0.0 nun s ) for isomer shift values of sample spectra. The typical Mossbauer spectrum of the 14.4 keV transition of Fe in natural iron (Fig. 4.10) represents a simple example of pure nuclear Zeeman effect. Because of the cubic symmetry of the iron lattice, there is no quadrupole shift of the nuclear energy levels. The relative intensities of the six magnetic dipole transitions are... 

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