To obtain a resonant gamma-ray absorption spectrum it is ncccssary to relate the transmission intensity to the instantaneous source-absorber velocity. In practice there arc a number of methods which can be used to accomplish this, and these can be roughly divided into two groups: those which employ constant velocity drives and those which employ continuously reliable velocity drives. The characteristics and advantages of cach of these systems have been discusscd in detail by Wcrthcim and elsewhere. In the present discussion wc shall not explore any of these mcdiods in detail, but will conccrn ourselves only with the details of the spcctra which arc obtained.
A typical Mossbauer spcctrum is a characteristic plot of the total number of events (counts) observed as a function of sourcc-absorbcr velocity (the standard convention is to represent motion of the source toward the absorber by positive velocities and motion of the source away from the absorber by negative vclocitics).
The profile of the resonance maximum obeys (ideally) the Lorcntz relationship:
constant (£-£<>) + /4 r2
in which Eo is the Dopplcr energy (in the velocity units) at resonance maximum. Since the rcsonancc line which is observed in a measure of the overlap of two lines of width r, the measured full width at half maximum is 2 r = r.
The parameters of the Mossbauer spcctrum which arc of greatest interest to a chemist arc the magnitude of the rcsonancc cflcct (e), the line width (f), the isomer shift (IS), the quadrupolc splitting (QS), magnetic hypcrfinc structure (mhfs), line asymmetry, and temperature coefficients of these parameters.