Absorber Optimization Mass Absorption and Thickness

The thickness of a Mossbauer sample affects not only the strength of the Mossbauer signal but also the intensity of the radiation arriving at the detector because the y-rays are inherently attenuated by the sample because of nonresonant mass absorption caused by the photo effect and Compton scattering as mentioned earlier. The counting rate C in the detector decreases exponentially with the density of the absorber, 

Particularly for thin Mossbauer absorbers with a low concentration of the resonance nuclide and high mass absorption, it may be problematic to apply the recommendation for sample preparation (f 0.2), because the resulting electronic absorption may be prohibitively high. In such a case, it may pay well to optimize the absorber thickness, i.e., the area density f. To this end, following the approach of Long et al. [33], we adopt the general expression  

Interestingly, the curve shape of SNR( ) does not depend explicitly on the concentration of Mossbauer nuclei in the sample. The function has a maximum at f = 2 (see Fig. 3.13), as one can easily verify from the root of the first derivative. 

The ideal thickness of a Mossbauer absorber is therefore given by [33]  

The derivation of the expressions (3.3)-(3.6) is found in Appendix A (cf. CD-ROM). Since for most Fe-spectra the level of nonresonant background counts, Ab, may be in the range of 10-30% of the total counts, the absorber thickness is usually best adjusted to a value between the limits given above. The maximum of SNR( ) is naturally rather broad, such that deviations from / opt of even dz50% are fairly immaterial. 

---