The y-decay scheme

The 14-41-keV y-transition is internally converted to a high degree during the decay. Many early papers used a value for the internal conversion 

The relative abundance of Fe in natural iron is rather low (219%) but the isotope has a large cross-section for capture of the 14-41-keV y-ray (theoretically 2-57 X 10 cm ), and isotopic enrichment is only needed when the iron content of the sample is very low, as for example in proteins or very dilute iron alloys. Enrichment with 90% Fe in the latter cases generally gives an adequate cross-section. The free-atom recoil energy of 1-95 X 10 eV is low and ensures a high proportion of recoilless events. 

---