Moessbauer line-shape parameters for sup 159 Tb in TbAl sub 2 and Tb sub 4 O sub 7

Precise line-shape analyses of Moessbauer spectra using the 58-keV transition in {sup 159}Tb have been made using an exact representation of the line shape in transmission. Spectra were made using a Dy{sub 2}O{sub 3} source at liquid-nitrogen temperature and absorbers of Tb{sub 4}O{sub 7} and TbAl{sub 2} at several temperatures from 81 to 297 K. We have measured the recoilless fraction as a function of temperature between 115 and 297 K for TbAl{sub 2} and at 81 and 294 K in Tb{sub 4}O{sub 7}. If the theoretical value of the internal conversion coefficient of 11 is accepted, we cannot fit our results to a Debye model, even in a rough approximation. The mean lifetime of the first excited state of {sup 159}Tb was determined to be 77(1) ps. We have found the value of the interference parameter for this transition to be {minus}0.0058(7), which appears larger in magnitude than predicted by theory.