Dispersion amplitudes in the Moessbauer lineshape of 237Np, 182W, and 195Pt

Measurements of the dispersion amplitude for the Moessbauer resonances in 237Np (60 keV), 182W (100 keV) and 195Pt (99 keV) are reported. For the E1 transition in 237Np a value of xi = -(3.4 + 0.2) x 10-2 was found. Theoretical estimates of xi for this case are of limited accuracy and the comparison with experimental data is inaccurate because of the considerable line broadening. Computer simulations show the influence of xi on 237Np hyperfine spectra. For the E2 transition in 182W the dependence of the effective value of xi on absorber thickness was studied. Extrapolations to zero thickness yield xi = -(1.0 +- 0.1) x 10-2 in good agreement with theoretical calculations. For 195Pt we found xi = -(1.1 +- 0.3) x 10-2 for the dispersion amplitude at zero absorber thickness. This is the first pure M1 transition for which a dispersion term has been observed. For all three resonances the dispersion is large enough to affect the determination of isomer shifts