Time-differential perturbed gamma-ray angular correlations have been measured for 148'15~ recoiling into vacuum and into helium (v/c= 1.3 ~o) in the pressure region from 0.75 to 25Torr. A model is presented of the perturbation mechanism for recoils travelling in vacuum, which allows for the hitherto hidden fact that a hard core of the attenuation exists. In addition, a higher order modification of the first order Abragam and Pound theory for random perturbations has to be taken into account at very low pressure.

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