Nuclear deorientation for heavy ions recoiling in vacuum and low pressure gas
Time-differential perturbed gamma-ray angular correlations have been measured for148, 150Nd(21+) recoiling into vacuum and into helium (ν/c=1.3%) in the pressure region from 0.75 to 25 Torr. 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.