Abstract
The bubble formation around a positronium atom has been studied in both 3He and 4He by lifetime measurements at temperatures 1.4-40K and at pressures up to 70 bar. Bubble formation is observed at all these temperatures and in all phases. The bubble is found to be almost as large in the solid as in the liquid. No qualitative difference is found between the two isotopes. The lifetime data are analysed using the square-well model, which is found to be valid in a well defined density region. The values obtained for the well depth correspond to the Ps-He scattering length as=0.75 AA. The bubble radii are about 15 AA at low pressures, decreasing to close to 7 AA at the highest pressures. The transition from free to bubble state with increasing density is found to occur without any abrupt change in the annihilation rate. The transition is reasonably well defined below 10K but becomes diffuse with increasing temperature.
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