| Description: |
The thermalization of positronium ( Ps) formed at a few eV in gases is investigated using timeresolved, Doppler broadening measurements of the annihilation photons. Magnetic quenching permits energy measurements about 40 ns after Ps is formed in H 2 , N 2 , He, Ne, Ar, isobutane, and neopentane. The thermalization rate is measured by changing the gas density, and a classical elastic scattering cross section and a Ps formation energy are determined. The impact of Ps thermalization on decay rate experiments using gases is also discussed. [S0031-9007(98) PACS numbers: 36.10. Dr, 34.50.Bw, 78.70.Bj Collisions between normal gas atoms and the exotic atom positronium (Ps, positron-electron bound state) are interesting and unique because Ps is so light relative to its target. Hence Ps, formed at typically a few eV in most gases, will thermalize very slowly if elastic scattering is the only available energy loss mechanism. In the elastic case, the fractional energy loss per collision is only of order m͞M ϳ 10 24 (m is the Ps mass, M is the atomic/molecular mass.) The low energy Ps-atom collision is also inherently quantum mechanical in nature since the de Broglie wavelength of Ps below 1 eV is greater than 9 Å, larger than the classical geometric atomic size. Moreover, it was recognized early [1] that cross section calculations must include the polarization/Van der Waals interaction and electron exchange. These features apparently complicate the calculations of cross sections, done presently including the exchange interaction for only oneand two-electron systems scattering Ps (H: [2]; H 2 : [3]; He: Positronium as a scattering probe offers a unique experimental advantage since its annihilation into two photons provides a mechanism for determining its velocity and hence the rate of thermalization and the momentum transfer cross section ͑s m ͒ for gas targets. Previous measurements of Ps thermalization in gases In this Letter, we report the measurement of Ps thermalization rates, formation energies, and momentum transfer ... |