Scattering positrons

Scattering Studies

Numerous aspects of positron interactions with atoms and molecules can be investigated using scattering techniques. The buffer-gas-trap-based, high-energy-resolution positron beam has enabled new capabilities. Since the beam is magnetically guided, new techniques were developed to carry out these measurements. Key results include the first measurements of state-resolved cross sections for electronic (atoms and molecules) and vibrational excitation (molecules) and refined measurements of positron-impact cross sections for elastic scattering, direct ionization and positronium formation.

The scattering technique involves passing the beam through a cell containing the test gas, then passing the transmitted beam (containing un-scattered and scattered particles) thorough a retarding potential analyzer (RPA). The adiabatic invariance of E⊥/B (i.e., energy tied up in cyclotron motion/magnetic field strength) can be used to measure elastic scattering if B is held constant along the beam path (M = 1 case above, where M is the ratio of B at the RPA relative to that in the scattering cell). If B is greatly reduced at the RPA relative to that at the scattering cell (e.g., M = 10 case above), then the RPA measures the total energy loss, enabling measurement of cross sections for inelastic processes [1, 2].

Highlights include the cross section for electronic excitation and Ps formation in N2 (right, above) [3], the key to BGT efficiency; and excitation of the asymmetric C-F stretch mode in CF4 (right, below), which enables rapid cooling in the BGT [4]. State-resolved study of molecular rotational excitations will be possible using the new CBT-based beam LINK.

Further Reading:

[1] J. P. Sullivan, et al., Phys. Rev. A 66, 042708 (2002).
[2] C. M. Surko, et al., J. Phys. B: At. Mol. Opt. Phys. 38, R57 (2005).
[3] J. P. Sullivan, et al., Phys. Rev. Lett. 87, 073201 (2001).
[4] J. P. Marler, et al., Phys. Rev. A 72, 062702 (2005).