Positron Research Group -|-|- St Olaf College, Northfield, MN, USA


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Positron Research Group
Science Center 155
1520 St. Olaf Avenue
Northfield, MN 55057

Dr. Jason Engbrecht
507-646-3968 FAX


Anna Legard '08

Dan Endean '09

David Green '09





Positronium scattering off of gas atoms.

When an energetic positron enters a gas environment it will begin colliding with the gas atoms/molecules and quickly (less than 1 ns) lose energy until it is in the energy regime of a few eV. At these energies it has a high likelihood of forming the atom positronium. This positronium will have energy of a few electron volts, meaning that it is approximately 100 times hotter than the particles surrounding it. As the positronium collides with the gas atoms/molecules that are more massive by factors of thousands it will slowly lose energy until it reaches thermal energies, a process called thermalization.

A crude simulation of this process can be seen by visiting our Java Applet of Thermalization.



Graph of the differing results for scattering data (He)

A sample of some of the differing results for scattering data of Ps in helium gas. Note the large disagreement between various results.

[Theoretical calculations]
[Experimental results]

In recent years, advances in both theoretical and experimental techniques have begun to provide insight into the positronium (Ps)-gas scattering process. However, calculation and measurement of these processes has proven to be challenging. The figure at right shows a sampling of the current research for Ps-He scattering at energies below 6.8 eV. As can be seen from the figure there are still large discrepancies in both theoretical calculations and experimental measurements. In addition to the He results shown here, theoretical and experimental progress has been made for Ps scattering in other gases. While the majority of theoretical work has focused on one and two electron systems (H, H2, & He), recent calculations on other noble gases have been completed [3,7]. Experimental work, however, has been able to look at these gases as well as other molecular gases such as N2, isobutane, and neopentane [6]. The purpose of our work here at St. Olaf is to develop a next-generation experiment using an Age-Momentum Correlation apparatus that will open up new areas of study in Ps-gas scattering.


Tell me about the apparatus! --->