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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.
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! --->
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