The goal of these experiments is to make absolute (no fitted paramters), low-energy (0.05 eV), high resolution (25 meV) measurements of the interactions of positrons with atoms and molecules.
On this page you can find information on our experimental techniques and on our recent results in these areas:

Rendering of scattering cell (left) and retarding potential analyser, RPA, (right) used for cross section measurements

The positrons from a ²²Na radioactive source are cooled (~25 meV) in the three stage buffer gas trap. They are then magnetically guided through a scattering cell (containing the gas to be studied) and then through a retarding potential analyser. Finally, positrons not lost from the beam hit into a plate at the end of the vaccumm chamber and the annihlation gamma rays are detected with a NaI crystal and photodiode. Conventional electrostatic scattering techniques, such as those used in most electron scattering experiments, cannot be used for these measurements because the cold positron beam is guided by a magnetic field. Instead, we make use of the properties of the positron beam in the magnetic field.

In the case of charged particles in a magnetic field, there are two components to the total kinetic energy, E_T.
1) The energy in the direction of propagation of the beam, E_||.
2) The energy perpendicular to the beam, E_^ i.e the energy associated with the cyclotron motion.
The following cartoons help describe what happens in the gas cell.

	In the incident positron beam, the Larmour radius of the particles is small, and to a good approximation, ET = E||. Keep in mind the the RPA can only measure the E|| energy distribution
	Case 1: When an elastic collision takes place (i.e. no energy loss or gain), some of the energy of the scattered positron is transferred to E^. The amount of this energy transfer is given by E^ = ET sin2(q), where q is the scattering angle. Thus after a scattering event, we have E|| = ET cos2(q). Using a retarding potential analyser, we can measure the distribution of E|| in the positron beam after it passes through the scattering cell containing the test gas.
	Case 2: If both elastic and inelastic scatterng take place, however, it is impossible to distinguish between particles scattered elastically and inelastically, by measuring only the E|| distribution. Recall the RPA can only measure E||.
	We use a technique called the "magnetic beach". This method makes use of the fact that for the magnetically guided positron beam, E^/B is an adiabatic invariant. Therefore, if we guide the particles to a region of lower magnetic field after they scatter in the gas cell, E^ is reduced, and the energy distribution looks like that shown here. This in effect allows the RPA to measure the final total energy of the particles. We lose information about the scattering angle associated with the collision, but we can use this technique to distinguish between elastic and inelastic collisions in order to measure the total inelastic scattering cross section.

Differential and total elastic positron scattering cross sections have been measured from argon at a variety of energies using cold pulses of positrons extracted from the new positron accumulator (energy width from 18 to 25 meV). Measurements of differential cross sections for positron scattering from Ar, Kr and Xe have been made.(read more)

The angular range we are able to measure is limited, depending on the incident energy and the energy spread of the positron beam. Angles close to both 0° and 90° are difficult to measure. Another complication of the experiment is that positrons scattered at greater than 90° (90 + a, for instance) are scattered back along the direction of the incident positron beam. They can then scatter off the potential barrier at the positron accumulator and again pass through the scattering cell. If the probability of a collision is low, then they can pass through the gas cell and are indistiguishable from particles scattered at less than 90° (90 - a).

INELASTIC SCATTERING

Inelastic positron scattering cross sections can be measured using the so-called "magnetic beach" described above. This technique has been used to study CF₄,CH₄, CO, H₂, and CO₂. A similiar procedure can be used to study electronic excitation in atoms. Results in Ar and Xe haev been published. It is also possble, albeit more complicated, to use this method to study electronic excitation in molecules. The difficulty comes from the fact that within each electronic excitation state there is a vibrational manifold. These states can be distinguished however by using the known Franck-Condon values and measurements of a few of the electronics levels in N₂ and H₂ have been published.

		J.P. Sullivan, S. J. Gilbert and C. M. Surko "Excitation of Molecular Vibrations by Positron Impact" Phys. Rev. Lett. 86 (2001), pp. 1494-7.
		J.P. Sullivan, J.P. Marler, S.J. Gilbert, S.J. Buckman and C.M. Surko "Excitation of Electronic States of Ar, H2 and N2 by Positron Impact" Phys. Rev. Lett. 87 (2001).

POSITRONIUM FORMATION and IONIZATION

The formation of positronium, Ps (i.e., the ``atom" consisting of a bound electron-positron pair), is relevant, for example, to applications in a variety of fields including material science and biophysics. Since there is no analog of positronium formation in electron scattering, the extensive understanding of electron interactions with atomic targets is of little help in developing procedures to treat this phenomenon theoretically. In particular, positronium formation requires the inclusion of an additional set of final states. This poses a serious challenge to theory that has not yet been solved in general, particularly at lower values of positron energy where simple perturbative approaches, such as the Born approximation, are invalid. When a Ps atom is made the neutral atom is no longer confined to the magnetic field lines and either self-annihilates or drifts out of the beam and annihilates on the chamber walls. Therefore Ps cross sections can be measured by noting a loss in positron beam current. A set of benchmark measurements have been made in the nobel gasses,Ne, Ar, Kr, and Xe,(shown below and compared to recent similar measurements made at UCL and a second independent analysis explained further in our paper). Similar measurements have also been made in the diatomic molecules: N₂, CO and O₂.

		J. P. Marler and C.M. Surko "Positron-impact ionization, positronium formation, and electronic excitation cross sections for diatomic molecules" Phys. Rev. A 72 (2005), 062713.
		J. P. Marler, J.P. Sullivan and C.M. Surko "Ionization and Positronium Formation in Noble Gases" Phys. Rev. A 71 (2005), 022701.

ELECTRON SCATTERING

Using the cold beam techniques described above electrons from the positron source/moderator can provide a source of cold electrons. By adapting the detector to be a charged particle detector instead of an annihilation photon detector, it is now possible to make the same measurements for electrons as for positrons in the same experimental apparatus. These electron cross section measurements are interesting for comparison to positron cross sections but in principle, also could make a valuable contribution to electron physics by making cross section measurements (for instance of vibrational cross sections) that have proven difficult using conventional electron scattering techniques. This technique has first been applied to CF4. The results of both positron and electron impact excitation of the v3 mode as well as a comparison to the Born-Dipole model prediction for this cross section is shown to the right.

		J. P. Marler, G. F. Gribakin and C. M. Surko "Comparison of positron -impact vibrational excitation cross sections with the Born-dipole model" NIM B 247 (2006) 87-91.
		J. P. Marler and C.M. Surko "Systematic comparison of positron- and electron-impact excitation of the v3 vibrational mode of CF4" Phys. Rev. A 72 (2005), 062702.

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