The positron trap, which is shown above, uses a Penning trap and a nitrogen buffer gas to collect and cool positrons from a radioactive source and moderator.
THE MODERATOR
A 22Na source (22Na is a positron emitter) is located behind a parabolic cone (shown above at the left hand side of the figure). This cone is cooled to 7 Kelvin using a two-stage refrigerator. By admitting neon gas into the source chamber, a layer of solid neon is deposited on the cone. This acts as a moderator for the positrons, which are emitted from the neon surface with an energy spread of approximately 1 eV (FWHM) at a mean energy of ~2 eV. This compares to a mean energy of ~0.2 MeV from the radiocative source, with the energy of the positrons ranging from 0 to 0.5 MeV. This process has an efficiency of 0.1%. Tungsten is another commonly used moderator which has the advantage of a much better energy spread (~0.2 eV after moderation) but has a greatly reduced efficiency (~0.005%). From the moderator, the positrons are guided magnetically to the first stage of the trap.
TRAPPING & COOLING
The positron accumulator consists of three electrode segments located inside a constant magnetic field of approximately 1.5 kG. The segments are at successively lower electric potentials and, using differential pumping, they are in successively lower pressures of nitrogen buffer gas.
Positrons lose energy by electronic excitation of the nitrogen molecules and they become trapped in the third stage in less than 1 ms. They continue to lose energy through vibrational and rotational excitation of the molecules, cooling to the electrode temperature (i.e., room temperature, ~ 25 meV) in approximately 1 second. This process is shown schematically below. To improve throughput, we add a small amount of CF4 to the third stage. This further reduces the cooling time to 0.1 seconds.
After the positrons are accumulated and cooled in the third stage, we are able to rapidly pump out the buffer gas and store the resulting positron plasma for long periods of time (lifetimes of 1500 seconds are achieved routinely). We have accumulated as many as 300 million positrons in this manner, corresponding to a plasma density of ~107 cm-3. The efficiency of the trap is approximately 25%, measured by comparing the number of positrons accumulated in the trap to the moderated positron flux from the radioactive source.
BEAM EXTRACTION
In the past, experiments were performed in situ using the cold positron plasma stored in the third stage (e.g. our early annihilation experiments). This process provided only zero energy, thermal positrons and was fairly time-consuming because it required prior removal of the buffer gasses. These shortcomings were remedied with the development of new beam extraction techniques. We can now extract positrons as an energy-tunable cold pulse or series of pulses for use in other experiments outside the trap. Furthermore, we no longer need to evacuate the buffer gasses.
A positron pulse is produced by raising the well in the third stage until all the positrons spill over a fixed potential barrier at the end of the trap. The voltage on that barrier defines the parallel energy of the positrons exiting the trap while the perpendicular energy is a thermal 25 meV. If the potential well is raised in steps rather than all at once, a series of smaller pulses is produced. Click here for publications on the trap operation.
APPLICATIONS
Much of our research is focussed on the study of positron interactions with matter (for more on our research in this area, click here). Other areas of research using this method of positron accumulation include investigation of the properties of the positron plasma and study of the interactions between the plasma and an electron beam (papers on these experiments can be found here).
The 22Na source and moderator can also be used as a source of electrons, which can be trapped and cooled in the same way as the positrons. This gives us the opportunity to do similar experiments with both positrons and electrons, which can provide interesting comparisons of the way these different particles interact with matter. More information about these experiments can be found here.
Ultimately, the buffer gas trap will be used to feed our high field trap. This will allow us to manipulate the positron plasmas in a number of new ways. In particular, we could compress them to a higher density, cool them to 10K, and store them in much greater quantities for longer periods of time. This will, in turn, facilitate further advances in our associated atomic and molecular experiments.
Group members working on positron plasmas are Jason Young, James Danielson, and Toby Weber.