We have built a dc beam of positrons. After moderation in solid neon the positrons are magnetically guided away from the source, through what will be our buffer gas trap, and onto a phosphor screen, located in front of a CCD camera.
Below is a typical image we have recorded of our positron beam.
The beam has a roughly doughnut-shape, which is determined by the shape of the neon moderator (the moderator has a hollow centre, and so there are fewer of the slow positrons that we detect in the centre of the beam). This beam structure is highlighted in the following figure which shows the recorded beam intensity along a line (shown in the above figure).
In this experiment we are recording around 2 million positrons per second, several metres downstream of the source.
This is an important step forward in our experiment. The next step will be to accumulate positrons in our trap, and then accelerate them onto a target for Ps production.
We now have our 22Na source installed in the chamber.
This photograph shows the source on the far left-hand side (encased in lead bricks). The emitted positrons are moderated in solid neon and are then magnetically guided through the vacuum chamber towards the trap (toward the right of the image, next to the gas cylinder).
This plot shows how the moderator is grown by injecting neon gas in the source chamber up to a pressure of around 10^-4 mbar (red, dashed). The 22Na source is housed on the top of a cold head, maintained at around 7 K (blue). This is cold enough for the neon to freeze, which it does inside an aperture directly in front of the source. This solid block of neon forms the moderator which slows down the emitted positrons to speeds where we can easily guide and trap them. The black curve shows the growth in detected slow positrons as the moderator is formed. The positrons are measured downstream of the source, directly in front of the trap.