The first step towards trapping positrons is to produce a low energy positron beam. This is accomplished using the combination of a sodium-22 radioactive source and a neon ice moderator. The neon ice is frozen in a conical geometry in front of the source which is cooled to approximately 5K using a cryocooler. The 22Na source strength is of the order 2GBq; hence it must be shielded by lead to make sure that the dose rate is low enough for personnel to work in the area. In the diagram above only the internal shielding is visible, but an outer layer of 16-20cm thickness is also used. The photographs below show the copper source holder and a dummy source (left) and the small conical opening mounted on the cryocooler (right). 



The slow positron beam is guided to the trap system using a magnetic field generated by a series of Helmholtz coils. These can be seen in the photograph below, in a dark red colour surrounding the vacuum chambers.

Positrons are initially collected in a buffer gas positron accumulator before being transferred to the cusp trap for anti-hydrogen production. The pre-accumulator consists of a long gas cell (on the left in the photograph below) and a high precision cylindrical multi-ringed electrode trap (right) which are located within a superconducting magnet. Both the gas cell and the MRE trap have a number of electrodes which may be individually biased to produce a trapping potential. An example trapping potential is shown in the figure below in red (y axis scale on the right hand side). By adjusting the internal diameter of the gas cell electrodes and adding pumping holes a spatially varying pressure distribution can be produced; an example of this distribution is shown in the figure below in black (y axis on the left hand side).



The principle of operation of the pre-accumulator is to use inelastic collisions with nitrogen molecules as an energy loss mechanism for the positrons. Specifically, the excitation of N2 by positron impact :

e+ + N2 → N2* + e+

In this process, the positron loses 8.5eV of energy to the internal energy of the nitrogen molecule. An electrical potential is created so that if a positron undergoes one such collision it is confined within the trap electrodes. Once confined to the trap system, the positron will undergo other such collisions, losing energy until it becomes confined to a small harmonic potential well in the MRE trap. The pressure distribution is arrange such that positrons entering the trap have the highest probability of colliding with a N2 molecule at the trap entrance and that the pressure is low enough at the position of the harmonic potential that positrons have a long lifetime against annihilation

Once a substantial number of positrons have been accumulated in the pre-accumulator, they are transferred to the cusp trap via a magnetic conduit. The transfer section consists of 3 magnets which guide the positron cloud via two 90 degree bends into the cusp trap.

This transfer process may be repeated multiple times, a process referred to as stacking. In this way a large number of positrons can be accumulated in the cusp ready for combination with pbars to produce antihydrogen.