Positron Source

Positrons in ALPHA originate from a radioactive isotope of sodium Na-22. The Na-22 source is manufactured at iThemba Labs in Cape Town, South Africa and then shipped in a special container to CERN following international protocols for the transport of radioactive substances.

Na-22 has a long half-life of 2.6 years, and therefore emits a reproducible quantity of positrons on a day-to-day basis. It undergoes radioactive decay as follows:

Na → 22Ne + e+ + νe

The emitted positrons typically have an energy range in the hundreds of keV (100 keV = 570 million km/h), and therefore need to be slowed down before they can be trapped . At ALPHA, this is accomplished in a multi-stage device known as the Positron Source and the Positron Accumulator.

In the first stage, the emitted positrons pass through a layer of solid neon (T = 8K) which serves as a moderator to slow down the positrons. While most of the positrons annihilate within the moderator volume, a small fraction (< 1%) make it through with significantly reduced kinetic energies (E = 50 eV). These lower energy positrons are then magnetically guided into the trap of the Positron Accumulator.