Cryogenic tunable microwave cavity at 13GHz for hyperfine spectroscopy of antiprotonic helium

For the precise measurement of the hyperfine structure of antiprotonic helium, microwave radiation of 12.9GHz frequency is needed, tunable over ±100MHz. A cylindrical microwave cavity is used whose front and rear faces are meshed to allow the antiprotons and laser beams to enter. The cavity is embedded in a cryogenic helium gas target. Frequency tuning of ∼300MHz with Q values of 2700–3000 was achieved using over-coupling and an external triple stub tuner. We also present Monte-Carlo simulations of the stopping distribution of antiprotons in the low-density helium gas using the GEANT4 package with modified energy loss routines.