Prototype aerogel ˇ Cerenkov counter for kaon/pion separation

For hypernuclear experiments, which are currently performed at GSI (HypHI) or proposed for FAIR (PANDA) and for MAMI (A1), an unambiguous kaon identification is essential. An aerogel Cerenkov counter was designed for the KAOS spectrometer at the Mainz Microtron MAMI for the discrimination between kaons and pions of momenta p ≥ 1GeV/c. Before the construction of the final detector a smaller prototype has been built, mounted inside the KAOS spectrometer, and tested. The prototype consisted of a 20×10× 10 cm light collection box with a single layer of aerogel of active area 20 × 10 cm. The inner walls were covered by Millipore paper and the photons were detected by two 2” PMTs, attached on opposite sides. Different refractive