Kaon tagging at 0 degree scattering angle for high-resolution decay-pion spectroscopy

At the Mainz Microtron MAMI hypernuclei can be studied by (e,e'K) reactions. By detecting the kaon which is emitted in forward direction, with the KAOS spectrometer placed at 0 degree scattering angle, reactions involving open strangeness production are tagged. High-resolution magnetic spectrometers are then used to coincidentally detect the monoenergetic decay-pions from mesonic two-body weak decays of light hypernuclei at rest. As a pioneering experiment has confirmed, the KAOS spectrometer is exposed to a large flux of background particles, mostly positrons from Bremsstrahlung pair production. In order to increase the efficiency of kaon identification the KAOS spectrometer was modified to suppress background particles at the cost of a high momentum resolution, which is less important for this experiment. This was achieved by placing up to 14 cm of lead absorbers in front of the detectors, in which positrons are blocked by forming electromagnetic showers while the effect on kaons is limited. An additional time-of-flight wall and a new threshold Cerenkov detector help to increase the detection efficiency of kaons. It has been shown, that with this set-up and a ⁹Be target a clean identification of kaons associated with electro-produced Λ hyperons at 0 degree scattering angle in coincidence with pions originating in weak decays of strange systems is possible