A 4π dilepton spectrometer: PEPSI

A novel positron-electron pair spectroscopy instrument (PEPSI) was designed to measure transitions in the energy region 10-40 MeV. It consists of Nd2Fe14B permanent magnets forming a compact 4 pi magnetic filter consisting of 12 positron and 20 electron mini-orange-like spectrometers. The response function of PEPSI has bern measured with mono-energetic beams of electrons from 5 to 20 MeV. The PEPSI spectrometer was used for measuring the internal pair conversion coefficient (alpha(pi)) of the 15.1 MeV M1 transition from a J(pi) = 1(+) state to the ground state in C-12. Our experimental value of alpha(pi) = (3.3+/-0.5) x 10(-3) is in good agreement with theoretical estimates