Efficiency determination for the ATRAP detector system

The ATRAP experiment at the CERN Antiproton decelerator (AD) aiming for a precise CPT test by studying the antihydrogen atom includes a detection system for the antiproton annihilation products. Antihydrogen production studies require the knowledge of the produced antihydrogen atoms as well as absolute antiproton losses during the development. In 2008 the ATRAP experiment has been modified by adding an additional solenoid to improve the p̄ trapping and has been operated at a lower main field 1 T. Furthermore some photomultipliers have been exchanged and the PM voltages have been adjusted. Therefore the efficiency for the detection of an antiproton annihilation has been determined by using cosmic rays and Monte Carlo studies. In order to get accurate values for the detection efficiency of a p̄ annihilation one should trap a single antiproton, release it at a well defined time, observe the registered detector signals and do this procedure a sufficient number of times to reach a low statistical uncertainty. The trapping of single antiprotons within the ATRAP setup has been achieved in 2008 but it has to be further developed before this experimental efficiency determination is available. Another possibility is the use of Monte Carlo studies which have been done by using the GEANT4 software package. As input for the simulation studies one needs a realistic geometry and the energy thresholds for the registration of a signal in the individual detector components. As a first step the efficiency of the different detector components for the detection of a minimum ionized particle is determined by measuring cosmic particles. The detector system is sketched in fig. 1. It consists of four fiber rings sur-