Spatially resolved investigation of thermally treated brass with a coincidence Doppler spectrometer

A spatially resolving coincidence Doppler spectrometer has been developed at the intense positron beam NEPOMUC at the nuclear research reactor FRMII in Garching near Munich. The positrons are focused onto the sample which is positioned in a vacuum chamber in order to achieve lateral resolution. The emitted annihilation radiation is detected by two facing germanium detectors in coincidence in order to enable background-free measurements. Since the measured spectra are not degraded by background, the high momentum region of the annihilation line can be precisely analysed. The positron beam structure is described and first measurements are presented in which thermally treated brass was investigated in order to determine the chemical environment of the positron at the annihilation site.