Improvements in the in-beam gamma-ray spectroscopy provided by an ancillary detector coupled to a Ge gamma-spectrometer: The DIAMANT-EUROGAM II example

For the first time the 4 pi gamma-ray spectrometer EUROGAM II has been coupled to a 4 pi light charged particle detector array, DIAMANT, during a test experiment on the reaction S-32 + Ni-58 at 120 MeV beam energy. A very large improvement in the peak-to-background ratio of the gamma-spectra has been found when EUROGAM II is triggered by DIAMANT to select an exit channel. A simple algebra has been developed which provides theoretical estimates in good agreement with these experimental results. It is demonstrated that, depending on both the gamma-spectrometer and ancillary detector performances, much better peak-to-background can be obtained by such a coupling. For the same peak-to-background ratio, the use of an ancillary detector allows for a lower gamma-ray coincidence level and therefore improves the statistics. Ways to select the most appropriate ancillary detector an given. The ability of the ancillary detector to provide a total Doppler shift correction is crucial for the improvement of the peak-to-background ratio.