Optimization of the detector-target geometry for in-beam singles and coincidence γ-ray spectroscopy

Abstract

The spectroscopy of γ-rays belonging to high multiplicity cascades requires special care to be taken of the summing-up effects in spectrometric detectors. For typical (heavy ion, xn γ) experiments these effects set rather low practical limits on the solid angle subtended by a Ge(Li) detector at a target. These limits are calculated to vary between about 1% and 5% for a standard coincidence experiment with current Ge(Li) detectors. The limit for quantitative yield determinations are much lower. Simple figures-of-merit are presented as functions of total detection efficiency and of γ-ray multiplicity. Detectors with substantially improved peak-to-total ratios or special purpose-oriented detectors for in-beam experiments are needed in order to improve the detection efficiency beyond the present low limits. Due to the dependence of the photopeak effiency on the γ-ray multiplicity there is a distortion introduced to the multiplicity distribution measured with a Ge(Li)NaI multidetector coincidence system. Formulae including the summing-up corrections in the Ge(Li) detector are presented for coincidence probabilities of various folds. The corrections are found to be important, particularly for higher moments of the multiplicity distribution. The incorporation of these corrections in practical data analysis is discussed.