Optimization of sample geometries in low-level gamma spectroscopy

In gamma spectrometric measurements of environmental samples it is often important to maximize the throughput without compromising the required detection limits. In order to do this one needs to know the optimum sample size and shape for counting with germanium detectors of different sizes. In this study, relative full energy peak efficiencies corresponding to different sample volumes were obtained by a computer method transferring a measured full energy peak efficiencies to other cylindrical geometries for the same detector. Two germanium detectors having 40 and 100% peak efficiencies relative to a 3 in. by 3 in. NaI(Tl) were studied. The investigated sample volumes ranged from 3 to 500 ml. The effect of sample density was also examined. Comparing the corresponding count rates of different beakers and filling heights, a Marinelli beaker configuration was almost always found to be the best choice. Even if there is a rather small amount of sample material available, it is usually profitable to dilute it to a larger volume and measure it in a Marinelli beaker.