the MEDUSA gamma ray logging system  Most “conventional” airborne gamma ray systems comprise a set of four 4l sodium iodine scintillator crystals (c.f. the GR820 from Exploranium, and others). In such systems, the measured gamma spectra are translated into potassium, uranium and thorium concentrations using (a derivative of) the so called “windows method”. In our system, we include two enhancements that allow for a drastic reduction in detector volume and thereby in detector weight. First of all, our patented data analysis algorithms use the full spectral information in determining the nuclide concentrations. Hendriks et. al. (Hendriks, 2001) have shown that a reduction in uncertainty per nuclide concentration of at least a factor of 2 can be achieved by applying Full Spectrum Analysis instead of the traditional Windows method. This allows a reduction in detector volume by a factor of 4(¹) without compromising the efficiency of the system (see also the figure below). Secondly, the type of detector material greatly influences the measurement efficiency. Traditional systems utilize NaI as a scintillator. We prefer to use CsI as a scintillator. CsI is about 2.5 times more efficient as NaI. Moreover, it is more robust and less hygroscopic. In conclusion, our enhanced FSA analysis combined with an optimized 4x4x16 inch CsI detector allows for at least the same efficiency and data quality as a pack of four 4x4x16inch NaI detectors, thereby greatly reducing weight, complexity of the analysis and cost. The relative uncertainty in the activity concentrations of 232Th spectra analysed according to the full-spectrum analysis (triangles) and traditional windows analysis (squares) as a function of counts in the spectrum (from Hendriks, 2001). Upon request extra detectors can be added to the system, to a maximum of four 4x4x16 units. Note the overall weight per detector of about 20 kg.