The Ludlum Model 44-172 YSO (Yttrium Oxyorthosilicate) detector is used primarily for the detection of low-energy gamma radiation, from approximately 5 keV to 145 keV. It will also detect beta energy as low as 49 keV. The detector consists of a 2.5 cm (1 in.) diameter by 1 mm (0.04 in.) thick YSO scintillator. The window area is approximately 5 cm2 and is constructed of 1.2 mg/cm2 metalized Mylar.