Large size LSO:Ce and YSO:Ce scintillators for 50 MeV range /spl gamma/-ray detector

LSO:Ce and LYSO:Ce scintillators are candidates for the pre-shower detector for the search for /spl mu//sup +//spl rarr/e/sup +/ /spl gamma/ decay with the branching-ratio sensitivity of 10/sup -14/ at PSI (Villigen, Switzerland). The detection involves 52.8 MeV /spl gamma/-rays. To study the response of the crystals to high energy /spl gamma/-rays we have chosen a Pu-Be neutron source which gives mono-energetic 4.439 MeV /spl gamma/-rays due to the /sup 9/Be(/spl alpha/,n) reaction. We have studied the response of a /spl phi/60/spl times/10 mm YSO:0.2%Ce crystal and a half disk of /spl phi/60/spl times/10 mm LSO:0.2%Ce. Despite the large background of /sup 176/Lu, the high energy /spl gamma/-rays can be detected efficiently. We have also studied the response to lower energy /spl gamma/-rays (from /sup 137/Cs and /sup 60/Co sources) and found the conditions of optimum work. The YSO crystal detects 4.439 MeV /spl gamma/-rays primarily by double 511 keV escapes that form a distinct peak in the energy spectrum.