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

LSO:Ce and LYSO:Ce has been proposed as a perspective pre-shower detector for search for μ+→e⁺ γ decay with the branching-ratio sensitivity of 10^-14 at PSI (Villigen, Switzerland). The detection involves 52.8 MeV γ-rays. To study the response of the crystals to high energy γ-rays we have chosen Pu-Be neutron source which due to a ⁹Be(α,n) reaction gives monoenergetic 4.439 MeV γ-rays. We have studied the response of φ60×10 mm YSO:0.2%Ce crystal and half disk of φ60×10 mm LSO:0.2%Ce. Despite the large background of ¹⁷⁶ Lu the high energy γ-rays can be detected efficiently. We have also studied the response to lower energy γ-rays (from ¹³⁷Cs and ⁶⁰Co sources) and found the conditions of optimum work, YSO crystal detects 4.439 MeV γ-rays primarily by double 511 keV escapes that form a distinct peak on energy spectrum