First we calibrated the spectrometer in atmosphere using the peak-channel of 241Am, Thc-c' standard radioactive source while the window diameter isφ3. 0 mm and gained following formula with respect to the energy (E/MeV) ofαand the channel (Ch): E=1. 507 55 + 0. 016 31×Ch+6. 235 24×10-6×Ch2 So, when E=5. 486 MeV, the Ch is equal to 221. 493. When changing the diameter of incident window toφ1.5 mm, we find the peak-channel of
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