A high efficiency electron pair spectrometer

Abstract a multi-detector high efficiency spectrometer for the detection of electron-positron pairs in the presence of high intensity gamma radiation has been designed, built and tested. The spectrometer employs six RCA 8850 photomultipliers. A “sandwich” made of a 0.5 mm thick anthracene glued to the front face of a 45 mm thick NE 102A plastic scintillator was mounted on each photomultiplier. A pulse shape discrimination circuit distinguishes electron from gamma signals, making use of the fact that electrons produce a relatively long scintillation pulse (≈ 25 nrmns ) in the anthracene, whereas gammas have a negligible probability to interact with the thin anthracene. The six scintillators cover a solid angle of 3.2 π sr around the target and their front surfaces form nine boundary lines. An annular Si(Li) detector (0.081 sr) used to detect particles backscattering from the target may be operated in coincidence with the electron detector. Proper adjustment of the pulse shape discriminators resulted in accepting the electrons from a 207 Bi source and in rejecting 98% of 60 Co radiation. The spectrometer was tested using the reaction 19 F(p, α) 16 O at 2.47 MeV proton bombarding energy. Approximately 90% of the 7.12 MeV gammas from the 1 − level in 16 O were rejected. The pairs from the 6.05 MeV O + level were detected with 28% efficiency and 12% energy resolution (fwhm).