The response of A 24cm x 36cm NaI(TI) spectrometer to γ rays between 50 and 300 MeV

The high-energy gamma-ray response function of a 24 cm diameter x 36 cm long NaI(Tl) detector surrounded by a 10 cm thick plastic anticoincidence shield was measured using a tagged-bremsstrahlung beam. Incident electrons of 300 and 350 MeV radiated in a 55 μm aluminum foil and were analyzed in a 1800 magnetic spectrometer to produce gamma rays at 50, 100, 200, and 300 MeV. For a 2.5 cm diameter collimation of the gamma ray beam, the NaI(Tl) resolution was found to be 5.5% fwhm at Eγ=50 MeV, increasing to 6.3% fwhm at Eγ=300 MeV. The energy resolution significantly improved by accepting only those events which were not coincident with the plastic shield. At Eγ=300 MeV, a low plastic threshold gave a resolution of 2.7% and reduced the NaI(Tl) efficiency to 17%. These measurements are compared with the results from a Monte Carlo calculation using the electron generating shower (EGS) code. (orig.)