An Absolutely Calibrated Source of 6.13 MeV Gamma-Rays

At the 340 keV resonance in the 19F(p, αγ)16O reaction, after kinematic corrections, there is a 1:1 correspondence between the 55/s=deg/ yield of alpha particles feeding the 6.13 MeV state and the yield of 6.13 MeV photons observed at 55° to the proton beam. The 55° intensities of alpha particles feeding the various states in 16O were found to be: α6.05: α6.13: α6.92: α7.12 = ≤0.14: 100: 0.08: 2.6 which implies the only non-negligible contaminant is a 7.12 MeV gamma-ray radiation with an intensity of 2.6%. After a number of small corrections, including those for photon attenuation in the target backing and air, the fluence of 6.13 MeV gamma-rays can be determined absolutely with an uncertainty of ~1%. Excellent agreement was obtained in a comparison of the calculated and measured efficiencies of a 5 × 4" NaI detector. The calibration of Ge(Li) detectors required an aluminium coated target to avoid a Doppler tail on the 6.13 MeV photon peaks caused by 16O nuclei recoiling from the surface of the CaF2 target.