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The photodisintegration of He3. Robertson, Lyle Purmal


The cross section for the photodisintegration of a three body nucleus, helium-3, has been accurately measured for the first time. The He³( ɣ,p)D reaction cross section has been measured at gamma ray energies of 6.14, 6.97 and 7.08 Mev and found to be 0.104 ± .007, 0.298 ± .050 and 0.308±.024 milli-barns respectively. The photodisintegration cross section agrees with that calculated from the D(p,ɣ)He³ reaction using the principle of detailed balance to within the experimental errors. This is an accurate verification of this principle as applied to a direct nuclear-photon interaction. The photodisintegration reaction was observed in a cylindrical gridded ionization chamber using a helium-3, methane, and argon gas mixture. The F¹⁹(p, αɣ)016 reactions at proton bombarding energies of 873.5 kev and 935 kev were used as the source of gamma rays of well defined energy. The energy resolution of the chamber was adequately good to give a reasonable separation of the 642 kev photodisintegration reaction product peak from the electron background and the 765 kev peak from the He³(n,p)T reaction. It was necessary to reduce the tritium contamination in the helium-3 gas in order to reduce the background arising from the beta decay of the tritium; purification to less than 1 part tritium in 10¹⁰ parts helium was achieved. Proposed measurements of the photodisintegration cross section at higher gamma ray energies using ionization chambers and other types of detectors are discussed.

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