Experimental Study of the ReactionHe3(γ, π+)H3up to 260 MeV

The differential cross section for the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}(\gamma , {\pi }^{+}){}_{}{}^3{}_{}{}^{}\mathrm{H}$ was measured for photon energies of 180 to 260 MeV. A target of gaseous ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at 20.4°K and approximately one atmosphere pressure was irradiated by a 270-MeV bremsstrahlung x-ray beam. The reaction was observed by detecting ${\pi }^{+}$-${}_{}{}^3{}_{}{}^{}\mathrm{H}$ coincidences. Tritons with energies between 9.7 and 24.5 MeV (momentum transfer 1.2 ${\mathrm{F}}^{-1\mathrm{}}$ to 1.9 ${\mathrm{F}}^{-1\mathrm{}}$) were observed at laboratory angles of 26°, 30°, 35°, and 40°. At 26° the cross sections in the center-of-mass system ranged from 2.55±0.34 μb/sr to 0.78±0.16 μb/sr, decreasing with increasing photon energy. Impulse-approximation calculations relate the cross section to the matter form factor of the 3-nucleon system under the assumption of identical wave functions for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{H}$. Cross sections found in this experiment lie from 25% to 50% below the values predicted by this theory with form factors from electron-scattering data inserted.