Spin-triplet strength in the sup 3 H( rvec p ,. gamma. ) sup 4 He reaction at E sub p =2 MeV

The cross section {sigma}({theta}) and analyzing power {ital A}{sub {ital y}}({theta}) of the {sup 3}H({ital {rvec p}},{gamma}){sup 4}He reaction have been measured at an incident proton energy {ital E}{sub {ital p}}=2.00 MeV. The angular distribution of the spin-dependent cross section {sigma}({theta}){ital A}{sub {ital y}}({theta}) shows that both odd parity ({ital E}1) and even parity ({ital E}2,{ital M}1) transitions contribute to this reaction. Detailed matrix element fits result in a spin-triplet total cross section which is 0.72(+0.29,{minus}0.18)% of the total {sup 3}H({ital {rvec p}},{gamma}){sup 4}He reaction cross section. A nucleon-only calculation does not reproduce the data, and predicts a spin-triplet total cross section which is only 0.01% of the total cross section. The inclusion of meson exchange currents into this calculation increases the spin-triplet cross section to 0.5% of the total cross section, in agreement with the present results. A comparison of the calculated and fitted matrix elements is presented.