A proton-proton bremsstrahlung experiment has been carried out at TRIUMF using a 280-MeV polarized proton beam impinging on a liquid-hydrogen target. All three outgoing particles were detected: the higher-energy proton in a magnetic spectrometer, the lower-energy proton with plastic scintillators, and the photon in lead-glass Cherenkov detectors. The experiment shows the first unambiguous evidence for off-shell effects in the free nucleon-nucleon interaction, in that the analyzing powers disagree strongly with the predictions of the soft-photon approximation (which incorporates only on-shell information) but are consistent with the results of calculations using the Bonn and Paris potentials.

Cross sections have been measured up to 35 MeV excitation for the 76 Se͑n,p͒ 76 As reaction at five laboratory angles between 0°and 15°. The incident neutron energy was 198 MeV. The distribution of Gamow-Teller ͑GT͒ strength at excitation energies less than 10 MeV was deduced by carrying out a multipole decomposition of the data. The GT strength below 6 MeV excitation was found to be less than 0.9 units, in qualitative agreement with a quasiparticle random-phase approximation calculation. The present results have been compared with data from the 76 Ge͑p,n͒ 76 As reaction to obtain an estimate of a lower limit for the lifetime of 76 Ge for ␤␤ decay. The estimate is in qualitative agreement with the measured lifetime. ͓S0556-2813͑97͒03206-8͔PACS number͑s͒: 25.40.Kv, 24.30.Cz, 27.50.ϩe, 23.40.Ϫs , ͑4͒but the summation now runs over intermediate states of all spins and parities. An estimate of M 0 is clearly a prerequisite for relating a lifetime measured in 0 ␤␤ decay to a *

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