Measurements of the He-4(gamma,pi(+)n)H-3 and He-4(gamma,pi(+)p)nnn reactions in quasifree pi production kinematic regimes have been performed using unpolarized and linearly polarized tagged photons in conjunction with large solid angle pi and n detectors at the MAMI-B accelerator, Mainz. Differential cross sections are presented for photon energies spanning the Delta(1232) excitation region. These data combined with previously presented results for the C-12(gamma,pi(+)n)B-11 reaction provide an extensive data set for future investigations of Delta-medium effects in nuclei. The He-4(gamma,pi(+)n)H-3 cross sections are significantly smaller than theoretical predictions based on the free proton amplitude for Delta excitation with a crude correction for final state interactions, and exhibit different trends, which could possibly be a consequence of Delta-medium effects. From a comparison with published C-12(gamma,pi(+)p)Be-11 data, it is concluded that the He-4(gamma,pi(+)p)nnn reaction mechanisms are probably two-step processes, although the possibility that part or all of the cross section arises from interactions with preexisting Delta(++) particles cannot be ruled out.

The C-12(<()over right arrow>, pp) and C-12(<()over right arrow>, pn) reactions have been studied for E-gamma = 160-350 MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry Sigma for missing energies E-m below 70 MeV, where direct emission nucleon pairs is expected, A strong peak at low E-m is observed in Sigma ((gamma ,pp)), but not in Sigma ((gamma ,pn)). Further differences between the two reaction channels are observed in the E-gamma dependence of Sigma and in the angular distributions of differential cross sections for photons polarized parallel, or perpendicular, to the reaction plane. Theoretical calculations using an unfactorized distorted wave treatment of direct two-nucleon emission do not agree with the magnitude of the photon asymmetry or the angular dependence of the differential cross sections for either channel. For E-m>70 MeV and E gamma >250 MeV, Sigma has a substantial negative value which is similar for both reaction channels. This is attributed to two-step reactions following initial quasifree pion production.

The ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pp)$ and ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pn)$ reactions have been studied for ${E}_{\ensuremath{\gamma}}=160\ensuremath{-}350$ MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry $\ensuremath{\Sigma}$ for missing energies ${E}_{m}$ below 70 MeV, where direct emission nucleon pairs is expected. A strong peak at low ${E}_{m}$ is observed in ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pp)},$ but not in ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pn)}.$ Further differences between the two reaction channels are observed in the ${E}_{\ensuremath{\gamma}}$ dependence of $\ensuremath{\Sigma}$ and in the angular distributions of differential cross sections for photons polarized parallel, or perpendicular, to the reaction plane. Theoretical calculations using an unfactorized distorted wave treatment of direct two-nucleon emission do not agree with the magnitude of the photon asymmetry or the angular dependence of the differential cross sections for either channel. For ${E}_{m}>70$ MeV and ${E}_{\ensuremath{\gamma}}>250$ MeV, $\ensuremath{\Sigma}$ has a substantial negative value which is similar for both reaction channels. This is attributed to two-step reactions following initial quasifree pion production.

The photon asymmetry (Σ) of the 12C(γ→,pn) reaction has been measured using linearly polarised tagged photons at the Mainz microtron MAMI for Eγ=180–340 MeV. The data have been analysed in separate missing energy (Em) regions corresponding to the ejection of nucleons from (1p)2 and (1p)(1s) shells. The measured Σ values for both Em regions are smaller in magnitude than corresponding 2H data, but the (1p)2 results have a similar Eγ dependence. Calculations of direct two-nucleon emission overestimate the magnitude of Σ at all photon energies.

The photon asymmetry (Sigma) of the C-12((gamma)over-right-arrow,pn) reaction has been measured using linearly polarised tagged photons at the Mainz microtron MAMI for E-gamma = 180-340 MeV. The data have been analysed in separate missing energy (E-m) regions corresponding to the ejection of nucleons from (lp)(2) and (lp)(ls) shells. The measured Sigma values for both E-m regions are smaller in magnitude than corresponding H-2 data, but the (lp)(2) results have a similar E-gamma dependence. Calculations of direct two-nucleon emission overestimate the magnitude of Sigma at all photon energies. (C) 1999 Elsevier Science B.V. All rights reserved.

A thin walled cell of 8cm length and 3cm diameter has been constructed for liquid 4He to be used as a target in a tagged photon beam. The target and the housing have been optimised to allow for detection of charged particles emitted on both sides of the beam. A reservoir of 3l liquid He and good thermal insulation permitted refilling cycles of 12h.