K. Kossert, M. Camen, F. Wissmann, J. Ahrens, H.-J. Arends, R. Beck, G. Caselotti, P. Grabmayr, P.D. Harty, O. Jahn, P. Jennewein, M.I. Levchuk, A.I. L’vov, J.C. McGeorge, A. Natter, V. Olmos de León, V.A. Petrun’kin, M. Schumacher, B. Seitz, F. Smend, A. Thomas, W. Weihofen, and F. Zapadtka 1 Zweites Physikalisches Institut, Universität Göttingen, D-37073 Göttingen, Germany 2 Institut für Kernphysik, Universität Mainz, D-55099 Mainz, Germany 3 Physikalisches Institut, Universität Tübingen, D-72076 Tübingen, Germany 4 Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK 5 B.I. Stepanov Institute of Physics, Belarussian Academy of Sciences, 220072 Minsk, Belarus 6 P.N. Lebedev Physical Institute, 117924 Moscow, Russia (February 8, 2008)

A comprehensive set of $^{4}\mathrm{He}$$(\ensuremath{\gamma},n)$ absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from $23<{E}_{\ensuremath{\gamma}}<70$ MeV were directed toward a liquid $^{4}\mathrm{He}$ target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983.

A comprehensive set of 4He(,n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E<70 MeV were directed toward a liquid 4He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983. (Less)

A near-threshold 4He(g,n) cross-section measurement has been performed at MAX-lab. Tagged photons from 23 < Eg < 42 MeV were directed toward a liquid 4He target, and neutrons were detected by time-of-flight in two liquid-scintillator arrays. Seven-point angular distributions were measured for eight photon energies. The results are compared to experimental data measured at comparable energies and Recoil-Corrected Continuum Shell Model, Resonating Group Method, and recent Hyperspherical-Harmonic Expansion calculations. The angle-integrated cross-section data is peaked at a photon energy of about 28 MeV, in disagreement with the value recommended by Calarco, Berman, and Donnelly in 1983.

Using the Mainz 48 cm 0 x 64 cm NaI(Tl) detector and the segmented Gottingen recoil detector SENECA in coincidence, Compton scattering by the proton at theta(gamma)(lab) = 136degrees has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities gamma(pi) = -38.7 +/- 1.8 and -27.2 +/- 3.1 (x10(-4) fm(4)), respectively.

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.

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θlabγ = 59°-155° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be \(\) = [11.9±0.5stat.±1.3syst.±0.3mod.] . 10-4fm3 and \(\) = [1.2±0.7stat.±0.3syst.±0.4mod.)] . 10-4fm3. These results confirm the Baldin sum rule which was re-evaluated to be \(\) + \(\) = [13.8±0.4] . 10-4fm3. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γπ beyond the t-channel π0-exchange.

R. O. Owens, 1 M. Sauer, 3 R. Schneider, 3 G. J. Wagner, 3 and T. T-H. Yau Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom Department of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom Physikalisches Institut, Universita ̈t Tübingen, D-72076 Tu ̈bingen, Germany Institut für Kernphysik, Universita ̈t Mainz, D-55099 Mainz, Germany ~Received 23 November 1999; published 21 June 2000 !

A measurement of the C-12(gamma,pi(+)n)B-11 reaction in quasifree pi-production kinematic regimes has been performed using tagged photons in conjunction with large solid angle pi and n detectors. The aim of the experiment was to investigate predicted modifications to the Delta excitation of nucleons and their subsequent propagation and decay, brought about by the nuclear medium. Differential cross sections an presented for photon energies spanning the Delta(1232) excitation region. The measurements are consistent with distorted wave impulse approximation calculations in which the amplitude for proton Delta excitation, followed by Delta propagation and decay to pi(+)+n, is reduced compared to that for a free p. However, because of uncertainties in the magnitudes of the final state interactions, it is concluded that improved calculations are required to obtain a quantitative estimate of Delta-medium effects.

The C-12( gamma,np) and C-12( gamma,pp) reactions have been studied using the Glasgow photon tagging spectrometer at the Mainz MAMI electron microtron for E gamma = 150-700 MeV over a kinematic range which extends well beyond the approximately back-to-back detector arrangements of previous work. For C-12(gamma,np) the general trends of the missing energy distributions are reproduced over a wide range of kinematics and photon energies by the theory developed by the Valencia group. The corresponding C-12(gamma,PP) channel is overestimated by a factor of similar to 3. Detailed comparisons of the experimental data with a Monte Carlo simulation of the direct 2N knockout process provide the first evidence above the Delta resonance for direct 2N knockout and show that this process dominates the C-12(gamma,np) reaction at low missing energies up to E gamma similar to 700 MeV. The C-12(gamma,pp) reaction is somewhat less well described by the Monte Carlo simulation. A possible explanation of the observed discrepancies within a direct 2N framework is presented. At high recoil momenta both 2N reaction channels exhibit an excess yield compared to the Monte Carlo prediction of direct 2N knockout. The excess yield in this region is compared with the predicted effects of short-range correlations and with the predicted contributions due to other reaction mechanisms.