B. Ziegler

180° Compton scattering by the proton below the pion threshold

Abstract Absolute cross sections for Compton scattering from protons have been determined at 180° for the backscattered photon at incident laboratory photon energies of 98 and 132 MeV. For the difference between the electric and the magnetic polarizability of the proton a value of (7.03 − 2.37 +2.49 − 2.05 +2.14 ) × 10 −4 fm 3 has been derived using the predictions from calculations based on relativistic dispersion relations.

The photodisintegration of deuterium at 0° for the outgoing protons

Abstract The absolute differential cross section for the photodisintegration of the deuteron at 0° for the outgoing protons has been measured at lab photon energies of 20, 30, 40, 75, 100 and 120 MeV using a magnetic spectrometer and a bremsstrahlung beam. The absolute cross-section values have an error of ±6% and were determined with an angular resolution of 1.5°. A serious discrepancy is found to exist between the measured 0° cross sections and those predicted by calculations using the Hamada-Johnston and the boundary condition potential models.

Nuclear photon scattering by 208Pb

Abstract Photon-scattering cross sections covering the energy range from 10 MeV to 100 MeV have been measured using monochromatic photons and bremsstrahlung. A comparison of both sets of data shows relatively small inelastic contributions, in addition to the elastic processes. The elastic processes yield parameters for the giant dipole, and the isovector giant quadrupole resonances. The partition of the total absorption cross section for 208 Pb into these resonances, as well as into the quasideuteron, and the mesonic absorption is deduced. To identify fully the multipolarities in the Δ-resonance region, the measurements must be extended to higher photon energies. From the scattering cross section at 60 MeV to 100 MeV photon energy, a value for the sum of the static electric and magnetic polarizabilities of an average bound nucleon has been deduced which is slightly larger than the presently accepted experimental value for the free proton. Using the classical form factor picture for a description of the effects of spatial extent, form factors could be deduced for the normal and the exchange part of the interacting electromagnetic current.

Photodisintegration of the deuteron at 15-25 MeV photon energy

Abstract The total cross section for the photodisintegration of the deuteron has been measured at 15, 20 and 25 MeV photon energy. The results agree with theory within the experimental errors of ± 5 % and do not confirm a 20 % discrepancy between experiment and theory as previously reported in the literature.

Absorption and scattering of photons by 208Pb

Abstract Experimental data on elastic scattering of photons in the energy range from 10 to 100 MeV for the nucleus 208 Pb are presented. These new data along with some previously published data on photoabsorption are tentatively analysed in the framework of a consistent description. Besides a value for the summed total strength, the partial strengths of some multipoles were obtained. An electric quadrupole resonance was found to be located at 24.3 MeV with a strength of 1.4 isovector sums. An experimental value was deduced for a parameter which can be regarded as a half-density radius for the spatial interaction strength density distribution, including exchange currents, in a nucleus. This radius parameter was found to be 4.9 ± 0.15 fm, considerably smaller than the electrical charge density distributions radius (6.6 fm), as obtained by elastic electron scattering.

Photon scattering from 12C and 208Pb in the Δ-region

Abstract The photon-scattering cross sections at 115° for 12 C and 208 Pb have been measured using bremsstrahlung energies E of 150, 200, 250, 300, 350 and 400 MeV. Scattered photons having energies in the range 0.9 E to E were accepted by the detector. The measured cross sections are much larger than the prediction of a simple model relating the scattering cross section at a large angle to the forward-scattering cross section and the form factor for elastic electron scattering. This discrepancy is discussed in terms of collective effects, inelastic scattering, or exchange-current distributions.

Polarizability of the neutron

Abstract A first measurement of quasi-free Compton scattering by the neutron bound in the deuteron has yielded a value for the polarizability of the neutron of αn=(11.7 −11.7+4.3)×10 −4fm 3. This means that a finite value for the polarizability has been obtained, though the result is still consistent with zero. The π0 decay constant Fπ 0γγ and the pion nucleon coupling constant gNNπ have opposite signs.

Nuclear photon scattering by 12C

Abstract Cross sections for photon scattering by the 12C nucleus have been measured in the energy range from 15 to 140 MeV. Quasimonochromatic tagged photons allowed the separation of elastic and inelastic processes. An additional measurement with continuous bremsstrahlung improved the accuracy of the elastic part of the cross section considerably. In analyzing the bremsstrahlung runs, the inelastic components were taken from the tagged photon results and from literature. The cross sections were analyzed in terms of giant resonances, proton-neutron substructures and subnuclear excitations. Form factors for charge and exchange current were applied to take into account the finite nucleon and exchange-current distribution in 12C. For the E2 resonance, a coupled-channel model was used, which is applicable to an isolated resonance superimposed on a continuum. The average sum of the static electric and magnetic polarizabilities of bound nucleons was found to be in agreement with the value of the free proton.

The forward, backward and 90° deuteron photodisintegration between 7 and 19 MeV photon energy

Abstract The 0° and 180°, as well as the 90° differential cross sections for the 2 H(γ, p)n reaction have been measured at lab photon energies between 7 and 19 MeV. Special attention was paid to the accuracy, in particular by measuring the forward Compton electron yield. For the extreme angles, the statistical error on our results amounts to 4–5% (0°) and 7–8% (180°), respectively, while the systematic uncertainty is at most 3%. The data confirm the existence of a minimum in the forward cross section and indicate beyond any doubt that the fore/aft ratio is larger than unity. Comparison shows reasonable agreement with the results from recent “conventional” theoretical approaches, including meson exchange and relativistic corrections, although the description of the c -coefficient, appearing in the Partovi expansion of the differential cross section, remains unsatisfactory. A possible source for this discrepancy could be the inadequate description of the El and/or the E2 transition operator.

Absolute photofission cross section of 209Bi in the energy range from 40 to 65 MeV

Abstract The absolute photofission cross section of 209 Bi has been measured with monoenergetic γ-radiation between 40 and 65 MeV photon energy. Cross-section data have been obtained with an accuracy between 9 and 20%. The experimental result is compared with the excitation function calculated on the basis of the statistical model. In order to reproduce the measured data on σ γf , the (γ, n) cross section must decrease with increasing photon energy faster than the experimental total (γ, n) cross section. This behaviour can possibly be explained by the assumption that after photon absorption a compound nucleus is formed only for a small, and with photon energy decreasing, fraction of all decays.