Table of contents

The mesonic-exchange-current contributions for the gamma d to pi ⁰d reaction are studied by using the model in which this effect is described by the elementary amplitude of double pion photoproduction on a nucleon, one of them being soft and absorbed by the other nucleon. The interference effects with the impulse and pion rescattering terms are also calculated. It is found that at the (3-3) resonance energy the whole contribution is between 5 and 10% at backward angles.

The analysing power A_y( theta ) for elastic and inelastic (2⁺, E*=1.78 MeV) neutron scattering from Si was measured at 14.1 MeV. The data were analysed together with published differential cross sections using the coupled-channels formalism. The parameters of the optical potential, especially for the spin-orbit part, could be definitely determined from the authors' accurate analysing power data.

The establishment of several excited 0⁺ states in ¹²⁴Te provides a sensitive test which shows that the collective levels in this nucleus can be described by the O(6) limiting symmetry of the interacting boson model. Unlike other O(6) nuclei the spin-dependent L.L term in the Hamiltonian is negative and causes the highest spin levels in each multiplet to lie at lowest energy. It is suggested that the even tellurium isotopes may provide an opportunity to study the Su(5) to O(6) transition.

The extraction of multipole resonance fission decay probabilities from inclusive electrofission cross sections is found to require data of greatly increased accuracy than those of the fission decay low-multipolarity electric resonances to electrofission cross sections. Data presently available are shown to be compatible with statistical fission decay of the giant dipole resonance.

The authors argue that the N_beta trajectory plays a role in nucleon-exchange reactions which, though small compared with N_alpha in pi N to N pi , provides a better understanding of SU(3)-related nucleon-exchange reactions, particularly those involving Sigma and eta production.

Optical-model analyses of 800 MeV elastic proton scattering by eleven nuclei are performed. Relations between the optical-model parameters and the nucleon-nucleon parameters of the Glauber model are derived. The results indicate that the nucleon-nucleon cross section in the Glauber calculations should be somewhat larger than the free one and that the slope parameters for the spin-dependent amplitude should be considerably larger than assumed in previous analyses.

Unitarily transformed deuteron wavefunctions of the kind recently used in a model-dependent derivation of the deuteron radius and in consideration of the deuteron parameter D₂ relevant to sub-Coulomb stripping reactions are used to calculate the total cross section for photodisintegration sigma _ph, the thermal n-p radiative capture cross section and the deuteron electric form factor. These wavefunctions, which have an S-wave node as suggested by a recent quark model, give a reduction in sigma _ph by up to about 6% compared with that from the untransformed wavefunctions. Current data do not allow discrimination between the two.

The authors investigate the merits of a simple generator coordinate method (GCM) description of the breathing mode. It yields very good excitation energies for ⁴⁰Ca, ⁹⁰Zr, ¹⁴⁰Ce and ²⁰⁸Pb. The results are compared with experiment and with other theories. The authors also consider collective formalisms derived from the GCM and find that they constitute an excellent approximation to the exact GCM.

The g factor of the 1.35 MeV ⁵/₂^- state in ¹⁹F has been measured by observing the hyperfine modulation of the gamma-ray anisotropy in highly stripped ions recoiling into vacuum following the ⁴He(¹⁹F, alpha )¹⁹F reaction. From the observed spatial frequency the g factor was deduced to the mod g mod =0.269(43). Simultaneous lifetime determinations for this state and the ¹³/₂⁺ state in ¹⁹F gave values of tau =4.17(6) ps (1.35 MeV) and tau =3.68(38) ps (4.65 MeV).

The number of K-shell vacancies produced per positron emission has been measured for the beta ⁺ decay branch of ⁵⁸Co. The result, (16.6+or-2.1)*10^-4 vacancies per positron, is in reasonable agreement with a recent independent measurement but is in severe disagreement with the latest theoretical prediction.

The beta-delayed fission of ²⁴⁰Bk was studied following the reaction ²⁴¹Am(³He,4n) at E_lab=33 MeV. From the experiment the author obtains a half-life T₁2/=(4.2+or-0.8) min and a ground-state cross section of (28+or-16) mu b; using these data the beta-delayed fission probability is determined as (1.3_-0.7^+1.8)*10^-5. The fission barrier parameters of ²⁴⁰Cm are obtained from an extrapolation of those for the adjacent ²⁴²Cm in accordance with the lifetimes. Assuming a Gaussian structure in the ²⁴⁰Bk beta strength function with a width of the order sigma =0.35 MeV, the centroid energy E_beta =2.1+or-0.1 MeV is obtained from a model calculation. The model includes WKB resonant penetrability, double-delayed fission and microscopic level densities. Using published experimental data the fission barrier heights of ^244,246,248Cf and ^248,250Fm are estimated assuming the same strength functions. The consistency between the estimated barriers and known systematics is improved considerably with the inclusion of the strength function structure.

The atmospheric correction factor for cosmic-ray antiproton measurements has been re-calculated using an approximation of the slab model. It is found that the effect of the p non-annihilation inelastic interaction is quite significant. Neglecting this effect has led to an overestimation of the expected p flux at low energies at the top of the atmosphere.

The authors provide information to enable experimenters to interpret measurements of Cerenkov pulse widths (FWHM) in terms of the distance along the axis to air-shower maximum. The relations were calculated from Monte Carlo simulations for protons using a scaling model and rising cross sections. Primary energies of 10¹⁵, 10¹⁶, 10¹⁷ eV and zenith angles from 0 degrees to 40 degrees are covered. The results will be useful for radii of observation from 200 to 350 m and refer to sea level. The relations are sensitive to marked deviations from scaling-model behaviour on the approach to maximum; however, reasonable aerosol variations to not invalidate the results. The procedure frequently used to obtain the true widths by correcting the measured widths for the impulse response of the detector can lead to appreciable errors near a radius of 200 m. An alternative method is described.

The longitudinal development of muons at atmospheric depths between 200 and 700 g cm^-2 has been studied by measuring the arrival time distributions of muons in air showers with primary energies from 1*10¹⁷ to a few times 10¹⁹ eV observed at Mount Chacaltaya. These arrival time distributions are consistent within those expected from very high multiplicity models of nuclear interactions, whether produced by primary protons or heavy nuclei. The longitudinal development of muons determined from equi-intensity cuts of muon size spectra of air showers is also consistent with the predictions of these models. However, the longitudinal developments of electrons expected from such models show much faster attenuation than is observed. A two-component model of air showers is proposed to explain this apparent discrepancy.