Previous experiments with positrons from radionuclides have demonstrated that positron beams are a rich source of information about the surface condition of solids. We have now demonstrated the possibility of producing very intense beams at the Lawrence Livermore 100 MeV electron linac and installed an apparatus that produces a variable energy positron beam at energies between 500 eV and 20 keV with sufficient intensity to perform a variety of new positron experiments. The positron beam is pulsed with 10 ns to 3 μs duration at rates up to 1440 pulses per second, with as many as 106 positrons available per pulse. Experiments that require either pulsed or steady currents are possible in an ultrahigh vacuum environment. For the first time two-dimensional angular correlation spectra of the surface positron and positronium annihilation at a single crystal sample have been obtained for copper.

The photoneutron cross sections for $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ have been measured from threshold to \ensuremath{\sim}25 MeV with monoenergetic photons from the annihilation in flight of fast positrons. These reactions include the two-body breakup of $^{3}\mathrm{H}$ and the three-body breakup of both $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$; these measurements for $^{3}\mathrm{H}$ are the first to span the energy region across the peaks of the cross sections. An efficient B${\mathrm{F}}_{3}$-tube-and-paraffin neutron detector and high-pressure gaseous samples were employed in these measurements. The results, when compared with each other and with results for the two-body breakup cross section for $^{3}\mathrm{He}$ from the literature, show that: (a) the two-body breakup cross sections for $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ have nearly the same shape, but the one for $^{3}\mathrm{He}$ lies lower in magnitude; (b) the three-body breakup cross section for $^{3}\mathrm{He}$ lies higher in magnitude, broader in the peak region, and also rises less sharply from threshold than that for $^{3}\mathrm{H}$; and (c) these differences between the cross sections for the breakup modes largely compensate in their sum, so that the total photon absorption cross sections for $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ are nearly the same in both size and shape at energies near and above their peaks. Theoretical results from the literature disagree with the experimental results to a certain extent over the entire photon-energy region for which the photoneutron cross sections were measured. Sum rule predictions also fail to reproduce the experimental results. These discrepancies constitute a challenge to the principle of charge symmetry of the nuclear force, but more complete theoretical calculations are needed to ascertain whether these discrepancies can be ascribed entirely to electromagnetic effects.NUCLEAR REACTIONS $^{3}\mathrm{H}$($\ensuremath{\gamma}$,$n$), $^{3}\mathrm{H}$($\ensuremath{\gamma}$,$2n$), $^{3}\mathrm{He}$($\ensuremath{\gamma}$,$n$); measured $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}})$, threshold to \ensuremath{\sim}25 MeV; monoenergetic photons, high-pressure gas samples; two-body breakup, three-body breakup, charge asymmetry.

The prompt neutron multiplicities for photofission of the four isotopes /sup 235/U, /sup 236/U, /sup 238/U, and /sup 232/Th have been measured with monoenergetic photons over the energy range from 5.5 to 18 MeV using the annihilation in flight of fast positrons. The delayed neutron yield has been measured for all four isotopes at 10.9- and 16.8-MeV photon energies. The ratio of first- to second-chance fission has been measured as a function of energy up to 17-MeV excitation energy for /sup 236/U and /sup 238/U photofission.

The photoneutron cross sections $\ensuremath{\sigma}(\ensuremath{\gamma},n)$ and $\ensuremath{\sigma}(\ensuremath{\gamma},2n)$, and total photofission cross sections $\ensuremath{\sigma}(\ensuremath{\gamma},F)$ have been measured for $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{232}\mathrm{Th}$ from threshold to 18.3 MeV using monoenergetic photons from the annihilation in flight of fast positrons and neutron-multiplicity detection in an efficient $4\ensuremath{\pi}$ neutron detector. Use of the ring-ratio technique allowed both the average photofission neutron energy for each nucleus to be obtained as a function of photon energy and, for $^{236}\mathrm{U}$ and $^{238}\mathrm{U}$, the determination of the partial cross sections for first-chance $\ensuremath{\sigma}(\ensuremath{\gamma},f)$ and second-chance $\ensuremath{\sigma}(\ensuremath{\gamma},nf)$ photofission as well. Information extracted from the data includes integrated cross sections and their moments, giant-resonance parameters, deformation and radius parameters, and relative and absolute neutron and fission probabilities.NUCLEAR REACTIONS $^{235,236,238}\mathrm{U}$ and $^{232}\mathrm{Th}$ ($\ensuremath{\gamma}$,$n$,$2n$,$F$), ${E}_{\ensuremath{\gamma}}=5\ensuremath{-}18.3$ MeV; measured $4\ensuremath{\pi}$ neutron yield, neutron multiplicities, and average energies for monoenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},2n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},F)$, integrated cross sections and moments, GDR parameters, nuclear shape parameters, neutron and fission probabilities.

Photoneutron cross sections, including $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},pn)]$, $\ensuremath{\sigma}(\ensuremath{\gamma},2n)$, and $\ensuremath{\sigma}(\ensuremath{\gamma},3n)$, were measured for $^{55}\mathrm{Mn}$ and $^{59}\mathrm{Co}$ from threshold to 36.5 MeV, with a photon energy resolution which varied from 80 keV at the lowest to 170 keV at the highest energies measured. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for ($\ensuremath{\gamma}$,$1n$) and ($\ensuremath{\gamma}$,$2n$) events were determined simultaneously with the cross-section data by the ring-ratio technique. The cross sections exhibit considerable but weak structure. Other nuclear information extracted from the data includes parameters of the giant dipole resonance, integrated cross sections and their moments, and nuclear symmetry energies. A comparison is made with previous experimental data for these nuclei as well as with theoretical predictions based upon hydrodynamic, vibrational, and dynamic collective models. None of these models fits the data for these odd-even nuclei satisfactorily; more theoretical work is needed for this nuclear mass region.NUCLEAR REACTIONS $^{55}\mathrm{Mn}$, $^{59}\mathrm{Co}$ ($\ensuremath{\gamma}$, $n$, $2n$, $3n$), ${E}_{\ensuremath{\gamma}}=10\ensuremath{-}36.5$ MeV; measured $4\ensuremath{\pi}$ neutron yield, multiplicities, average energies for monoenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}}, 1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}}, 2n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}}, 3n)$, GDR parameters, integrated cross sections and moments, nuclear symmetry energies.

The photoneutron cross sections for $^{13}\mathrm{C}$ have been measured from near threshold to over 40 MeV using monoenergetic photons from positron in-flight annihilation. Several sharp features below the giant resonance were distinguished. The results both for this "pygmy-resonance" region and for the giant resonance near 24 MeV differ markedly from previously reported measurements and provide a much better quantitative comparison with recent theoretical calculations of the photoneutron reaction in $^{13}\mathrm{C}$. Comparison of the measured total photoneutron cross section with recent data on the ground-state photoreaction and with average photoneutron energies provides evidence for the isospin splitting of the giant resonance for this nucleus.NUCLEAR REACTIONS: $^{13}\mathrm{C}$($\ensuremath{\gamma}$,$n$), ${E}_{\ensuremath{\gamma}}=7.6\ensuremath{-}41.8$ MeV; measured $4\ensuremath{\pi}$ neutron yield for monoenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},2n)$, integrated cross sections, isospin splitting of the giant resonance.