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.

Measurements of the two-body and three-body photodisintegration cross sections for tritium are reported. The measurements were done with monoenergetic photons, high-pressure gas samples, and neutron-multiplicity detection. Presently available theoretical calculations are not adequate to explain the results.

Photoneutron cross sections, including $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},pn)]$, $\ensuremath{\sigma}[(\ensuremath{\gamma},2n)+(\ensuremath{\gamma},p2n)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},3n)$, were measured for $^{188}\mathrm{Os}$, $^{189}\mathrm{Os}$, $^{190}\mathrm{Os}$, and $^{192}\mathrm{Os}$ from 7 to 30 MeV and for $^{186}\mathrm{Os}$ from 11 to 20 MeV, with a photon energy resolution of about 300 keV. 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 each multiplicity were determined simultaneously with the cross-section data by the ring-ratio technique. Nuclear information extracted from the data includes parameters of the giant dipole and giant quadrupole resonances, integrated cross sections and their moments, nuclear symmetry energies, and nuclear deformation parameters and intrinsic quadrupole moments. No fewer than eight kinds of evidence point to a sudden change of behavior between $^{189}\mathrm{Os}$ and $^{190}\mathrm{Os}$, which could be interpreted as a phase transition from a statically deformed prolate nucleus to a $\ensuremath{\gamma}$-unstable one, in general (but not detailed) agreement with the prediction of a dynamic-collective-model calculation.NUCLEAR REACTIONS $^{186,188,189,190,192}\mathrm{Os}$($\ensuremath{\gamma}$,$n$,$2n$,$3n$), ${E}_{\ensuremath{\gamma}}=7\ensuremath{-}30$ MeV; measured $4\ensuremath{\pi}$ neutron yield, multiplicities, average energies for monenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},2n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},3n)$, GDR parameters, nuclear shape parameters, integrated cross sections and moments, GQR parameters, nuclear phase transition.

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