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[en] The vacuum system is described for the beam-transport lines used with the Lawrence Livermore Laboratory 100-MeV electron-positron linear accelerator. It is operated in the high (10^-4 to 10^-5 Pa) range using standard ion pumps with turbomolecular pumps for roughdown. The important factors considered during the initial design phases are reviewed; these factors contributed to the very reliable performance of the vacuum system since its startup five years ago. The cleaning and handling procedures used on the beam-transport lines, magnet vacuum chambers, and other vacuum components are discussed. (auth)

[en] We have studied the spin relaxation in diluted spin ice Ho_2-x Y_x Ti₂O₇ by means of neutron spin echo spectroscopy. Remarkably, the geometrical frustration is not relieved by doping with non-magnetic Y, and the dynamics of the freezing is unaltered in the spin echo time window up to x ≅ 1.6. At higher doping with non-magnetic Y (x (ge) 1.6) a new relaxation process at relatively high temperature (up to at least T ≅ 55 K) appears which is more than 10 times faster than the thermally activated main relaxation process. We find evidence that over the whole range of composition all Ho spins participate in the dynamics. These results are compared to a.c. susceptibility measurements of the diluted Ho and Dy spin ice systems. X-ray absorption fine structure (EXAFS) spectra and x-ray diffraction show that the samples are structurally well ordered