The effect of finite resistance in the vacuum-tank walls on the longitudinal stability of an intense beam of particles in an accelerator is investigated theoretically. We show that even if the particle frequency is an increasing function of particle energy, the wall resistance can render the beam unstable against longitudinal bunching. In the absence of frequency spread in the unperturbed beam, the instability occurs with a growth rate that is proportional to (N/{sigma}){sup 1/2}, where N is the number of particles in the beam and {sigma} is the conductivity of the surface material. By means of the Vlasov equation a criterion for beam stability is obtained. In the limit of highly conducting walls the criterion involves the frequency spread in the unperturbed beam, the number of particles N, the beam energy, geometrical properties of the accelerator, but not the conductivity {sigma}. A numerical example presented indicates that certain observations of beam behavior in the MURA 40-Mev-electron accelerator may be related to the phenomenon we investigated.

The isotope Re/sup 189/ was produced by fsst neutron irradiation of osmium and by the ( alpha ,p) reaction on tungsten. The rhenium was separated chemically from the target material, and beta, gamma, and internal conversion spectra were measured. The mass assignment is confirmed by the observation of eleven electromagnetic transitions in the Os/sup 189/ daughter, including the 30.8-kev isomeric transition (8 hr), all of which were known from the decay of Ir/ sup 189/. Re/sup 189/ has a half-life of 23.4 plus or minus 1.0 hr and emits betaray groups with end-point energies 1000, 780, and 725 kev, and probably others. Results of coincidence measurements lead to some new information about the level scheme of Os/sup 189/ Evidence was found for strong rotation-particle coupling between low-lying K =1/2 and K = 3/2 bands in Os/sup 189/. 40 references. (auth)