Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive Krypton 85. We find that the general morphology of a positive streamer discharge in high purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding Krypton 85, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates. Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.Comment: 30 pages, 13 figure

Possible viscosity effects in neutron-induced fission of /sup 232/Th and /sup 238/U. [Yields]

Fission yields induced in the /sup 238/U(n,f) and /sup 232/Th(n,f) reactions were determined as a function of incident neutron energy (E/sub n/). The ratio of /sup 115/Cd-to-/sup 140/Ba yields as a function of E/sub n/ is analyzed by means of the equation Y/sub 1//Y/sub 2/ = exp(2(a/sub 1/(E/sub n/+E/sub 1/)/sup 1/2/ -2(a/sub 2/(E/sub n/+E/sub 2/)/sup 1/2/) to give values of a/sub i/, the level density parameter, and E/sub i/, the excitation energy for E/sub n/=0. The energies E/sub i/ are interpreted on the basis of the liquid drop model with shell and pairing corrections. Values are deduced for the energy dissipated by viscosity effects in the descent from the saddle point to the point where masses are fixed in the fissioning nucleus. These values are 1.7 MeV for /sup 232/Th(n,f) and 4.8 MeV for /sup 238/U(n,f). These values are consistent with the experimental observation that anti ..nu../sub p/ is approx. 0.6 neutron greater for /sup 239/U fission than for /sup 233/Th fission and that strong odd--even (nucleon pairing) effects are found in the fragment total kinetic energy distribution for /sup 230/Th fission but not for /sup 234/U fission. The low dissipation energy values together with the low values of pre-scission kinetic energy deduced by Guet, et al., (Nucl. Phys. A134 (1971)1) indicate a shorter path from the saddle point of the fissioning nucleus to scission than is generally assumed in theoretical calculations. 31 references

Nuclear spectroscopic studies of the 23.7 h 248Bk

Main features of the decay of the 23.7 h 248Bk have been determined by spectroscopy with scintillation and semiconductor detectors. The results are as follows: half-life 23.7 +/- 0.2 h; branching is (30 +/- 5)% to 248Cm ([approximate] 23% EC decay to 0+ ground state and [approximate] 7% to first 2+ state) and (70 +/- 5)%[beta]- decay to 248Cf(5 % to 592 keV K[pi] = 2- state and (65 +/- 5)% to the ground-state band). The [beta]- decay energy, Q[beta]-, has been measured to be 860 +/- 20 keV, and the electron capture decay energy, QEC, has been derived from closed cycle to be 705 +/- 25 keV. The deduced logft values of [beta]- and EC transitions restrict the spin of the 248Bk ground state to 1, with configuration assignment ; . It has also been deduced that the long lived isomer lies 65 +/- 40 keV abovethe 23.7h 248Bk ground state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22573/1/0000118.pd