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The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil
Full text linkThe critical-velocity behavior of oscillatory superfluid Helium-4 flow
through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil
has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up
to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during
which the frequency remained below 400 Hz, the critical velocity was a
nearly-linearly decreasing function of increasing temperature throughout the
region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi
phase slips could be observed at the onset of dissipation. In runs with
frequencies higher than 400 Hz, downward curvature was observed in the decrease
of critical velocity with increasing temperature. In addition, above 500 Hz an
alteration in supercritical behavior was seen at the lower temperatures,
involving the appearance of large energy-loss events. These irregular events
typically lasted a few tens of half-cycles of oscillation and could involve
hundreds of times more energy loss than would have occurred in a single
complete 2 Pi phase slip at maximum flow. The temperatures at which this
altered behavior was observed rose with frequency, from ~ 0.6 K and below, at
500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203
Experiments concerning the low-energy states of the O19 nucleus
Get PDFAngular distributions have been measured for three groups of protons from the O18(d, p)O19 reaction, those leaving O19 in its states at 0, 0.096, and 1.47 Mev. Deuteron energies of 1.74 and 2.50 Mev in the laboratory system were used. The distributions of protons leaving O19 in its ground state and in its 1.47-Mev state are characteristic of stripping and indicate the formation of the ground state by an l=2 neutron and of the 1.47-Mev state by an l=0 neutron. However, the distribution of protons leaving O19 in its 0.096-Mev state does not lend itself to a stripping interpretation.
It has been found that the γ decay of the 1.47-Mev state of O19, following the formation of this state in the O18(d, p)O19 reaction, proceeds mostly to the 0.096-Mev state. The mean life of the 0.096-Mev state has been measured by observing the decay in flight of recoiling excited O19 nuclei and is found to be 1.75(1±0.16)×10^-9 second. These observations restrict the likely assignments of spin and parity for the 0.096-Mev state to 3/2± or 5/2+
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