Cap Omega Research Articles

Universal criterion for the onset of superconductivity in granular films.

In a granular film, modeled by a Josephson-coupled lattice, virtual tunneling of quasiparticles results in a critical value R/sub D'Alembertian//sup c/ for the normal-state sheet resistance. R/sub D'Alembertian//sup c/ is a universal upper bound for zero-temperature superconductivity and is computed in descending powers of z, the number of grain neighbors. Mean-field theory for a square lattice gives the leading term R/sub D'Alembertian//sup c/ = 5.7 K ..cap omega... For the fractional correction from the next term in the expansion, the Bethe approximation gives -3/5z/sup 2/, or -4% for z = 4.

Observation of Omega - production in e+e- annihilation at 29 GeV.

Inclusive ..cap omega../sup -/ production in e/sup +/e/sup -/ annihilation at 29 GeV has been measured with the Mark II detector. From an integrated luminosity of 207 pb/sup -1/, we determine a production rate of 0.014 +- 0.006 +- 0.004 ..cap omega../sup -/,Omega-bar/sup +/ per hadronic event. This is roughly 35 times the Lund-model prediction of 0.0004 ..cap omega../sup -/,Omega-bar/sup +/ per hadronic event, but comparable to the Webber-model prediction of 0.006 ..cap omega../sup -/,Omega-bar/sup +/ per hadronic event. The large rate of ..cap omega../sup -/ production, compared with production rates for other baryons, and with theoretical predictions based on diquark models, indicates that spin suppression does not hold for ..cap omega../sup -/ production.

Coincidence spectroscopy of highly charged xenon ions by electron impact.

Spectroscopic studies of highly charged xenon ions (up to Xe/sup 9+/), produced by electron--xenon-atom collisions, have been carried out in coincidence with electrons ejected at 90/sup 0/ to the incident electron direction and having energies between 20 and 550 eV. Relative values of partial-double-differential ionization cross sections, d/sup 2/sigma/sup (n)//de d..cap omega.., for Xe/sup +/ to Xe/sup 8+/ have been measured for several incident electron energies between 1.0 and 8.0 keV.

Systematics of angular-dependent neutron production by 585 MeV protons on targets with 12

The intranuclear cascade-evaporation model of a high-energy nucleon-meson transport code has been used to calculate neutron production from nonelastic interactions of 585 MeV protons with C, Al, Fe, Nb, In, Ta, Pb, and U targets at emission angles of 30/sup 0/, 90/sup 0/, and 150/sup 0/. These model predictions are compared with measurements of the double differential cross sections d/sup 2/sigma/d..cap omega..dT/sub n/ of neutrons with kinetic energies between 0.9 and 585 MeV. In general, the model predicts approximately the correct neutron production in the evaporation region, whereas in the high energy region (E/sub n/approx. >20 MeV) systematic discrepancies between measurements and model predictions occur. These increase rapidly with increasing neutron energy and increasing emission angle.

Elastic and inelastic scattering of 362 MeV polarized protons from 40Ca.

Scattering of 362 MeV polarized protons from /sup 40/Ca to the ground, 3/sup -/ (3.74 MeV), 2/sup +/ (3.90 MeV), and 5/sup -/ (4.49 MeV) states has been studied from 3/sup 0/ to 65/sup 0/ in the laboratory. Measured values of dsigma/d..cap omega.. and A/sub y/ are compared with standard relativistic and nonrelativistic calculations using transition densities from random-phase-approximation calculations and electron scattering data.

( pi,p) reactions at low excitation energy.

Results of the first experimental search for the excitation of discrete final states following exclusive (..pi../sup -/,p) reactions are reported. The measurements include differential cross section at theta/sub lab/ = 25/sup 0/ for (..pi../sup +- /,p) reactions on /sup 24/Mg, /sup 27/Al, /sup 40/Ca, and /sup 58/Ni at T/sub ..pi../ = 120 MeV and (..pi../sup -/,p) on /sup 12/C at T/sub ..pi../ = 145 MeV. The (..pi../sup -/,p) reactions yield peaks with dsigma/d..cap omega..less than or equal to1 ..mu..b/sr compared to 7--22 ..mu..b/sr for peaks from (..pi../sup +/,p) reactions. The shape of the continuum in an excitation energy range of 10--40 MeV was found to be independent of pion charge and target. The ratios of (..pi../sup +/,p)/(..pi../sup -/,p) averaged over excitation energy and the ratios of the same reaction on different nuclei are presented. The magnitude of the proton yield in the low excitation continuum is more than 20 times larger for ..pi../sup +/ than for ..pi../sup -/, which supports a two-nucleon absorption model including pion charge exchange.

Theory for the low-temperature dependence of the ultrasonic attenuation in heavy-fermion metals.

A new formula is derived for the low-temperature sound attenuation GAMMA in heavy-fermion metals. As a result of the coupling between the sound wave and induced electronic temperature fluctuations, which are significant for large Grueneisen constant ..cap omega../sub u//sub ,//sub G/, the theory predicts at low temperatures a peak in the attenuation coefficient GAMMA(T) of longitudinal sound, as has been recently observed for UPt/sub 3/. The anomalous temperature dependence results from ..cap omega../sub u//sub ,//sub G/(T). This result of our theory is supported by the fact that ..cap omega../sub u//sub ,//sub G/ is proportional to the thermopower P which exhibits a peak at about 8 K where GAMMA(T) peaks.

Proton radiative capture by deuterium between 100 and 200 MeV.

Differential cross sections, (dsigma/d..cap omega..)(theta), and analyzing powers, A/sub y/(theta), have been measured for the reaction /sup 2/H(p-arrow-right,..gamma..) /sup 3/He at laboratory energies of 99.1, 150.3, and 200.7 MeV. The cross sections obtained are found to be in very good agreement with those of other authors. Both the size and shape of the angular distributions for the cross sections are found to be well accounted for by a simple ''quasideuteron'' model. Further support for this model is provided by the measured analyzing powers. Both the shape and size of the angular distributions for A/sub y/ are found to be very similar to those predicted for n(p-arrow-right,..gamma..)d at the same E/sub ..gamma..//sup c.m./.

Simultaneous Junction Formation Using a Directed Energy Light Source

not Available.

Calculation of the mobility of electrons injected in liquid xenon

A model calculation is carried out in which we evaluate the mobility of electrons injected in liquid argon. Scattering by both phonons and static density fluctuations is taken into account. The calculation for the mobility limited by phonon scattering differs from the usual calculation in crystals by considering both the local changes in the deformation potential and the changes of the amplitude of the phonons that are caused by the existence of density fluctuations. The calculation of the mobility limited by scattering from density fluctuations is carried out with the assumption that they give rise to a square-well (or barrier) potential that will scatter the electrons. The above perturbation ..delta..V/sub 0/ is related to a density fluctuation ..delta..n by ..delta..V/sub 0/ = V/sub 0/(n-bar+..delta..n)-V/sub 0/(n-bar). The scattering volumes ..cap omega.., where the density fluctuation ..delta..n is located, are weighted by exp(-r/xi) where xi is the correlation length and r is the radius of ..cap omega... The magnitude of the different density fluctuations is weighted by exp(-(..delta..n)/sup 2/..cap omega../2nS(0)), where S(0) = nk/sub B/TK/sub T/, K/sub T/ is the isothermal compressibility. The calculation of the mean free path is carried out using partial waves. Both scattering mechanisms, scattering by phonons andmore » static density fluctuations, give comparable contributions to the mobility.« less

Calculation of the mobility of electrons injected in liquid argon.

A model calculation is carried out in which we evaluate the mobility of electrons injected in liquid argon. Scattering by both phonons and static density fluctuations is taken into account. The calculation for the mobility limited by phonon scattering differs from the usual calculation in crystals by considering both the local changes in the deformation potential and the changes of the amplitude of the phonons that are caused by the existence of density fluctuations. The calculation of the mobility limited by scattering from density fluctuations is carried out with the assumption that they give rise to a square-well (or barrier) potential that will scatter the electrons. The above perturbation ..delta..V/sub 0/ is related to a density fluctuation ..delta..n by ..delta..V/sub 0/ = V/sub 0/(n-bar+..delta..n)-V/sub 0/(n-bar). The scattering volumes ..cap omega.., where the density fluctuation ..delta..n is located, are weighted by exp(-r/xi) where xi is the correlation length and r is the radius of ..cap omega... The magnitude of the different density fluctuations is weighted by exp(-(..delta..n)/sup 2/..cap omega../2nS(0)), where S(0) = nk/sub B/TK/sub T/, K/sub T/ is the isothermal compressibility. The calculation of the mean free path is carried out using partial waves. Both scattering mechanisms, scattering by phonons andmore » static density fluctuations, give comparable contributions to the mobility.« less

Pion production: A probe for coherence in medium-energy heavy-ion collisions.

The production of neutral pions has been studied in reactions of 35 MeV/nucleon /sup 14/N+ /sup 27/Al,Ni,W and 25 MeV/nucleon /sup 16/O+ /sup 27/Al,Ni. Inclusive pion differential distributions dsigma/dT/sub ..pi../, dsigma/d..cap omega.., dsigma/dy, dsigma/dp/sub perpendicular/, and d/sup 2/sigma/dy dp/sub perpendicular/ have been measured by detecting the two pion-decay ..gamma.. rays in a setup of 20 lead glass Cerenkov detector telescopes. Special care was taken to understand and suppress background events. Effects of pion reabsorption are discussed and it is found that the cross sections presented here are substantially affected by such final state interactions. The comparatively large experimental cross sections and the shape of the spectral distributions cannot be accounted for in single nucleon-nucleon collision or statistical models; they rather call for a coherent pion production mechanism.

Inclusive photoproduction of strange baryons at 20 GeV.

Cross sections are presented for the inclusive photoproduction of ${K}_{S}^{0}$, \ensuremath{\Lambda}, \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, ${\ensuremath{\Xi}}^{\mathrm{\ensuremath{-}}}$, \ensuremath{\Xi}\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{\mathrm{\ensuremath{-}}}$, ${\ensuremath{\Sigma}}^{0}$, and ${\ensuremath{\Sigma}}^{\mathrm{*}\ifmmode\pm\else\textpm\fi{}}$(1385) at 20 GeV. An upper limit to ${\ensuremath{\Omega}}^{\mathrm{\ensuremath{-}}}$ production is also given. The data come from 284 000 hadronic events photoproduced in the SLAC 1-m hydrogen-bubble-chamber hybrid facility exposed to a nearly monochromatic, polarized 20-GeV backscattered photon beam. A comparison of the ${K}_{S}^{0}$, \ensuremath{\Lambda}, \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, and ${\ensuremath{\Xi}}^{\mathrm{\ensuremath{-}}}$ rates per inelastic event to ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$p data show that \ensuremath{\gamma}p rates are consistent with being higher than the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$p rates, providing evidence of an ss\ifmmode\bar\else\textasciimacron\fi{} component of the photon. The pair cross sections for ${K}_{S}^{0}$${K}_{S}^{0}$, ${K}_{S}^{0}$\ensuremath{\Lambda}, ${K}_{S}^{o}$\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, and \ensuremath{\Lambda}\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{} are presented. The ${x}_{F}$ distributions of the \ensuremath{\Lambda}, \ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{}, and ${\ensuremath{\Xi}}^{\mathrm{\ensuremath{-}}}$ are compared to a quark-diquark fusion model, giving information on strange-baryon photoproduction mechanisms.

Neutrino reactions in 13C and the behavior of the axial current form factor.

The total and differential cross sections sigma(..nu../sub ..mu../+ /sup 13/C..-->../sup 13/N g.s.+..mu../sup -/) and dsigma/d..cap omega..(..nu../sub ..mu../+/sup 13/C..-->../sup 13/N g.s.+..mu../sup -/) are calculated from threshold to E/sub ..nu../ = 260 MeV. The dependence of the cross section on the axial current form factor F/sub A/(q/sup 2/) is obtained for a range of assumptions. The dependence of the cross section on the pseudoscalar form factor is found to be negligible.

Characterization of electron-beam deposited tungsten films on sapphire and silicon

In this work, the electrical and structural properties of e-beam deposited tungsten films on sapphire are described. Tungsten thin films (200–400 nm) were evaporated on sapphire substrates at different substrate temperature Ts. Film resistivity varied from 95 μΩ cm deposited at room temperature to 5.5 μΩ cm deposited at Ts between 440 and 500 °C. The resistivity value of 5.5 μΩ cm is comparable to the bulk resistivity value of 5.50–5.65 μΩ cm. The crystalline structure of these films was characterized by x-ray diffraction and scanning transmission electron microscopy (STEM) (Kikuchi lines). The films show a distinct increase in the degree of crystal orientation as Ts approaches 400 °C. The films deposited at 400 °C and above shows (100) epitaxial growth on the (11̄02) plane or r plane of sapphire, which has a distorted fourfold symmetry. Film stresses which vary from tensile (7.3×109 dyn/cm2) to compressive (−4.4×109 dyn/cm2) between 100 and 500 °C were measured by the interference fringe technique. The intrinsic stress was determined to vary from 8.3×109 dyn/cm2 at Ts=100 °C to 2.4×109 dyn/cm2 at Ts=500 °C. The observed intrinsic stresses (tensile) in epitaxial films are regarded as produced by the lattice misfit at the film–substrate interface. The minimum total film stress was obtained between Ts=350 and 400 °C, where tensile intrinsic stress was compensated by the compressive thermal stress. The properties of tungsten films on silicon, silicon oxide, and polysilicon substrates are also discussed.

Design of Radio Frequency Systiem for the Riken Separated Sector Cyclotron

The radio-frequency (RF) system of the RIKEN SSC (K=540) is required to work in a frequency range of 20 to 45 MHz and to generate the maximum acceleration voltage 250 kV. A new movable box type variable frequency resonator is designed for that purpose. This resonator is a compact half wave length coaxial type ((2.1 m(H) x 3.5 m(W) x 1.6 m(D)). The delta shaped dee whose radial length is 2.7 m is supported in median plane by vertical stems from the both sides. The resonant frequency is varied by moving the boxes surrounding the stems. The performance of this resonator is studied on a one-fourth scale model. The maximum power loss is estimated to be 250 kW for the required dee voltages and the radially increasing voltage distributions are obtained. The RF power is fed into the resonator through a 50 ..cap omega.. coaxial feeder line ( about 1.8 m length) which is coupled with the resonator in good impedance matching by means of a variable capacitive coupler. The final amplifier is of grounded grid configuration; load resistance matching is made by a variable capacitor inserted in series at its output port. The final amplifier and its components aremore » studied on full sized models. The result shows the movable box type resonator and power amplifier satisfy the design aim.« less

Generation and Distribution of Radio-Frequency Power in LEP

The beam energy in LEP Phase I at maximum luminosity is 55 GeV. With a circulating current of 3 mA per beam, an effective cavity shunt impedance of 40 M..cap omega../m and RF power losses in the waveguide system of 7.5%, a total of 14.9 MW RF generator power is required for achieving this energy, the respective values for injection being 20 GeV and about 0.26 MW. This RF power is generated by 16 klystrons and supplied to the 128 accelerating structures via a WR 2300 waveguide system. Two klystrons, together with 16 accelerating structures, the controls and associated waveguide system, form an RF unit.

Fermilab Tevatron High Level RF Accelerating Systems

Eight tuned rf cavities have been installed and operated in the F0 straight section of the Tevatron. Their mechanical placement along the beam line enables them to be operated for colliding beams as two independent groups of four cavities, group 1-4 accelerating antiprotons and group 5-8 accelerating protons. The only difference is that the spacing between cavities 4 and 5 was increased to stay clear of the F0 colliding point. The cavities can easily be rephased by switching cables in a low-level distribution system (fan-out) so that the full accelerating capability of all eight cavities can be used during fixed target operations. Likewise, the cables from capacitive probes on each cavity gap can be switched to proper lengths and summed in a fan-back system to give an rf signal representing the amplitude and phase as ''seen by the beam,'' separately for protons and antiprotons. Such signals have been used to phase lock the Tevatron to the Main Ring for synchronous transfer. A cavity consists of two quarter-wave resonators placed back to back with a coaxial drift tube separating the two accelerating gaps by ..pi.. radians. The cavities are very similar to the prototype which has been previously described/sup 3/ andmore » is operating as Station 8 in the Tevatron. Only additional water cooling around the high current region of the drift tube supports and a double loop used to monitor the unbalance current through the Hipernom mode damping resistor have been added. Each cavity has a Q of about7100, a shunt impedance of 1.2 M..cap omega.., and is capable of running cw with a peak accelerating voltage of 360« less

Theory of Feedthroughs: Wave Transmission and Voltage Characteristics of High-Power Current Feeds for ICRH

Using circuit theory and solutions of a scalar twodimensional Laplace equation for the electrostatic potential, we examine various feedthroughs of interest for voltage holding, voltage standing wave ratio (VSWR) and impedance characteristics. Suitable inhomogeneous Dirichlet, scalar boundary conditions are imposed for the potentials for the calculation of the electrostatic fields. The inhomogeneous vector Newman boundary conditions on the surface of the dielectric are dispatched by use of a conformal mapping. Several feedthroughs are examined: old Princeton Large Torus (PLT), JULICH designs, and several Oak Ridge National Laboratory (ORNL) designs that have been actually fabricated and exhaustively tested. Some of these feedthrough configurations have been optimized to provide constant 50-..cap omega.. impedance or minimum VSWR.

Determination of two-nucleon spectroscopic amplitudes from (d, alpha ) analyzing powers.

Tensor and vector analyzing powers and cross sections have been measured for low-lying levels populated by the sup89Y(d,..cap alpha..) sup87Sr reaction at 9.0, 12.0, and 16.0 MeV. Angular distributions of dsigma/d..cap omega.., Asuby, Asubyy, Asubxx, and Asubxz have been analyzed using full finite-range distorted-wave Born approximation calculations including ..cap alpha..-particle D-state effects. It is shown that the sign and amplitude of two-nucleon spectroscopic amplitudes corresponding to different L and J transfers can be determined from the angular distributions of the tensor analyzing powers.