Search

Tethered bilayer lipid membranes (tBLM) are formed on 1) pure tether lipid triethyleneoxythiol cholesterol (EO(3)C) or on 2) mixed self-assembled monolayers (SAMs) of EO(3)C and 6-mercaptohexanol (6MH). While EO(3)C is required to form a tBLM with high resistivity, 6MH dilutes the cholesterol content in the lower leaflet of the bilayer forming ionic reservoirs required for submembrane hydration. Here we show that these ionic reservoirs are required for ion transport through gramicidin or valinomycin, most likely due to the thermodynamic requirements of ions to be solvated once transported through the membrane. Unexpectedly, electrochemical impedance spectroscopy (EIS) shows an increase of capacitance upon addition of gramicidin, while addition of valinomycin decreases the membrane resistance in the presence of K(+) ions. We hypothesise that this is due to previously reported phase separation of EO(3)C and 6MH on the surface. This results in ionic reservoirs on the nanometre scale, which are not fully accounted for by the equivalent circuits used to describe the system.

Since the discovery that muonic deuterium at energies near a few eV could travel distances of the order of 1 mm in condensed hydrogen, and in particular that muonic tritium and muonic deuterium could emerge from the surface of a solid hydrogen layer, the advantages of solid targets have enabled the study of several processes important in muon catalyzed fusion. A review of the results is presented, emphasizing the strengths and limitations of the use of solid hydrogen layer targets.

Recent advances in the study of Aurivillius phases (Bi2An-1BnO3n+3, n = 1−5) as oxide ion conductors are presented. The structures of modified Aurivillius phases containing extrinsic oxygen vacancies as well as Aurivillius phases containing intrinsic oxygen vacancies are surveyed. A detailed discussion of the conductivity behavior of these Aurivillius phases is given.

Recent advances in the study of oxygen-deficient perovskites and brownmillerite phases as oxygen ion conductors are reviewed. The structures of these phases containing oxygen vacancies are surveyed and the role of the oxygen vacancies with respect to the ionic conductivity in these materials is discussed in detail.

BaBi4Ti3MO14.5 (M=Ga, In and Sc), compounds that consist of intergrowths between the Aurivillius phase Bi4Ti3O12 and brownmilleritelike-layers of BaMO2.5 were synthesized and their oxygen ion conductivities were measured using complex impedance. All three materials display a discontinuity in Arrhenius plots of their ionic conductivity. A phase transition at this discontinuity is confirmed by differential thermal analysis and is consistent with an oxygen vacancy order-disorder transition. For BaBi4Ti3ScO14.5 there is an observed hysteresis for this phase transition. At 900°C the conductivities are 2.5×102 S·cm −1 for BaBi4Ti3ScO14.5 and 4.9×10−2 S·cm−1 for BaBi4Ti3InO14.5 and 4.5×10−2S·cm−1 for BaBi 4Ti3GaO14.5.

The ionic conductivity of oxygen deficient Bi{sub 2}Sr{sub 2}M {prime}{sub 2}M{double_prime}{sub 2}O{sub 11.5} and the non-oxygen deficient Bi{sub 2}Sr{sub 2}M {prime}{sub 2}TiO{sub 12} (M{prime} = Nb, Ta and M{double_prime} = Al,Ga) has been investigated. Phase transitions are described.

Muonic hydrogen isotopes (μ− p, μ− d, and μ−t) are simple quantum mechanical systems ideally suited for studies of numerous fundamental phenomena in electroweak and strong interactions as well as in applied areas such as muon chemistry or muon catalyzed fusion.

An intrusion detection evaluation test bed was developed which generated normal traffic similar to that on a government site containing 100's of users on 1000's of hosts. More than 300 instances of 38 different automated attacks were launched against victim UNIX hosts in seven weeks of training data and two weeks of test data. Six research groups participated in a blind evaluation and results were analyzed for probe, denial-of-service (DoS) remote-to-local (R2L), and user to root (U2R) attacks. The best systems detected old attacks included in the training data, at moderate detection rates ranging from 63% to 93% at a false alarm rate of 10 false alarms per day. Detection rates were much worse for new and novel R2L and DoS attacks included only in the test data. The best systems failed to detect roughly half these new attacks which included damaging access to root-level privileges by remote users. These results suggest that further research should focus on developing techniques to find new attacks instead of extending existing rule-based approaches.

Intrusion detection systems monitor the use of computers and the network over which they communicate, searching for unauthorized use, anomalous behavior, and attempts to deny users, machines or portions of the network access to services. Potential users of such systems need information that is rarely found in marketing literature, including how well a given system finds intruders and how much work is required to use and maintain that system in a fully functioning network with significant daily traffic. Researchers and developers can specify which prototypical attacks can be found by their systems, but without access to the normal traffic generated by day-to-day work, they can not describe how well their systems detect real attacks while passing background traffic and avoiding false alarms. This information is critical: every declared intrusion requires time to review, regardless of whether it is a correct detection for which a real intrusion occurred, or whether it is merely a false alarm. To meet the needs of researchers, developers and ultimately system administrators we have developed the first objective, repeatable, and realistic measurement of intrusion detection system performance. Network traffic on an Air Force base was measured, characterized and subsequently simulated on an isolated network on which a few computers were used to simulate thousands of different Unix systems and hundreds of different users during periods of normal network traffic. Simulated attackers mapped the network, issued denial of service attacks, illegally gained access to systems, and obtained super-user privileges. Attack types ranged from old, well-known attacks, to new, stealthy attacks. Seven weeks of training data and two weeks of testing data were generated, filling more than 30 CD-ROMs. Methods and results from the 1998 DARPA intrusion detection evaluation will be highlighted, and preliminary plans for the 1999 evaluation will be presented.

A dark matter (DM) search experiment was flown on the IMAX balloon payload to search for a possible minor component of the dark matter in the Galactic halo: ionizing massive particles (IMPs) (mx≳104 GeV/c2) that cannot penetrate the atmosphere due to their low‐velocities and high energy‐loss. The DM search experiment consisted of a delayed coincidence between four large plastic scintillation detectors arranged in a vertical stack. In order to search for ultra‐slow particles which do not slow down in the IMAX telescope, the experiment contained TDCs which measured the time‐delay Ti,i+1∈(0.3, 14.0) μs between hits in successive counters to ∼2% precision. We present IMP flux limits for non‐slowing IMPs and also for IMPs which slow down significantly within the IMAX telescope. This experiment effectively closes much of a previously unconstrained ‘‘window’’ in the mass/cross‐section joint parameter spaces for massive particles as the dominant halo DM.

Oxygen-deficient layered bismuth oxides, Bi2Sr2M'2M”O11.5 [(M' = Nb, Ta) and (M” = Al, Ga)] were synthesized. Powder X-ray diffraction and transmission electron microscopy were usedto characterize the new materials' structures. The ionic conductivity was measuredusing impedance spectroscopy which indicated the existence of multiple conductive states in the new oxygen-deficient materials. Arrhenius plots of the conductivity showed discontinuities which were attributed to transitions between different conductive states. At 800ºC, Bi2Sr2Nb2GaO11.5 and Bi2Sr2Nb2A1O1.5, have ionic conductivities of 2.0×10−2 S/cm and 1.2×10−2 S/cm, respectively. Differential thermal analysis showed phase transitions in the oxygen-deficient materials. These transitionsoccurred at temperatures similar to those at which discontinuities were observed in the Arrhenius plots of the conductivity and are attributed to oxygen vacancy order/disorder transitions. The transference number was calculated for some of the samples by measuring both the EMF and the conductivity as a function of oxygen partial pressure. Under atmospheric conditions the new materials are predominantly ionic conductors.

The muonium atom ${\mathrm{\ensuremath{\mu}}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ is a simple system from which to gain information about the e-\ensuremath{\mu} interaction both to test the existing theory of QED and to search for physics beyond the standard model. A systematic study has been carried out of the production and properties of thermal muonium from a fine silica powder. The velocity and angular distributions are examined, as well as the experimental conditions resulting in the optimum production rate. The particular properties of silica powder that give it its muonium-producing capability are investigated. These studies are carried out in the context of a diffusion model, the validity of which is examined.

A search is described for mixing of muonium ({mu}{sup +}{ital e}{sup {minus}}) and antimuonium ({mu}{sup {minus}}{ital e}{sup +}). Thermal muonium was produced by stopping muons in a SiO{sub 2} powder target. As a conversion signature, a {mu}{sup {minus}} from antimuonium would create {sup 184}Ta in an adjacent tungsten foil. The surface layer of the sample was chemically extracted and counted in a low-background germanium spectrometer; no conversion events were observed. The resulting upper limit on the probability that a muonium atom spontaneously converts to antimuonium is 2.1{times}10{sup {minus}6} (90% confidence). This corresponds to a limit of 0.29{ital G}{sub {ital F}} on the effective four-fermion coupling constant between muonium and antimuonium.

We describe a dc volume H− source, employing multicusp plasma confinement, which has been constructed for use with the TRIUMF cyclotron. An extracted H− current density of 12 mA/cm2 has been obtained. Beam emittance and brightness have been measured as a function of current density and beam fraction. Beam brightness values for normalized beam emittances of the order of 0.2 πmm mrad at 81% beam fraction are typically 10 mA/(mm mrad)2 [equivalent rms B=223 mA/(mm mrad)2].

Etude de la distribution d'energie de muons de faible energie transmis a travers de minces solides moderateurs

A search is described for muonium (..mu../sup +/e/sup -/) conversion to antimuonium (..mu../sup -/e/sup +/) with a coupling strength approximately equal to that of the weak interaction. A silica powder target produced thermal muonium in vacuum. A unique sequence of radioactivity following the capture of a ..mu../sup -/ in tungsten was the signal for conversion. No events were seen, implying an upper limit on the effective four-fermion weak-coupling constant for conversion of 0.88G/sub F/ (90% confidence level), corresponding to a branching ratio of less than 2 x 10/sup -5/ with respect to normal muon decay.

Recent investigations have concentrated on the parameters affecting the quality of the H− beam from a newly developed dc volume multicusp ion source with a single‐stage‐post‐acceleration (SSPA) extraction system for injection into the TRIUMF cyclotron. The experimental results indicate that the beam emittance is nearly independent of the arc current in the range of 10 A to 40 A. Also described are modifications to the extraction system which have significantly reduced the electron contamination in the H− beam.

Muonium atoms have been observed in vacuum after emission from a layer of finely divided silica powder. By extrapolation of the decay positron track both the time and position of muon decay are measured, confirming thermal emission at room temperature. The yields range up to (19 +- 6)% of muons stopping in the layer, depending on its thickness. The result is used to recalculate the upper limit for conversion of muonium to antimuonium.

An improved upper limit is reported for muonium (μ+e−) conversion to antimuonium (μ−e+) which corresponds to a coupling strength approximately equal to that of the weak interaction. Muonium was produced in vacuum from a thin silica powder layer. The conversion signal was radioactivity induced by negative muon capture in tungsten in an adjacent surface. No events were seen, establishing an upper limit on the effective four fermion weak coupling constant for conversion of 1.1 GF (90% confidence level). The corresponding lower limit on the mass of a possibly doubly charged scalar boson in a left‐right symmetric model is approximately the W boson mass.

We observe the emission of slow positive muons (..mu../sup +/) from solid neon, argon, krypton, and xenon moderators exposed to a 4.2-MeV incident ..mu../sup +/ beam. The time-of-flight spectra for all of the targets studied exhibit a narrow distribution with no delayed component. Energy spectra obtained from the time-of-flight data indicate a maximum below approx.10 eV with a tail extending to higher energies. The data suggest a slowly thermalizing muon emission mechanism, implying a long diffusion length for low-energy ..mu../sup +/ in these solids. Of the targets measured, argon was observed to produce the highest yield (approx.10/sup -5/ slow ..mu../sup +/ per incident ..mu../sup +/), providing a useful flux for further experimentation.