5,987 research outputs found
Creating long-lived neutral-atom traps in a cryogenic environment
We describe techniques for creating long-lived magneto-optical and magnetostatic traps for neutral atoms. These traps exist in a sealed cryogenic environment with a temperature near 4 K, where the background gas pressure can be extremely low. To date we have observed cesium magneto-optical traps with background-limited lifetimes in excess of 1 h, and magnetostatic traps with lifetimes of nearly 10 min. From these observations we use the known He-Cs van der Waals collision cross section to infer typical background gas pressures in our apparatus below 4Ă10^(-12) Torr. With hardware improvements we expect this pressure can be made much lower, extending the magnetostatic-trap lifetimes one to two orders of magnitude. Furthermore, with a cryogenic system one can use superconducting magnets and SQUID detectors to trap and to nondestructively sense spin-polarized atoms. With superconducting microstructures one can achieve very large magnetic-field gradients and curvatures, as high as âŒ10^6 G/cm and âŒ10^9 G/cm^2, respectively, for use in magnetic and magneto-optical traps
Stability of Magneto-optical Traps with Large Field Gradients: Limits on the Tight Confinement of Single Atoms
We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than âŒ1 kG/cm, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than âŒ5 ÎŒm. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT
Dynamics of Human Walking
The problem of biped locomotion at steady speeds is discussed through a
Lagrangian formulation developed for velocity-dependent, body driving forces.
Human walking on a level surface is analyzed in terms of the data on the
resultant ground-reaction force and the external work. It is shown that the
trajectory of the center of mass is due to a superposition of its rectilinear
motion with a given speed and a backward rotation along a shortened
hypocycloid. A stiff-to-compliant crossover between walking gaits is described
and the maximum speed for human walking, given by an instability of the
trajectory, is predicted.
Key words: locomotion, integrative biology, muscles, bipedalism, human
walking, biomechanics.Comment: 9 pages, 4 figure
Electrically detected magnetic resonance using radio-frequency reflectometry
The authors demonstrate readout of electrically detected magnetic resonance
at radio frequencies by means of an LCR tank circuit. Applied to a silicon
field-effect transistor at milli-kelvin temperatures, this method shows a
25-fold increased signal-to-noise ratio of the conduction band electron spin
resonance and a higher operational bandwidth of > 300 kHz compared to the kHz
bandwidth of conventional readout techniques. This increase in temporal
resolution provides a method for future direct observations of spin dynamics in
the electrical device characteristics.Comment: 9 pages, 3 figure
Saprolegnia diclina IIIA and S. parasitica employ different infection strategies when colonizing eggs of Atlantic salmon, Salmo salar L.
Acknowledgements The work has been funded by the European Commission through the EU Marie Curie ITN project SAPRO (238550) (MMS, AW). We would also like to acknowledge support from the BBSRC and the University of Aberdeen (PvW) and Landcatch and AquaGen for providing salmon eggs. Elin Rolen's assistance with sequencing of the strains is highly appreciated.Peer reviewedPublisher PD
Forming simulation of a thermoplastic commingled woven textile on a double dome
This paper presents thermoforming experiments and FE simulations of a commingled glass-PP woven composite on a double dome geometry, with the aim of assessing the correspondence of predicted and experimental shear angles. Large local deformations - especially in-plane shear, i.e. relative rotation between the two yarn families â occur when draping a textile on a three dimensional part and eventually unwanted phenomena like wrinkling or tearing may occur. The macroscopic drape behaviour of a weave is generally subdivided into: 1) The high tensile resistance along the yarn directions, expressed as non-linear stress-strain curves, and 2) The shear resistance, expressed as non-linear shear force versus shear angle curves. The constitutive model is constituted of a dedicated non-orthogonal hypo-elastic shear resistance model, previously described in [1, 2], combined with truss elements that represent the high tensile resistance along the yarn directions. This model is implemented in a user subroutine of the ABAQUS explicit FE solver. The material parameters have been identified via textile biaxial tensile tests at room temperature and bias extension tests at 200°. Thermoforming experiments are performed on a rectangular blank with the warp direction along the second symmetry plane of the tool, with a preheating temperature of 200°C, a constant mold temperature of about 70°C, and a blankholder ring. It was concluded that the shear angles were fairly well predicted for this particular case study, which could be expected in view of the fact that no wrinkles had formed during the thermoforming experiment
A new combined DTP-HBV HiB vaccine- strategy for incorporation of HIB vaccination into childhood immunisation programmes
Objectives. To evaluate the immunogenicity and reactogenicity of a pentavalent vaccine prepared by extemporaneously mixing diphtheria-tetanus pertussis-hepatitis B vaccine (DTPHBV) and lyophilised Haemophilus influenzae type B (Hib)tetanus conjugate vaccines in the same syringe, compared with the same vaccines given as separate, concomitant administrations.Design. Open, randornised comparative study.Setting. Durban, South Africa.Subjects. A total of 120 healthy male and female infants were enrolled in the trial and randomised into two groups; group 1 received the combined administration (DTP-HBV-Hib), and group 2 received separate administrations of DTP-HBV and Hib vaccines. Vaccines were given as a three-dose primary vaccination course at 6,10 and 14 weeks of age.Outcome measures. Antibody levels were measured using standard techniques and local and general solicited symptoms were recorded using diary cards.Results. All subjects had seroprotective titres against diphtheria and tetanus; and antipolyribose-ribitol phosphate (PRP) titres â„ 0.15 )ÎŒg/ ml 1 month after the final dose. A vaccine response (defined as post-vaccination titres â„ 15 ELISA (EL).U/ ml in initially seronegative subjects; and as post- vaccination titres â„ pre-vaccination titres in ini tially seropositive subjects) against the pertussis component was seen in 83% and 85% of subjects in the groups receiving combined and separate administration. No differences were  seen in any of the geometric mean titres (GMTs) between the two administrations either 2 months after the second dose or 1 month after the final dose. There was no observed increase in reactogenicity in the group receiving the mixed administration.Conclusions. The results demonstrate that combined DTPHBV- Hib vaccine is well tolerated and immunogeni
Differential activation of lumbar and sacral motor pools during walking at different speeds and slopes
Organization of spinal motor output has become of interest for investigating differential activation of lumbar and sacral motor pools during locomotor tasks. Motor pools are associated with functional grouping of motoneurons of the lower limb muscles. Here we examined how the spatiotemporal organization of lumbar and sacral motor pool activity during walking is orchestrated with slope of terrain and speed of progression. Ten subjects walked on an instrumented treadmill at different slopes and imposed speeds. Kinetics, kinematics, and electromyography of 16 lower limb muscles were recorded. The spinal locomotor output was assessed by decomposing the coordinated muscle activation profiles into a small set of common factors and by mapping them onto the rostrocaudal location of the motoneuron pools. Our results show that lumbar and sacral motor pool activity depend on slope and speed. Compared with level walking, sacral motor pools decrease their activity at negative slopes and increase at positive slopes, whereas lumbar motor pools increase their engagement when both positive and negative slope increase. These findings are consistent with a differential involvement of the lumbar and the sacral motor pools in relation to changes in positive and negative center of body mass mechanical power production due to slope and speed.NEW & NOTEWORTHY In this study, the spatiotemporal maps of motoneuron activity in the spinal cord were assessed during walking at different slopes and speeds. We found differential involvement of lumbar and sacral motor pools in relation to changes in positive and negative center of body mass power production due to slope and speed. The results are consistent with recent findings about the specialization of neuronal networks located at different segments of the spinal cord for performing specific locomotor tasks
An investigation of eddy-current damping of multi-stage pendulum suspensions for use in interferometric gravitational wave detectors
In this article we discuss theoretical and experimental investigations of the use of eddy-current damping for multi-stage pendulum suspensions such as those intended for use in Advanced LIGO, the proposed upgrade to LIGO (the US laser interferometric gravitational-wave observatory). The design of these suspensions is based on the triple pendulum suspension design developed for GEO 600, the German/UK interferometric gravitational wave detector, currently being commissioned. In that detector all the low frequency resonant modes of the triple pendulums are damped by control systems using collocated sensing and feedback at the highest mass of each pendulum, so that significant attenuation of noise associated with this so-called local control is achieved at the test masses. To achieve the more stringent noise levels planned for Advanced LIGO, the GEO 600 local control design needs some modification. Here we address one particular approach, namely that of using eddy-current damping as a replacement or supplement to active damping for some or all of the modes of the pendulums. We show that eddy-current damping is indeed a practical alternative to the development of very low noise sensors for active damping of triple pendulums, and may also have application to the heavier quadruple pendulums at a reduced level of damping
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