13,876 research outputs found
Comment on: Diffusion through a slab
Mahan [J. Math. Phys. 36, 6758 (1995)] has calculated the transmission
coefficient and angular distribution of particles which enter a thick slab at
normal incidence and which diffuse in the slab with linear anisotropic,
non-absorbing, scattering. Using orthogonality relations derived by McCormick &
Kuscer [J. Math. Phys. 6, 1939 (1965); 7, 2036 (1966)] for the eigenfunctions
of the problem, this calculation is generalised to a boundary condition with
particle input at arbitrary angles. It is also shown how to use the
orthogonality relations to relax in a simple way the restriction to a thick
slab.Comment: 3 pages, LaTeX, uses RevTe
Failure of non-vacuum steam sterilization processes for dental handpieces
Background:
Dental handpieces are used in critical and semi-critical operative interventions. Although a number of dental professional bodies recommend that dental handpieces are sterilized between patient use there is a lack of clarity and understanding of the effectiveness of different steam sterilization processes. The internal mechanisms of dental handpieces contain narrow lumens (0·8-2·3mm) which can impede the removal of air and ingress of saturated steam required to achieve sterilization conditions.
Aim:
To identify the extent of sterilization failure in dental handpieces using a non-vacuum process.
Methods:
In-vitro and in-vivo investigations were conducted on commonly used UK benchtop steam sterilizers and three different types of dental handpieces. The sterilization process was monitored inside the lumens of dental handpieces using thermometric (TM) methods (dataloggers), chemical indicators (CI) and biological indicators (BI).
Findings:
All three methods of assessing achievement of sterility within dental handpieces that had been exposed to non-vacuum sterilization conditions demonstrated a significant number of failures (CI=8/3,024(fails/n tests); BI=15/3,024; TM=56/56) compared to vacuum sterilization conditions (CI=2/1,944; BI=0/1,944; TM=0/36). The dental handpiece most likely to fail sterilization in the non-vacuum process was the surgical handpiece. Non-vacuum sterilizers located in general dental practice had a higher rate of sterilization failure (CI=25/1,620; BI=32/1,620; TM=56/56) with no failures in vacuum process.
Conclusion:
Non-vacuum downward/gravity displacement, type-N steam sterilizers are an unreliable method for sterilization of dental handpieces in general dental practice. The handpiece most likely to fail sterilization is the type most frequently used for surgical interventions
Investigating steam penetration using thermometric methods in dental handpieces with narrow internal lumens during sterilizing processes with non-vacuum or vacuum processes
Background:
Dental handpieces are required to be sterilized between patient use. Vacuum steam sterilization processes with fractionated pre/post-vacuum phases or unique cycles for specified medical devices, are required for hollow instruments with internal lumens to assure successful air removal. Entrapped air will compromise achievement of required sterilization conditions. Many countries and professional organisations still advocate non-vacuum sterilization processes for these devices.
Aim:
To investigate non-vacuum downward/gravity displacement, type-N steam sterilization of dental handpieces, using thermometric methods to measure time to achieve sterilization temperature at different handpiece locations.
Methods:
Measurements at different positions within air turbines were undertaken with thermocouples and dataloggers. Two examples of commonly used UK benchtop steam sterilizers were tested; a non-vacuum benchtop sterilizer (Little Sister 3, Eschmann, UK) and a vacuum benchtop sterilizer (Lisa, W&H, Austria). Each sterilizer cycle was completed with three handpieces and each cycle in triplicate.
Findings:
A total of 140 measurements inside dental handpiece lumens were recorded. We demonstrate that the non-vacuum process fails (time range 0-150 seconds) to reliably achieve sterilization temperatures within the time limit specified by the International standard (15 seconds equilibration time). The measurement point at the base of the handpiece failed in all test runs (n=9) to meet the standard. No failures were detected with the vacuum steam sterilization type B process with fractionated pre-vacuum and post-vacuum phases.
Conclusion:
Non-vacuum downward/gravity displacement, type-N steam sterilization processes are unreliable in achieving sterilization conditions inside dental handpieces and the base of the handpiece is the site most likely to fail
On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts
A simple model for variability in relativistic plasma outflows is studied, in
which nonthermal electrons are continuously and uniformly injected in the
comoving frame over a time interval dt. The evolution of the electron
distribution is assumed to be dominated by synchrotron losses, and the energy-
and time-dependence of the synchrotron and synchrotron self-Compton (SSC)
fluxes are calculated for a power-law electron injection function with index s
= 2. The mean time of a flare or pulse measured at photon energy E with respect
to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for
synchrotron and SSC processes, respectively, until the time approaches the
limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is
the Doppler factor. This dependence is in accord with recent analyses of blazar
and GRB emissions, and suggests a method to discriminate between external
Compton and SSC models of high-energy gamma radiation from blazars and GRBs.
The qualititative behavior of the X-ray spectral index/flux relation observed
from BL Lac objects can be explained with this model. This demonstrates that
synchrotron losses are primarily responsible for the X-ray variability behavior
and strengthens a new test for beaming from correlated hard X-ray/TeV
observations.Comment: 10 pages, 2 figures, accepted for publication in Astrophysical
Journal Letters; uses aaspp4.sty, epsf.st
Particle acceleration at ultrarelativistic shocks: an eigenfunction method
We extend the eigenfunction method of computing the power-law spectrum of
particles accelerated at a relativistic shock fronts to apply to shocks of
arbitrarily high Lorentz factor. In agreement with the findings of Monte-Carlo
simulations, we find the index of the power-law distribution of accelerated
particles which undergo isotropic diffusion in angle at an ultrarelativistic,
unmagnetized shock is s=4.23 (where s=-d(ln f)/dp with f the Lorentz invariant
phase-space density and p the momentum). This corresponds to a synchrotron
index for uncooled electrons of a=0.62 (taking cooling into account a=1.12),
where a=-d(ln F)/dn, F is the radiation flux and n the frequency. We also
present an approximate analytic expression for the angular distribution of
accelerated particles, which displays the effect of particle trapping by the
shock: compared with the non-relativistic case the angular distribution is
weighted more towards the plane of the shock and away from its normal. We
investigate the sensitivity of our results to the transport properties of the
particles and the presence of a magnetic field. Shocks in which the ratio of
Poynting to kinetic energy flux upstream is not small are less compressive and
lead to larger values of .Comment: Minor additions on publicatio
Magnetically suspended flywheel system study
A program to study the application of a graphite/epoxy, magnetically suspended, pierced disk flywheel for the combined function of spacecraft attitude control and energy storage (ACES) is described. Past achievements of the program include design and analysis computer codes for the flywheel rotor, a magnetically suspended flywheel model, and graphite/epoxy rotor rings that were successfully prestressed via interference assembly. All hardware successfully demonstrated operation of the necessary subsystems which form a complete ACES design. Areas of future work include additional rotor design research, system definition and control strategies, prototype development, and design/construction of a UM/GSFC spin test facility. The results of applying design and analysis computer codes to a magnetically suspended interference assembled rotor show specific energy densities of 42 Wh/lb (92.4 Wh/kg) are obtained for a 1.6 kWh system
X-Ray Spectral Variability of Extreme BL Lac AGN H1426+428
Between 7 March 2002 and 15 June 2002, intensive X-ray observations were
carried out on the extreme BL Lac object H1426+428 with instruments on board
the Rossi X-ray Timing Explorer (RXTE). These instruments provide measurements
of H1426+428 in the crucial energy range that characterizes the first peak of
its spectral energy distribution. This peak, which is almost certainly due to
synchrotron emission, has previously been inferred to be in excess of 100 keV.
By taking frequent observations over a four-month campaign, which included
450 ksec of RXTE time, studies of flux and spectral variability on
multiple timescales were performed, along with studies of spectral hysteresis.
The 3-24 keV X-ray flux and spectra exhibited significant variability, implying
variability in the location of the first peak of the spectral energy
distribution. Hysteresis patterns were observed, and their characteristics have
been discussed within the context of emission models.Comment: accepted for publication in Astrophysical Journa
- …