2,533 research outputs found
Channeling Effect and Improvement of the Efficiency of Charged Particle Registration with Crystal Scintillators
The importance is emphasized of taking into account the channeling along the
low index crystallographic axes and planes of a part of low-energy (1-10 keV)
recoil ions in measurements of their parameters with crystal scintillators of
the type of NaI(Tl) etc. Because the nucleus stopping power in channels is low
as compared with electronic stopping power, the light yield of the scintillator
must be, accordingly, higher in the given case than that for ions having higher
energy (tens keV and more), which lose most part of their energy via nuclear
collisions outside channels. Hence, in particular, it follows that the DAMA/NaI
observations in Gran Sasso of the annual modulation of the signal frequency in
a narrow range of scintillations with an amplitude of 2-6 keV electron
equivalent may be due to incidence onto the Earth of exceedingly massive
particles (of the type of Planckian objects) from elongated Earth-crossing
heliocentric orbits at a velocity of 30-50 km/s. In NaI(Tl), these particles
create the iodine recoil ions with just the energy of 2-6 keV.Comment: 10 page
Practice pointer: Using the new UK-WHO growth charts
The new UK growth charts for children aged 0-4 years (designed using data from the new WHO standards) describe the optimal pattern of growth for all children, rather than the prevailing pattern in the UK (as with previous charts).
The new charts are suitable for all ethnic groups and set breast feeding as the norm.
UK children match the new charts well for length and height, but after age 6 months fewer children will be below the 2nd centile for weight or show weight faltering, and more will be above the 98th centile.
The new charts look different: they have a separate preterm section, no lines between 0 and 2 weeks, and the 50th percentile is no longer emphasised.
The charts give clear instructions on gestational correction, and there is a new chart for infants born before 32 weeks’ gestation.
The instructions advise on when and how to measure and when a measurement or growth pattern is outside the normal range.
The charts include a “look-up” tool for determining the body mass index centile from height and weight centiles without calculation and aid for predicting adult height.
The charts and supporting educational materials can be downloaded from www.growthcharts.rcpch.ac.u
Optics: general-purpose scintillator light response simulation code
We present the program optics that simulates the light response of an
arbitrarily shaped scintillation particle detector. Predicted light responses
of pure CsI polygonal detectors, plastic scintillator staves, cylindrical
plastic target scintillators and a Plexiglas light-distribution plate are
illustrated. We demonstrate how different bulk and surface optical properties
of a scintillator lead to specific volume and temporal light collection
probability distributions. High-statistics optics simulations are calibrated
against the detector responses measured in a custom-made cosmic muon tomography
apparatus. The presented code can also be used to track particles intersecting
complex geometrical objects.Comment: RevTeX LaTeX, 37 pages in e-print format, 12 Postscript Figures and 1
Table, also available at
http://pibeta.phys.virginia.edu/public_html/preprints/optics.p
Full-vector analysis of a realistic photonic crystal fiber
We analyze the guiding problem in a realistic photonic crystal fiber using a
novel full-vector modal technique, a biorthogonal modal method based on the
nonselfadjoint character of the electromagnetic propagation in a fiber.
Dispersion curves of guided modes for different fiber structural parameters are
calculated along with the 2D transverse intensity distribution of the
fundamental mode. Our results match those achieved in recent experiments, where
the feasibility of this type of fiber was shown.Comment: 3 figures, submitted to Optics Letter
Magic wavelengths for the np-ns transitions in alkali-metal atoms
Extensive calculations of the electric-dipole matrix elements in alkali-metal
atoms are conducted using the relativistic all-order method. This approach is a
linearized version of the coupled-cluster method, which sums infinite sets of
many-body perturbation theory terms. All allowed transitions between the lowest
ns, np_1/2, np_3/2 states and a large number of excited states are considered
in these calculations and their accuracy is evaluated. The resulting
electric-dipole matrix elements are used for the high-precision calculation of
frequency-dependent polarizabilities of the excited states of alkali-metal
atoms. We find magic wavelengths in alkali-metal atoms for which the ns and
np_1/2 and np_3/2 atomic levels have the same ac Stark shifts, which
facilitates state-insensitive optical cooling and trapping.Comment: 12 pages, 8 figure
First measurement of low intensity fast neutron background from rock at the Boulby Underground Laboratory
A technique to measure low intensity fast neutron flux has been developed.
The design, calibrations, procedure for data analysis and interpretation of the
results are discussed in detail. The technique has been applied to measure the
neutron background from rock at the Boulby Underground Laboratory, a site used
for dark matter and other experiments, requiring shielding from cosmic ray
muons. The experiment was performed using a liquid scintillation detector. A
6.1 litre volume stainless steel cell was filled with an in-house made liquid
scintillator loaded with Gd to enhance neutron capture. A two-pulse signature
(proton recoils followed by gammas from neutron capture) was used to identify
the neutron events from much larger gamma background from PMTs. Suppression of
gammas from the rock was achieved by surrounding the detector with high-purity
lead and copper. Calibrations of the detector were performed with various gamma
and neutron sources. Special care was taken to eliminate PMT afterpulses and
correlated background events from the delayed coincidences of two pulses in the
Bi-Po decay chain. A four month run revealed a neutron-induced event rate of
1.84 +- 0.65 (stat.) events/day. Monte Carlo simulations based on the GEANT4
toolkit were carried out to estimate the efficiency of the detector and the
energy spectra of the expected proton recoils. From comparison of the measured
rate with Monte Carlo simulations the flux of fast neutrons from rock was
estimated as (1.72 +- 0.61 (stat.) +- 0.38 (syst.))*10^(-6) cm^(-2) s^(-1)
above 0.5 MeV.Comment: 37 pages, 24 figures, to be published in Astroparticle Physic
Low-loss criterion and effective area considerations for photonic crystal fibers
We study the class of endlessly single-mode all-silica photonic crystal
fibers with a triangular air-hole cladding. We consider the sensibility to
longitudinal nonuniformities and the consequences and limitations for realizing
low-loss large-mode area photonic crystal fibers. We also discuss the
dominating scattering mechanism and experimentally we confirm that both macro
and micro-bending can be the limiting factor.Comment: Accepted for Journal of Optics A - Pure and Applied Optic
The radical character of the acenes: A density matrix renormalization group study
We present a detailed investigation of the acene series using high-level
wavefunction theory. Our ab-initio Density Matrix Renormalization Group
algorithm has enabled us to carry out Complete Active Space calculations on the
acenes from napthalene to dodecacene correlating the full pi-valence space.
While we find that the ground-state is a singlet for all chain-lengths,
examination of several measures of radical character, including the natural
orbitals, effective number of unpaired electrons, and various correlation
functions, suggests that the longer acene ground-states are polyradical in
nature.Comment: 10 pages, 8 figures, supplementary material, to be published in J.
Chem. Phys. 127, 200
Measurement of the quenching factor of Na recoils in NaI(Tl)
Measurements of the quenching factor for sodium recoils in a 5 cm diameter
NaI(Tl) crystal at room temperature have been made at a dedicated neutron
facility at the University of Sheffield. The crystal has been exposed to 2.45
MeV mono-energetic neutrons generated by a Sodern GENIE 16 neutron generator,
yielding nuclear recoils of energies between 10 and 100 keVnr. A cylindrical
BC501A detector has been used to tag neutrons that scatter off sodium nuclei in
the crystal. Cuts on pulse shape and time of flight have been performed on
pulses recorded by an Acqiris DC265 digitiser with a 2 ns sampling time.
Measured quenching factors of Na nuclei range from 19% to 26% in good agreement
with other experiments, and a value of 25.2 \pm 6.4% has been determined for 10
keV sodium recoils. From pulse shape analysis, the mean times of pulses from
electron and nuclear recoils have been compared down to 2 keVee. The
experimental results are compared to those predicted by Lindhard theory,
simulated by the SRIM Monte Carlo code, and a preliminary curve calculated by
Prof. Akira Hitachi.Comment: 21 pages, 13 figure
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