1,877 research outputs found
Live-timer method of automatic dead-time correction for precision counting
Automatic correction for dead time losses in nuclear counting experiments is implemented by a simple live timer arrangement in which each counting interval is extended for compensation for the dead time during that interval. this method eliminates repetitious manual calculations, source of error, and dependence upon paralysis shifts
Observed Faraday Effects in Damped Lyman-Alpha Absorbers and Lyman Limit Systems: The Magnetised Environment of Galactic Building Blocks at Redshift=2
Protogalactic environments are typically identified using quasar absorption
lines, and these galactic building blocks can manifest as Damped Lyman-Alpha
Absorbers (DLAs) and Lyman Limit Systems (LLSs). We use radio observations of
Faraday effects to test whether DLAs and LLSs host a magnetised medium, by
combining DLA and LLS detections throughout the literature with 1.4 GHz
polarization data from the NRAO VLA Sky Survey (NVSS). We obtain a control, a
DLA, and a LLS sample consisting of 114, 19, and 27 lines-of-sight respectively
- all of which are polarized at to ensure Rician bias is
negligible. Using a Bayesian framework, we are unable to detect either coherent
or random magnetic fields in DLAs: the regular coherent magnetic fields within
the DLAs must be G, and the lack of depolarization is consistent
with the weakly magnetised gas in DLAs being non-turbulent and quiescent.
However, we find mild suggestive evidence that LLSs have coherent magnetic
fields: after controlling for the redshift-distribution of our data, we find a
71.5% probability that LLSs have a higher RM than a control sample. We also
find strong evidence that LLSs host random magnetic fields, with a 95.5%
probability that LLS lines-of-sight have lower polarized fractions than a
control sample. The regular coherent magnetic fields within the LLSs must be
G, and the magnetised gas must be highly turbulent with a typical
scale on the order of -20 pc, which is similar to that of the Milky
Way. This is consistent with the standard dynamo pedagogy, whereby magnetic
fields in protogalaxies increase in coherence and strength as a function of
cosmic time. Our results are consistent with a hierarchical galaxy formation
scenario, with the DLAs, LLSs, and strong magnesium II (MgII) systems exploring
three different stages of magnetic field evolution in galaxies.Comment: Submitted to Ap
The effective potential, critical point scaling and the renormalization group
The desirability of evaluating the effective potential in field theories near
a phase transition has been recognized in a number of different areas. We show
that recent Monte Carlo simulations for the probability distribution for the
order parameter in an equilibrium Ising system, when combined with low-order
renormalization group results for an ordinary system, can be used to
extract the effective potential. All scaling features are included in the
process.Comment: REVTEX file, 22 pages, three figures, submitted to Phys. Rev.
Reaction-Diffusion Process Driven by a Localized Source: First Passage Properties
We study a reaction-diffusion process that involves two species of atoms,
immobile and diffusing. We assume that initially only immobile atoms, uniformly
distributed throughout the entire space, are present. Diffusing atoms are
injected at the origin by a source which is turned on at time t=0. When a
diffusing atom collides with an immobile atom, the two atoms form an immobile
stable molecule. The region occupied by molecules is asymptotically spherical
with radius growing as t^{1/d} in d>=2 dimensions. We investigate the survival
probability that a diffusing atom has not become a part of a molecule during
the time interval t after its injection and the probability density of such a
particle. We show that asymptotically the survival probability (i) saturates in
one dimension, (ii) vanishes algebraically with time in two dimensions (with
exponent being a function of the dimensionless flux and determined as a zero of
a confluent hypergeometric function), and (iii) exhibits a stretched
exponential decay in three dimensions.Comment: 7 pages; version 2: section IV is re-written, references added, 8
pages (final version
High resolution Ge/Li/ spectrometer reduces rate-dependent distortions at high counting rates
Modified spectrometer system with a low-noise preamplifier reduces rate-dependent distortions at high counting rates, 25,000 counts per second. Pole-zero cancellation minimizes pulse undershoots due to multiple time constants, baseline restoration improves resolution and prevents spectral shifts
Casimir versus Helmholtz forces: Exact results
Recently, attention has turned to the issue of the ensemble dependence of
fluctuation induced forces. As a noteworthy example, in systems the
statistical mechanics underlying such forces can be shown to differ in the
constant magnetic canonical ensemble (CE) from those in the
widely-studied constant grand canonical ensemble (GCE). Here, the
counterpart of the Casimir force in the GCE is the \textit{Helmholtz} force in
the CE. Given the difference between the two ensembles for finite systems, it
is reasonable to anticipate that these forces will have, in general, different
behavior for the same geometry and boundary conditions. Here we present some
exact results for both the Casimir and the Helmholtz force in the case of the
one-dimensional Ising model subject to periodic and antiperiodic boundary
conditions and compare their behavior. We note that the Ising model has
recently being solved in Phys.Rev. E {\bf 106} L042103(2022), using a
combinatorial approach, for the case of fixed value of its order parameter.
Here we derive exact result for the partition function of the one-dimensional
Ising model of spins and fixed value using the transfer matrix method
(TMM); earlier results obtained via the TMM were limited to and even.
As a byproduct, we derive several specific integral representations of the
hypergeometric function of Gauss. Using those results, we rigorously derive
that the free energies of the CE and grand GCE are related to each other via
Legendre transformation in the thermodynamic limit, and establish the leading
finite-size corrections for the canonical case, which turn out to be much more
pronounced than the corresponding ones in the case of the GCE.Comment: 33 pages, 7 figures. The derivations in Appendix C are simplifie
Polymer desorption under pulling: a novel dichotomic phase transition
We show that the structural properties and phase behavior of a self-avoiding
polymer chain on adhesive substrate, subject to pulling at the chain end, can
be obtained by means of a Grand Canonical Ensemble (GCE) approach. We derive
analytical expressions for the mean length of the basic structural units of
adsorbed polymer, such as loops and tails, in terms of the adhesive potential
and detachment force, and determine values of the universal exponents which
govern their probability distributions. Most notably, the hitherto
controversial value of the critical adsorption exponent is found to
depend essentially on the interaction between different loops. The chain
detachment transition turns out to be of the first order, albeit dichotomic,
i.e., no coexistence of different phase states exists. These novel theoretical
predictions and the suggested phase diagram of the adsorption-desorption
transformation under external pulling force are verified by means of extensive
Monte Carlo simulations.Comment: 10 pages, 4 figure
Cassiopeia A: dust factory revealed via submillimetre polarimetry
If Type-II supernovae - the evolutionary end points of short-lived, massive
stars - produce a significant quantity of dust (>0.1 M_sun) then they can
explain the rest-frame far-infrared emission seen in galaxies and quasars in
the first Gyr of the Universe. Submillimetre observations of the Galactic
supernova remnant, Cas A, provided the first observational evidence for the
formation of significant quantities of dust in Type-II supernovae. In this
paper we present new data which show that the submm emission from Cas A is
polarised at a level significantly higher than that of its synchrotron
emission. The orientation is consistent with that of the magnetic field in Cas
A, implying that the polarised submm emission is associated with the remnant.
No known mechanism would vary the synchrotron polarisation in this way and so
we attribute the excess polarised submm flux to cold dust within the remnant,
providing fresh evidence that cosmic dust can form rapidly. This is supported
by the presence of both polarised and unpolarised dust emission in the north of
the remnant, where there is no contamination from foreground molecular clouds.
The inferred dust polarisation fraction is unprecedented (f_pol ~ 30%) which,
coupled with the brief timescale available for grain alignment (<300 yr),
suggests that supernova dust differs from that seen in other Galactic sources
(where f_pol=2-7%), or that a highly efficient grain alignment process must
operate in the environment of a supernova remnant.Comment: In press at MNRAS, 10 pages, print in colou
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
The shape of invasion perclation clusters in random and correlated media
The shape of two-dimensional invasion percolation clusters are studied
numerically for both non-trapping (NTIP) and trapping (TIP) invasion
percolation processes. Two different anisotropy quantifiers, the anisotropy
parameter and the asphericity are used for probing the degree of anisotropy of
clusters. We observe that in spite of the difference in scaling properties of
NTIP and TIP, there is no difference in the values of anisotropy quantifiers of
these processes. Furthermore, we find that in completely random media, the
invasion percolation clusters are on average slightly less isotropic than
standard percolation clusters. Introducing isotropic long-range correlations
into the media reduces the isotropy of the invasion percolation clusters. The
effect is more pronounced for the case of persisting long-range correlations.
The implication of boundary conditions on the shape of clusters is another
subject of interest. Compared to the case of free boundary conditions, IP
clusters of conventional rectangular geometry turn out to be more isotropic.
Moreover, we see that in conventional rectangular geometry the NTIP clusters
are more isotropic than TIP clusters
- …