119 research outputs found
North-South Distribution of Solar Flares during Cycle 23
In this paper, we investigate the spatial distribution of solar flares in the
northern and southern hemisphere of the Sun that occurred during the period
1996 to 2003. This period of investigation includes the ascending phase, the
maximum and part of descending phase of solar cycle 23. It is revealed that the
flare activity during this cycle is low compared to previous solar cycle,
indicating the violation of Gnevyshev-Ohl rule. The distribution of flares with
respect to heliographic latitudes shows a significant asymmetry between
northern and southern hemisphere which is maximum during the minimum phase of
the solar cycle. The present study indicates that the activity dominates the
northern hemisphere in general during the rising phase of the cycle
(1997-2000). The dominance of northern hemisphere is shifted towards the
southern hemisphere after the solar maximum in 2000 and remained there in the
successive years. Although the annual variations in the asymmetry time series
during cycle 23 are quite different from cycle 22, they are comparable to cycle
21.Comment: 6 pages, 2 figures, 1 table; Accepted for the publication in the
proceedings of international solar workshop held at ARIES, Nainital, India on
"Transient Phenomena on the Sun and Interplanetary Medium" in a special issue
of "Journal of Astrophysics and Astronomy (JAA)
Study of Distribution and Asymmetry of Solar Active Prominences During Solar Cycle 23
In this paper we present the results of a study of the spatial distribution
and asymmetry of solar active prominences (SAP) for the period 1996-2007 (solar
cycle 23). For more meaningful statistical analysis we have analysed the
distribution and asymmetry of SAP in two subdivisions viz. Group1 (ADF, APR,
DSF, CRN, CAP) and Group2 (AFS, ASR, BSD, BSL, DSD, SPY, LPS). The north-south
(N-S) latitudinal distribution shows that the SAP events are most prolific in
the 21-30degree slice in the northern and southern hemispheres and east-west
(E-W) longitudinal distribution study shows that the SAP events are most
prolific (best visible) in the 81-90degree slice in the eastern and western
hemispheres. It has been found that the SAP activity during this cycle is low
compared to previous solar cycles. The present study indicates that during the
rising phase of the cycle the number of SAP events were roughly equal on the
north and south hemispheres. However, activity on the southern hemisphere has
been dominant since 1999. Our statistical study shows that the N-S asymmetry is
more significant then the E-W asymmetry.Comment: 21 pages 5 figures; Published online; 02 October, 2009; Solar Physics
Journa
Light-ion production in the interaction of 96 MeV neutrons with oxygen
Double-differential cross sections for light-ion (p, d, t, He-3 and alpha)
production in oxygen, induced by 96 MeV neutrons are reported. Energy spectra
are measured at eight laboratory angles from 20 degrees to 160 degrees in steps
of 20 degrees. Procedures for data taking and data reduction are presented.
Deduced energy-differential and production cross sections are reported.
Experimental cross sections are compared to theoretical reaction model
calculations and experimental data at lower neutron energies in the literature.
The measured proton data agree reasonably well with the results of the model
calculations, whereas the agreement for the other particles is less convincing.
The measured production cross sections for protons, deuterons, tritons and
alpha particles support the trends suggested by data at lower energies.Comment: 21 pages, 13 figures, submitted to Phys. Rev.
Discrimination of gamma rays due to inelastic neutron scattering in AGATA
Possibilities of discriminating neutrons and gamma rays in the AGATA
gamma-ray tracking spectrometer have been investigated with the aim of reducing
the background due to inelastic scattering of neutrons in the high-purity
germanium crystals. This background may become a serious problem especially in
experiments with neutron-rich radioactive ion beams. Simulations using the
Geant4 toolkit and a tracking program based on the forward tracking algorithm
were carried out by emitting neutrons and gamma rays from the center of AGATA.
Three different methods were developed and tested in order to find
'fingerprints' of the neutron interaction points in the detectors. In a
simulation with simultaneous emission of six neutrons with energies in the
range 1-5 MeV and ten gamma rays with energies between 150 and 1450 keV, the
peak-to-background ratio at a gamma-ray energy of 1.0 MeV was improved by a
factor of 2.4 after neutron rejection with a reduction of the photopeak
efficiency at 1.0 MeV of only a factor of 1.25.Comment: Accepted for publication in Nuclear Instruments and Methods in
Physics Research, A, 26 May 2009; 13 pages, 5 tables, 12 figure
Nucleon-induced reactions at intermediate energies: New data at 96 MeV and theoretical status
Double-differential cross sections for light charged particle production (up
to A=4) were measured in 96 MeV neutron-induced reactions, at TSL laboratory
cyclotron in Uppsala (Sweden). Measurements for three targets, Fe, Pb, and U,
were performed using two independent devices, SCANDAL and MEDLEY. The data were
recorded with low energy thresholds and for a wide angular range (20-160
degrees). The normalization procedure used to extract the cross sections is
based on the np elastic scattering reaction that we measured and for which we
present experimental results. A good control of the systematic uncertainties
affecting the results is achieved. Calculations using the exciton model are
reported. Two different theoretical approches proposed to improve its
predictive power regarding the complex particle emission are tested. The
capabilities of each approach is illustrated by comparison with the 96 MeV data
that we measured, and with other experimental results available in the
literature.Comment: 21 pages, 28 figure
Compilation of Giant Electric Dipole Resonances Built on Excited States
Giant Electric Dipole Resonance (GDR) parameters for gamma decay to excited
states with finite spin and temperature are compiled. Over 100 original works
have been reviewed and from some 70 of which more than 300 parameter sets of
hot GDR parameters for different isotopes, excitation energies, and spin
regions have been extracted. All parameter sets have been brought onto a common
footing by calculating the equivalent Lorentzian parameters. The current
compilation is complementary to an earlier compilation by Samuel S. Dietrich
and Barry L. Berman (At. Data Nucl. Data Tables 38(1988)199-338) on
ground-state photo-neutron and photo-absorption cross sections and their
Lorentzian parameters. A comparison of the two may help shed light on the
evolution of GDR parameters with temperature and spin. The present compilation
is current as of January 2006.Comment: 31 pages including 1 tabl
Character of particle-hole excitations in 94 Ru deduced from gamma-ray angular correlation and linear polarization measurements
Signatures of a dissipative phase transition in photon correlation measurements
This work was supported by the Swiss National Science Foundation (SNSF) through the National Centre of Competence in Research - Quantum Science and Technology (NCCR QSIT). A.S., C.S., and S.H. acknowledge support by the State of Bavaria and the DFG within the Project Schn1376/3-1.Understanding and characterizing phase transitions in driven-dissipative systems constitutes a new frontier for many-body physics[1-8]. A generic feature of dissipative phase transitions is a vanishing gap in the Liouvillian spectrum [9], which leads to long-lived deviations from the steady state as the system is driven towards the transition. Here, we show that photon correlation measurements can be used to characterize the corresponding critical slowing down of non-equilibrium dynamics. We focus on the extensively studied phenomenon of optical bistability in GaAs cavity polaritons [10,11], which can be described as a first-order dissipative phase transition [12-14]. Increasing the excitation strength towards the bistable range results in an increasing photon-bunching signal along with a decay time that is prolonged by more than nine orders of magnitude as compared with that of single polaritons. In the limit of strong polariton interactions leading to pronounced quantum fluctuations, the mean-field bistability threshold is washed out. Nevertheless, the functional form with which the Liouvillian gap closes as the thermodynamic limit is approached provides a signature of the emerging dissipative phase transition. Our results establish photon correlation measurements as an invaluable tool for studying dynamical properties of dissipative phase transitions without requiring phase-sensitive interferometric measurements.PostprintPeer reviewe
A closer look at neuron interaction with track-etched microporous membranes
Microporous membranes support the growth of neurites into and through micro-channels, providing a different type of neural growth platform to conventional dish cultures. Microporous membranes are used to support various types of culture, however, the role of pore diameter in relation to neurite growth through the membrane has not been well characterised. In this study, the human cell line (SH-SY5Y) was differentiated into neuron-like cells and cultured on track-etched microporous membranes with pore and channel diameters selected to accommodate neurite width (0.8 µm to 5 µm). Whilst neurites extended through all pore diameters, the extent of neurite coverage on the non-seeded side of the membranes after 5 days in culture was found to be directly proportional to channel diameter. Neurite growth through membrane pores reduced significantly when neural cultures were non-confluent. Scanning electron microscopy revealed that neurites bridged pores and circumnavigated pore edges – such that the overall likelihood of a neurite entering a pore channel was decreased. These findings highlight the role of pore diameter, cell sheet confluence and contact guidance in directing neurite growth through pores and may be useful in applications that seek to use physical substrates to maintain separate neural populations whilst permitting neurite contact between cultures
Ultrafast all-optical switching by single photons
An outstanding goal in quantum optics is the realization of fast optical
non-linearities at the single-photon level. Such non-linearities would allow
for the realization of optical devices with new functionalities such as a
single-photon switch/transistor or a controlled-phase gate, which could form
the basis of future quantum optical technologies. While non-linear optics
effects at the single-emitter level have been demonstrated in different
systems, including atoms coupled to Fabry-Perot or toroidal micro-cavities,
super-conducting qubits in strip-line resonators or quantum dots (QDs) in
nano-cavities, none of these experiments so far has demonstrated single-photon
switching on ultrafast timescales. Here, we demonstrate that in a strongly
coupled QD-cavity system the presence of a single photon on one of the
fundamental polariton transitions can turn on light scattering on a transition
from the first to the second Jaynes-Cummings manifold with a switching time of
20 ps. As an additional device application, we use this non-linearity to
implement a single-photon pulse-correlator. Our QD-cavity system could form the
building-block of future high-bandwidth photonic networks operating in the
quantum regime
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