17,831 research outputs found
Nationalism and internationalism in science: the case of the discovery of cosmic rays
The discovery of cosmic rays, a milestone in science, comprised scientists in
Europe and the US and took place during a period characterised by nationalism
and lack of communication. Many scientists that took part in this research a
century ago were intrigued by the penetrating radiation and tried to understand
the origin of it. Several important contributions to the discovery of the
origin of cosmic rays have been forgotten and in particular that of Domenico
Pacini, who in June 1911 demonstrated by studying the decrease of radioactivity
with an electroscope immersed in water that cosmic rays could not come from the
crust of the Earth. Several historical, political and personal facts might have
contributed to the substantial disappearance of Pacini from the history of
science.Comment: To be published in European Physical Journal H - Version revised
after interaction with the editor and the referee
Spatiotemporal correlations of earthquakes in the continuum limit of the one-dimensional Burridge-Knopoff model
Spatiotemporal correlations of the one-dimensional spring-block
(Burridge-Knopoff) model of earthquakes, either with or without the viscosity
term, are studied by means of numerical computer simulations. The continuum
limit of the model is examined by systematically investigating the model
properties with varying the block-size parameter a toward a\to 0. The Kelvin
viscosity term is introduced so that the model dynamics possesses a sensible
continuum limit. In the presence of the viscosity term, many of the properties
of the original discrete BK model are kept qualitatively unchanged even in the
continuum limit, although the size of minimum earthquake gets smaller as a gets
smaller. One notable exception is the existence/non-existence of the
doughnut-like quiescence prior to the mainshock. Although large events of the
original discrete BK model accompany seismic acceleration together with a
doughnut-like quiescence just before the mainshock, the spatial range of the
doughnut-like quiescence becomes narrower as a gets smaller, and in the
continuum limit, the doughnut-like quiescence might vanish altogether. The
doughnut-like quiescence observed in the discrete BK model is then a phenomenon
closely related to the short-length cut-off scale of the model
Multiple giant resonances in nuclei: their excitation and decay
The excitation of multiphonon giant resonances with heavy ions is discussed.
The conventional theory, based on the use of the virtual photon number method
in conjunction with the harmonic model is presented and its shortcomings are
discussed. The recently developed model that invoke the Brink-Axel mechanism as
an important contribution to the cross-section is discussed and compared to the
conventional, harmonic model. The decay properties of these multiple giant
resonances are also discussed within the same coherent + fluctuation model in
conjunction with the hybrid decay model. It is demonstrated that the Brink-Axel
mechanism enhances the direct decay of the states, as data seem to require.
Comparison of our model with other recent theoretical works is presented.Comment: 12 pages, four figures, two tables. Invited talk at the International
Conference on Collective Motion in Nuclei Under Extreme Conditions (COMEX1),
Paris, France, 10-13 June 200
Simulation study of the two-dimensional Burridge-Knopoff model of earthquakes
Spatiotemporal correlations of the two-dimensional spring-block
(Burridge-Knopoff) model of earthquakes are extensively studied by means of
numerical computer simulations. The model is found to exhibit either
``subcritical'' or ``supercritical'' behavior, depending on the values of the
model parameters. Transition between these regimes is either continuous or
discontinuous. Seismic events in the ``subcritical'' regime and those in the
``supercritical'' regime at larger magnitudes exhibit universal scaling
properties. In the ``supercritical'' regime, eminent spatiotemporal
correlations, {\it e.g.}, remarkable growth of seismic activity preceding the
mainshock, arise in earthquake occurrence, whereas such spatiotemporal
correlations are significantly suppressed in the ``subcritical'' regime.
Seismic activity is generically suppressed just before the mainshock in a close
vicinity of the epicenter of the upcoming event while it remains to be active
in the surroundings (the Mogi doughnut). It is also observed that, before and
after the mainshock, the apparent -value of the magnitude distribution
decreases or increases in the ``supercritical'' or ``subcritical'' regimes,
respectively. Such distinct precursory phenomena may open a way to the
prediction of the upcoming large event
142Nd/144Nd Inferences on the nature and origin of the source of high 3He/4He magmas
High-precision measurements of 142Nd/144Nd in picrites from the Baffin Bay region that contain the highest 3He/4He ratios yet measured in terrestrial mantle-derived rocks are indistinguishable from the value measured in the terrestrial standard and other modern mantle-derived rocks. The Baffin Island lavas are distinguished from other hotspot lavas by their unusually high 3He/4He and 182W/184W ratios, but their Sr, 142Nd, 143Nd, Hf, and Pb isotopic signatures overlap the values measured in North Atlantic MORB. These features imply either that the mantle source region of high 3He/4He magmas carries the lithophile isotopic signatures of incompatible element depletion, or that the He isotope signature of this source is decoupled from the lithophile isotope tracers in the magmas. The coupled 142Nd–143Nd data are consistent with the magma source acquiring the incompatible element depletion during, or shortly after, Earth formation if the bulk-Earth has a 142Nd/144Nd ratio more similar to the average measured for enstatite chondrites than modern terrestrial rocks. If Earth's initial 142Nd/144Nd was higher than the average of enstatite chondrites, the data are consistent with the traditional interpretation that the depleted-mantle reservoir was formed through the extraction of an incompatible-element-rich reservoir, such as continental crust, after the circa 4 Ga extinction of 146Sm. This explanation, however, fails to account for the high 3He/4He. The Nd isotopic composition of the picrites could reflect a dominant contribution from the incompatible element depleted source of North Atlantic MORB, overprinted by a small (10–20%) contribution from a mantle source with He concentrations at least ten times higher than the depleted mantle along with W isotopic compositions substantially higher than typical of mantle-derived rocks
Accurate photometry of extended spherically symmetric sources
We present a new method to derive reliable photometry of extended spherically
symmetric sources from {\it HST} images (WFPC2, ACS/WFC and NICMOS/NIC2
cameras), extending existing studies of point sources and marginally resolved
sources. We develop a new approach to accurately determine intrinsic sizes of
extended spherically symmetric sources, such as star clusters in galaxies
beyond the Local Group (at distances <~ 20 Mpc), and provide a detailed
cookbook to perform aperture photometry on such sources, by determining
size-dependent aperture corrections (ACs) and taking sky oversubtraction as a
function of source size into account. In an extensive Appendix, we provide the
parameters of polynomial relations between the FWHM of various input profiles
and those obtained by fitting a Gaussian profile (which we have used for
reasons of computational robustness, although the exact model profile used is
irrelevant), and between the intrinsic and measured FWHM of the cluster and the
derived AC. Both relations are given for a number of physically relevant
cluster light profiles, intrinsic and observational parameters. AC relations
are provided for a wide range of apertures. Depending on the size of the source
and the annuli used for the photometry, the absolute magnitude of such extended
objects can be underestimated by up to 3 mag, corresponding to an error in mass
of a factor of 15. We carefully compare our results to those from the more
widely used DeltaMag method, and find an improvement of a factor of 3--40 in
both the size determination and the AC.Comment: The paper is accepted for publication in A&A, Section 13
(Observational Techniques, published electronically). The published version
contains one example table per appendix. A version of the paper containing
all tables as well as all data in electronical form are available
http://www.astro.physik.uni-goettingen.de/~galev/panders/Sizes_AC
SU(2,1) Dynamics of Multiple Giant Dipole Resonance Coulomb Excitation
We construct a three-dimensional analytically soluble model of the nonlinear
effects in Coulomb excitation of multiphonon Giant Dipole Resonances (GDR)
based on the SU(2,1) algebra. The full 3-dimensional model predicts further
enhancement of the Double GDR (DGDR) cross sections at high bombarding
energies. Enhancement factors for DGDR measured in thirteen different processes
with various projectiles and targets at different bombarding energies are well
reproduced with the same value of the nonlinearity parameter with the exception
of the anomalous case of Xe which requires a larger value.Comment: 10 pages, 3 Postscript figures, late
Decay theory of double giant resonances
The decay theory of double giant resonances incorporating fluctuation
contributions of the Brink-Axel type is developed. The gamma and neutron
emission decay of Double Giant Dipole Resonances (DGDR) in 208Pb is discussed
in connection with a recent measurement.Comment: 5 pages, Late
Static three- and four-quark potentials
We present results for the static three- and four-quark potentials in SU(3)
and SU(4) respectively. Using a variational approach, combined with multi-hit
for the time-like links, we determine the ground state of the baryonic string
with sufficient accuracy to test the and ans\"atze for the
baryonic Wilson area law. Our results favor the ansatz, where the
potential is the sum of two-body terms.Comment: Lattice2001(heavyquark
Target-mass corrections and the Bloom-Gilman duality of the nucleon structure function
The occurrence of the Bloom-Gilman local duality in the low-order moments of
the nucleon structure function is investigated for values of the squared
four-momentum transfer Q**2 between ~ 0.5 and 10 (GeV/c)**2. At variance with
previous analyses truncated Cornwall-Norton moments, limited to the
nucleon-resonance production regions, are considered. The role played by
target-mass corrections is illustrated, showing that target-mass effects are
necessary (but not sufficient) for producing the observed Bloom-Gilman duality
of the nucleon structure function. The possibility of a local duality between
the unphysical region at large values of the Nachtmann variable and the nucleon
elastic peak contribution is analyzed. It is found that the proton magnetic
form factor extracted assuming local duality is significantly below the
experimental data at low and intermediate values of Q**2.Comment: final version with minor modifications, to appear in Phys. Lett.
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