7,786 research outputs found
A probability-conserving cross-section biasing mechanism for variance reduction in Monte Carlo particle transport calculations
In Monte Carlo particle transport codes, it is often important to adjust
reaction cross sections to reduce the variance of calculations of relatively
rare events, in a technique known as non-analogous Monte Carlo. We present the
theory and sample code for a Geant4 process which allows the cross section of a
G4VDiscreteProcess to be scaled, while adjusting track weights so as to
mitigate the effects of altered primary beam depletion induced by the cross
section change. This makes it possible to increase the cross section of nuclear
reactions by factors exceeding 10^4 (in appropriate cases), without distorting
the results of energy deposition calculations or coincidence rates. The
procedure is also valid for bias factors less than unity, which is useful, for
example, in problems that involve computation of particle penetration deep into
a target, such as occurs in atmospheric showers or in shielding
Magnetic properties of PrCu at high pressure
We report a study of the low-temperature high-pressure phase diagram of the
intermetallic compound PrCu, by means of molecular-field calculations and
Cu nuclear-quadrupole-resonance (NQR) measurements under pressure.
The pressure-induced magnetically-ordered phase can be accounted for by
considering the influence of the crystal electric field on the electron
orbitals of the Pr ions and by introducing a pressure-dependent exchange
interaction between the corresponding local magnetic moments. Our experimental
data suggest that the order in the induced antiferromagnetic phase is
incommensurate. The role of magnetic fluctuations both at high and low
pressures is also discussed.Comment: 7 pages, 6 figures, submitted to Eur. Phys. J.
Solvent influence on the magnetic field effect of polymethylene‐linked photogenerated radical ion pairs.
The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene‐linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N‐dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance‐dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation
Improved meteorological measurements from buoys and ships (IMET) : preliminary comparison of precipitation sensors
Rainfall data obtained from an optical rain gauge and a capacitive siphon rain
gauge are analyzed and discussed. These sensors were developed for unattended use
and are being considered for use at sea on ships and buoys.Funding was provided by the National Science Foundation under
Grant No. OCE-87-0961
Solvent effects on the magnetic-field-dependent reaction yields of photogenerated radical ion pairs.
A pronounced solvent viscosity and polarity dependence of the magnetic field effect was found for polymethylene-linked radical ion pairs generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH2)n-N,N-dimethylaniline, with n = 7–16. This is interpreted in terms of the general radical pair mechanism, i.e. the nuclear hyperfine interaction mechanism including a spin-exchange interaction, modulated by the stochastic folding motion of the linking CH2 chain which leads to a “motional deformation” of the reaction yield spectra
A compilation of moored current meter data and wind recorder data from the Severe Environment Surface Mooring (SESMOOR) volume XLIII
A Severe Environment Surface Mooring (SESMOOR) was designed to make long term meteorological and near surface
oceanographic measurements in areas where harsh envionmental conditions prevail. SESMOOR was deployed in the North
Atlantic Ocean approximately 300 km southeast of Halifax, Nova Scotia for 141 days during the winter of 1988-89.
Meterological data were acquired from two Vector Averaging Wind Recorders (VAWR) located on top of a specially designed
buoy mast and included air temperature, relative humidity, barometric pressure, wind velocity, solar and longwave radiation. Sea
surface temperature was also acquired by the VAWR. Current velocities and sea temperatures were obtained from two Vector Measuring Current Meters (VMCM) at 20 and 50 meters below the sea surface.
This report discusses instrument performance, data quality, pre-and post-deployment calibrations, data telemetry, data
processing procedures. This report also presents the data in a variety of displays.Funding was provided by the Office of Naval Research under Contract No. N00014-84-C-0134, NR 083-400 and Grant No. N00014-90-J-1423
An Algorithm for Computing Screened Coulomb Scattering in Geant4
An algorithm has been developed for the Geant4 Monte-Carlo package for the
efficient computation of screened Coulomb interatomic scattering. It explicitly
integrates the classical equations of motion for scattering events, resulting
in precise tracking of both the projectile and the recoil target nucleus. The
algorithm permits the user to plug in an arbitrary screening function, such as
Lens-Jensen screening, which is good for backscattering calculations, or
Ziegler-Biersack-Littmark screening, which is good for nuclear straggling and
implantation problems. This will allow many of the applications of the TRIM and
SRIM codes to be extended into the much more general Geant4 framework where
nuclear and other effects can be included.Comment: 19 pages, 6 figures; corrected to rerferee comments, typo in equation
5 fixe
Coherent photonuclear reactions for isotope transmutation
Coherent photonuclear isotope transmutation (CPIT) produces exclusively
radioactive isotopes (RIs) by coherent photonuclear reactions via E1 giant
resonances. Photons to be used are medium energy photons produced by laser
photons backscattered off GeV electrons. The cross sections are as large as 0.2
- 0.6 b, being independent of individual nuclides. A large fraction of photons
is effectively used for the photonuclear reactions, while the scattered GeV
electrons remain in the storage ring to be re-used. CPIT with medium energy
photons provides specific/desired RIs with the high rate and the high density
for nuclear science, molecular biology and for nuclear medicines.Comment: 8 pages, 2 figure
Stable ultrahigh-density magneto-optical recordings using introduced linear defects
The stability of data bits in magnetic recording media at ultrahigh densities
is compromised by thermal `flips' -- magnetic spin reversals -- of nano-sized
spin domains, which erase the stored information. Media that are magnetized
perpendicular to the plane of the film, such as ultrathin cobalt films or
multilayered structures, are more stable against thermal self-erasure than
conventional memory devices. In this context, magneto-optical memories seem
particularly promising for ultrahigh-density recording on portable disks, and
bit densities of 100 Gbit inch have been demonstrated using recent
advances in the bit writing and reading techniques. But the roughness and
mobility of the magnetic domain walls prevents closer packing of the magnetic
bits, and therefore presents a challenge to reaching even higher bit densities.
Here we report that the strain imposed by a linear defect in a magnetic thin
film can smooth rough domain walls over regions hundreds of micrometers in
size, and halt their motion. A scaling analysis of this process, based on the
generic physics of disorder-controlled elastic lines, points to a simple way by
which magnetic media might be prepared that can store data at densities in
excess of 1 Tbit inch.Comment: 5 pages, 4 figures, see also an article in TRN News at
http://www.trnmag.com/Stories/041801/Defects_boost_disc_capacity_041801.htm
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