136 research outputs found
Angle resolved IBIC analysis of 4H-SiC Schottky diodes
We present a new experimental procedure based on the ion beam induced charge collection (IBIC) to characterise semiconductor detectors and devices. It consists in measuring the charge collection efficiency (q) as a function of the angle of incidence (eta) of a strongly penetrating MeV ion beam focussed onto a partially depleted semiconductor detector. The unidimensional model based on the drift-diffusion model derived from the Shockley-Ramo-Gunn's theorem gives the theoretical background to fit the eta(alpha) curve and to estimate both the extension of the depletion layer, the dead layer thickness and the minority carrier diffusion length. To illustrate the analytical capability of this technique, a 2 MeV proton beam was focussed at different incident angles onto a 4H-SiC Schottky diode; the experimental results and the theoretical approach are presented and discussed. (c) 2006 Published by Elsevier B.V
Subthreshold rho^0 photoproduction on 3He
A large reduction of the rho^0 mass in the nuclear medium is reported,
inferred from dipion photoproduction spectra in the 1 GeV region, for the
reaction 3He(gamma,pi+ pi-)X with a 10% duty factor tagged-photon beam and the
TAGX multi-particle spectrometer. The energy range covered (800 < E(gamma) <
1120 MeV) lies mostly below the free rho^0 production threshold, a region which
is believed sensitive to modifications of light vector-meson properties at
nuclear-matter densities. The rho^0 masses extracted from the MC fitting of the
data, m*(rho^0) = 642 +/- 40, 669 +/- 32, and 682 +/- 56 MeV/c^2 for E(gamma)
in the 800-880, 880-960, and 960-1040 MeV regions respectively, are
independently corroborated by a measured, assumption-free, kinematical
observable. This mass shift, far exceeding current mean-field driven
theoretical predictions, may be suggestive of rho^0 decay within the range of
the nucleonic field.Comment: 40 pages, 13 figures, submitted to Phys. Rev.
Nuclear Transparency to Intermediate-Energy Protons
Nuclear transparency in the (e,e'p) reaction for 135 < Tp < 800 MeV is
investigated using the distorted wave approximation. Calculations using
density-dependent effective interactions are compared with phenomenological
optical potentials. Nuclear transparency is well correlated with proton
absorption and neutron total cross sections. For Tp < 300 MeV there is
considerable sensitivity to the choice of optical model, with the empirical
effective interaction providing the best agreement with transparency data. For
Tp > 300 MeV there is much less difference between optical models, but the
calculations substantially underpredict transparency data and the discrepancy
increases with A. The differences between Glauber and optical model
calculations are related to their respective definitions of the semi-inclusive
cross section. By using a more inclusive summation over final states the
Glauber model emphasizes nucleon-nucleon inelasticity, whereas with a more
restrictive summation the optical model emphasizes nucleon-nucleus
inelasticity; experimental definitions of the semi-inclusive cross section lie
between these extremes.Comment: uuencoded gz-compressed tar file containing revtex and bbl files and
5 postscript figures, totalling 31 pages. Uses psfi
Nuclear transparency from quasielastic A(e,e'p) reactions uo to Q^2=8.1 (GeV/c)^2
The quasielastic (e,ep) reaction was studied on targets of
deuterium, carbon, and iron up to a value of momentum transfer of 8.1
(GeV/c). A nuclear transparency was determined by comparing the data to
calculations in the Plane-Wave Impulse Approximation. The dependence of the
nuclear transparency on and the mass number was investigated in a
search for the onset of the Color Transparency phenomenon. We find no evidence
for the onset of Color Transparency within our range of . A fit to the
world's nuclear transparency data reflects the energy dependence of the free
proton-nucleon cross section.Comment: 11 pages, 6 figure
Relativistic calculation of nuclear transparency in (e,e'p) reactions
Nuclear transparency in (e,e'p) reactions is evaluated in a fully
relativistic distorted wave impulse approximation model. The relativistic mean
field theory is used for the bound state and the Pauli reduction for the
scattering state, which is calculated from a relativistic optical potential.
Results for selected nuclei are displayed in a Q^2 range between 0.3 and 1.8
(GeV/c)^2 and compared with recent electron scattering data. For Q^2 = 0.3
(GeV/c)^2 the results are lower than data; for higher Q^2 they are in
reasonable agreement with data. The sensitivity of the model to different
prescriptions for the one-body current operator is investigated. The off-shell
ambiguities are rather large for the distorted cross sections and small for the
plane wave cross sections.Comment: 8 pages, 3 figure
Probing the DeltaNN component of 3He
The 3He(gamma,pi^+/- p) reactions were measured simultaneously over a tagged
photon energy range of 800<E_gamma<1120 MeV, well above the Delta resonance
region. An analysis was performed to kinematically isolate Delta knockout
events from conventional Delta photoproduction events, and a statistically
significant excess of pi+p events was identified, consistent with Delta++
knockout. Two methods were used to estimate the DeltaNN probability in the 3He
ground state, corresponding to the observed knockout cross section. The first
gave a lower probability limit of 1.5+/-0.6+/-0.5%; the second yielded an upper
limit of about 2.6%.Comment: 14 page
Activation and Deactivation of a Robust Immobilized Cp*Ir-Transfer Hydrogenation Catalyst: A Multielement in Situ X-ray Absorption Spectroscopy Study
A highly robust immobilized [Cp*IrCl2]2 precatalyst on Wang resin for transfer hydrogenation, which can be recycled up to 30 times, was studied using a novel combination of X-ray absorption spectroscopy (XAS) at Ir L3-edge, Cl K-edge, and K K-edge. These culminate in in situ XAS experiments that link structural changes of the Ir complex with its catalytic activity and its deactivation. Mercury poisoning and âhot filtrationâ experiments ruled out leached Ir as the active catalyst. Spectroscopic evidence indicates the exchange of one chloride ligand with an alkoxide to generate the active precatalyst. The exchange of the second chloride ligand, however, leads to a potassium alkoxideâiridate species as the deactivated form of this immobilized catalyst. These findings could be widely applicable to the many homogeneous transfer hydrogenation catalysts with Cp*IrCl substructure
Measurement of nuclear transparency ratios for protons and neutrons
This paper presents, for the first time, measurements of neutron transparency ratios for nuclei relative to C measured using the (e,eâČn) reaction, spanning measured neutron momenta of 1.4 to 2.4 GeV/c. The transparency ratios were extracted in two kinematical regions, corresponding to knockout of mean-field nucleons and to the breakup of Short-Range Correlated nucleon pairs. The extracted neutron transparency ratios are consistent with each other for the two measured kinematical regions and agree with the proton transparencies extracted from new and previous (e,eâČp) measurements, including those from neutron-rich nuclei such as lead. The data also agree with and confirm the Glauber approximation that is commonly used to interpret experimental data. The nuclear-mass-dependence of the extracted transparencies scales as Aα with α=â0.289±0.007, which is consistent with nuclear-surface dominance of the reactions
Evidence for the Onset of Color Transparency in Electroproduction off Nuclei
We have measured the nuclear transparency of the incoherent diffractive
process in C and Fe targets relative to H
using a 5 GeV electron beam. The nuclear transparency, the ratio of the
produced 's on a nucleus relative to deuterium, which is sensitive to
interaction, was studied as function of the coherence length (),
a lifetime of the hadronic fluctuation of the virtual photon, and the
four-momentum transfer squared (). While the transparency for both
C and Fe showed no dependence, a significant
dependence was measured, which is consistent with calculations that included
the color transparency effects.Comment: 6 pages and 4 figure
- âŠ