3,817 research outputs found
Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron
Measuring Compton scattered photons and recoil neutrons in coincidence,
quasi-free Compton scattering by the neutron has been investigated at MAMI
(Mainz) at in an energy range from 200 to 400 MeV.
From the data a polarizability difference of in units of has been
determined. In combination with the polarizability sum deduced from photo absorption data, the neutron electric and
magnetic polarizabilities, and ,
are obtained
Incoherent pion photoproduction on the deuteron in the first resonance region
Incoherent pion photoproduction on the deuteron is studied in the first
resonance region. The unpolarized cross section, the beam asymmetry, and the
vector and tensor target asymmetries are calculated in the framework of a
diagrammatic approach. Pole diagrams and one-loop diagrams with scattering
in the final state are taken into account. An elementary operator for pion
photoproduction on the nucleon is taken in various on-shell forms and
calculated using the SAID and MAID multipole analyses. Model dependence of the
obtained results is discussed in some detail. A comparison with predictions of
other works is given. Although a reasonable description of many available
experimental data on the unpolarized total and differential cross sections and
photon asymmetry has been achieved, in some cases a significant disagreement
between the theory and experiment has been found. Invoking known information on
the reactions and we predict the total
photoabsorption cross section for deuterium. We find that our values strongly
overestimate experimental data in the vicinity of the peak.Comment: 22 pages, 23 figure
Quasi-free Photoproduction from the Bound Nucleon
Differential cross-sections for quasi-free photoproduction from the
proton and neutron bound in the deuteron have been measured for MeV at usind the Glasgow photon
tagger at MAMI, the Mainz 48 cm 64 cm NaI(Tl) photon
detector and the G\"ottingen SENECA recoil detector. For the proton
measurements made with both liquid deuterium and liquid hydrogen targets allow
direct comparison of "free" photoproduction cross-sections as extracted
from the bound proton data with experimental free cross sections which are
found to be in reasonable agreement below 320 MeV. At higher energies the
"free" cross sections extracted from quasifree data are significantly smaller
than the experimental free cross sections and theoretical predictions based on
multipole analysis. For the first time, "free" neutron cross sections have been
extracted in the -region. They are also in agreement with the
predictions from multipole analysis up to 320 MeV and significantly smaller at
higher photon energies
An Analysis of the Chemical Composition of the Atmosphere of Venus on an AMS of the Venera-12 Using a Gas Chromatograph
Eight analyses of the atmosphere of Venus were made beginning at an altitude of 42 km right down to the surface of the planet. The following were detected in the atmosphere of Venus: nitrogen in concentrations of 2.5 plus or minus 0.5 volumetric %, argon ir concentrations (4 plus or minus 2) x 10 to the minus 3 power volumetric %, CO--(2.8 plus or minus 1.4) x 10 to the minus 3 power volumetric % and SO2 in concentrations (1.3 plus or minus 0.6) x 10 to the minus 2 power volumetric %. The upper limits were estimated for the content of oxygen and water equal to 2 x 10 to the minus 3 power and 10 to the minus 2 power volumetric %, respectively
Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons
The general structure of the cross section of scattering with
polarized photon and/or nucleon in initial and/or final state is systematically
described and exposed through invariant amplitudes. A low-energy expansion of
the cross section up to and including terms of order is given which
involves ten structure parameters of the nucleon (dipole, quadrupole,
dispersion, and spin polarizabilities). Their physical meaning is discussed in
detail. Using fixed-t dispersion relations, predictions for these parameters
are obtained and compared with results of chiral perturbation theory. It is
emphasized that Compton scattering experiments at large angles can fix the most
uncertain of these structure parameters. Predictions for the cross section and
double-polarization asymmetries are given and the convergence of the expansion
is investigated. The feasibility of the experimental determination of some of
the struture parameters is discussed.Comment: 41 pages of text, 9 figures; minor revisions prior to publication in
Phys. Rev.
Dispersion Effects in Nucleon Polarisabilities
We present a formalism to extract the dynamical nucleon polarisabilities
defined via a multipole expansion of the structure amplitudes in nucleon
Compton scattering. In contradistinction to the static polarisabilities,
dynamical polarisabilities gauge the response of the internal degrees of
freedom of a composed object to an external, real photon field of arbitrary
energy. Being energy dependent, they therefore contain additional information
about dispersive effects induced by internal relaxation mechanisms, baryonic
resonances and meson production thresholds of the nucleon. We give explicit
formulae to extract the dynamical electric and magnetic dipole as well as
quadrupole polarisabilities from low energy nucleon Compton scattering up to
the one pion production threshold and discuss the connection to the definition
of static nucleon polarisabilities. As a concrete example, we examine the
results of leading order Heavy Baryon Chiral Perturbation Theory for the four
leading spin independent iso-scalar polarisabilities of the nucleon. Finally,
we consider the possible r{\^o}le of energy dependent effects in low energy
extractions of the iso-scalar dipole polarisabilities from Compton scattering
on the deuteron.Comment: 17 pages LaTeX2e with 2 figures, using includegraphicx (5 .eps
files). Minor corrections, references updated. Contents identical to version
to appear in Phys. Rev. C 65, spelling differen
Quasi-free Compton Scattering and the Polarizabilities of the Neutron
Differential cross sections for quasi-free Compton scattering from the proton
and neutron bound in the deuteron have been measured using the Glasgow/Mainz
tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48
cm 64 cm NaI(Tl) photon detector and the G\"ottingen SENECA
recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV
at . Liquid deuterium and hydrogen targets
allowed direct comparison of free and quasi-free scattering from the proton.
The neutron detection efficiency of the SENECA detector was measured via the
reaction . The "free" proton Compton scattering cross
sections extracted from the bound proton data are in reasonable agreement with
those for the free proton which gives confidence in the method to extract the
differential cross section for free scattering from quasi-free data.
Differential cross sections on the free neutron have been extracted and the
difference of the electromagnetic polarizabilities of the neutron have been
obtained to be
in units . In combination with the polarizability sum deduced from photoabsorption data, the neutron electric and
magnetic polarizabilities, and
are obtained. The backward spin polarizability of the neutron was determined to
be
Dirac-Foldy term and the electromagnetic polarizability of the neutron
We reconsider the Dirac-Foldy contribution to the neutron electric
polarizability. Using a Dirac equation approach to neutron-nucleus scattering,
we review the definitions of Compton continuum (), classical
static (), and Schr\"{o}dinger () polarizabilities
and discuss in some detail their relationship. The latter is the
value of the neutron electric polarizability as obtained from an analysis using
the Schr\"{o}dinger equation. We find in particular , where is the magnitude of the magnetic moment
of a neutron of mass . However, we argue that the static polarizability
is correctly defined in the rest frame of the particle, leading to
the conclusion that twice the Dirac-Foldy contribution should be added to
to obtain the static polarizability .Comment: 11 pages, RevTeX, to appear in Physical Review
Quasi-free Compton Scattering from the Deuteron and Nucleon Polarizabilities
Cross sections for quasi-free Compton scattering from the deuteron were
measured for incident energies of 236--260 MeV at the laboratory angle -135
degrees. The recoil nucleons were detected in a liquid-scintillator array
situated at 20 degrees. The measured differential cross sections were used,
with the calculations of Levchuk et al., to determine the polarizabilities of
the bound nucleons. For the bound proton, the extracted values were consistent
with the accepted value for the free proton. Combining our results for the
bound neutron with those from Rose et al., we obtain one-sigma constraints of
alpha_n = 7.6-14.0 and beta_n = 1.2-7.6.Comment: 4 pages, 3 figures, accepted in PR
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