7,794 research outputs found
Influence of misalignments on performance of externally occulted solar coronagraphs. Application to PROBA-3/ASPIICS
ASPIICS is a novel externally occulted coronagraph that will be launched
onboard the PROBA-3 mission of ESA. The external occulter (EO) will be placed
on one satellite ~150 m ahead of the second satellite with an optical
instrument. During part of each orbit, the satellites will fly in a precise
formation, constituting a giant externally occulted coronagraph. Large distance
between the EO and the primary objective will allow observations of the
white-light solar corona starting already from ~1.1RSun. We analyze influence
of shifts of the satellites and misalignments of optical elements on diffracted
light. Based on the quantitative influence of misalignments on diffracted
light, we will provide a "recipe" for choosing the size of the internal
occulter (IO) to achieve a trade-off between the minimal height of observations
and sustainability to possible misalignments. We implement a numerical model of
the diffracted light and its propagation through the optical system, and
compute intensities of diffracted light throughout the instrument. Our
numerical model extends axi-symmetrical model of Rougeot et al. 2017 to
non-symmetrical cases. The computations fully confirm main properties of the
diffracted light obtained from semi-analytical consideration. Results: relative
influences of various misalignments are significantly different. We show that:
the IO with R=1.1RSun is large enough to compensate possible misalignments in
ASPIICS, apodizing the edge of the IO leads to additional suppression of the
diffracted light. Conclusions: the most important misalignment is the tilt of
the telescope WRT the line connecting the center of the EO and the entrance
aperture. Special care should be taken to co-align the EO and the coronagraph,
i.e. co-aligning the diffraction fringe from the EO and the IO. We suggest that
the best orientation strategy is to point the coronagraph to the center of the
EO.Comment: 13 pages, 15 figure
Exploring manifestation and nature of a dineutron in two-neutron emission using a dynamical dineutron model
Emission of two neutrons or two protons in reactions and decays is often
discussed in terms of "dineutron" or "diproton" emission. The discussion often
leans intuitively on something described by Migdal-Watson approximation. In
this work we propose a way to formalize situations of dineutron emission. It is
demonstrated that properly formally defined dineutron emission may reveal
properties which are drastically different from those traditionally expected,
and properties which are actually observed in three-body decays.Comment: 11 pages, 11 Figure
Elastic Scattering of He based on a Cluster Description
Elastic scattering observables (differential cross section and analyzing
power) are calculated for the reaction He(p,p)He at projectile energies
starting at 71 MeV/nucleon. The optical potential needed to describe the
reaction is derived describing He in terms of a He-core and two
neutrons. The Watson first order multiple scattering ansatz is extended to
accommodate the internal dynamics of a composite cluster model for the He
nucleus scattering from a nucleon projectile. The calculations are compared
with the recent experiments at the projectile energy of 71 MeV/nucleon. In
addition, differential cross sections and analyzing powers are calculated at
selected higher energies.Comment: To be published in Phy. Rev.
Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph
Context: ASPIICS is a novel externally occulted solar coronagraph, which will
be launched onboard the PROBA-3 mission of the European Space Agency. The
external occulter will be placed on the first satellite approximately 150 m
ahead of the second satellite that will carry an optical instrument. During 6
hours per orbit, the satellites will fly in a precise formation, constituting a
giant externally occulted coronagraph. Large distance between the external
occulter and the primary objective will allow observations of the white-light
solar corona starting from extremely low heights 1.1RSun. Aims: To analyze
influence of optical ghost images formed inside the telescope and develop an
algorithm for their removal. Methods: We implement the optical layout of
ASPIICS in Zemax and study the ghost behaviour in sequential and non-sequential
regimes. We identify sources of the ghost contributions and analyze their
geometrical behaviour. Finally we develop a mathematical model and software to
calculate ghost images for any given input image. Results: We show that ghost
light can be important in the outer part of the field of view, where the
coronal signal is weak, since the energy of bright inner corona is
redistributed to the outer corona. However the model allows to remove the ghost
contribution. Due to a large distance between the external occulter and the
primary objective, the primary objective does not produce a significant ghost.
The use of the Lyot spot in ASPIICS is not necessary.Comment: 14 pages, 13 figure
Two-proton radioactivity and three-body decay. IV. Connection to quasiclassical formulation
We derive quasiclassical expressions for the three-body decay width and
define the ``preexponential'' coefficients for them. The derivation is based on
the integral formulae for the three-body width obtained in the semianalytical
approach with simplified three-body Hamiltonian [L.V. Grigorenko and M.V.\
Zhukov, arXiv:0704.0920v1]. The model is applied to the decays of the first
excited state of Ne and ground state of Fe.
Various qualitative aspects of the model and relations with the other
simplified approaches to the three-body decays are discussed.Comment: 9 Pages, 2 figure
Decay mechanism and lifetime of Kr
The lifetime of the recently discovered emitter Kr was recently
found considerably below the lower limit predicted theoretically. This
communication addresses this issue.Different separation energy systematics are
analyzed and different mechanisms for emission are evaluated. It is found
that the most plausible reason for this disagreement is a decay mechanism of
Kr, which is not "true " emission, but "transition dynamics" on the
borderline between true and sequential decay mechanisms. If this is
true, this imposes stringent limits MeV on the ground state
energy of Br relative to the Se- threshold.Comment: 7 pages, 8 figure
Possibility to study a two-proton halo in Ne
The nuclide Ne is studied theoretically in a three-body
O++ model. We demonstrate that the experimental condition for
existence of a proton halo in Ne can be reasonably quantified in terms
of configuration mixing. We discuss experimental evidences for a proton
halo in Ne. We define which kind of experimental data could elucidate
this issue.Comment: 5 pages, 5 figure
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