566 research outputs found
Characterization of an INVS Model IV Neutron Counter for High Precision () Cross-Section Measurements
A neutron counter designed for assay of radioactive materials has been
adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum
efficiency make it well suited for () cross-section measurements near
the neutron emission threshold. A high precision characterization of the
counter has been made using neutrons from several sources. Using a combination
of measurements and simulations, the absolute detection efficiency of the
neutron counter was determined to an accuracy of 3% in the neutron energy
range between 0.1 and 1 MeV. It is shown that this efficiency characterization
is generally valid for a wide range of targets.Comment: 22 pages, 13 figure
Low-Energy Photodisintegration of the Deuteron and Big-Bang Nucleosynthesis
The photon analyzing power for the photodisintegration of the deuteron was
measured for seven gamma-ray energies between 2.39 and 4.05 MeV using the
linearly polarized gamma-ray beam of the High-Intensity Gamma-ray Source at the
Duke Free-Electron Laser Laboratory. The data provide a stringent test of
theoretical calculations for the inverse reaction, the neutron-proton radiative
capture reaction at energies important for Big-Bang Nucleosynthesis. Our data
are in excellent agreement with potential model and effective field theory
calculations. Therefore, the uncertainty in the baryon density obtained from
Big-Bang Nucleosynthesis can be reduced at least by 20%.Comment: 5 pages, 5 figure
Natural and sail-displaced doubly-symmetric Lagrange point orbits for polar coverage
This paper proposes the use of doubly-symmetric, eight-shaped orbits in the circular restricted three-body problem for continuous coverage of the high-latitude regions of the Earth. These orbits, for a range of amplitudes, spend a large fraction of their period above either pole of the Earth. It is shown that they complement Sun-synchronous polar and highly eccentric Molniya orbits, and present a possible alternative to low thrust pole-sitter orbits. Both natural and solar-sail displaced orbits are considered. Continuation methods are described and used to generate families of these orbits. Starting from ballistic orbits, other families are created either by increasing the sail lightness number, varying the period or changing the sail attitude. Some representative orbits are then chosen to demonstrate the visibility of high-latitude regions throughout the year. A stability analysis is also performed, revealing that the orbits are unstable: it is found that for particular orbits, a solar sail can reduce their instability. A preliminary design of a linear quadratic regulator is presented as a solution to stabilize the system by using the solar sail only. Finally, invariant manifolds are exploited to identify orbits that present the opportunity of a ballistic transfer directly from low Earth orbit
Characterization of an INVS model IV neutron counter for high precision cross-section measurements
A neutron counter designed for assay of radioactive materials has been adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum efficiency make it well suited for (γ, n) cross-section measurements near the neutron emission threshold. A high precision characterization of the counter has been made using neutrons from several sources. Using a combination of measurements and simulations, the absolute detection efficiency of the neutron counter was determined to an accuracy of ± 3% in the neutron energy range between 0.1 and 1 MeV. It is shown that this efficiency characterization is generally valid for a wide range of targets
Cross-section measurement of 9Be(γ,n)8Be and implications for α+α+n→9Be in the r process
Models of the r process are sensitive to the production rate of 9Be because, in explosive environments rich in neutrons, α(αn,γ) 9Be is the primary mechanism for bridging the stability gaps at A=5 and A=8. The α(αn,γ)9Be reaction represents a two-step process, consisting of α+α→8Be followed by 8Be(n,γ)9Be. We report here on a new absolute cross-section measurement for the 9Be(γ,n)8Be reaction conducted using a highly efficient, 3He-based neutron detector and nearly monoenergetic photon beams, covering energies from Eγ=1.5 MeV to Eγ=5.2 MeV, produced by the High Intensity γ-ray Source of Triangle Universities Nuclear Laboratory. In the astrophysically important threshold energy region, the present cross sections are 40% larger than those found in most previous measurements and are accurate to ±10% (95% confidence). The revised thermonuclear α(αn,γ)9Be reaction rate could have implications for the r process in explosive environments such as type II supernovae
Extension of Earth-Moon libration point orbits with solar sail propulsion
This paper presents families of libration point orbits in the Earth-Moon system that originate from complementing the classical circular restricted three-body problem with a solar sail. Through the use of a differential correction scheme in combination with a continuation on the solar sail induced acceleration, families of Lyapunov, halo, vertical Lyapunov, Earth-centred, and distant retrograde orbits are created. As the solar sail circular restricted three-body problem is non-autonomous, a constraint defined within the differential correction scheme ensures that all orbits are periodic with the Sun’s motion around the Earth-Moon system. The continuation method then starts from a classical libration point orbit with a suitable period and increases the solar sail acceleration magnitude to obtain families of orbits that are parametrised by this acceleration. Furthermore, different solar sail steering laws are considered (both in-plane and out-of-plane, and either fixed in the synodic frame or fixed with respect to the direction of sunlight), adding to the wealth of families of solar sail enabled libration point orbits presented. Finally, the linear stability properties of the generated orbits are investigated to assess the need for active orbital control. It is shown that the solar sail induced acceleration can have a positive effect on the stability of some orbit families, especially those at the L2 point, but that it most often (further) destabilises the orbit. Active control will therefore be needed to ensure long-term survivability of these orbits
The three-nucleon bound state using realistic potential models
The bound states of H and He have been calculated using the Argonne
plus the Urbana three-nucleon potential. The isospin state
have been included in the calculations as well as the - mass difference.
The H-He mass difference has been evaluated through the charge
dependent terms explicitly included in the two-body potential. The calculations
have been performed using two different methods: the solution of the Faddeev
equations in momentum space and the expansion on the correlated hyperspherical
harmonic basis. The results are in agreement within 0.1% and can be used as
benchmark tests. Results for the CD-Bonn interaction are also presented. It is
shown that the H and He binding energy difference can be predicted
model independently.Comment: 5 pages REVTeX 4, 1 figures, 6 table
Three-Nucleon Forces from Chiral Effective Field Theory
We perform the first complete analysis of nd scattering at
next-to-next-to-leading order in chiral effective field theory including the
corresponding three-nucleon force and extending our previous work, where only
the two-nucleon interaction has been taken into account. The three-nucleon
force appears first at this order in the chiral expansion and depends on two
unknown parameters. These two parameters are determined from the triton binding
energy and the nd doublet scattering length. We find an improved description of
various scattering observables in relation to the next-to-leading order results
especially at moderate energies (E_lab = 65 MeV). It is demonstrated that the
long-standing A_y-problem in nd elastic scattering is still not solved by the
leading 3NF, although some visible improvement is observed. We discuss
possibilities of solving this puzzle. The predicted binding energy for the
alpha-particle agrees with the empirical value.Comment: 36 pp, 20 figure
A new form of three-body Faddeev equations in the continuum
We propose a novel approach to solve the three-nucleon (3N) Faddeev equation
which avoids the complicated singularity pattern going with the moving
logarithmic singularities of the standard approach. In this new approach the
treatment of the 3N Faddeev equation becomes essentially as simple as the
treatment of the two-body Lippmann-Schwinger equation. Very good agreement of
the new and old approaches in the application to nucleon-deuteron elastic
scattering and the breakup reaction is found.Comment: 20 pages, 3 eps figure
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
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