249 research outputs found
A Cautionary Tale: The Coulomb Modified ANC for the State in O
We discuss the impact of the uncertainty ( keV) in the excitation
energy of the astrophysically important 6.356 MeV state of O
on the precision with which the Coulomb reduced ANC () for the
\left<^{17}\mathrm{O}(1/2^+_2) \mid \protect{^{13}\mathrm{C}} + \alpha
\right> overlap can be extracted from direct reaction data. We find a linear
dependence of on the binding energy, the value extracted
varying by a factor of 4 over the range --
MeV. This represents an intrinsic limit on the precision with which
can be determined which cannot be improved unless or until
the uncertainty in is reduced
Strong coupling effects in near-barrier heavy-ion elastic scattering
Accurate elastic scattering angular distribution data measured at bombarding
energies just above the Coulomb barrier have shapes that can markedly differ
from or be the same as the expected classical Fresnel scattering pattern
depending on the structure of the projectile, the target or both. Examples are
given such as 18O + 184W and 16O + 148,152Sm where the expected rise above
Rutherford scattering due to Coulomb-nuclear interference is damped by coupling
to the target excited states, and the extreme case of 11Li scattering, where
coupling to the 9Li + n + n continuum leads to an elastic scattering shape that
cannot be reproduced by any standard optical model parameter set. The recent
availability of high quality 6He, 11Li and 11Be data provides further examples
of the influence that coupling effects can have on elastic scattering.
Conditions for strong projectile-target coupling effects are presented with
special emphasis on the importance of the beam-target charge combination being
large enough to bring about the strong coupling effects. Several measurements
are proposed that can lead to further understanding of strong coupling effects
by both inelastic excitation and nucleon transfer on near-barrier elastic
scattering. A final note on the anomalous nature of 8B elastic scattering is
presented as it possesses a more or less normal Fresnel scattering shape
whereas one would a priori not expect this due to the very low breakup
threshold of 8B. The special nature of 11Li is presented as it is predicted
that no matter how far above the Coulomb barrier the elastic scattering is
measured, its shape will not appear as Fresnel like whereas the elastic
scattering of all other loosely bound nuclei studied to date should eventually
do so as the incident energy is increased, making both 8B and 11Li truly
"exotic".Comment: Review articl
Influence of single-neutron stripping on near-barrier <sup>6</sup>He+<sup>208</sup>Pb and <sup>8</sup>He+<sup>208</sup>Pb elastic scattering
The influence of single-neutron stripping on the near-barrier elastic scattering angular distributions for the 6,8He+208Pb systems is investigated through coupled reaction channels (CRC) calculations fitting recently published data to explore the differences in the absorptive potential found in the scattering of these two neutron-rich nuclei. The inclusion of the coupling reduces the elastic cross section in the Coulomb-nuclear interference region for 8He scattering, whereas for 6He its major impact is on the large-angle elastic scattering. The real and imaginary dynamic polarization potentials are obtained by inverting the CRC elastic scattering S-matrix elements. These show that the main absorptive features occur between 11 and 12 fm for both projectiles, while the attractive features are separated by about 1 fm, with their main structures occurring at 10.5 fm for 6He and 11.5 fm for 8He
Multi-neutron transfer in He induced reactions near the Coulomb barrier
The measured inclusive He and He production cross sections of G.
Marqu{\'i}nez-Dur{\'a}n {\em et al.}, Phys.\ Rev.\ C {\bf 98}, 034615 (2018)
are reexamined and the conclusions concerning the relative importance of 1n and
2n transfer to the production of He arising from the interaction of a 22
MeV He beam with a Pb target revised. A consideration of the
kinematics of the 2n-stripping reaction when compared with the measured He
total energy versus angle spectrum places strict limits on the allowed
excitation energy of the Pb residual, so constraining distorted wave
Born approximation calculations that the contribution of the 2n stripping
process to the inclusive He production can only be relatively small. It is
therefore concluded that the dominant He production mechanism must be 1n
stripping followed by decay of the He ejectile. Based on this result we
present strong arguments in favor of direct, one step four-neutron (4n)
stripping as the main mechanism for He production.Comment: 7 pages, 2 figure
Galactic Cosmic Ray Simulation at the NASA Space Radiation Laboratory
The external Galactic Cosmic Ray (GCR) spectrum is significantly modified when it passes through spacecraft shielding and astronauts. One approach for simulating the GCR space radiation environment at ground based accelerators would use the modified spectrum, rather than the external spectrum, in the accelerator beams impinging on biological targets. Two recent workshops have studied such GCR simulation. The first workshop was held at NASA Langley Research Center in October 2014. The second workshop was held at the NASA Space Radiation Investigators' workshop in Galveston, Texas in January 2015. The results of these workshops will be discussed in this paper
Galactic Cosmic Ray Simulator at the NASA Space Radiation Laboratory
The external Galactic Cosmic Ray (GCR) spectrum is significantly modified when it passes through spacecraft shielding and astronauts. One approach for simulating the GCR space radiation environment is to attempt to reproduce the unmodified, external GCR spectrum at a ground based accelerator. A possibly better approach would use the modified, shielded tissue spectrum, to select accelerator beams impinging on biological targets. NASA plans for implementation of a GCR simulator at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory will be discussed
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