292 research outputs found
DISTRIBUTIONS ISOTOPIQUES DES PRODUITS DE TRANSFERTS TRÈS INÉLASTIQUES ENTRE IONS LOURDS
On compare les distributions isotopiques expérimentales des produits de transfert très inélastiques obtenus par les réactions 40Ca (284 MeV) + 40Ca et 40Ar (295 MeV) + 232Th
Nuclear break-up of 11Be
The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be
from the GANIL facility on a 48Ti target by measuring correlations between the
10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to
the emission of a neutron at large angle in the laboratory frame is identified
with the towing mode through its characteristic n-fragment correlation. The
experimental spectra are compared with a model where the time dependent
Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A
good agreement is found between experiment and theory for the shapes of neutron
experimental energies and angular distributions. The spectroscopic factor of
the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission
from the 1p and 1d orbitals is also studied
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
Probing pre-formed alpha particles in the ground state of nuclei
In this Letter, we report on alpha particle emission through the nuclear
break-up in the reaction 40Ca on a 40Ca target at 50A MeV. It is observed that,
similarly to nucleons, alpha particles can be emitted to the continuum with
very specific angular distribution during the reaction. The alpha particle
properties can be understood as resulting from an alpha cluster in the daughter
nucleus that is perturbed by the short range nuclear attraction of the
collision partner and emitted. A time-dependent theory that describe the alpha
particle wave-function evolution is able to reproduce qualitatively the
observed angular distribution. This mechanism offers new possibilities to study
alpha particle properties in the nuclear medium.Comment: 4 pages, 3 figure
Search for low lying dipole strength in the neutron rich nucleus Ne
Coulomb excitation of the exotic neutron-rich nucleus Ne on a
Pb target was measured at 58 A.MeV in order to search for low-lying E1
strength above the neutron emission threshold. Data were also taken on an
Al target to estimate the nuclear contribution. The radioactive beam
was produced by fragmentation of a 95 A.MeV Ar beam delivered by the
RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged
fragment hodoscope and a neutron wall. Using the invariant mass method in the
Ne+n channel, we observe a sizable amount of E1 strength between 6 and
10 MeV. The reconstructed Ne angular distribution confirms its E1
nature. A reduced dipole transition probability of B(E1)=0.490.16
is deduced. For the first time, the decay pattern of low-lying
strength in a neutron-rich nucleus is obtained. The results are discussed in
terms of a pygmy resonance centered around 9 MeV
Eta-Helium Quasi-Bound States
The cross section and tensor analysing power t_20 of the d\vec{d}->eta 4He
reaction have been measured at six c.m. momenta, 10 < p(eta) < 90 MeV/c. The
threshold value of t_20 is consistent with 1/\sqrt{2}, which follows from
parity conservation and Bose symmetry. The much slower momentum variation
observed for the reaction amplitude, as compared to that for the analogous
pd->eta 3He case, suggests strongly the existence of a quasi-bound state in the
eta-4He system and optical model fits indicate that this probably also the case
for eta-3He.Comment: LaTeX, uses elsart.sty, 10 pages, 3 Postscript figures, Submitted to
Physics Letters
HIGH EXCITATION ENERGY STRUCTURES IN HEAVY ION COLLISIONS ON A208Pb TARGET
Energy spectra of fragments from the 36Ar +208Pb reaction at 11 MeV/n and 20Ne + 208Pb reaction at 30 MeV/n were measured using a time of flight spectrometer. Structures ranging up to 130 MeV excitation energy are observed in the inelastic spectra. These structures are shown to be due to an excitation of the208Pb target nucleus
He Structure and Mechanisms of He Backward Elastic Scattering
The mechanism of He backward elastic scattering is studied.
It is found that the triangle diagrams with the subprocesses He,
He and He, where and
denote the singlet deuteron and diproton pair in the state,
respectively, dominate in the cross section at 0.3-0.8 GeV, and their
contribution is comparable with that for a sequential transfer of a pair
at 1-1.5 GeV.
The contribution of the , estimated on the basis of the spectator
mechanism of the He reaction, increases the HeHe cross section by one order of magnitude as compared to the
contribution of the deuteron alone.
Effects of the initial and final states interaction are taken into account.Comment: 17 pages, Latex, 4 postscript figures, expanded version, accepted by
Physical Review
Decay Pattern of Pygmy States Observed in Neutron-Rich 26 Ne
Coulomb excitation of the exotic neutron-rich nucleus Ne26 on a Pb208 target
was measured at 58 MeV/u in order to search for low-lying E1 strength above the
neutron emission threshold. This radioactive beam experiment was carried out at
the RIKEN Accelerator Research Facility. Using the invariant mass method in the
Ne25+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV
excitation energy. By performing a multipole decomposition of the differential
cross section, a reduced dipole transition probability of B(E1)=0.49+-0.16e2fm2
is deduced, corresponding to 4.9+-1.6% of the Thomas-Reiche-Kuhn sum rule. For
the first time, the decay pattern of low-lying strength in a neutron-rich
nucleus is measured. The extracted decay pattern is not consistent with several
mean-field theory descriptions of the pygmy states.Comment: arXiv admin note: text overlap with arXiv:0705.175
Has charge symmetry breaking been observed in the d d -> alpha pi0 reaction?
Estimates are made of the d d -> alpha gamma gamma production cross sections
in a model where each neutron-proton pair in the beam and target initiates an n
p -> d gamma reaction. This approach, which successfully reproduces observables
in two-pion production at intermediate energies, suggests that direct
two-photon production could provide a very significant background to the
measurement of the charge-symmetry-breaking (CSB) reaction d d -> alpha pi0. A
non-vanishing CSB cross section has been reported which might be confused with
such two-photon production under the given experimental conditions.Comment: 9 pages, ReVTEX, 2 figures, submitted to Phys. Rev. Let
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