193 research outputs found
Pion radii in nonlocal chiral quark model
The electromagnetic radius of the charged pion and the transition radius of
the neutral pion are calculated in the framework of the nonlocal chiral quark
model. It is shown in this model that the contributions of vector mesons to the
pion radii are noticeably suppressed in comparison with a similar contribution
in the local Nambu--Jona-Lasinio model. The form-factor for the process
gamma*pi+pi- is calculated for the -1 GeV^2<q^2<1.6 GeV^2. Our results are in
satisfactory agreement with experimental data.Comment: 7 pages, 7 figure
Measurements of sideward flow around the balance energy
Sideward flow values have been determined with the INDRA multidetector for
Ar+Ni, Ni+Ni and Xe+Sn systems studied at GANIL in the 30 to 100 A.MeV incident
energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in
agreement with previous experimental results and theoretical calculations.
Negative sideward flow values have been measured. The possible origins of such
negative values are discussed. They could result from a more important
contribution of evaporated particles with respect to the contribution of
promptly emitted particles at mid-rapidity. But effects induced by the methods
used to reconstruct the reaction plane cannot be totally excluded. Complete
tests of these methods are presented and the origins of the
``auto-correlation'' effect have been traced back. For heavy fragments, the
observed negative flow values seem to be mainly due to the reaction plane
reconstruction methods. For light charged particles, these negative values
could result from the dynamics of the collisions and from the reaction plane
reconstruction methods as well. These effects have to be taken into account
when comparisons with theoretical calculations are done.Comment: 27 pages, 15 figure
Space and Time pattern of mid-velocity IMF emission in peripheral heavy-ion collisions at Fermi energies
The emission pattern in the V_perp - V_par plane of Intermediate Mass
Fragments with Z=3-7 (IMF) has been studied in the collision 116Sn + 93Nb at
29.5 AMeV as a function of the Total Kinetic Energy Loss of the reaction. This
pattern shows that for peripheral reactions most of IMF's are emitted at
mid-velocity. Coulomb trajectory calculations demonstrate that these IMF's are
produced in the early stages of the reaction and shed light on geometrical
details of these emissions, suggesting that the IMF's originate both from the
neck and the surface of the interacting nuclei.Comment: 4 pages, 3 figures, RevTex 3.1, submitted to Phys. Rev. Letter
Response of CsI(Tl) scintillators over a large range in energy and atomic number of ions (Part I): recombination and delta -- electrons
A simple formalism describing the light response of CsI(Tl) to heavy ions,
which quantifies the luminescence and the quenching in terms of the competition
between radiative transitions following the carrier trapping at the Tl
activator sites and the electron-hole recombination, is proposed. The effect of
the delta rays on the scintillation efficiency is for the first time
quantitatively included in a fully consistent way. The light output expression
depends on four parameters determined by a procedure of global fit to
experimental data.Comment: 28 pages, 6 figures, submitted to Nucl. Inst. Meth.
Multifragmentation process for different mass asymmetry in the entrance channel around the Fermi energy
The influence of the entrance channel asymmetry upon the fragmentation
process is addressed by studying heavy-ion induced reactions around the Fermi
energy. The data have been recorded with the INDRA 4pi array. An event
selection method called the Principal Component Analysis is presented and
discussed. It is applied for the selection of central events and furthermore to
multifragmentation of single source events. The selected subsets of data are
compared to the Statistical Multifragmentation Model (SMM) to check the
equilibrium hypothesis and get the source characteristics. Experimental
comparisons show the evidence of a decoupling between thermal and compresional
(radial flow) degrees of freedom in such nuclear systems.Comment: 28 pages, 15 figures, article sumitted to Nuclear Physics
Multiplicity correlations of intermediate-mass fragments with pions and fast protons in 12C + 197Au
Low-energy pi+ (E < 35 MeV) from 12C+197Au collisions at incident energies
from 300 to 1800 MeV per nucleon were detected with the Si-Si(Li)-CsI(Tl)
calibration telescopes of the INDRA multidetector. The inclusive angular
distributions are approximately isotropic, consistent with multiple
rescattering in the target spectator. The multiplicity correlations of the
low-energy pions and of energetic protons (E > 150 MeV) with intermediate-mass
fragments were determined from the measured coincidence data. The deduced
correlation functions 1 + R \approx 1.3 for inclusive event samples reflect the
strong correlations evident from the common impact-parameter dependence of the
considered multiplicities. For narrow impact-parameter bins (based on
charged-particle multiplicity), the correlation functions are close to unity
and do not indicate strong additional correlations. Only for pions at high
particle multiplicities (central collisions) a weak anticorrelation is
observed, probably due to a limited competition between these emissions.
Overall, the results are consistent with the equilibrium assumption made in
statistical multifragmentation scenarios. Predictions obtained with
intranuclear cascade models coupled to the Statistical Multifragmentation Model
are in good agreement with the experimental data.Comment: 9 pages, 11 figures, subm. to EPJ
Excitation and decay of projectile-like fragments formed in dissipative peripheral collisions at intermediate energies
Projectile-like fragments (PLF:15<=Z<=46) formed in peripheral and
mid-peripheral collisions of 114Cd projectiles with 92Mo nuclei at E/A=50 MeV
have been detected at very forward angles, 2.1 deg.<=theta_lab<=4.2 deg.
Calorimetric analysis of the charged particles observed in coincidence with the
PLF reveals that the excitation of the primary PLF is strongly related to its
velocity damping. Furthermore, for a given V_PLF*, its excitation is not
related to its size, Z_PLF*. For the largest velocity damping, the excitation
energy attained is large, approximately commensurate with a system at the
limiting temperatureComment: 5 pages, 6 figure
Multifragmentation of a very heavy nuclear system (I): Selection of single-source events
A sample of `single-source' events, compatible with the multifragmentation of
very heavy fused systems, are isolated among well-measured 155Gd+natU 36AMeV
reactions by examining the evolution of the kinematics of fragments with Z>=5
as a function of the dissipated energy and loss of memory of the entrance
channel. Single-source events are found to be the result of very central
collisions. Such central collisions may also lead to multiple fragment emission
due to the decay of excited projectile- and target-like nuclei and so-called
`neck' emission, and for this reason the isolation of single-source events is
very difficult. Event-selection criteria based on centrality of collisions, or
on the isotropy of the emitted fragments in each event, are found to be
inefficient to separate the two mechanisms, unless they take into account the
redistribution of fragments' kinetic energies into directions perpendicular to
the beam axis. The selected events are good candidates to look for bulk effects
in the multifragmentation process.Comment: 39 pages including 15 figures; submitted to Nucl. Phys.
Multifragmentation in Xe(50A MeV)+Sn Confrontation of theory and data
We compare in detail central collisions Xe(50A MeV) + Sn, recently measured
by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in
order to identify the reaction mechanism which leads to multifragmentation. We
find that QMD describes the data quite well, in the projectile/target region as
well as in the midrapidity zone where also statistical models can be and have
been employed. The agreement between QMD and data allows to use this dynamical
model to investigate the reaction in detail. We arrive at the following
observations: a) the in medium nucleon nucleon cross section is not
significantly different from the free cross section, b) even the most central
collisions have a binary character, c) most of the fragments are produced in
the central collisions and d) the simulations as well as the data show a strong
attractive in-plane flow resembling deep inelastic collisions e) at midrapidity
the results from QMD and those from statistical model calculations agree for
almost all observables with the exception of . This
renders it difficult to extract the reaction mechanism from midrapidity
fragments only. According to the simulations the reaction shows a very early
formation of fragments, even in central collisions, which pass through the
reaction zone without being destroyed. The final transverse momentum of the
fragments is very close to the initial one and due to the Fermi motion. A
heating up of the systems is not observed and hence a thermal origin of the
spectra cannot be confirmed.Comment: figures 1 and 2 changed (no more ps -errors
Charge and current-sensitive preamplifiers for pulse shape discrimination techniques with silicon detectors
New charge and current-sensitive preamplifiers coupled to silicon detectors
and devoted to studies in nuclear structure and dynamics have been developed
and tested. For the first time shapes of current pulses from light charged
particles and carbon ions are presented. Capabilities for pulse shape
discrimination techniques are demonstrated.Comment: 14 pages, 12 figures, to be published in Nucl. Inst. Meth.
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