302 research outputs found
UHECR observations and lensing in the magnetic field of the Virgo cluster
We discuss how lensing by magnetic fields in galaxy clusters affects
ultrahigh energy cosmic ray (UHECR) observations. As specific example, we use
Virgo together with the cluster magnetic fields obtained earlier in a
constrained simulation of structure formation including MHD processes. We find
that, if M87 is the single source of UHECRs from Virgo, the emitted flux is
strongly anisotropic in the most interesting energy range, (50-100)EeV, and
differs from the average value by a factor five or more for a significant
fraction of observers. Since magnetic lensing is energy dependent, the external
energy spectrum as seen by different observers varies strongly too. These
anisotropies are averaged out in the case that all active galactic nuclei in
Virgo emit UHECRs. In both cases, the anisotropies of the emitted UHECR flux
may introduce an important bias in the interpretation of UHECR data like, e.g.,
the determination of the source density n_s and the source energy spectrum of
UHECRs.Comment: 12 pages, 15 eps figures; v2: extended discussion of modifications in
external energy spectrum, matches version to be publishe
Elements Discrimination in the Study of Super-Heavy Elements using an Ionization Chamber
Dedicated ionization chamber was built and installed to measure the energy
loss of very heavy nuclei at 2.7 MeV/u produced in fusion reactions in inverse
kinematics (beam of 208Pb). After going through the ionization chamber,
products of reactions on 12C, 18O targets are implanted in a Si detector. Their
identification through their alpha decay chain is ambiguous when their
half-life is short. After calibration with Pb and Th nuclei, the ionization
chamber signal allowed us to resolve these ambiguities. In the search for rare
super-heavy nuclei produced in fusion reactions in inverse or symmetric
kinematics, such a chamber will provide direct information on the nuclear
charge of each implanted nucleus.Comment: submitted to NIMA, 10 pages+4 figures, Latex, uses elsart.cls and
grahpic
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
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
Effect of the intermediate velocity emissions on the quasi-projectile properties for the Ar+Ni system at 95 A.MeV
The quasi-projectile (QP) properties are investigated in the Ar+Ni collisions
at 95 A.MeV taking into account the intermediate velocity emission. Indeed, in
this reaction, between 52 and 95 A.MeV bombarding energies, the number of
particles emitted in the intermediate velocity region is related to the overlap
volume between projectile and target. Mean transverse energies of these
particles are found particularly high. In this context, the mass of the QP
decreases linearly with the impact parameter from peripheral to central
collisions whereas its excitation energy increases up to 8 A.MeV. These results
are compared to previous analyses assuming a pure binary scenario
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 of a very heavy nuclear system (II): bulk properties and spinodal decomposition
The properties of fragments and light charged particles emitted in
multifragmentation of single sources formed in central 36AMeV Gd+U collisions
are reviewed. Most of the products are isotropically distributed in the
reaction c.m. Fragment kinetic energies reveal the onset of radial collective
energy. A bulk effect is experimentally evidenced from the similarity of the
charge distribution with that from the lighter 32AMeV Xe+Sn system. Spinodal
decomposition of finite nuclear matter exhibits the same property in simulated
central collisions for the two systems, and appears therefore as a possible
mechanism at the origin of multifragmentation in this incident energy domain.Comment: 28 pages including 14 figures; submitted to Nucl. Phys.
Study of intermediate velocity products in the Ar+Ni collisions between 52 and 95 A.MeV
Intermediate velocity products in Ar+Ni collisions from 52 to 95 A.MeV are
studied in an experiment performed at the GANIL facility with the 4
multidetector INDRA. It is shown that these emissions cannot be explained by
statistical decays of the quasi-projectile and the quasi-target in complete
equilibrium. Three methods are used to isolate and characterize intermediate
velocity products. The total mass of these products increases with the violence
of the collision and reaches a large fraction of the system mass in mid-central
collisions. This mass is found independent of the incident energy, but strongly
dependent on the geometry of the collision. Finally it is shown that the
kinematical characteristics of intermediate velocity products are weakly
dependent on the experimental impact parameter, but strongly dependent on the
incident energy. The observed trends are consistent with a
participant-spectator like scenario or with neck emissions and/or break-up.Comment: 37 pages, 13 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
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