49 research outputs found
Determination of the freeze-out temperature by the isospin thermometer
The high-resolution spectrometer FRS at GSI Darmstadt provides the full
isotopic and kinematical identification of fragmentation residues in
relativistic heavy-ion collisions. Recent measurements of the isotopic
distribution of heavy projectile fragments led to a very surprising new
physical finding: the residue production does not lose the memory of the N/Z of
the projectile ending up in a universal de-excitation corridor; an ordering of
the residues in relation to the neutron excess of the projectile has been
observed. These unexpected features can be interpreted as a new manifestation
of multifragmentation. We have found that at the last stage of the reaction the
temperature of the big clusters subjected to evaporation is limited to a
universal value. The thermometer to measure this limiting temperature is the
neutron excess of the residues.Comment: 8 pages, 6 figures, corrected some misprints in the abstract, to be
published in "Yadernaya Fizika" as a proceeding of the "VII International
School Seminar on Heavy-Ion Phyics", Dubna (Russia), May 27 - June 1, 200
Time-Dependent Hartree-Fock simulation of the expansion of abraded nuclei
A recent interpretation of the caloric curve based on the expansion of the
abraded spectator nucleus is re-analysed in the framework of the Time-Dependent
Hartree-Fock (TDHF) evolution. It is shown that the TDHF dynamics is more
complex than a single monopolar collective motion at moderate energy. The
inclusion of other important collective degrees of freedom may lead to the
dynamical creation of hollow structure. Then, low density regions could be
locally reached after a long time by the creation of these exotic density
profiles. In particular the systematic of the minimum density reached during
the expansion (the so-called turning points) appears to be different.Comment: 30 Latex pages including 9 figure
Fission-Residues Produced in the Spallation Reaction 238U+p at 1 A GeV
Fission fragments from 1 A GeV 238U projectiles irradiating a hydrogen target
were investigated by using the fragment separator FRS for magnetic selection of
reaction products including ray-tracing and DE-ToF techniques. The momentum
spectra of 733 identified fragments were analysed to provide isotopic
production cross sections, fission-fragment velocities and recoil momenta of
the fissioning parent nuclei. Besides their general relevance, these quantities
are also demanded for applications. Calculations and simulations with codes
commonly used and recently developed or improved are compared to the data.Comment: 60 pages, 21 figures, 4 tables, 2 appendices (15 pages
Evaluation of the total photoabsorption cross sections for actinides from photofission data and model calculations
We have calculated the fission probabilities for 237-Np, 233,235,238-U,
232-Th, and nat-Pb following the absorption of photons with energies from 68
MeV to 3.77 GeV using the RELDIS Monte-Carlo code. This code implements the
cascade-evaporation-fission model of intermediate-energy photonuclear
reactions. It includes multiparticle production in photoreactions on
intranuclear nucleons, pre-equilibrium emission, and the statistical decay of
excited residual nuclei via competition of evaporation, fission, and
multifragmentation processes. The calculations show that in the GeV energy
region the fission process is not solely responsible for the entire total
photoabsorption cross section, even for the actinides: ~55-70% for 232-Th,
\~70-80% for 238-U, and ~80-95% for 233-U, 235-U, and 237-Np. This is because
certain residual nuclei that are created by deep photospallation at GeV photon
energies have relatively low fission probabilities. Using the recent
experimental data on photofission cross sections for 237-Np and 233,235,238-U
from the Saskatchewan and Jefferson Laboratories and our calculated fission
probabilities, we infer the total photoabsorption cross sections for these four
nuclei. The resulting cross sections per nucleon agree in shape and in
magnitude with each other. However, disagreement in magnitude with
total-photoabsorption cross-section data from previous measurements for nuclei
from C to Pb calls into question the concept of a ``Universal Curve'' for the
photoabsorption cross section per nucleon for all nuclei.Comment: 39 pages including 11 figure
Mutual heavy ion dissociation in peripheral collisions at ultrarelativistic energies
We study mutual dissociation of heavy nuclei in peripheral collisions at
ultrarelativistic energies. Earlier this process was proposed for beam
luminosity monitoring via simultaneous registration of forward and backward
neutrons in zero degree calorimeters at Relativistic Heavy Ion Collider.
Electromagnetic dissociation of heavy ions is considered in the framework of
the Weizsacker-Williams method and simulated by the RELDIS code. Photoneutron
cross sections measured in different experiments and calculated by the GNASH
code are used as input for the calculations of dissociation cross sections. The
difference in results obtained with different inputs provides a realistic
estimation for the systematic uncertainty of the luminosity monitoring method.
Contribution to simultaneous neutron emission due to grazing nuclear
interactions is calculated within the abrasion model. Good description of CERN
SPS experimental data on Au and Pb dissociation gives confidence in predictive
power of the model for AuAu and PbPb collisions at RHIC and LHC.Comment: 46 pages with 7 tables and 13 figures, numerical integration accuracy
improved, next-to-leading-order corrections include
Particle-hole state densities with non-equidistant single-particle levels
The correct use of energy-dependent single-particle level (s.p.l.) densities
within particle-hole state densities based on the equidistant spacing model
(ESM) is analysed. First, an analytical expression is obtained following the
convolution of energy-dependent excited-particle and hole densities. Next, a
comparison is made with results of the ESM formula using average s.p.l.
densities for the excited particles and holes, respectively. The Fermi-gas
model (FGM) s.p.l. densities calculated at the corresponding average excitation
energies are used in both cases. The analysis concerns also the density of
particle-hole bound states. The pairing correlations are taken into account
while the comparison of various effects includes the exact correction for the
Pauli exclusion principle. Quantum-mechanical s.p.l. densities and the
continuum effect can also match a corresponding FGM formula, suitable for use
within the average energy-dependent partial state density in multistep reaction
models.Comment: 29 pages, ReVTeX, 11 postscript figures, submitted to Phys.Rev.
Discovery and Cross-Section Measurement of Neutron-Rich Isotopes in the Element Range from Neodymium to Platinum at the FRS
With a new detector setup and the high-resolution performance of the fragment
separator FRS at GSI we discovered 57 new isotopes in the atomic number range
of 60: \nuc{159-161}{Nb}, \nuc{160-163}{Pm}, \nuc{163-166}Sm,
\nuc{167-168}{Eu}, \nuc{167-171}{Gd}, \nuc{169-171}{Tb}, \nuc{171-174}{Dy},
\nuc{173-176}{Ho}, \nuc{176-178}{Er}, \nuc{178-181}{Tm}, \nuc{183-185}{Yb},
\nuc{187-188}{Lu}, \nuc{191}{Hf}, \nuc{193-194}{Ta}, \nuc{196-197}{W},
\nuc{199-200}{Re}, \nuc{201-203}{Os}, \nuc{204-205}{Ir} and \nuc{206-209}{Pt}.
The new isotopes have been unambiguously identified in reactions with a
U beam impinging on a Be target at 1 GeV/u. The isotopic production
cross-section for the new isotopes have been measured and compared with
predictions of different model calculations. In general, the ABRABLA and COFRA
models agree better than a factor of two with the new data, whereas the
semiempirical EPAX model deviates much more. Projectile fragmentation is the
dominant reaction creating the new isotopes, whereas fission contributes
significantly only up to about the element holmium.Comment: 9 pages, 4 figure
Electromagnetic Counterparts of Compact Object Mergers Powered by the Radioactive Decay of R-process Nuclei
The most promising astrophysical sources of kHz gravitational waves (GWs) are
the inspiral and merger of binary neutron star(NS)/black hole systems.
Maximizing the scientific return of a GW detection will require identifying a
coincident electro-magnetic (EM) counterpart. One of the most likely sources of
isotropic EM emission from compact object mergers is a supernova-like transient
powered by the radioactive decay of heavy elements synthesized in ejecta from
the merger. We present the first calculations of the optical transients from
compact object mergers that self-consistently determine the radioactive heating
by means of a nuclear reaction network; using this heating rate, we model the
light curve with a one dimensional Monte Carlo radiation transfer calculation.
For an ejecta mass ~1e-2 M_sun[1e-3 M_sun] the resulting light curve peaks on a
timescale ~ 1 day at a V-band luminosity nu L_nu ~ 3e41[1e41] ergs/s (M_V =
-15[-14]); this corresponds to an effective "f" parameter ~3e-6 in the
Li-Paczynski toy model. We argue that these results are relatively insensitive
to uncertainties in the relevant nuclear physics and to the precise early-time
dynamics and ejecta composition. Due to the rapid evolution and low luminosity
of NS merger transients, EM counterpart searches triggered by GW detections
will require close collaboration between the GW and astronomical communities.
NS merger transients may also be detectable following a short-duration
Gamma-Ray Burst or "blindly" with present or upcoming optical transient
surveys. Because the emission produced by NS merger ejecta is powered by the
formation of rare r-process elements, current optical transient surveys can
directly constrain the unknown origin of the heaviest elements in the Universe.Comment: 14 pages, 7 figures; accepted to MNRAS; title changed to highlight
r-process connection and new figure added
Complex nuclear-structure phenomena revealed from the nuclide production in fragmentation reactions
Complex structural effects in the nuclide production from the projectile
fragmentation of 1 A GeV 238U nuclei in a titanium target are reported. The
structure seems to be insensitive to the excitation energy induced in the
reaction. This is in contrast to the prominent structural features found in
nuclear fission and in transfer reactions, which gradually disappear with
increasing excitation energy. Using the statistical model of nuclear reactions,
relations to structural effects in nuclear binding and in the nuclear level
density are demonstrated.Comment: 19 pages, 14 figures, background information on
http://www-w2k.gsi.de/kschmidt
Electromagnetic-induced fission of 238U projectile fragments, a test case for the production of spherical super-heavy nuclei
Isotopic series of 58 neutron-deficient secondary projectiles (205,206At,
205-209Rn, 208-212,217,218Fr, 211-223Ra, 215-226Ac, 221-229Th, 226-231Pa,
231-234U) were produced by projectile fragmentation using a 1 A GeV 238U beam.
Cross sections of fission induced by nuclear and electromagnetic interactions
in a secondary lead target were measured. They were found to vary smoothly as a
function of proton and neutron number of the fissioning system, also for nuclei
with large ground-state shell effects near the 126-neutron shell. No
stabilization against fission was observed for these nuclei at low excitation
energies. Consequences for the expectations on the production cross sections of
super-heavy nuclei are discussed.Comment: 20 pages, 13 figure