Statistical Decay Model Research Articles

Excitation and decay of the Gamow-Teller giant resonance in90Nb

The proton decay of spin-isospin strength in ${}^{90}$Nb was investigated after excitation via the reaction ${}^{90}$Zr${(}^{6}$Li,${}^{6}$He)${}^{90}$Nb at 156 MeV bombarding energy. The Gamow-Teller (GT) strength distribution and corresponding $B$(GT) values were extracted from singles spectra measured at extreme forward angles including zero degree. Protons in coincidence with ${}^{6}$He ejectiles were detected using a multidetector arrangement of semiconductor strip detectors. Relative branching ratios and ${}^{6}$He-$p$ angular correlations have been measured for a proton decay to well defined low lying single hole states in ${}^{89}$Zr. Evidence for a dominant statistical decay has been found in the whole observed region of excitation energy; a small direct decay component, however, cannot be ruled out. In the region of the Gamow-Teller giant resonance between 8.0 and 12.0 MeV excitation energy additional strength with higher mulitpolarities $L>~1$ was identified from a comparison with predictions in the statistical decay model. At higher excitation energies above the giant resonance region the decay characteristics confirm the presence of a flat distribution of Gamow-Teller strength.

Fragmentation analysis of size selected sodium clusters

The fragmentation processes of small sodium clusters induced by photoionization are investigated for a size selected neutral cluster beam. We use the collision induced momentum transfer in a crossed beam experiment to achieve the separation of the masses. Photoions produced with photon energies up to 4.66 eV are mass analyzed in a time-of-flight mass spectrometer. A considerable amount of fragmentation is observed leading exclusively to population of ionic channels with an even number of electrons. The measured fragmentation probabilities for cluster sizes up to n=8 are compared to a statistical unimolecular decay model. This gives a satisfactory agreement also for the photon energy dependence of the fragmentation probabilities if one assumes a significant contribution from cluster isomers.

Statistical model for nonadiabatic decay of an exciplex strongly coupled to a dissociative continuum

We present a statistical model for the decay of an exciplex that is strongly coupled to repulsive ground-state potential energy surface. The motion of the exciplex is assumed to be governed by the excited electronic state adiabatic potential surface, and transitions to the ground state are caused by nonadiabatic transitions at a seam of avoided crossing of two strongly coupled diabatic states underlying the adiabatic representation. The model assumes that motion in the exciplex is ergodic. The predictions of the model are tested against accurate quantum dynamics and trajectory surface hopping calculations for Na*+H2→NaH2*→Na+H2.

Multifragmentation through Exotic Shape Nuclei in (5 GeV/u) + Au Collisions

We simulate the fragmentation processes in the {$\alpha$~$+$~Au~} collisions at a bombarding energy of 5 GeV/u using the simplified RQMD approach plus the statistical decay model. We find from the simulation that the angular-distribution of the intermediate mass fragments has a sideward peak, more strongly in the transverse direction than in the beam-direction, when the intermediate nucleus formed by the dynamical process has an annular eclipse shape, which explains the experimental results.

Time scale of the preequilibrium process in intermediate-energy nucleon-induced reactions

The time scale required to reach thermal equilibrium in intermediate-energy nucleon-induced preequilibrium reactions was investigated by quantum molecular dynamics (QMD). The average kinetic energy of two colliding particles in their center-of-mass system was taken to be the measure of such thermal equilibration. The corresponding time was found to be of the order of 20 fm/c (\ensuremath{\sim}7 \ifmmode\times\else\texttimes\fi{} ${10}^{\mathrm{\ensuremath{-}}23}$ sec), a time scale that is much shorter than the time believed for the compound reaction, i.e., \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}18}$ sec. An interpretation of the reason why the kinetic energy brought by the projectile is dumped so fastly to thermal equilibrium is discussed. The equilibration time of 20 fm/c gives a justification of our ``hybrid'' approach of using the QMD plus the statistical decay model (SDM) which are connected at a time scale of \ensuremath{\sim}100 fm/c. \textcopyright{} 1996 The American Physical Society.

Analysis of proton-induced fragment production cross sections by the quantum molecular dynamics plus statistical decay model.

The production cross sections of various fragments from proton-induced reactions on $^{56}$Fe and $^{27}$Al have been analyzed by the Quantum Molecular Dynamics (QMD) plus Statistical Decay Model (SDM). It was found that the mass and charge distributions calculated with and without the statistical decay have very different shapes. These results also depend strongly on the impact parameter, showing an importance of the dynamical treatment as realized by the QMD approach. The calculated results were compared with experimental data in the energy region from 50 MeV to 5 GeV. The QMD+SDM calculation could reproduce the production cross sections of the light clusters and intermediate-mass to heavy fragments in a good accuracy. The production cross section of $^{7}$Be was, however, underpredicted by approximately 2 orders of magnitude, showing the necessity of another reaction mechanism not taken into account in the present model.

Fragment emission from modestly excited nuclear systems

Abstract Fragment emission patterns occurring in nuclear systems of modest excitation are studied. Exclusive measurement of fragment emission in 14 N+ 197 Au reactions at E A = 100, 130 and 156 MeV allows selection of central collisions where a single source dominates the decay. Low threshold measurement of IMF emission for these events allows investigation of the influence of detector threshold effects. The time scale of fragment emission is deduced using fragment-fragment velocity correlations. Comparisons are made to the predictions of a statistical decay model.

Study of the decay of hot nuclei formed inLa139-induced reactions atEA=45MeV by a hybrid dynamical-statistical calculation

The reactions {sup 139}La + {sup 27}Al and {sup 139}La + {sup 65}Cu at {ital E}/{ital A} = 45 MeV have been modeled by combining a Boltzmann-Nordheim-Vlasov dynamical model calculation with a statistical sequential binary decay model code. For the reaction {sup 139}La + {sup 27}Al, the major features of the experimental data are adequately described by the model calculations. These features include the inclusive fragment cross sections and the total charge and source velocity distributions of multiple fragment events. Other finer features, such as charge-Dalitz plots and the branching ratios between events of different multiplicity, are not reproduced by the calculation. The failure of the calculations is even greater for the reaction {sup 139}La + {sup 65}Cu, in which only the source velocity distributions of multiple fragment events are reproduced. Because the source velocity can be a measure of how much of the target is incorporated into the projectile in inverse kinematics reactions, this suggests that the earliest stages of the reaction, described by the dynamical calculation, are adequately characterized by the model. It is the later stages of the reaction, when fragments are emitted, where the model calculation appears to fail. However, there are some indications that statisticalmore » decay in the reaction {sup 139}La + {sup 65}Cu has occurred, whether by sequential binary decays or some type of prompt multifragmentation. {copyright} {ital 1996 The American Physical Society.}« less

Dilepton decay from excited states in 28Si populated via different isospin entrance channels.

Nuclear states in $^{28}\mathrm{Si}$, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction $^{4}\mathrm{He}$${+}^{24}$Mg and the mixed-isospin $^{3}\mathrm{He}$${+}^{25}$Mg reaction. In both reactions the dilepton (${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ spectrum, over the converted photon yield, is observed in the energy region 17--22 MeV in the $^{3}\mathrm{He}$ induced reaction, possibly indicating an entrance channel effect. \textcopyright{} 1996 The American Physical Society.

Dynamics of heavy-ion fusion probed by d/p double ratios from a cross bombardment.

The particle decay ensuing from the reactions 86.0 MeV $^{16}\mathrm{O}$ + $^{148}\mathrm{Sm}$ and 239.1 MeV $^{64}\mathrm{Ni}$ + $^{100}\mathrm{Mo}$ was studied. These reactions each form $^{164}\mathrm{Yb}$ compound nuclei excited to \ensuremath{\approxeq} 54 MeV. Particle decay from compound nucleus producing reactions was selected by gating on the gamma-ray fold and the angular region of the particle emission. While there are no discernable differences in the dominant decay channels between the two reactions, there are fewer deuterons from the more symmetric system. This difference can be interpreted two ways: as a suppression of the emission of energetically expensive clusters during the time required for shape equilibration (which is predicted to be longer for the more symmetric entrance channel), or as an enhancement of the emission of energetically expensive clusters from the more asymmetric system at the very early stage of the collision when the initial energy deposited is only available to a reduced number of nucleons. The first explanation is identical to that used in recent high energy photon work while the second could be identified as the result of the emission of clusters on the multistep compound branch leading to the fusion of the low energy heavy ions. If the first explanation is adopted, the observed suppression is larger than predicted by a standard statistical decay model coupled to a dynamical fusion model, but consistent with work using high energy photons as a probe of fusion dynamics.

Analysis of the (N,xN') reactions by quantum molecular dynamics plus statistical decay model.

We propose a model based on quantum molecular dynamics (QMD) incorporated with a statistical decay model (SDM) to describe various nuclear reactions in a unified way. In this first part of the work, the basic ingredients of the model are defined and the model is applied systematically to the nucleon- (N-)induced reactions. It has been found that our model can give a remarkable agreement in the energy-angle double differential cross sections of (N,${\mathit{xN}}^{\ensuremath{'}}$) type reactions for incident energies from 100 MeV to 3 GeV with a fixed parameter set. A unified description of the three major reaction mechanisms of (N,${\mathit{xN}}^{\ensuremath{'}}$) reactions, i.e., compound, preequilibrium, and spallation processes, is given with our model.

Damping of unbound single-particle modes.

The (\ensuremath{\alpha}${,}^{3}$He-n) reaction has been investigated at 120 MeV incident energy on $^{64}\mathrm{Ni}$, $^{90}\mathrm{Zr}$, and $^{120}\mathrm{Sn}$ target nuclei. Neutrons in coincidence with $^{3}\mathrm{He}$ particles emitted at 0\ifmmode^\circ\else\textdegree\fi{} were detected using the multidetector array EDEN, in order to get information about the decay of single-particle states embedded in the (\ensuremath{\alpha}${,}^{3}$He) continuum. Neutron angular correlations, multiplicity values, and branching ratios to low-lying states of the final nuclei have been compared with the predictions of the statistical decay model. Evidence for a significant nonstatistical decay branch has been observed in the three nuclei below about 15 MeV excitation energy. Direct branching ratios in $^{91}\mathrm{Zr}$ deduced from this analysis are compared with the predictions of two nuclear structure models. At higher excitation energy, the decay characteristics of the (\ensuremath{\alpha}${,}^{3}$He) continuum are shown to be mainly statistical.

Relativistic effects in simulations of the fragmentation process with the microscopic framework

We simulate the fragmentation processes in the Ca + Ca collisions at the bombarding energy 1.05 GeV/u using the Lorentz covariant RQMD and the non covariant QMD approaches, incorporated with the statistical decay model. By comparing the results of RQMD with those of QMD, we examine the relativistic effects and find that the multiplicity of the α particle after the statistical decay process is sensitive to the relativistic effects. It is shown that the Lorentz covariant approach is necessary to analyze the fragmentation process even at the energy around E lab= 1 GeV/u as long as we are concerned with the final observables of the mass distribution, particularly, the light fragments around A = 3–4.

Heavy residues from very mass-asymmetric heavy-ion reactions.

The isotopic production cross sections and momenta of all residues with nuclear charge (Z) greater than 39 from the reaction of 26, 40, and 50 MeV/nucleon {sup 129}Xe + Be, C, and Al were measured. The isotopic cross sections, the momentum distribution for each isotope, and the cross section as a function of nuclear charge and momentum are presented here. The new cross sections are consistent with previous measurements of the cross sections from similar reaction systems. The shape of the cross section distribution, when considered as a function of Z and velocity, was found to be qualitatively consistent with that expected from an incomplete fusion reaction mechanism. An incomplete fusion model coupled to a statistical decay model is able to reproduce many features of these reactions: the shapes of the elemental cross section distributions, the emission velocity distributions for the intermediate mass fragments, and the Z versus velocity distributions. This model gives a less satisfactory prediction of the momentum distribution for each isotope. A very different model based on the Boltzman-Nordheim-Vlasov equation and which was also coupled to a statistical decay model reproduces many features of these reactions: the shapes of the elemental cross section distributions, the intermediate massmore » fragment emission velocity distributions, and the Z versus momentum distributions. Both model calculations over-estimate the average mass for each element by two mass units and underestimate the isotopic and isobaric widths of the experimental distributions. It is shown that the predicted average mass for each element can be brought into agreement with the data by small, but systematic, variation of the particle emission barriers used in the statistical model. The predicted isotopic and isobaric widths of the cross section distributions can not be brought into agreement with the experimental data using reasonable parameters for the statistical model.« less

Source characteristics of evaporation residues and intermediate-mass fragments in Kr-induced reactions at 26.4 and 34.4 MeV/nucleon

Abstract Fragments from Z = 3 up to Z = 37, produced in Kr + C, Al, nat Ti reactions at 26.4 and 34.4 MeV/nucleon were detected. Velocity, energy, mass and atomic number of fragments were measured. Analysis of the velocity spectra and angular distributions of the intermediate-mass fragments shows the presence of an isotropic component with the characteristics of an asymmetric binary splitting of a hot nucleus. A comparison of the source characteristics associated to these fragments and to evaporation residues indicates that the measurement of intermediate-mass fragments allows to select the most dissipative collisions. The results are discussed in the framework of the statistical decay model following complete and incomplete fusion.

Fusion cross sections in systems leading to 170Hf at near-barrier energies.

In an effort to study the effect of the entrance channel mass asymmetry on the fusion process at near-barrier energies, we have measured the fusion cross section and its distribution according to the different evaporation residues for the $^{28}\mathrm{Si}$${+}^{142}$Ce, $^{32}\mathrm{S}$${+}^{138}$Ba, and $^{48}\mathrm{Ti}$${+}^{122}$Sn systems. All these systems lead to the same compound nucleus $^{170}\mathrm{Hf}$. The measurements were performed using a delayed x-ray technique. For the last two systems we have also measured the fission cross sections in the same bombarding energy range. This experimental information can be used to restrict the free parameters of the statistical model used to account for the relative yield. A constrained and realistic statistical decay model is useful in reducing the uncertainties in the determination of the spin distribution from measurements of gamma multiplicities for these systems. The excitation function for the fusion cross section can be described using a schematic coupled channels calculation with realistic coupling strengths. Our results show no umambiguous effect that can be associated with the entrance channel mass asymmetry.

Nuclear photofissility at intermediate and high energies.

The nuclear photofissility of actinides, at intermediate and high energies, was studied by means of a formalism which makes salient the aspects of preequilibrium emissions, compound nucleus formation, and fission decay of the equilibrated system. This formalism was applied in the interpretation of the recently measured photofissilities of [sup 232]Th and [sup 238]U, in the energy range 300--1200 MeV, using (a) an intranuclear cascade Monte Carlo calculation to describe the preequilibrium and thermalization processes, and (b) the statistical model for the fission decay of the compound nucleus. It was found that the nonsaturation of the [sup 232]Th photofissility, even at energies up to 1200 MeV, could be explained as a consequence of its higher nuclear transparency, when compared with heavier actinides. Also, the question about a possible competition between statical and direct fission in [sup 232]Th and [sup 238]U was addressed.

Deexcitation of primary projectile-like fragments in the reaction 40Ca+natCu at 35 MeV/nucleon: Comparison with sequential binary decay and percolation models.

This article presents a new analysis of primary projectile-like fragments produced in peripheral and mid-peripheral collisions in the reaction $^{40}\mathrm{Ca}$${+}^{\mathrm{nat}}$Cu at 35 MeV/nucleon. The data were obtained using the charged particle amphora array. The distributions of multiplicity, inclusive charge, and heaviest fragment charge of the deexcitation products as well as the moments characterizing the single event charge partitions are remarkably well reproduced by a statistical sequential binary decay model even up to excitation energies of more than 8 MeV/nucleon. The results produced by the sequential binary decay model and a simple bond percolation model are qualitatively very similar. However the sequential binary decay mechanism leads to charge fluctuations which are in better agreement with the data than those predicted by the percolation model.

Explosive multifragmentation in the 32S+27Al reaction at 37.5 MeV/nucleon.

We describe an experimental study of the [sup 32]S+[sup 27]Al reaction at 37.5 MeV/nucleon carried out with the AMPHORA multidetector. A small fraction of events in which the total charge was detected and which are shown to originate mainly from central collisions have been isolated and compared with statistical decay model predictions. Dynamical properties provide strong evidence for a radial directed collective component (blast wave) in the fragment energies which dominates the interfragment Coulomb repulsion and the thermal motion.

Complex Fragment Emission for the Study of the Fermi Energy Domain Nuclear Collisions

Complex fragment production in the Fermi energy domain heavy ion collisions is studied. The physics used for the description of the various remnants produced in the entrance channel is based on fireball model taking into account the kinematic and dynamic features of the collision partners, and the decay processes are simulated by a version of the statistical binary decay model. Applications are shown to the target-like products for reactions of copper with 24.0, 31.1, 38.5 and 46.4 MeV/u 12C. Absolute comparisons with experimental data give a very well overall agreement.