Statistical Decay Model Research Articles

Effect of fragment emission time on the temperature of momentum quadrupole fluctuations

A systematic study of a momentum quadrupole fluctuation thermometer has been presented for heavy-ion collisions at 35 MeV/nucleon via the isospin-dependent quantum molecular dynamics model accompanied by the statistical decay model GEMINI. It is determined that the fragment momentum fluctuation temperature indeed reflects the average temperature of the excitation source in the fragmentation process and the secondary decay. We find that the divergence between the initial temperature and the final measurement temperature is different for protons, tritons, and $^{3}\mathrm{He}$. The maximum divergence of the temperature is about $20%$ probed by protons. As an example of using protons as the probe particle, we find that the isospin dependence of the nuclear temperature is consistent between the initial temperature and the final measurement temperature.

Further study on mechanism of production of light complex particles in nucleon-induced reactions

The Improved Quantum Molecular Dynamics (ImQMD) model incorporated with the statistical decay model is used to investigate the intermediate energy nucleon-induced reactions. In our last work, the description on light complex particle emission has been great improved with a phenomenological mechanism called surface coalescence and emission introduced into ImQMD model. In this work, taking account of different specific binding energies and separation energies for various light complex particles, the phase space parameters in surface coalescence model are readjusted. By using the new phase space parameters set with better physical fundament, the double differential cross sections of emitted light complex particles are found to be in better agreement with experimental data.

Improvement of photonuclear reaction model below 140 MeV in the PHITS code

The photonuclear reaction model in the particle and heavy ion transport code system (PHITS) code is improved for incident photon energies below 140 MeV. Japanese Evaluated Nuclear Data Library (JENDL) Photonuclear Data File 2004 (JENDL/PD-2004) is adopted to determine the total reaction cross section. The statistical decay model after excitation of the nucleus in PHITS is improved by modifying the decay widths for light nuclei under the isospin selection rule to reasonably reproduce the proton and neutron emission in the de-excitation process. The quasideuteron disintegration process is newly introduced into PHITS to handle the photonuclear reaction up to 140 MeV of incident photon energy. The accuracy of the improvements was verified by comparison with the experimental literature data. The improved PHITS can contribute to various practical applications such as neutron dose estimation in X-ray therapy.

Mechanism of the production of light complex particles in nucleon-induced reactions

The improved quantum molecular dynamics (ImQMD) model incorporated with the statistical decay model is successful in describing emission of the nucleons in the intermediate energy spallation reactions, but not good enough in describing productions of the light complex particles, i.e. d, t, 3He and 4He. To improve the description on emission of light complex particles, a phenomenological mechanism called surface coalescence and emission is introduced into the ImQMD model: nucleon ready to escape from the compound nuclei can coalesce with the other nucleon(s) to form light complex particle and be emitted. With updated ImQMD model, the description on the experimental data of light complex particles produced in nucleon-induced reactions are great improved.

Alpha cluster structure in 56Ni

The inelastic α-scattering experiment on 56Ni in inverse kinematics was performed at an incident energy of 50 MeV/u at GANIL. A very high multiplicity for α-particle emission was observed with our phase-space limited experimental set-up. The maximum observed multiplicity, which cannot be explained by means of the statistical decay model, amounted to seven. The ideal classical gas model at kT = 3 MeV fairly well reproduced the experimental momentum distribution and multiplicity of alpha particles. This result strongly suggests that an alpha-gas state in 56Ni may be excited via inelastic alpha scattering.

Analysis of fragmentation excitation functions of lead by carbon ions up to 400 MeV/u

Fragmentation reaction cross-sections of lead bombarded by carbon ions below 400MeV/u are measured and compared with those simulated with Monte-Carlo radiation transport code PHITS to pinpoint the problems of simulation based on the quantum molecular dynamics model and the statistical decay model. Cross-sections for 32 fragments from A=24 to 85 and those from A=121 to 175 are measured within a reasonable uncertainty. Contribution of secondary particles to production of measured fragments are generally less than 20%. Comparison shows production cross-sections for fragments from A=20 to 60 and those from A=110 to 130 are underestimated by one order of magnitude. Further simulation shows production of each group of fragments in simulation are attributed to evaporation of fission fragments and to evaporation of highly excited prefragments, respectively. The lower the energy is, the more this trend is pronounced. Observed underestimation indicates presence of nuclear reaction mechanisms not considered in the current simulation such as multi-fragmentation.

Isotopic dependence of nuclear temperatures

A systematic study of isotope temperatures has been presented for heavy-ion collisions at 600 MeV/nucleon via the isospin-dependent quantum molecular dynamics model in the company of the statistical decay model (GEMINI). We find that the isospin dependence of the isotope temperatures in multifragmentation is weak; however, this effect is still visible over a wide isotopic range. The isotope temperatures for the neutron-rich projectiles are larger than those for the neutron-poor projectiles. We also find that the isotope temperatures calculated by the model decrease with increasing nuclear mass.

Event Generator Models in the Particle and Heavy Ion Transport Code System; PHITS

We present the event generator rnodels incorporated in the particle and heavy ion transport code system PHITS. For the high energy nuclear reactions, we discuss the QMD model and the INC model followed by the statistical decay model. For low energy neutron transport by using the nuclear data, we propose a new model, in which we combine the evaluated nuclear data and the reaction models so as to describe all ejectiles of collision keeping the energy and momentum conservation. By this new model, we can estimate new quantities which are related to the higher order correlations beyond one-body observable, for an example, the deposit energy distribution in a cell, which cannot be obtained by the transport calculation based on the Boltzmann equation with the evaluated nuclear data.

Excitation energy and nuclear dissipation probed with evaporation-residue cross sections

Using a Langevin equation coupled with a statistical decay model, we calculate the excess of evaporation-residue cross sections over its standard statistical-model value as a function of nuclear dissipation strength for $^{200}\mathrm{Hg}$ compound nuclei (CNs) under two distinct types of initial conditions for populated CNs: (i) high excitation energy but low angular momentum (produced via proton-induced spallation reactions at GeV energies and via peripheral heavy-ion collisions at relativistic energies) and (ii) high angular momentum but low excitation energy (produced through fusion mechanisms). We find that the conditions of case (ii) not only amplify the effect of dissipation on the evaporation residues, but also substantially increase the sensitivity of this excess to nuclear dissipation. These results suggest that, in experiments, to obtain accurate information of presaddle nuclear dissipation strength by measuring evaporation-residue cross sections, it is best to choose the heavy-ion-induced fusion reaction approach to yield excited compound nuclei.

NEAR BARRIER REACTIONS INDUCED BY WEAKLY BOUND CLUSTER NUCLEI

Experimental excitation functions are presented for complete fusion and transfer reactions in the interaction of 6 He and 6,8,9 Li with 206 Pb , 209 Bi , and nat Pt targets. The data on fusion in the 6 He -induced reactions at energies close to the Coulomb barrier differ from predictions within the framework of the statistical model for compound nucleus decay. For these exit channels a strong enhancement has been observed. Enhancement of the cross section for neutron transfer (with the 6 He and 8,9 Li beams) and deuteron transfer (with the 6 Li beam) reactions is observed at deep sub-barrier energies. The results are discussed from the viewpoint of how the nuclear cluster structure influences the probability of interaction at near-barrier energies.

Fissility effects on evaporation residue spin distributions

Based on a dynamical Langevin equation coupled with a statistical decay model, we calculate the spin distribution of evaporation residue cross sections of nuclei 190Os and 210Po which have the same neutron-to-proton ratio N / Z but have a difference in fissility. We find that a large fissility enhances the sensitivity of the residue spin distribution to pre-saddle friction substantially. The results suggest that on the experimental side, to obtain accurate information of pre-saddle dissipation strength by measuring the evaporation residue spin distribution it is optimal to populate among various compound systems with equal N / Z those with high fissility.

STATISTICAL DECAY OF EXCITED ΛΛ HYPERNUCLEI AND γ-SPECTROSCOPY AT $\rm \bar P \rm ANDA$

Hypernuclear physics is currently attracting renewed attention. Thanks to the use of stored [Formula: see text] beams, copious production of double Λ hypernuclei is expected at the [Formula: see text] experiment which will enable high precision γ-spectroscopy of such nuclei for the first time. In the present work we have studied the population of particle stable, excited states in double hypernuclei after the capture of a Ξ- within a statistical decay model. In order to check the feasibility of producing and performing γ-spectroscopy of double hypernuclei at [Formula: see text], an event generator based on these calculations has been implemented in the [Formula: see text] simulation framework PANDAROOT.

System size effects on probing nuclear dissipation with neutrons

Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 190Os, 200Hg, and 210Po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size.

Effects of N/Z on Post-Saddle γ Emission as an Observable of Post-Saddle Nuclear Friction

Based on a dynamical Langevin equation coupled with a statistical decay model, we calculate the variation of the post-saddle giant dipole resonance (GDR) γ-ray multiplicity of the heavy nuclei 240Cf, 246Cf, 252Cf, and 240U with the post-saddle friction strength (β). We find that the sensitivity of the post-saddle γ emission to β decreases considerably with increasing the neutron-to-proton ratio (N/Z) of the system. Moreover, for 240U, the γ emission is no longer sensitive to β. We suggest that to accurately obtain information of the post-saddle friction strength by measuring pre-scission GDR γ-ray multiplicities, it is optimal to choose among the various compound systems those with low N/Z.

Determining the energy barrier for decay out of superdeformed bands

An asymptotically exact quantum mechanical calculation of the matrix elements for tunneling through an asymmetric barrier is combined with the two-state statistical model for decay out of superdeformed bands to determine the energy barrier (as a function of spin) separating the superdeformed and normal-deformed wells for several nuclei in the 190 and 150 mass regions. The spin-dependence of the barrier leading to sudden decay out is shown to be consistent with the decrease of a centrifugal barrier with decreasing angular momentum. Values of the barrier frequency in the two mass regions are predicted.

Roadmap for double hypernuclei spectroscopy at PANDA

Hypernuclear Physics is currently attracting renewed attention. Thanks to the use of stored beams, copious production of double Λ hypernuclei is expected at the anda experiment which will enable high precision γ–spectroscopy of such nuclei for the first time. In the present work we have studied the population of particle stable, excited states in double hypernuclei after the capture of a Ξ− within a statistical decay model. In order to check the feasibility of producing and performing γ–spectroscopy of double hypernuclei at ANDA, an event generator based on these calculations has been implemented in the ANDA simulation framework PANDAROOT.

Quantum molecular dynamics study of the mass distribution of products in7.0A MeVU238+U238collisions

Within the improved quantum molecular dynamics (ImQMD) model incorporating the statistical decay model, the reactions of $^{238}\mathrm{U}+^{238}\mathrm{U}$ at an energy of $7.0A$ MeV have been studied. The charge, mass, and excitation energy distributions of primary fragments are investigated within the ImQMD model, and de-excitation processes of these primary fragments are described by a statistical decay model. The mass distribution of the final products in $^{238}\mathrm{U}+^{238}\mathrm{U}$ collisions is obtained and compared with the recently obtained experimental data.

Isospin effects on light charged particles as probes of nuclear dissipation

The multiplicities of postsaddle protons and {alpha} particles of the heavy systems {sup 234}Cf, {sup 240}Cf, {sup 246}Cf, and {sup 240}U as functions of the postsaddle dissipation strength are calculated in the framework of a dynamical Langevin model coupled with a statistical decay model. It is found that with increasing isospin of the Cf system, the sensitivity of the postsaddle proton and {alpha}-particle multiplicity to the dissipation strength decreases substantially, and it disappears for the {sup 240}U system. We suggest that on the experimental side, to accurately probe the postsaddle dissipation strength by measuring the prescission proton and {alpha}-particle multiplicity, it is best to populate heavy compound systems with low isospin.

Isospin effects on neutrons as a probe of nuclear dissipation

Based on a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the prescission neutron multiplicity of the heavy nuclei $^{240}\mathrm{Cf}$, $^{246}\mathrm{Cf}$, $^{252}\mathrm{Cf}$, and $^{240}\mathrm{U}$ over that predicted by the standard statistical model as a function of the postsaddle dissipation strength. We find that with increasing isospin of the system, the sensitivity of the excess to the dissipation strength decreases substantially. Moreover, for $^{240}\mathrm{U}$, this excess is no longer sensitive to the nuclear dissipation. These results suggest that, on the experimental side, to accurately obtain information of the postsaddle dissipation strength by measuring the neutron multiplicity evaporated during the fission process of heavy nuclei, it is best to populate those compound systems with low isospin.

THE DYNAMICAL ISOSPIN EFFECT ON PROTON-INDUCED REACTIONS ON Sn ISOTOPES

In this talk we present a model, Improved Quantum Molecular Dynamics (ImQMD05) model incorporated with a Statistical Decay Model (SDM), to describe intermediate energy proton induced spallation reactions. A good agreement with experimental data of double differential cross sections of emitted neutrons is obtained. We further apply this model to study the isospin effect in proton induced spallation reactions on a series Sn isotope targets. We find that the systematic behavior of the reaction cross sections for Sn isotope targets deviates from the empirical expression obtained by fitting the experimental data for proton induced spallation reactions on target nuclei along β-stability line. The extent of the deviation depends on the density dependence of the symmetry energy strongly. We also find an obvious shift of the elastic scattering angular distribution of emitted protons when the symmetry energy is taken into account for neutron-rich Sn isotope targets and the angle shifted strongly depends on the stiffness of the symmetry energy. The attractive effect of the symmetry potential of target on incident proton directly influences the motion of the incident proton leading to strong isospin effect on the reaction dynamics and thus on these reaction observables. We conclude that the measurement of reaction cross sections and the elastic scattering angular distributions in proton induced spallation reactions on Sn isotopes can provide clear constraint for the density dependence of symmetry energy.