Cascade Code Research Articles

Towards a solution of theΓn∕Γppuzzle in the nonmesonic weak decay ofΛhypernuclei

One of the main open problems in the physics of $\ensuremath{\Lambda}$ hypernuclei is the lack of a sound theoretical interpretation of the large experimental values for the ratio ${\ensuremath{\Gamma}}_{n}∕{\ensuremath{\Gamma}}_{p}\ensuremath{\equiv}\ensuremath{\Gamma}(\ensuremath{\Lambda}n\ensuremath{\rightarrow}nn)∕\ensuremath{\Gamma}(\ensuremath{\Lambda}p\ensuremath{\rightarrow}np)$. To approach the problem, we have incorporated a one-meson-exchange model for the $\ensuremath{\Lambda}N\ensuremath{\rightarrow}nN$ transition in finite nuclei in an intranuclear cascade code for the calculation of single- and double-coincidence nucleon distributions corresponding to the nonmesonic weak decay of $_{\ensuremath{\Lambda}}^{5}\mathrm{He}$ and $_{\ensuremath{\Lambda}}^{12}\mathrm{C}$. Due to the reduction of interferences, two-nucleon coincidences are expected to give a cleaner determination of ${\ensuremath{\Gamma}}_{n}∕{\ensuremath{\Gamma}}_{p}$ than single-nucleon observables. Single-nucleon distributions are found to be less sensitive to variations of ${\ensuremath{\Gamma}}_{n}∕{\ensuremath{\Gamma}}_{p}$ than double-coincidence spectra. The comparison of our results with preliminary KEK coincidence data allows us to extract a ${\ensuremath{\Gamma}}_{n}∕{\ensuremath{\Gamma}}_{p}$ ratio for $_{\ensuremath{\Lambda}}^{5}\mathrm{He}$ of $0.39\ifmmode\pm\else\textpm\fi{}0.11$ when multinucleon induced channels are omitted.

KEWPIE: A dynamical cascade code for decaying exited compound nuclei

A new dynamical cascade code for decaying hot nuclei is proposed and specially adapted to the synthesis of super-heavy nuclei. For such a case, the interesting channel is of the tiny fraction that will decay through particles emission, thus the code avoids classical Monte-Carlo methods and proposes a new numerical scheme. The time dependence is explicitely taken into account in order to cope with the fact that fission decay rate might not be constant. The code allows to evaluate both statistical and dynamical observables. Results are successfully compared to experimental data.

Nucleon-nucleon coincidence spectra in the nonmesonic weak decay of lambda hypernuclei and the gamma(n)/gamma(p) puzzle.

By combining a one-meson-exchange model for the LambdaN-->nN transition in finite nuclei with an intranuclear cascade code, we have obtained nucleon-nucleon (angular and energy) coincidence distributions from the nonmesonic weak decay of (5)(Lambda)He and (12)(Lambda)C hypernuclei. Although, due to the elimination of interferences, two-nucleon coincidences are expected to give a cleaner extraction (with respect to single nucleon observables) of the ratio Gamma(n)/Gamma(p) identical with Gamma(Lambdan-->nn)/Gamma(Lambdap-->np), we show that the effect of the final state interactions is still important even when applying favorable energy and angular cuts. The agreement of our results for the ratio N(nn)/N(np) between the number of nn and np emitted pairs with preliminary KEK coincidence data allows us to conclude that Gamma(n)/Gamma(p) for (5)(Lambda)He should be small and close to the value of 0.46 predicted by our meson-exchange model.

Neutron spallation source and the Dubna Cascade Code

Neutron multiplicity per incident proton,n/p, in collision of high energy proton beam with voluminous Pb and W targets has been estimated from the Dubna Cascade Code and compared with the available experimental data for the purpose of benchmarking of the code. Contributions of various atomic and nuclear processes for heat production and isotopic yield of secondary nuclei are also estimated to assess the heat and radioactivity conditions of the targets. Results obtained from the code show excellent agreement with the experimental data at beam energy,E < 12 GeV and differ maximum up to 25% at higher energy

Experimental and Theoretical Study of the Residual Product Nuclide Yields in 100-2600 MeV Proton-Irradiated Thin Targets

The work is aimed at experimental determining and computer simulating the independent and cumulative yields of residual product nuclei in the target and structure materials of the transmutation facilities driven by high-current accelerators. The ITEP U-10 accelerator was used in 48 experiments to obtain more than 4000 values of the yields of radioactive residual product nuclei in 0.1-2.6 GeV proton-irradiated thin 182, 183, 184, 186W, natW, 56Fe, 58Ni, 93Nb, 232Th, 232Th, natU, 99Tc, 59Co, 63, 65Cu, natHg, 208Pb, and 27Al targets. The results of verifying the LAHET, CEM95, CEM2k, CASCADE, CASCADE/INPE, YIELDX, HETC, INUCL, and other simulation codes are presented.

QMD and JAM Calculations for High Energy Nucleon-Nucleus Collisions

We describe the two simulation codes, QMD and JAM, for high energy nuclear reactions. QMD can treat the nucleus-nucleus reactions as well as nucleon-nucleus reactions based on the molecular dynamics. We have applied the QMD code intensively to nucleon-nucleus reactions and checked its validity. The cross sections obtained by the QMD are now used for evaluation of high energy nuclear data in JAERI. JAM is a hadronic cascade code including the resonance and string model for the hadron-hadron collisions at high energy up to 200 GeV. We have developed a high energy particle transport code NMTC/JAM by including the JAM code for the intra-nuclear cascade part.

Induced radioactivity problem for high-power heavy ion accelerators: Experimental investigation and long-time predictions

The experimental results of the activation spectra, dose rate measurements, and the residual nuclide production cross sections obtained after the irradiation of the NatCu and 59Co targets by 12C ion beams at ITEP and GSI are presented in this paper. These results are compared with simulations by the CASCADE and LAQGSM codes.

Pion double charge exchange on4He

The doubly differential cross sections for the $^4$He$(\pi^+,\pi^-) 4p$ reaction were calculated using both a two-nucleon sequential single charge exchange model and an intranuclear cascade code. Final state interactions between the two final protons which were the initial neutrons were included in both methods. At incident pion energies of 240 and 270 MeV the low-energy peak observed experimentally in the energy spectrum of the final pions can be understood only if the contribution of pion production is included. The calculated cross sections are compared with data.

Spallation neutron production and the current intra-nuclear cascade and transport codes

A recent renascent interest in energetic proton-induced production of neutrons originates largely from the inception of projects for target stations of intense spallation neutron sources, like the planned European Spallation Source (ESS), accelerator-driven nuclear reactors, nuclear waste transmutation, and also from the application for radioactive beams. In the framework of such a neutron production, of major importance is the search for ways for the most efficient conversion of the primary beam energy into neutron production. Although the issue has been quite successfully addressed experimentally by varying the incident proton energy for various target materials and by covering a huge collection of different target geometries --providing an exhaustive matrix of benchmark data-- the ultimate challenge is to increase the predictive power of transport codes currently on the market. To scrutinize these codes, calculations of reaction cross-sections, hadronic interaction lengths, average neutron multiplicities, neutron multiplicity and energy distributions, and the development of hadronic showers are confronted with recent experimental data of the NESSI collaboration. Program packages like HERMES, LCS or MCNPX master the prevision of reaction cross-sections, hadronic interaction lengths, averaged neutron multiplicities and neutron multiplicity distributions in thick and thin targets for a wide spectrum of incident proton energies, geometrical shapes and materials of the target generally within less than 10% deviation, while production cross-section measurements for light charged particles on thin targets point out that appreciable distinctions exist within these models.

Equilibrium and Pre-Equilibrium Studies in Some Alpha Induced Reactions on Rhodium

Excitation functions of the reactions (α, 2 n ), (α, 3 n ), and (α, 4 n ) for 103 Rh have been measured with stacked foil activation technique for the projectile energies below 50 MeV α-particles. A large volume HPGe detector was used. The experimental data were compared with the calculated values obtained from the CASCADE code based on statistical model and ALICE-91 code based on Hybrid and Geometry Dependent Hybrid Models. It is found that the pre-equilibrium contribution is more pronounced in the high energy region of the excitation functions and the experimental data are explained only when the pre-equilibrium emission phenomenon is also taken into account along with the equilibrium decay. The percentage of pre-equilibrium fraction has also been calculated.

Comment on “Spin and excitation energy dependence of fission survival for the19F+175Lusystem”

In a recent paper Hui et al. fit their evaporation residue data with CASCADE calculations equally well with and without nuclear viscosity by scaling the fission barrier with different scaling factors. I find that the need for scaling the fission barrier is an artifact of the level density calculation incorporated into the original CASCADE code. Contrary to the findings of Hui et al. the evaporation residue cross section is a very sensitive measure of the nuclear viscosity, if the calculations are done without scaling the fission barrier using a more recent level density method.

Code development for the design study of the OMEGA Program accelerator-driven transmutation systems

The Japan Atomic Energy Research Institute carries out R&D on accelerator-driven transmutation systems (ADTS) under the national OMEGA program (Options Making Extra Gains from Actinides and fission products). The code system named ATRAS was developed to analyze neutronics and burnup characteristics of ADTS. It has a unique function of burnup analysis taking into account the effect of the spallation neutron source. ATRAS consists of a hadronic cascade code, a neutron transport code and a burnup analysis code. Utility programs for fuel exchange, pre-processing and post-processing were also incorporated. The neutronics design study of a spallation target as well as the analysis of the neutronics and burnup characteristics of the sodium-cooled ADTS were performed using ATRAS.

Transmutation of 129I and 237Np using spallation neutrons produced by 1.5, 3.7 and 7.4 GeV protons

Small samples of 129I and 237Np, two long-lived radwaste nuclides, were exposed to spallation neutron fluences from relatively small metal targets of lead and uranium, that were surrounded with a 6 cm thick paraffin moderator, and irradiated with 1.5, 3.7 and 7.4 GeV protons. The (n,γ) transmutation rates were determined for these nuclides. Conventional radiochemical La- and U-sensors and a variety of solid-state nuclear track detectors were irradiated simultaneously with secondary neutrons. Compared with results from calculations with well-known cascade codes (LAHET from Los Alamos and DCM/CEM from Dubna), the observed secondary neutron fluences are larger.

Review of the European project — Impact of Accelerator-Based Technologies on nuclear fission safety (IABAT)

The IABAT project — Impact of Accelerator Based Technologies on Nuclear Fission Safety — started in 1996 in the frame of 4 th Framework Programme of the European Union, R&D specific programme “Nuclear fission safety 1994–1998”, area A.2 “Exploring innovative approaches/Fuel cycle concepts,” as one of the first common European activities in ADS. The project was completed October 31, 1999. The overall objective of the IABAT project has been a preliminary assessment of the potential of Accelerator-Driven Systems (ADS) for transmutation of nuclear waste and for nuclear energy production with minimum waste generation. Moreover, more specific topics related to nuclear data and code development for ADS have been studied in more detail. Four ADSs have been studied for different fuel/coolant combinations: liquid metal coolant and solid fuel, liquid metal coolant and dispersed fuel, and fast and thermal molten salt systems. Target studies comprised multiple target solutions and radiation damage problems in a target environment. In a tool development part of the project a methodology of subcriticality monitoring has been developed based on Feynman-alpha and Rossi-alpha methods. Moreover, a new Monte-Carlo burnup code taking full advantage of continuous neutron cross-section data has been developed and benchmarked. Impact on the risk from high-level waste repositories from radiotoxicity reduction using ADS has been assessed giving no crystal-clear benefits of ADS for repository radiotoxicity reduction but concluding some important prerequisites for effective transmutation. In proliferation studies important differences between critical reactors and ADS have been underlined and non-proliferation measures have been proposed. In assessment of accelerator technology costing models have been created that allow the circular and linear accelerator options to be compared and the effect of parameter variations examined. The calculations reported show that cyclotron systems would be more economical, due mainly to the advantage of the cost of RF power supplies. However, the accelerator community regards with skepticism the possibility of transporting and extracting more than a 10mA beam current from a 1GeV cyclotron and therefore technical factors may limit the application of cyclotrons. Finally, this review summarizes development of nuclear data in the energy region between 20 Mev and 150 MeV. Neutron and proton transport data files for Fe, Ni, Pb, Th, 238U and 239Pu have been created. The high-energy part of the data files consists completely of results from model calculations, which are benchmarked against the available experimental data. Although there is obviously future work left regarding fine-tuning of several parts of the data files, the representation of nuclear reaction information up to 150 MeV is already better than can be attained with intranuclear cascade codes.

Prompt light particle emission in the 36Ar + 58Ni reaction at 95 MeV/nucleon

The prompt component at intermediate velocity of light charged particles is investigated. An improved coalescence model coupled to the intra-nuclear cascade code ISABEL is used to obtain light complex particle energy spectra and multiplicities as a function of impact parameter. The results are compared with experimental data from the 36Ar + 58Ni experiment at 95 MeV/nucleon, performed with the INDRA 4π detection system. The calculated prompt component is found to rather well reproduce proton spectra. For complex light charged particles the calculated components well populate the high energy part of spectra. Prompt emission can therefore explain the large transverse energies experimentally observed at mid-rapidity.

Predicted vacancy cluster structures in MgO and their interaction with helium

Atomistic simulation calculations were performed to predict defect energies of vacancy clusters and dissociation energies of clusters with helium. The computation technique was based on pair potentials and the Mott–Littleton methodology was used as implemented in the CASCADE code. Both the full charge and the partial charge model were employed. The partial charge model shows better agreement with experimental data reported in the literature. The energetically most favourable spatial configurations for clusters composed of up to eight vacancies were determined and the associated formation energies calculated. The results indicate that the three-dimensional growth of vacancy clusters in MgO is energetically favourable. Next, the activation energies for dissociation of helium atoms from these vacancy clusters were calculated. The activation energy for dissociation of a helium atom from vacancy clusters larger than a monovacancy is approximately 3.6 eV for the partial charge model. Remarkably, the activation energy for dissociation of a helium atom from a monovacancy was found to be higher, 3.9 eV for the partial charge model. This last energy is enhanced by lattice relaxation around the monovacancy. The dissociation energies from large vacancy clusters is in good agreement with the permeation energy of 3.3 ± 0.3 eV obtained experimentally with a permeation experiment in which helium release was monitored with neutron depth profiling (NDP).

Rescattering of vector meson daughters in high energy heavy ion collisions

We consider the role of hadronic rescattering of daughter kaons on the observed mass spectra from $\phi$ meson decays in ultra-relativistic heavy ion collisions. A hadronic cascade code (RQMD v2.4) shows that $\sim$26% of all $\phi$'s decaying to $K^+K^-$ in central Pb+Pb collisions at SPS energies ($E_{beam} = 158 GeV/A$) have a rescattered or absorbed daughter. This significantly affects the reconstructed invariant mass of the pair and shifts $\phi$ mesons out of the mass peak. Kaon rescattering depletes the low velocity region, hardening and broadening the observed phi $m_t$ and rapidity distributions respectively, relative to the dilepton channel. This effect produces an apparent change in the experimentally determined branching ratio not necessarily related to chiral symmetry restoration. Comparisons to recent experimental measures at CERN energies reveal a possible mechanism to account for the shape of the observed spectra, though not their absolute relative magnitude.

The GDR width in the excited [formula omitted] Eu compound nucleus at high angular momentum

High-energy γ -rays emitted in the decay of the hot compound nucleus 147 Eu have been measured in coincidence with low-energy γ -rays. The γ transitions from the different residual nuclei were detected by a multiplicity filter and by Ge detectors. The employed reaction was 37 Cl + 110 Pd at bombarding energies of 160, 165 and 170 MeV. The measured high-energy γ -ray spectra were analysed within the framework of the statistical model using the Cascade code. The GDR width in the angular momentum interval between 35 and 50 ℏ was found to increase weakly and to be rather well predicted by the thermal shape fluctuation model. Also the deformation parameter β as a function of the average angular momentum extracted from the data was found to be in general agreement with the model.

Alpha particle evaporation as a probe for dynamical deformations

Evaporation of alpha particles in heavy ion reactions followed by fusion has proved to be a powerful probe for the properties of emitting nuclei. Detailed experimental data and different model calculations allow us to probe whether the foundation of the statistical model holds for the compound nucleus populated in these reactions. It has been observed that in the case of composite nuclei at moderate energies and angular momenta, evaporation spectra are well explained in terms of the standard statistical model CASCADE code employing optical model transmission coefficients in the description of particle evaporation. However, it has been observed that experimental particle spectra from heavy ion induced fusion reactions are no longer consistent with the predictions of such models. It has been predicted by some authors that in these systems the emission barriers are lower than those expected from optical model transmission coefficients calculated for the respective inverse absorption channels. Some authors claim that these spectra may be well explained in terms of the statistical model incorporating only spin dependent level density and without lowering the emission barriers. The field is not yet free from the controversies. Furthermore, the assumption of the very short formation time in statistical model analysis is one extreme of the general evolution process which in fact is a continuous relaxation process, leading to the composite system from the entrance channel to the equilibrated configuration. Recent dynamical description of heavy ion collisions do not support this assumption in many cases. In symmetric entrance channels and for collisions where centre of mass energy is well above the Coulomb barrier, formation time can be even larger than decay time of the resulting composite system. In such cases realistic approach will be to couple the dynamical evolution of the intrinsic excitation of the composite system to a time dependent statistical model calculation. The above question has been addressed in the light of the alpha particle spectra taken in coincidence with the evaporation residues for the asymmetric 28Si+51V and the symmetric 28Si+27Al systems. The experimental data have been interpreted in the framework of dynamical trajectory model calculations.

Thermal excitation of heavy nuclei with 5–15 GeV/c antiproton, proton and pion beams

Excitation-energy distributions have been derived from measurements of 5.0–14.6 GeV/c antiproton, proton and pion reactions with 197 Au target nuclei, using the ISiS 4 π detector array. The maximum probability for producing high excitation-energy events is found for the 8 GeV/c antiproton beam relative to other hadrons, 3 He and p̄ beams from LEAR. For protons and pions, the excitation-energy distributions are nearly independent of hadron type and beam momentum above about 8 GeV/c. The excitation energy enhancement for p̄ beams and the saturation effect are qualitatively consistent with intranuclear cascade code predictions. For all systems studied, maximum cluster sizes are observed for residues with E ∗/A∼6 MeV.