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We tabulate the atomic mass excesses and binding energies, ground-state shell-plus-pairing corrections, ground-state microscopic corrections, and nuclear ground-state deformations of 9318 nuclei ranging from $^{16}$O to $A=339$. The calculations are based on the finite-range droplet macroscopic model and the folded-Yukawa single-particle microscopic model. Relative to our FRDM(1992) mass table in {\sc Atomic Data and Nuclear Data Tables} [{\bf 59} 185 (1995)], the results are obtained in the same model, but with considerably improved treatment of deformation and fewer of the approximations that were necessary earlier, due to limitations in computer power. The more accurate execution of the model and the more extensive and more accurate experimental mass data base now available allows us to determine one additional macroscopic-model parameter, the density-symmetry coefficient $L$, which was not varied in the previous calculation, but set to zero. Because we now realize that the FRDM is inaccurate for some highly deformed shapes occurring in fission, because some effects are derived in terms of perturbations around a sphere, we only adjust its macroscopic parameters to ground-state masses. The values of ten constants are determined directly from an optimization to fit ground-state masses of 2149 nuclei ranging from $^{16}$O to $^{265}_{106}$Sg and $^{264}_{108}$Hs. The error of the mass model is 0.5595~MeV. We also provide masses in the FRLDM, which in the more accurate treatments now has an error of 0.6618 MeV. But in contrast to the FRDM, it is suitable for studies of fission and has been extensively so applied elsewhere, with FRLDM(2002) constants. The FRLDM(2012) fits 31 fission barrier heights from $^{70}$Se to $^{252}$Cf with a root-mean-square deviation of 1.052 MeV., 233 pages, 41 figures. arXiv admin note: text overlap with arXiv:nucl-th/9308022

The SPectrometer for Ion DEtermination in fission Research (SPIDER) has been developed for measuring mass yield distributions of fission products from spontaneous and neutron-induced fission. The 2 E –2 v method of measuring the kinetic energy ( E ) and velocity ( v ) of both outgoing fission products has been utilized, with the goal of measuring the mass of the fission products with an average resolution of 1 atomic mass unit (amu). The SPIDER instrument, consisting of detector components for time-of-flight, trajectory, and energy measurements, has been assembled and tested using 229 Th and 252 Cf radioactive decay sources. For commissioning, the fully assembled system measured fission products from spontaneous fission of 252 Cf. Individual measurement resolutions were met for time-of-flight (250 ps FWHM), spacial resolution (2 mm FHWM), and energy (92 keV FWHM for 8.376 MeV). Mass yield results measured from 252 Cf spontaneous fission products are reported from an E – v measurement.

The Hauser-Feshbach statistical model is applied to the de-excitation of primary fission fragments using input mass yields calculated with macroscopic-microscopic models of the potential energy surface. We test the sensitivity of the prompt fission observables to the input mass yields for two important reactions, $^{235}$U$(n_\mathrm{th},f)$ and $^{239}$Pu$(n_\mathrm{th},f)$, for which good experimental data exist. General traits of the mass yields, such as the location of the peaks and their widths, can impact both the prompt neutron and $\gamma$-ray multiplicities, as well as their spectra. Specifically, we use several mass yields to determine a linear correlation between the calculated prompt neutron multiplicity $\bar{\nu}$ and the average heavy-fragment mass $\langle A_h\rangle$ of the input mass yields $\partial\bar{\nu}/\partial\langle A_h\rangle = \pm 0.1\,n/f/\mathrm{u}$. The mass peak width influences the correlation between the total kinetic energy of the fission fragments and the total number of prompt neutrons emitted $\bar{\nu}_T(\mathrm{TKE})$. Typical biases on prompt particle observables from using calculated mass yields instead of experimental ones are: $\delta \bar{\nu} = 4\%$ for the average prompt neutron multiplicity, $\delta \bar{M}_\gamma = 1\%$ for the average prompt $\gamma$-ray multiplicity, $\delta \bar{\epsilon}_n^\mathrm{LAB} = 1\%$ for the average outgoing neutron energy, $\delta \bar{\epsilon}_\gamma = 1\%$ for the average $\gamma$-ray energy, and $\delta \langle\mathrm{TKE}\rangle = 0.4\%$ for the average total kinetic energy of the fission fragments., Comment: 12 pages, 8 figures, 2 tables

The ability to describe production of light fragments (LF) is important for many applications, such as cosmic-ray-induced single event upsets (SEUs), radiation protection, and cancer therapy with proton and heavy-ion beams. The cascade-exciton model (CEM) and the Los Alamos version of the quark-gluon string model (LAQGSM) event generators in the LANL transport code MCNP6, describe quite well the spectra of fragments with sizes up to 4 He across a broad range of target masses and incident energies (up to ∼ 5 GeV for CEM and up to ∼ 1 TeV/A for LAQGSM). However, they do not predict the high-energy tails of LF spectra heavier than 4 He well. Most LF with energies above several tens of MeV are emitted during the precompound stage of a reaction. The current versions of our event generators do not account for precompound emission of LF larger than 4 He. The aim of our work is to generalize the precompound model to include such processes, leading to increased predictive power of LF production. Extending the model in this way provides preliminary results that have much better agreement with experimental data.

The liquid-drop model has its origins in the first mainstream model of the binding energy of nuclei, sometimes referred to as the semiempirical mass formula, which emerged in the mid 1930s. It is a beautiful example of a model that fulfills the criteria of what a theoretical model is and what an arbitrary parameterization of some data set is not: (1) it has a simple intuitive interpretation, (2) it was of enormous and immediate practical utility in interpreting nuclear experimental data such as radioactive decay and nuclear reactions, (3) it could predict binding energies of nuclei to which its parameters had not been adjusted, (4) it could be generalized to describe new, unanticipated phenomena such as fission, and (5) deviations of its predictions from experimental data yielded insight into nuclear structure and guided the development of more sophisticated models. Generalized liquid-drop models remain important because of the development of macroscopic–microscopic models which give important quantitative insight into ground-state structure and binding energies (nuclear masses) and many details of nuclear fission. We review these points and some associated historical milestones.

We extend the cascade-exciton model (CEM), and the Los Alamos version of the quark-gluon string model (LAQGSM), event generators of the Monte Carlo $N$-particle transport code version 6 (MCNP6), to describe production of energetic light fragments (LF) heavier than $^{4}\mathrm{He}$ from various nuclear reactions induced by particles and nuclei at energies up to about 1 TeV/nucleon. In these models, energetic LF can be produced via Fermi breakup, preequilibrium emission, and coalescence of cascade particles. Initially, we study several variations of the Fermi breakup model and choose the best option for these models. Then, we extend the modified exciton model (MEM) used by these codes to account for a possibility of multiple emission of up to 66 types of particles and LF (up to $^{28}\mathrm{Mg})$ at the preequilibrium stage of reactions. Then, we expand the coalescence model to allow coalescence of LF from nucleons emitted at the intranuclear cascade stage of reactions and from lighter clusters, up to fragments with mass numbers $A\ensuremath{\le}7$, in the case of CEM, and $A\ensuremath{\le}12$, in the case of LAQGSM. Next, we modify MCNP6 to allow calculating and outputting spectra of LF and heavier products with arbitrary mass and charge numbers. The improved version of CEM is implemented into MCNP6. Finally, we test the improved versions of CEM, LAQGSM, and MCNP6 on a variety of measured nuclear reactions. The modified codes give an improved description of energetic LF from particle- and nucleus-induced reactions; showing a good agreement with a variety of available experimental data. They have an improved predictive power compared to the previous versions and can be used as reliable tools in simulating applications involving such types of reactions.

Isotope production cross sections from 112Sn + 112Sn and 124Sn + 124Sn reactions at 1 GeV/nucleon, which were measured recently at GSI using the heavy-ion accelerator SIS18 and the Fragment Separator (FRS), have been analyzed with the latest Los Alamos Monte-Carlo transport code MCNP6 using the LAQGSM03.03 event generator. MCNP6 reproduces reasonably well all the measured cross sections. Comparison of the MCNP6 results with the measured data and with calculations by a modification of the Los Alamos version of the Quark-Gluon String Model allowing for multifragmentation processes in the framework of the Statistical Multifragmentation Model (SMM) by Botvina and coauthors, as realized in the code LAQGSM03.S1, does not suggest unambiguous evidence of a multifragmentation signature.

We describe a new cumulated fission product yield (FPY) evaluation for fission spectrum neutrons on plutonium that updates the ENDF/B-VI evaluation by England and Rider, for the forthcoming ENDF/B-VII.1 database release. We focus on FPs that are needed for high accuracy burnup assessments; that is, for inferring the number of fissions in a neutron environment. Los Alamos conducted an experiment in the 1970s in the Bigten fast critical assembly to determine fission product yields as part of the Interlaboratory Reaction Rate (ILRR) collaboration, and this has defined the Laboratory's fission standard to this day. Our evaluation includes use of the LANL-ILRR measurements (not previously available to evaluators) as well as other Laboratory FPY measurements published in the literature, especially the high-accuracy mass spectrometry data from Maeck and others. Because the measurement database for some of the FPs is small - especially for {sup 99}Mo - we use a meta-analysis that incorporates insights from other accurately-measured benchmark FP data, using R-value ratio measurements. The meta-analysis supports the FP measurements from the LANL-ILRR experiment. Differences between our new evaluations and ENDF/B-VI are small for some FPs (less than 1-2%-relative for {sup 95}Zr, {sup 140}Ba, {sup 144}Ce), but are larger for {sup 99}Momore » (4%-relative) and {sup 147}Nd (5%-relative, at 1.5 MeV) respectively. We present evidence for an incident neutron energy dependence to the {sup 147}Nd fission product yield that accounts for observed differences in the FPY at a few-hundred keV average energy in fast reactors versus measurements made at higher average neutron energies in Los Alamos' fast critical assemblies. Accounting for such FPY neutron energy dependencies is important if one wants to reach a goal of determining the number of fissions to accuracies of 1-2%. An evaluation of the energy-dependence of fission product yields is given for all A values based on systematical trends in the measured data, with a focus on the energy dependence over the fast neutron energy range from 0.2-2 MeV. Based on these trends, we present an evaluation of the FPY data at 0.5 and 2.0 MeV average incident neutron energies. This new set of ENDF/B-VII data will enable users to linearly interpolate between the pooled FPY data at {approx}0.5 MeV and our new data at 2 MeV to obtain FPYs at other energies.« less

The developmental version of MCNP5 has recently been extended to provide for continuous-energy transport of high-energy protons. This enhancement involves the incorporation of several significant new physics models into the code. Multiple Coulomb scattering is treated with an advanced model that takes account of projectile and nuclear target form factors. In the next version, this model will provide a coupled sampling of both angular deflection and collisional energy loss, including straggling. The proton elastic scattering model is also new, based on recent theoretical work. Charged particle transport in the presence of magnetic fields is accomplished either by using transfer maps from the COSY INFINITY code (in void regions) or by using an algorithm adapted from the MARS code (in void regions or in scattering materials). Work is underway to validate and implement the latest versions of the Cascade-Exciton Model and the Los Alamos Quark-Gluon String Model, which will process inelastic nuclear interactions and generate secondary particles.

We discuss the relation between the fission-fusion potential-energy surfaces of very heavy nuclei and the formation process of these nuclei in cold-fusion reactions. In the potential-energy surfaces, we find a pronounced valley structure, with one valley corresponding to the cold-fusion reaction, the other to fission. As the touching point is approached in the cold-fusion entrance channel, an instability towards dynamical deformation of the projectile occurs, which enhances the fusion cross section. These two 'cluster effects' enhance the production of superheavy nuclei in cold-fusion reactions, in addition to the effect of the low compound-system excitation energy in these reactions. Heavy-ion fusion reactions have been used extensively to synthesize heavy elements beyond actinide nuclei. In order to proceed further in this direction, we need to understand the formation process more precisely, not just the decay process. The dynamics of the formation process are considerably more complex than the dynamics necessary to interpret the spontaneous-fission decay of heavy elements. However, before implementing a full dynamical description it is useful to understand the basic properties of the potential-energy landscape encountered in the initial stages of the collision. The collision process and entrance-channel landscape can conveniently be separated into two parts, namely the early-stage separatedmore » system before touching and the late-stage composite system after touching. The transition between these two stages is particularly important, but not very well understood until now. To understand better the transition between the two stages we analyze here in detail the potential energy landscape or 'collision surface' of the system both outside and inside the touching configuration of the target and projectile. In Sec. 2, we discuss calculated five-dimensional potential-energy landscapes inside touching and identify major features. In Sec. 3, we present calculated 'collision surfaces' for still separated targets and projectiles. Implications for SHE formation are discussed. Section 4 is a short summary of the present analysis.« less

We calculate in a macroscopic-microscopic model fission potential-energy surfaces for neutron-rich actinides and fission-fusion potential-energy surfaces relevant to the analysis of heavy-ion reactions employed to form heavy-element evaporation residues. We study the latter multidimensional potential-energy surfaces both inside and outside the touching point. Inside the point of contact we define the potential on a multi-million-point grid in 5D deformation space where elongation, merging projectile and target spheroidal shapes, neck radius and projectile/target mass asymmetry are independent shape variables. The same deformation space and the corresponding potential-energy surface also describe the shape evolution from the nuclear ground-state to separating fragments in fission, and the fast-fission trajectories in incomplete fusion. For separated nuclei we study the macroscopic-microscopic potential energy, that is the "collision surface" between a spheroidally deformed target and a spheroidally deformed projectile as a function of three coordinates which are: the relative location of the projectile center-of-mass with respect to the target center-of-mass and the spheroidal deformations of the target and the projectile. We limit our study to the most favorable relative positions of target and projectile, namely that the symmetry axes of the target and projectile are collinear. We also calculate fission barriers for a long sequence of uranium isotopes. The aim is to understand another attempted production mechanism for superheavy elements, namely neutron-capture in thermonuclear explosions.

The CEM2k and LAQGSM codes have been recently developed at Los Alamos National Laboratory to simulate nuclear reactions for a number of applications. We have benchmarked our codes against most available measured data at incident particle energies from 10 MeV to 800 GeV and have compared our results with predictions of other current models used by the nuclear community. Here, we present a brief description of our codes and show illustrative results to show that CEM2k and LAQGSM can be used as reliable event generators for space-radiation-shielding, cosmic-ray-propagation, and other astrophysical applications. Finally, we show the use of our calculated cross sections together with experimental data from our LANL T-16 compilation to produce evaluated files which we use in the GALPROP model of galactic particle propagation to better constrain the size of the CR halo., 10 pages, 9 figures, LaTeX, talk given at the World Space Congress 2002, 34th COSPAR Scientific Assembly, Houston, Texas, USA, 10-19 October 2002, to appear in Advances in Space Research

A qualitative and quantitative comparison of the recent GSI measurements of the nuclide yields from 208 Pb and 238U at 1 GeV/nucleon and 197Au at 800 MeV/nucleon interactions with protons with the codes LAHET (with both ISABEL and Bertini options), CEM95, CEM97, CEM2k, CASCADE, CASCADE/INPE, YIELDX, HETC, and INUCL is presented The predictive power of these codes is reasonable for nuclides in the near spallation region but is worse for deep spallation and much worse in the fission region. None of these codes agree well with the data in the whole mass region of product nuclides and all must be improved to become reliable tools for accelerator-driven applications.

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.

We present calculations based on a realistic theoretical model of the multi-dimensional potential-energy surface of a fissioning nucleus. This surface guides the nuclear shape evolution from the ground state, over inner and outer saddle points, to the final configurations of separated fission fragments. Until recently, no calculation has properly explored a shape parameterization of sufficient dimensionality to permit the corresponding potential-energy surface to exhibit the multiple minima, valleys, saddle points and ridges that correspond to characteristic observables of the fission process. Here we calculate and analyze five-dimensional potential-energy landscapes based on grids of several million deformation points. We find that observed fission features such as different energy thresholds for symmetric and asymmetric fission and fission-fragment mass and kinetic-energy distributions are very closely related to properties of the valleys and mountain passes present in the calculated five-dimensional en...

An extended version of the cascade-exciton model (CEM) of nuclear reactions is applied to analyze nucleon-induced fission cross sections for {sup 209}Bi and {sup 208}Pb nuclei in the 45- to 500-MeV energy range. The available data on linear momentum transfer are analyzed as well. The results are compared with analytical approximations resulting from a comparative critical analysis of all available experimental data. Systematic discrepancies between calculations and experimental data are revealed. A modification of the CEM is proposed, which significantly improves the model predictions for projectile energies above 100 MeV.

We have used an extended version of the Cascade-Exciton Model (CEM) to analyze more than 600 excitation functions for proton induced reactions on 19 targets ranging from 12C to 197Au, for incident energies ranging from 10 MeV to 5 GeV. We have compared the calculations to available data, to calculations using approximately two dozen other models, and to predictions of several phenomenological systematics. We present here our conclusions concerning the relative roles of different reaction mechanisms in the production of specific final nuclides. We comment on the strengths and weaknesses of the CEM and suggest possible further improvements to the CEM and to other models.

Different reaction mechanisms contribute to the production of light fragments (LF) from nuclear reactions. Available models cannot accurately predict emission of LF from arbitrary reactions. However, the emission of LF is important for many applications, such as cosmic-ray-induced single event upsets, radiation protection, and cancer therapy with proton and heavy-ion beams, to name just a few. The cascade-exciton model (CEM) and the Los Alamos version of the quark-gluon string model (LAQGSM), as implemented in the CEM03.03 and LAQGSM03.03 event generators used in the Los Alamos transport code MCNP6, describe quite well the spectra of fragments with sizes up to 4He across a broad range of target masses and incident energies. However, they do not predict high-energy tails for LF heavier than 4He. The standard versions of CEM and LAQGSM do not account for preequilibrium emission of LF larger than 4He. The aim of our work is to extend the preequilibrium model to include such processes. We do this by including the emission of fragments heavier than 4He at the preequilibrium stage, and using an improved version of the Fermi Break-up model, providing improved agreement with various experimental data., 4 pages, 4 figures, Proc. International Nuclear Physics Conference INPC 2013, June 2-7, 2013, Florence, Italy, to be published in EPJ Web of Conferences

The following slides were presented as part of the LDRD 20120077DR Progress Appraisal Review held Tuesday, February 4, 2014. This is part of an ongoing project assessment the previous of which was documented in LA-UR-13-21182. This presentation documents the progress made against the goals agreed to as part of the 2013 review.

Emission of light fragments (LF) from nuclear reactions is an open question. Different reaction mechanisms contribute to their production; the relative roles of each, and how they change with incident energy, mass number of the target, and the type and emission energy of the fragments is not completely understood. None of the available models are able to accurately predict emission of LF from arbitrary reactions. However, the ability to describe production of LF (especially at energies $\gtrsim 30$ MeV) from many reactions is important for different applications, such as cosmic-ray-induced Single Event Upsets (SEUs), radiation protection, and cancer therapy with proton and heavy-ion beams, to name just a few. The Cascade-Exciton Model (CEM) version 03.03 and the Los Alamos version of the Quark-Gluon String Model (LAQGSM) version 03.03 event generators in Monte Carlo N-Particle Transport Code version 6 (MCNP6) describe quite well the spectra of fragments with sizes up to $^{4}$He across a broad range of target masses and incident energies (up to $\sim 5$ GeV for CEM and up to $\sim 1$ TeV/A for LAQGSM). However, they do not predict the high-energy tails of LF spectra heavier than $^4$He well. Most LF with energies above several tens of MeV are emitted during the precompound stage of a reaction. The current versions of the CEM and LAQGSM event generators do not account for precompound emission of LF larger than $^{4}$He. The aim of our work is to extend the precompound model in them to include such processes, leading to an increase of predictive power of LF-production in MCNP6. Extending our models to include emission of fragments heavier than $^4$He at the precompound stage has already provided preliminary results that have much better agreement with experimental data.

MCNP6 has been Validated and Verified (V&V) against intermediate- and high-energy fission cross-section experimental data. An error in the calculation of fission cross sections of 181Ta and a few nearby target nuclei by the CEM03.03 event generator in MCNP6 and a "bug: in the calculation of fission cross sections with the GENXS option of MCNP6 while using the LAQGSM03.03 event generator were detected during our V&V work. After fixing both problems, we find that MCNP6 using CEM03.03 and LAQGSM03.03 calculates fission cross sections in good agreement with available experimental data for reactions induced by nucleons, pions, and photons on both subactinide and actinide nuclei at incident energies from several tens of MeV to about 1 TeV., 3 pages, 3 figures, Proc. 2013 International Conference on Nuclear Data for Science & Technology (ND2013), March 4-8, 2013, New York, USA, to be published in Nuclear Data Sheets

For the past several years, a major effort has been undertaken at Los Alamos National Laboratory (LANL) to develop the transport code MCNP6, the latest LANL Monte-Carlo transport code representing a merger and improvement of MCNP5 and MCNPX. We emphasize a description of the latest developments of MCNP6 at higher energies to improve its reliability in calculating rare-isotope production, high-energy cumulative particle production, and a gamut of reactions important for space-radiation shielding, cosmic-ray propagation, and accelerator applications. We present several examples of validation and verification of MCNP6 compared to a wide variety of intermediate- and high-energy experimental data on reactions induced by photons, mesons, nucleons, and nuclei at energies from tens of MeV to about 1 TeV/nucleon, and compare to results from other modern simulation tools., Comment: 4 pages, 3 figures, Proc. 11th Conference on the Intersections of Particle and Nuclear Physics (CIPANP 2012), St. Petersburg, FL, May 28 - June 3, 2012

The double-humped structure of many actinide fission barriers is well established both experimentally and theoretically. There is also evidence, both experimental and theoretical, that some actinide nuclei have barriers with a third minimum, outside the second, fission-isomeric minimum. We perform a large-scale, systematic calculation of actinide fission barriers to identify which actinide nuclei exhibit third minima. We find that only a relatively few nuclei accessible to experiment exhibit third minima in their barriers: approximately nuclei with proton number $Z$ in the range $88\ensuremath{\le}Z\ensuremath{\le}94$ and nucleon number $A$ in the range $230\ensuremath{\le}A\ensuremath{\le}236$. We find that the third minimum is less than 1 MeV deep for light Th and U isotopes. This is consistent with some previous experimental and theoretical results, but it differs from some others. We discuss possible origins of these incompatible results and what are the most realistic predictions of where third minima are observable.

The recently measured probabilities of the $\ensuremath{\beta}$-delayed fission for ${}^{178,180}$Tl are used to deduce the fission-barrier heights of the daughter isotopes ${}^{178,180}$Hg, undergoing low-energy fission. Four alternative $\ensuremath{\beta}$-decay strength functions and four variants of the statistical model of de-excitation of the daughter nucleus are used to determine the fission-barrier height for ${}^{180}$Hg. Depending on the choice of the model, the deduced fission-barrier heights appear to be between 10$%$ and 40$%$ smaller than theoretical estimates. This observation is verified also for fission-barrier heights extracted using the probability of $\ensuremath{\beta}$-delayed fission of ${}^{178}$Tl. The spread in extracted fission-barrier heights results mainly from uncertainties in the magnitude of the pairing gap at the saddle configuration.

Fission-fragment mass distributions are asymmetric in fission of typical actinide nuclei for nucleon number $A$ in the range $228 \lnsim A \lnsim 258$ and proton number $Z$ in the range $90\lnsim Z \lnsim 100$. For somewhat lighter systems it has been observed that fission mass distributions are usually symmetric. However, a recent experiment showed that fission of $^{180}$Hg following electron capture on $^{180}$Tl is asymmetric. We calculate potential-energy surfaces for a typical actinide nucleus and for 12 even isotopes in the range $^{178}$Hg--$^{200}$Hg, to investigate the similarities and differences of actinide compared to mercury potential surfaces and to what extent fission-fragment properties, in particular shell structure, relate to the structure of the static potential-energy surfaces. Potential-energy surfaces are calculated in the macroscopic-microscopic approach as functions of fiveshape coordinates for more than five million shapes. The structure of the surfaces are investigated by use of an immersion technique. We determine properties of minima, saddle points, valleys, and ridges between valleys in the 5D shape-coordinate space. Along the mercury isotope chain the barrier heights and the ridge heights and persistence with elongation vary significantly and show no obvious connection to possible fragment shell structure, in contrast to the actinide region, where there is a deep asymmetric valley extending from the saddle point to scission. The mechanism of asymmetric fission must be very different in the lighter proton-rich mercury isotopes compared to the actinide region and is apparently unrelated to fragment shell structure. Isotopes lighter than $^{192}$Hg have the saddle point blocked from a deep symmetric valley by a significant ridge. The ridge vanishes for the heavier Hg isotopes, for which we would expect a qualitatively different asymmetry of the fragments., 8 pages, 9 figures

The recent unexpected discovery of asymmetric fission of ${}^{180}$Hg following the electron-capture decay of ${}^{180}$Tl has led to intense interest in experimentally mapping the fission-yield properties over more extended regions of the nuclear chart and compound-system energies. We present here a first calculation of fission-fragment yields for neutron-deficient Hg isotopes, using the recently developed Brownian Metropolis shape-motion treatment. The results for ${}^{180}$Hg are in approximate agreement with the experimental data. For ${}^{174}$Hg the symmetric yield increases strongly with decreasing energy, an unusual feature, which would be interesting to verify experimentally.

Random walks on five-dimensional potential-energy surfaces were recently found to yield fission-fragment mass distributions that are in remarkable agreement with experimental data. Within the framework of the Smoluchowski equation of motion, which is appropriate for highly dissipative evolutions, we discuss the physical justification for that treatment and investigate the sensitivity of the resulting mass yields to a variety of model ingredients, including in particular the dimensionality and discretization of the shape space and the structure of the dissipation tensor. The mass yields are found to be relatively robust, suggesting that the simple random walk presents a useful calculational tool. Quantitatively refined results can be obtained by including physically plausible forms of the dissipation, which amounts to simulating the Brownian shape motion in an anisotropic medium., Comment: 14 pages, 11 ps figures

We present calculations of fission properties for heavy elements. The calculations are based on the macroscopic-microscopic finite-range liquid-drop model with a 2002 parameter set. For each nucleus we have calculated the potential energy in three different shape parametrizations: (1) for 5 009 325 different shapes in a five-dimensional deformation space given by the three-quadratic-surface parametrization, (2) for 10 850 different shapes in a three-dimensional deformation space spanned by epsilon(2), epsilon(4), and gamma in the Nilsson perturbed-spheroid parametrization, supplemented by a densely spaced grid in epsilon(2), epsilon(3), epsilon(4), and epsilon(6) for axially symmetric deformations in the neighborhood of the ground state, and (3) an axially symmetric multipole expansion of the shape of the nuclear surface using beta(2), beta(3), beta(4), and beta(6) for intermediate deformations. For a fissioning system, it is always possible to define uniquely one saddle or fission threshold on the optimum trajectory between the ground state and separated fission fragments. We present such calculated barrier heights for 1585 nuclei from Z=78 to Z=125. Traditionally, actinide barriers have been characterized in terms of a "double-humped" structure. Following this custom we present calculated energies of the first peak, second minimum, and second peak in the barrier for 135 actinide nuclei from Th to Es. However, for some of these nuclei which exhibit a more complex barrier structure, there is no unique way to extract a double-humped structure from the calculations. We give examples of such more complex structures, in particular the structure of the outer barrier region near Th-232 and the occurrence of multiple fission modes. Because our complete results are too extensive to present in a paper of this type, our aim here is limited: (1) to fully present our model and the methods for determining the structure of the potential-energy surface, (2) to present fission thresholds for a large number of heavy elements, (3) to compare our results with the two-humped barrier structure deduced from experiment for actinide nuclei, and (4) to compare to additional fission-related data and other fission models. . (Less)

To determine which nuclei may exhibit shape isomerism, we use a well-benchmarked macroscopic-microscopic model to calculate potential-energy surfaces as functions of spheroidal (epsilon(2)), hexadecapole (epsilon(4)), and axial-asymmetry (gamma) shape coordinates for 7206 nuclei from A=31 to A=290. We analyze these and identify the deformations and energies of all minima deeper than 0.2 MeV. These minima may correspond to characteristic experimentally observable shape-isomeric states. Shape isomers mainly occur in the A=80 region, the A=100 region, and in an extended region centered around Pb-208. We compare our model to experimental results for Kr isotopes. Moreover, in a plot versus N and Z we show for each of the 7206 nuclei the calculated number of minima. The results reveal one fairly unexplored region of shape isomerism, which is experimentally accessible, namely the region northeast of Pb-208(82). (Less)

We introduce a droplet model to survey the interactions and reactions of blobs of QCD plasma in a mixed phase with low-density hadronic gas. We extend earlier work on the statistical mechanism of pionic emission by the droplets to include the fission of a large droplet into two smaller ones, and give quantitative estimates for the fission rate. We also consider the collisions of plasma droplets. By extending the proximity model to the case when the droplets are composed of relativistic particles, we show that collisions almost always lead to fusion of the plasma droplets, and give a simple estimate of the conditions necessary for fusion.

The MARS code is under continuous development and has recently undergone substantial improvements that further increase its reliability and predictive power in numerous shielding, accelerator, detector and space applications. The major developments and new features of the MARS15 (2004) version described in this paper concern an extended list of elementary particles and arbitrary heavy ions and their interaction cross sections, inclusive and exclusive nuclear event generators, module for modelling particle electromagnetic interactions, enhanced geometry and histogramming options, improved MAD-MARS Beam Line Builder, enhanced graphical user interface and an MPI-based parallelisation of the code.

An improved version of the Cascade-Exciton Model (CEM) of nuclear reactions realized in the code CEM2k and the Los Alamos version of the Quark-Gluon String Model (LAQGSM) have been developed recently at LANL to describe reactions induced by particles and nuclei for a number of applications. Our CEM2k and LAQGSM merged with the GEM2 evaporation/fission code by Furihata have predictive powers comparable to other modern codes and describe many reactions better than other codes; therefore both our codes can be used as reliable event generators in transport codes for applications. During the last year, we have made a significant improvements to the intranuclear cascade parts of CEM2k and LAQGSM, and have extended LAQGSM to describe photonuclear reactions at energies to 10 GeV and higher. We have produced in this way improved versions of our codes, CEM03.01 and LAQGSM03.01. We present a brief description of our codes and show illustrative results obtained with CEM03.01 and LAQGSM03.01 for different reactions compared with predictions by other models, as well as examples of using our codes as modeling tools for nuclear applications., 12 pages, 10 figures, to be published in Journal of Physics: Conference Series: Proc. Europhysics Conf. on New Trends in Nuclear Physics Applications and Technologies (NPDC19), Pavia, Italy, September 5-9, 2005

We estimate the fusion-barrier height ${B}_{\mathrm{fu}}^{\text{(two-body)}}$ for approaching ions in cold-fusion reactions in a model where the projectile deformation and quadrupole zero-point vibrational energy are taken into account. This barrier height is defined as the barrier energy at the target and projectile separation distance where an original oblate deformation of projectile and/or target caused by a repulsive Coulomb force turns into a large prolate deformation caused by the attractive nuclear force as the target and projectile come closer. The instability develops before touching because the attractive short-range nuclear force overcomes the repulsive Coulomb force and the shape-stabilizing effect of shell structure. The shell structure of the doubly magic $^{208}\mathrm{Pb}$ target is sufficiently strong that its shape remains very close to spherical in all cases studied here. The fusion potential for approaching ions in the two-body channel is calculated in the macroscopic-microscopic model with the quadrupole vibrational zero-point energy obtained in the WKB approximation. We compare our results with data from 10 experimental cold-fusion reactions and with the Bass barriers. Differences and similarities between the Yukawa-plus-exponential model and the Bass model are discussed. We also calculate five-dimensional potential-energy surfaces for the single compound system and show that well-established fission and fusion valleys are both present. For heavy systems, ${B}_{\mathrm{fu}}^{\text{(two-body)}}$ becomes lower than the fission barrier just beyond the ground state of the compound system. In the vicinity of this transition, the optimum collision energy for formation of evaporation residues can be expected to depend in a delicate fashion on the interplay among ${B}_{\mathrm{fu}}^{\text{(two-body)}}$, the fusion valley, the fission barrier of the compound system, and the one- and two-neutron separation energies ${S}_{1n}$ and ${S}_{2n}$. We discuss these issues in detail and calculate fission-barrier heights. Except for reactions in which the projectile is doubly magic or near doubly magic, the calculated quantities are consistent with the observed optimal energies for evaporation-residue formation.

An improved version of the Cascade-Exciton Model (CEM) of nuclear reactions implemented in the codes CEM2k and the Los Alamos version of the Quark-Gluon String Model (LAQGSM) has been developed recently at LANL to describe reactions induced by particles and nuclei at energies up to hundreds of GeV/nucleon for a number of applications. We present several improvements to the intranuclear cascade models used in CEM2k and LAQGSM developed recently to better describe the physics of nuclear reactions. First, we incorporate the photonuclear mode from CEM2k into LAQGSM to allow it to describe photonuclear reactions, not previously modeled there. Then, we develop new approximations to describe more accurately experimental elementary energy and angular distributions of secondary particles from hadron-hadron and photon-hadron interactions using available data and approximations published by other authors. Finally, to consider reactions involving very highly excited nuclei (E* > 2-3 MeV/A), we have incorporated into CEM2k and LAQGSM the Statistical Multifragmentation Model (SMM), as a possible reaction mechanism occurring after the preequilibrium stage. A number of other refinements to our codes developed recently are also listed., 5 pages, pdf, 6 figures, to be published in Proc. Int. Conf. on Nuclear Data for Science & Technology (ND2004), Santa Fe, USA, Sept. 26 - Oct. 1, 2004

Isotopically enriched 129I (85% 129I and 15% 127I) targets were irradiated with a beam of 660‐MeV protons at the JINR DLNP Phasotron and cross sections of formation of 74 residual products were determined using the γ‐spectrometry method. Here, we analyze all these data using eleven different models, realized in eight codes: LAHET (Bertini, ISABEL, INCL+ABLA, and INCL+RAL options), CASCADE, CEM95, CEM2k, LAQGSM+GEM2, CEM2k+GEM2, LAQGSM+GEMINI, and CEM2k+GEMINI, in order to validate the tested models against the experimental data and to understand better the mechanisms for production of residual nuclei. We find that most of the codes are fairly reliable in predicting cross sections for nuclides not too far away in mass from the targets, but differ greatly in the deep spallation, fission, and fragmentation regions. None of the codes tested here except GEMINI allow fission of nuclei as light as iodine, therefore the best agreement with the 129I data, especially in the A=40–90 region, is shown by the codes CEM2k and LAQGSM when they are merged with GEMINI. We conclude that none of the codes tested here are able to reproduce well all these data and all of them need to be further improved; development of a better universal evaporation/fission model should be of a high priority.

The MARS code system, developed over 30 years, is a set of Monte Carlo programs for detailed simulation of hadronic and electromagnetic cascades in an arbitrary geometry of accelerator, detector and spacecraft components with particle energy ranging from a fraction of an electron volt up to 100 TeV. The new MARS15 (2004) version is described with an emphasis on modeling physics processes. This includes an extended list of elementary particles and arbitrary heavy ions, their interaction cross-sections, inclusive and exclusive nuclear event generators, photo-hadron production, correlated ionization energy loss and multiple Coulomb scattering, nuclide production and residual activation, and radiation damage (DPA). In particular, the details of a new model for leading baryon production and implementation of advanced versions of the Cascade-Exciton Model (CEM03), and the Los Alamos version of Quark-Gluon String Model (LAQGSM03) are given. The applications that are motivating these developments, needs for better nuclear data, and future physics improvements are described.

Double-differential cross sections of neutron production at angles from 0 to 110 degrees from many reactions induced by light and medium nuclei on targets from 12-C to 208-Pb, at several incident energies from 95 to 600 MeV/A have been measured recently at the Institute of Physical and Chemical Research (RIKEN) Ring Cyclotron in Japan and at the Heavy-Ion Medical Accelerator of the National Institute of Radiological Science in Chiba, Japan using the time-of-flight technique. We have analyzed all these new measurements using the Quantum Molecular Dynamics (QMD) model, the Oak Ridge intranuclear cascade model HIC, the ISABEL intranuclear cascade model from LAHET, and the Los Alamos version of the Quark-Gluon String Model code LAQGSM03. On the whole, all four models used here describe reasonably well most of the measured neutron spectra, although different models agree differently with data from specific reactions and some serious discrepances are observed for some reactions. We present here some illustrative results from our study, discuss possible reasons for some of the observed discrepancies and try to outline ways to further improve the tested codes in order to address these problems., Comment: 5 pages, pdf, 3 figures, to be published in Proc. Int. Conf. on Nuclear Data for Sciience & Technology (ND2004), Santa Fe, USA, Sept. 26 - Oct. 1, 2004

Recently we have performed large-scale calculations of fission barriers in the actinide region based on five-dimensional deformation spaces with more than 3,000,000 deformation points for each potential-energy surface. We have determined new model constants. We have also extended our model to axially asymmetric shapes. We apply these techniques to the calculations of fission barriers of heavy nuclei from the line of beta stability to the r-process line. The aim is to study fission near the end of the r-process. We have also extended our model of {beta}-decay so that allowed Gamow-Teller transitions are treated in a quasi-particle random-phase approximation as earlier, but we now also consider first-forbidden transitions in the statistical gross theory. We discuss the properties of this enhanced model and present results of global calculations.

We present fission-barrier-height calculations for nuclei throughout the periodic table based on a realistic macroscopic-microscopic model. Compared to other calculations (i) we use a deformation space of a sufficiently high dimension, sampled densely enough to describe the relevant topography of the fission potential, (ii) we unambiguously find the physically relevant saddle points in this space, and (iii) we formulate our model so that we obtain continuity of the potential energy at the division point between a single system and separated fission fragments or colliding nuclei, allowing us to (iv) describe both fission-barrier heights and ground-state masses throughout the periodic table.

We calculate in a macroscopic‐microscopic model fission‐fusion potential‐energy surfaces relevant to the analysis of heavy‐ion reactions employed to form heavy‐element evaporation residues. We study these multidimensional potential‐energy surfaces both inside and outside the touching point.Inside the point of contact we define the potential on a multi‐million‐point grid in 5D deformation space where elongation, merging projectile and target spheroidal shapes, neck radius and projectile/target mass asymmetry are independent shape variables. The same deformation space and the corresponding potential‐energy surface also describe the shape evolution from the nuclear ground‐state to separating fragments in fission, and the fast‐fission trajectories in incomplete fusion.For separated nuclei we study the macroscopic‐microscopic potential energy, that is the “collision surface” between a spheroidally deformed target and a spheroidally deformed projectile as a function of three coordinates which are: the relative ...

We present fission-barrier-height calculations for nuclei throughout the periodic table based on a realistic macroscopic-microscopic model. Compared to other calculations (i) we use a deformation space of a sufficiently high dimension, sampled densely enough to describe the relevant topography of the fission potential, (ii) we unambiguously find the physically relevant saddle points in this space, and (iii) we formulate our model so that we obtain continuity of the potential energy at the division point between a single system and separated fission fragments or colliding nuclei, allowing us to (iv) describe both fission-barrier heights and ground-state masses throughout the periodic table.

Recent developments of the Cascade-Exciton Model (CEM) of nuclear reactions are described. The improved cascade-exciton model as implemented in the code CEM97 differs from the CEM95 version by incorporating new approximations for the elementary cross sections used in the cascade, using more precise values for nuclear masses and pairing energies, using corrected systematics for the level-density parameters, and several other refinements. We have improved algorithms used in many subroutines, decreasing the computing time by up to a factor of 6 for heavy targets. We describe a number of further improvements and changes to CEM97, motivated by new data on isotope production measured at GSI. This leads us to CEM2k, a new version of the CEM code. CEM2k has a longer cascade stage, less preequilibrium emission, and evaporation from more highly excited compound nuclei compared to earlier versions. CEM2k also has other improvements and allows us to better model neutron, radionuclide, and gas production in ATW spallation targets. The increased accuracy and predictive power of the code CEM2k are shown by several examples. Further necessary work is outlined., Invited talk, Int. Conf. on Nuclear Data for Science and Technology (ND2001), October 7-12, 2001, Tsukuba, Ibaraki, Japan, 6 pages, 7 figures, only pdf file

114 cross sections for nuclide production in a 1.0 GeV proton-irradiated thin 208Pb target have been measured by the direct gamma spectrometry method using a high-resolution Ge detector. The gamma spectra were processed by the GENIE-2000 code. The ITEP-developed SIGMA code was used together with the PCNUDAT nuclear decay database to identify the gamma lines and to determine the cross sections. The 27Al(p,x)22Na reaction was used to monitor the proton flux. Results of a feasibility study of the auxiliary 27Al(p,x)24Na and 27Al(p,x)7Be monitor reactions in the 0.07-2.6 GeV proton-energy range are presented as well. Most of the experimental data have been analyzed by the LAHET (with ISABEL and Bertini options), CEM95, CEM2k, INUCL, CASCADE, CASCADE/INPE, and YIELDX codes that simulate hadron-nucleus interactions.

Considering the prospects of using the W-Na target assemblies in ADS facilities, the experiments were made to study the nuclear-physics characteristics of W and Na, and the composite structures thereof in their interactions with 0.8-GeV and 1.6-GeV protons. The neutron and proton-induced reaction rates were measured inside, and on the surface of, a cylinder-shaped heterogeneous W-Na assembly together with the double-differential spectra of secondary neutrons emitted from different-depth W and Na discs. The measurement results were simulated by the LAHET, CEM2k, and KASKAD-S codes in terms of the latest versions of nuclear databases., Comment: Proc. Int. Conf. on Nuclear Data for Science and Technology (ND2001), October 7-12, 2001, Tsukuba, Ibaraki, Japan, 4 pages, 5 figures, only pdf file

Nuclei undergoing fission can be described by a multi-dimensional potential-energy surface that guides the nuclear shape evolution--from the ground state, through intermediate saddle points and finally to the configurations of separated fission fragments. Until now, calculations have lacked adequate exploration of the shape parameterization of sufficient dimensionality to yield features in the potential-energy surface (such as multiple minima, valleys, saddle points and ridges) that correspond to characteristic observables of the fission process. Here we calculate and analyse five-dimensional potential-energy landscapes based on a grid of 2,610,885 deformation points. We find that observed fission features--such as the distributions of fission fragment mass and kinetic energy, and the different energy thresholds for symmetric and asymmetric fission--are very closely related to topological features in the calculated five-dimensional energy landscapes.

The Monte Carlo particle transport code MCNPX and its associated data have been the focus of a major development effort at Los Alamos for several years. The system has reached a mature state, and has become a significant tool for many intermediate and high-energy particle transport applications. A recent version has been released to the Radiation Safety Information Computational Center (RSICC). A recent report provides an overview of the code and an extensive set of references for the component physics modules used in the code. In this paper we review the status of the developmental version of MCNPX, and describe some important new enhancements, including the use of evaluated nuclear data files for proton transport; the use of photonuclear reaction data; improved elastic and inelastic react ion cross sections for nucleons, antinucleons, pions, and kaons; and two new modes of operation of the code. We also illustrate the use of the new proton and photonuclear data in two representative applications.