S. G. Mashnik

Initial MCNP6 Release Overview

MCNP6 is simply and accurately described as the merger of MCNP5 and MCNPX capabilities, but it is much more than the sum of those two computer codes. MCNP6 is the result of five years of effort by the MCNP5 and MCNPX code development teams. These groups of people, residing in Los Alamos National Laboratory’s (LANL) X Computational Physics Division, Monte Carlo Codes Group (XCP-3), and Decision Applications Division, Radiation Transport and Applications Team (D-5), respectively, have combined their code development efforts to produce the next evolution of MCNP. While maintenance and bug fixes will continue for MCNP5 1.60 and MCNPX 2.7.0 for upcoming years, new code development capabilities only will be developed and released in MCNP6. In fact, the initial release of MCNP6 contains 16 new features not previously found in either code. These new features include the abilities to import unstructured mesh geometries from the finite element code Abaqus, to transport photons down to 1.0 eV, to transport electrons down to 10.0 eV, to model complete atomic relaxation emissions, and to generate or read mesh geometries for use with the LANL discrete ordinates code Partisn. The first release of MCNP6, MCNP6 Beta 2, is now available through the Radiation Safety Information Computational Center, and the first production release is expected in calendar year 2012. High confidence in the MCNP6 code is based on its performance with the verification and validation test suites, comparisons to its predecessor codes, the regression test suite, its code development process, and the underlying high-quality nuclear and atomic databases.

Challenging cosmic-ray propagation with antiprotons : evidence for a “fresh” nuclei component?

Recent measurements of the cosmic-ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary/primary nuclei ratios (e.g., boron/carbon) produce too few antiprotons. Matching both the secondary/primary nuclei ratio and the antiproton flux requires artificial breaks in the diffusion coefficient and the primary injection spectrum, suggesting the need for other approaches. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh, local, ‘‘ unprocessed ’’ component at low energies, perhaps associated with the Local Bubble, thus decreasing the measured secondary/primary nuclei ratio. The independent evidence for supernova activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z � 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three ‘‘ radioactive clock ’’ isotopes in CRs, 10 Be, 26 Al, and 36 Cl, are all consistent and indicate a halo size zh � 4 kpc, based on the most accurate data taken by the ACE spacecraft. Subject headings: cosmic rays — diffusion — elementary particles — Galaxy: general — ISM: general — nuclear reactions, nucleosynthesis, abundances

CEM03 and LAQGSM03—new modeling tools for nuclear applications

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. For special studies, we have also merged our two codes with the GEMINI code by Charity and with the SMM code of Botvina. 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.

CEM03.03 and LAQGSM03.03 Event Generators for the MCNP6, MCNPX, and MARS15 Transport Codes

A calorimetric-time-of-flight technique was used for real-time, high-precision measurement of neutron spectra at an angle of 175 from the initial proton beam direction, which hits a face plane of a cylindrical lead target of 20 cm in diameter and 25 cm thick. A comparison was performed between the neutron spectra predicted by the MARS, RTS&T, MCNP6, and the MCNPX 2.6.0 transport codes and that measured for 200, 400, 600, 800, and 1000 MeV protons. Neutron spectra were measured within the energy range from 0.7 to 250 MeV almost continuously. The transport codes tested here describe with different success the measured spectra, depending on the energy of the detected neutrons and on the incident proton energy, but all the models agree reasonably well with our data. 2010 Elsevier B.V. All rights reserved.

Recent developments of the cascade-exciton model of nuclear reactions

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.

Cross-sections for nuclide production in 1-GeV proton-irradiated Pb-208

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.

Development and Validation of the 7Li(p, n) Nuclear Data Library and Its Application in Monitoring of Intermediate Energy Neutrons

Systematics have been created for neutron spectra from the 7Li(p, n) reaction at 0° in the 50-200 MeV proton energy region. The available experimental data in the continuum part of the spectra show satisfactory overall agreement with a representation based on the phase-space distribution corresponding to the three-body breakup process 7Li(p, n 3He)α, with empirical correction factors, which depend regularly on incident energy. Validation of the systematics included folding of the predicted neutron spectra with standard 238U neutron fission cross section. Modeled in this way distributions of neutron-induced fission events agree reasonably with experimental data.

Proton-induced cross sections relevant to production of 225Ac and 223Ra in natural thorium targets below 200 MeV

Cross sections for (223,)(225)Ra, (225)Ac and (227)Th production by the proton bombardment of natural thorium targets were measured at proton energies below 200 MeV. Our measurements are in good agreement with previously published data and offer a complete excitation function for (223,)(225)Ra in the energy range above 90 MeV. Comparison of theoretical predictions with the experimental data shows reasonable-to-good agreement. Results indicate that accelerator-based production of (225)Ac and (223)Ra below 200 MeV is a viable production method.

225Ac and 223Ra production via 800 MeV proton irradiation of natural thorium targets

Cross sections for the formation of (225,227)Ac, (223,225)Ra, and (227)Th via the proton bombardment of natural thorium targets were measured at a nominal proton energy of 800 MeV. No earlier experimental cross section data for the production of (223,225)Ra, (227)Ac and (227)Th by this method were found in the literature. A comparison of theoretical predictions with the experimental data shows agreement within a factor of two. Results indicate that accelerator-based production of (225)Ac and (223)Ra is a viable production method.

Cross sections for nuclide production in a Fe-56 target irradiated by 300, 500, 750, 1000, 1500, and 2600 MeV protons compared with data on a hydrogen target irradiated by 300, 500, 750, 1000, and 1500 MeV/nucleon Fe-56 ions

This work presents the cross sections for radioactive nuclide production in Fe-56( p, x) reactions determined in six experiments using 300, 500, 750, 1000, 1500, and 2600 MeV protons of the externa ...