Improved modelling of helium and tritium production for spallation targets

Reliable predictions of light charged particle production in spallation reactions are important to correctly assess gas production in spallation targets. In particular, the helium production yield is important for assessing damage in the window separating the accelerator vacuum from a spallation target, and tritium is a major contributor to the target radioactivity. Up to now, the models available in the MCNPX transport code, including the widely used default option Bertini-Dresner and the INCL4.2-ABLA combination of models, were not able to correctly predict light charged particle yields. The work done recently on both the intranuclear cascade model INCL4, in which cluster emission through a coalescence process has been introduced, and on the de-excitation model ABLA allows correcting these deficiencies. This paper shows that the coalescence emission plays an important role in the tritium and $^3He$ production and that the combination of the newly developed versions of the codes, INCL4.5-ABLA07, now lead to good predictions of both helium and tritium cross sections over a wide incident energy range. Comparisons with other available models are also presented.Comment: 6 pages, 9 figure

Transport mechanism through metal-cobaltite interfaces

The resistive switching (RS) properties as a function of temperature were studied for Ag/La$_{1-x}$Sr$_x$CoO$_3$ (LSCO) interfaces. The LSCO is a fully-relaxed 100 nm film grown by metal organic deposition on a LaAlO$_3$ substrate. Both low and a high resistance states were set at room temperature and the temperature dependence of their current-voltage (IV) characteristics was mea- sured taking care to avoid a significant change of the resistance state. The obtained non-trivial IV curves of each state were well reproduced by a circuit model which includes a Poole-Frenkel element and two ohmic resistances. A microscopic description of the changes produced by the RS is given, which enables to envision a picture of the interface as an area where conductive and insulating phases are mixed, producing Maxwell-Wagner contributions to the dielectric properties.Comment: 13 pages, 5 figures, to be published in APL. Corresponding author: C. Acha ([email protected]

Influence of nuclear de-excitation on observables relevant for space exploration

The composition of the space radiation environment inside spacecrafts is modified by the interaction with shielding material, with equipment and even with the astronauts' bodies. Accurate quantitative estimates of the effects of nuclear reactions are necessary, for example, for dose estimation and prediction of single-event-upset rates. To this end, it is necessary to construct predictive models for nuclear reactions, which usually consist of an intranuclear-cascade or quantum-molecular-dynamics stage, followed by a nuclear-de-excitation stage. While it is generally acknowledged that it is necessary to accurately simulate the first reaction stage, transport-code users often neglect or underestimate the importance of the choice of the de-excitation code. The purpose of this work is to prove that the de-excitation model is in fact a non-negligible source of uncertainty for the prediction of several observables of crucial importance for space applications. For some particular observables, the systematic uncertainty due to the de-excitation model actually dominates the total uncertainty. Our point will be illustrated by making use of nucleon-nucleus calculations performed with several intranuclear-cascade/de-excitation models, such as the Li\`{e}ge Intranuclear Cascade model (INCL) and Isabel (for the cascade part) and ABLA07, Dresner, GEM, GEMINI++ and SMM (on the de-excitation side).Comment: 12 pages, 6 figures. Presented at the 38th COSPAR Scientific Assembly (Bremen, Germany, 18-25 July 2010). Submitted to Advances in Space Researc

Shell structure and few-nucleon removal in intranuclear cascade

It is well known that intranuclear-cascade models generally overestimate the cross sections for one-proton removal from heavy, stable nuclei by a high-energy proton beam, but they yield reasonable predictions for one-neutron removal from the same nuclei and for one-nucleon removal from light targets. We use simple shell-model calculations to investigate the reasons of this deficiency. We find that a correct description of the neutron skin and of the energy density in the nuclear surface is crucial for the aforementioned observables. Neither ingredient is sufficient if taken separately.Comment: Presented at the 11th International Spring Seminar on Nuclear Physics. To be published in Journal of Physics: Conference Serie

Extension of the Li\`ege Intranuclear-Cascade model to reactions induced by light nuclei

The purpose of this paper is twofold. First, we present the extension of the Li\`ege Intranuclear Cascade model to reactions induced by light ions. Second, we describe the C++ version of the code, which it is physics-wise equivalent to the legacy version, is available in Geant4 and will serve as the basis for all future development of the model. We describe the ideas upon which we built our treatment of nucleus-nucleus reactions and we compare the model predictions against a vast set of heterogeneous experimental data. In spite of the discussed limitations of the intranuclear-cascade scheme, we find that our model yields valid predictions for a number of observables and positions itself as one of the most attractive alternatives available to Geant4 users for the simulation of light-ion-induced reactions.Comment: Submitted to Phys. Rev.

Improving proton-induced one-nucleon removal in intranuclear cascade

It is a well-established fact that intranuclear-cascade models generally fail to consistently reproduce the cross sections for one-proton and one-neutron removal from stable nuclei by a high-energy proton beam. We use simple shell-model calculations to investigate the reasons of this deficiency. We find that a refined description of the neutron skin and of the energy density in the nuclear surface is crucial for the aforementioned observables, and that neither ingredient is sufficient if taken separately. As a by-product, the predictions for removal of several nucleons are also improved by the refined treatment.Comment: 15 pages, 11 figures. Submitted to Phys. Rev.

Continuum elastic sphere vibrations as a model for low-lying optical modes in icosahedral quasicrystals

The nearly dispersionless, so-called "optical" vibrational modes observed by inelastic neutron scattering from icosahedral Al-Pd-Mn and Zn-Mg-Y quasicrystals are found to correspond well to modes of a continuum elastic sphere that has the same diameter as the corresponding icosahedral basic units of the quasicrystal. When the sphere is considered as free, most of the experimentally found modes can be accounted for, in both systems. Taking into account the mechanical connection between the clusters and the remainder of the quasicrystal allows a complete assignment of all optical modes in the case of Al-Pd-Mn. This approach provides support to the relevance of clusters in the vibrational properties of quasicrystals.Comment: 9 pages without figure

Comparison of phenolic and volatile profiles of edible and toxic forms of Detarium senegalense J. F. GMEL

In Senegal, Detarium senegalense J.F. Gmel. (ditax in Wolof) is one of the most important important forest fruit species. However, exploitation of the edible fruit is based on local people's knowledge. Only trees whose fruits are consumed by animals are exploited. To identify them, a chemical comparison of edible and toxic forms was done in order to highlight differences between both forms. Dichloromethane leaf extracts from toxic and edible trees were analyzed by gas chromatography. Phenolic profile and volatile compounds from fruits extracts were studied respectively by High Performance Liquid Chromatography-mass spectrometry (HPLC-MS) and Gas Chromatography Mass Spectrometry (GC-MS). Cytotoxicity effect of fruits extracts was evaluated on murine macrophage cells J774 A1. GC-analysis of dichloromethane leaf extracts revealed the presence of lupenone and lupeol only in toxic extracts. 6'-O-galloyl-epiheterodendrin and isovaleronitrile were detected in toxic pulp. However, no cytotoxic effect was found in our conditions. This study has given the opportunity to identify within the same species, compounds which could differentiate both edible and toxic forms. Nevertheless further studies are needed to better understand which compounds are responsible for toxicity in the toxic form. (Résumé d'auteur