The impact-induced deposition of Al13 clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis–Sinclair potentials. The incident kinetic energy (Ein) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However, the original icosahedral structure was transformed to the fcc-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 11 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying.

Nuclear instruments and methods in physics research section B : beam interactions with materials and atoms

We prepare high quality actinide targets for studies of neutron induced and charged particle induced fission. I report on our efforts to measure fragment energy loss in the target backings and to diagnose the crud problem frequently found in 248Cm and 252Cf sources and targets. I discuss the preparation of multi-isotopic targets for the Fission TPC and our efforts to measure the pointing resolution of this device. The issues of target uniformity, chemical composition and radiation stability of the targets are discussed along with problems of high/low specific activity regions in a single target.

High Quality Actinide Targets

From nuclear materials accountability to detection of special nuclear material, SNM, the need for better modeling of fission has grown over the past decades. Current radiation transport codes compute average quantities with great accuracy and performance, but performance and averaging come at the price of limited interaction-by-interaction modeling. For fission applications, these codes often lack the capability of modeling interactions exactly: energy is not conserved, energies of emitted particles are uncorrelated, prompt fission neutron and photon multiplicities are uncorrelated. Many modern applications require more exclusive quantities than averages, such as the fluctuations in certain observables (e.g. the neutron multiplicity) and correlations between neutrons and photons. The new computational model, FREYA (Fission Reaction Event Yield Algorithm), aims to meet this need by modeling complete fission events. Thus it automatically includes fluctuations as well as correlations resulting from conservation of energy and momentum. FREYA has been integrated into the LLNL Fission Library, and will soon be part of MCNPX2.7.0 , MCNP6 , TRIPOLI-4.9 , and Geant4.10 . Program summary Program title: FREYA 1.0 Catalogue identifier: AEVS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEVS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Special LLNL licence No. of lines in distributed program, including test data, etc.: 187792 No. of bytes in distributed program, including test data, etc.: 2585089 Distribution format: tar.gz Programming language: Fortran 90, C++. Computer: Any computer with a Fortran 90 and a C++ compiler, tested with (a) Intel Xeon CPU X5660, 2.8 GHz, 48 GB RAM, (b) Intel(R) Xeon(R) CPU E5-2620, 2 GHz, 64 GB RAM, (c) Intel Xeon CPU W3520, 2.67 GHz, 6 GB RAM. Operating system: (a) Red Hat Enterprise Linux Server release 6.5 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)), (b) CentOS release 6.4 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)), (c) MacBook Pro OSX 10.6.8 (GNU Fortran and g++-mp-4.7 (MacPorts gcc47 4.7.3 5) 4.7.3). RAM: 6 GB Classification: 17.8. Nature of problem: Modeling of fission events. Solution method: Simulation of complete fission events, production of secondary fission fragments, fission neutrons and photons. Restrictions: Restricted to spontaneous fission of 238 U, 240 Pu, 244 Cm, 252 Cf; neutron-induced fission of 233 U, 235 U, 239 Pu, for incident neutron energies less than 20 MeV. Running time: 8 s for 1M events

Fission Reaction Event Yield Algorithm -- FREYA -- For Event-by -Event Simulation of Fission

. A new evaluation of spectrum averaged cross sections (SACS) of 235 U, 238 U, and 239 Pu measured in the 252 Cf(sf) reference neutron ﬁeld is presented and found to be consistent with the original Mannhart SACS evaluation in IRDF-2002. The comprehensive vetted experimental database that includes SACS ratio and absolute SACS measurements of major actinides is being used to update the SACS database employed as input of the new GMApy code to derive the Neutron Standards. An update of the current neutron standards based on Time Projection Chamber (TPC) shape data, a new comprehensive uncertainty quantiﬁcation, and revised SACS experimental database is proposed which result in a 0.7% increase of the evaluated 239 Pu(n,f) / 235 U(n,f) cross-section ratio in the 1–5 MeV energy region. The increase is due to a 0.3% reduction of the standard 235 U(n,f) cross section and a 0.4% increase of the 239 Pu(n,f) reference cross section in the 1–5 MeV energy region. Those changes are well within estimated USU ﬁssion cross-section uncertainties of 1.2%, but are relevant for the evaluated mean values.

Experimental spectrum averaged cross sections (SACS) in 252 Cf(sf) neutron ﬁeld and its impact on the evaluation of neutron standards

While nuclear data play an important role in nuclear physics applications, it has become important to have a better understanding of the data and try to minimize the uncertainties. In particular, there is a need for precision neutron-induced fission cross section measurements on fissile nuclei. Neutron-induced fission cross sections are typically measured as ratios, with a well-known standard in the denominator. While the 235U(n,f) reaction is a well measured standard, some light particle reactions are also well-known and their use as reference can provide information to remove shared systematic uncertainties that are present in an actinide-only ratio. A recent measurement of the 235U(n,f) reaction using as a reference the standard 6Li(n,t) reaction, was conducted at the Los Alamos Neutron Science Center using the NIFFTE collaboration’s fission time projection chamber (fissionTPC). The fissionTPC is a 2×2π charged particle tracker designed for measuring neutron-induced fission. Detailed 3D track reconstruction of the reaction products enables evaluation of systematic effects and corresponding uncertainties which are less directly accessible by other measurement techniques. This work focuses on the analysis for the event identification of the 6Li(n,t)α reaction in the fissionTPC.

pdf/6li-n-t-a-reaction-event-identification-for-the-235u-n-f-6li-ywcjdnp5.pdf

6Li(n,t)α reaction event-identification for the 235U(n,f)/6Li(n,t) cross section ratio measurement in the NIFFTE fissionTPC

The $^{239}$Pu(n,f)/$^{235}$U(n,f) cross-section ratio has been measured with the fission Time Projection Chamber (fissionTPC) from 100 keV to 100 MeV. The fissionTPC provides three-dimensional reconstruction of fission-fragment ionization profiles, allowing for a precise quantification of measurement uncertainties. The measurement was performed at the Los Alamos Neutron Science Center which provides a pulsed white source of neutrons. The data are recommended to be used as a cross-section ratio shape. A discussion of the status of the absolute normalization and comparisons to ENDF evaluations and previous measurements is included.

Measurement of the $^{239}$Pu(n,f)/$^{235}$U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber

We present the results of a measurement of isotopic concentrations and atomic number ratio of a double-sided actinide target with alpha-spectroscopy and mass spectrometry. The double-sided actinide target, with primarily Pu-239 on one side and U-235 on the other, was used in the fission Time Projection Chamber (fissionTPC) for a measurement of the neutron-induced fission cross-section ratio between the two isotopes. The measured atomic number ratio is intended to provide an absolute normalization of the measured fission cross-section ratio. The Pu-239/U-235 atom number ratio was measured with a combination of mass spectrometry and alpha-spectroscopy with a planar silicon detector with uncertainties of less than 1%.

Measurement of material isotopics and atom number ratio with $\alpha$-particle spectroscopy for the NIFFTE fission Time Projection Chamber actinide target

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Measurement of the $^{239}$Pu(n,f)/$^{235}$U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber

Measurement of material isotopics and atom number ratio with $\alpha$-particle spectroscopy for the NIFFTE fission Time Projection Chamber actinide target