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The Atomic Nucleus
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
The chance that a given atomic nucleus will be struck by a particle passing through a material depends on the ‘target area’ which the nucleus presents to the bombardment. From the diameters of nuclei we expect such target areas, called cross-sections, to be based on the order of magnitude of 10−28 m2. This in fact is the barn, the unit of nuclear cross-section. Consider a material with a cross-section σ m2 exposed to a flux of particles ϕ m−2 s−1, over a time t. The proportion of its nuclei which suffer collisions is then σϕt. The fission cross-section for natural uranium exposed to slow neutrons (kinetic energy ≃ 0.025 eV) is about 4 barns. Hence, in 1 month (≃ 2.6 × 106 sec) in a power reactor (e.g. ϕ=1017m−2s−1) about 1 atom in 104 in natural uranium undergoes fission. Each fission produces 2 atoms, about one-tenth of which are noble gases.
Nuclear Cross Sections, Reaction Probabilities, and Reaction Rates
Published in Robert E. Masterson, Introduction to Nuclear Reactor Physics, 2017
Cross sections can be defined for any type of process that involves a collision between two nuclear or subatomic particles, including the scattering of photons, the scattering of electrons, and the absorption of a neutron by an atomic nucleus. To determine the value of a nuclear cross section, one simply points a beam of particles with a well-defined energy at an object and measures the attenuation of the beam as it passes through the object. The object is then referred to as the “target.” By convention, most targets are constructed from materials 1 cm thick and the attenuation of the beam depends on the type of material that is used.
Understanding the Atom and the Nucleus
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
In nuclear physics, the probability that a nuclear particle interacts with another nuclear particle is called a “nuclear cross section.” For historical reasons, the probability of absorption or emission is measured in nuclear units called “barns.” Hence, nuclear cross sections are also measured in “barns.” Figure 1.5 shows some nuclear interaction cross sections as a function of energy (or indirectly the speed of a neutron) for both fast and slow neutrons in the vicinity of the nucleus of a boron atom. In general, slow moving neutrons are approximately 100 times more likely to be absorbed than very fast moving neutrons are. Nuclear reaction probabilities were historically measured in the units of barns because people believed that getting two protons or neutrons to interact together was like trying to hit “the broad side of a barn.” This terminology became very popular in the 1940s, and it has continued to exist until the current day. From a reactor designer’s point of view, the control of neutrons, including their creation, reabsorption, and reemission from the nuclei of a couple of very specific atoms, is the most important facet of building an operational nuclear reactor. Before we discuss how neutrons can be manipulated (by changing their speed), we would like to mention that protons and neutrons are not fundamental particles—at least as far as the universe is concerned.
Strategical Approach for the Neutronics in the European Fusion Programme
Published in Fusion Science and Technology, 2021
D. Leichtle, U. Fischer, C. Bachmann
A dedicated programme on nuclear data development and experimental validation is implemented to satisfy the needs for high-quality fusion nuclear data. It addresses the evaluation, processing, and benchmarking of nuclear cross-section data with support by cross-section measurements and integral validation experiments. The required tools for model calculations and sensitivity/uncertainty assessments are being continuously improved. After successfully passing a thorough benchmarking and validation process, the cross-section data evaluations are eventually fed into the Joint Evaluated Fusion and Fission File18 (JEFF), which serves as the reference neutron cross-section data library for the DEMO project. As an alternative, the FENDL library19 is adopted in alignment with the ITER project; it is the reference library for the IFMIF-DONES project. The reference neutron activation data library is JEFF-3.3 (Ref. 20), which is based entirely on the TENDL-2017 library,21 superseding the previous EAF-2007/2010 (Ref. 22). Further special data libraries are provided for gas production and displacement damage calculations. These kinds of data serve obviously as response functions, thus defining nuclear analysis output quantities.
A Study on Interactions of 14.7-MeV Protons and 3.6-MeV Alphas in 93Nb Target
Published in Fusion Science and Technology, 2023
The most important quantity for nuclear reaction investigations is the nuclear cross section. The cross section in the nuclear reaction codes depends explicitly on the chosen input parameters. The TALYS code allows for making cross-section calculations based on different nuclear input selections, such as nuclear masses, level densities, gamma-strength functions, excitation spectra, the width fluctuation correction factor, and the optical model potential.