Absorption Branching Ratios Research Articles

Neutronics simulations for the design of neutron flux monitors in SPARC.

This paper presents the development and application of high-fidelity neutronic models of the SPARC tokamak for the design of neutron flux monitors (NFM) for application during plasma operations. NFMs measure the neutron flux in the tokamak hall, which is related to fusion power via calibration. We have explored Boron-10 gamma-compensated ionization chambers (ICs) and parallel-plate Uranium-238 fission chambers (FCs). We plan for all NFMs to be located by the wall in the tokamak hall and directly exposed to neutrons streaming through a shielded opening in a midplane port. Our simulations primarily use a constructive solid geometry-based OpenMC model based on the true SPARC geometry. The OpenMC model is benchmarked against a detailed CAD-based MCNP6 model. The B10 ICs are equipped with high-density polyethylene (HDPE) sleeves, borated HDPE housings, and borated aluminum covers to shield out scattered neutrons, optimize detector response levels, and make calibration robust against changes in the tokamak hall. The B10 neutron absorption branching ratio may cause the detectors' responses to be non-linear to neutron flux >200keV. However, our simulations unveil that, in the SPARC environment and with the proposed housings and sleeves, >99% of the detector responses are induced by <100keV neutrons. U238's insensitivity to slow neutrons makes this FC a promising candidate for direct fusion neutron measurements. Along with a borated HDPE sleeve, about 60% of the FCs' responses are induced by direct neutrons.

Magnetic Circular Dichroism in X-Ray Absorption and Core-Level Photoemission

ABSTRACTHere is reported observation of magnetic circular dichroism in both x-ray absorption and core-level photoemission of ultra thin magnetic films using circularly polarized x-rays. Iron films (1–4 ML) grown on a Cu (001) substrate at 150 K and magnetized perpendicular to the surface show dramatic changes in the L2,3 absorption branching ratio for different x-ray polarizations. For linearly-polarized x-rays perpendicular to the magnetic axis of the sample the branching ratio was 0.75. For films ≥ 2 ML, this ratio varied from 0.64 to 0.85 for photon spin parallel and anti-parallel, respectively, to the magnetic axis. This effect was observed either by changing the x-ray helicity for a fixed magnetic axis, or by reversing the magnetic axis for a fixed x-ray helicity. Our observation can be analyzed within a simple one-electron picture, if the raw branching ratios are normalized so that the linear value becomes statistical. Furthermore, warming the films to -300 K eliminated this effect, indicating a loss of magnetization in the film over a temperature range of -30 K. Finally, reversing the relative orientation of the photon spin and the magnetic axis from parallel to anti-parallel allowed measurement with photoemission of the exchange splitting of the Fe 2p and 3p core levels which were found to be 03 eV and 0.2 eV, respectively. These results are consistent with earlier studies, but the use of off-plane circularly-polarized x-rays from a bending magnet monochromator offers -2 orders of magnitude greater intensity than typical spin-polarization measurements. Finally, we have performed preliminary x-ray absorption studies of UFe2, demonstrating the feasibilty of MCD Measurements in 5f as well as 3d Materials.

Photoacoustic spectra of rare-earth 1-tyrosine hydrazide, 8-hydroxyquinoline and ammonium metavanadate

The photoacoustic spectra of 1-tyrosine hydrazide, 8-hydroxyquinoline and ammonium metavanadate complexes containing Pr, Nd, Er and Ho have been recorded in the 400–800 nm region using a single beam photoacoustic spectrometer. The main spectral features in the photoacoustic spectra are found to be due to triply ionized rare-earth atoms and the assignments reveal a red shift in their spectral features as compared to their positions in the free ions. These shifts have been used to calculate the nephelauxetic parameters for these complexes. The photoacoustic branching ratios have also been determined and compared with absorption branching ratios for some of these complexes.

The X-ray absorption branching ratio of transition metal compounds

The origin of non-statistical branching ratios in spin-orbit split X-ray absorption spectra is explained. Atomic calculations for transition metals show a systematic change which is due to initial state spin-orbit splitting and electrostatic interactions between core hole and valence electrons.The results of these atomic calculations can be formulated in general rules, which are also applicable to solids. In the free atom the branching ratio reaches a maximum for the Hund's rule ground state and its value decreases gradually for S, L and J. levels of higher energy. The presence of a crystal field results in a lower branching ratio when it produces a low-spin ground state. The rules can be used to assess the spin state and the spin-orbit splitting from the experimental branching ratio in transition metal- and rare earth compounds.

Linear relation between x-ray absorption branching ratio and valence-band spin-orbit expectation value.

The fraction of the total line strength of one of the core-hole spin-orbit-split manifolds in x-ray absorption is proved to be related to the expectation value of the valence-band spin-orbit operator. The relation obtained, by angular momentum algebra, is valid if the total angular momentum of the core hole is a good quantum number, which is true for deep core levels. Therefore, the branching ratio of deep-core-hole manifolds is probably the most direct probe to measure the valence-band spin-orbit interactions, especially in transition-metal compounds. The branching ratio measures the angular part of the spin-orbit operator and is complementary to magnetic measurements which determine the Land\'e g factor.

Measurements of Lifetimes and Oscillator Strengths of Neutral and Singly Ionized Atoms. Experimental Results of the Last Five Years

After a short description of the standard methods to determine f-values and atomic lifetimes, the most important development of the last years - the application of pulsed dye lasers - is discussed and some results are compared. The further development in the field of f-value measurement is described by some typical activities: emission and absorption measurements with thermal light sources and the application of emission and absorption branching ratio measurements. The general accuracy which can be obtained with the different measuring methods is discussed.The article does not comprehend the whole work which has been done in this field since 1977. There appeared a large number of articles which are not mentioned because of the necessarily limited volume of such a survey. Especially, the treatment of the beam foil technique which yields a lot of lifetimes for singly ionized atoms is omitted completely.