Density functional theory calculations on magnetic properties of actinide compounds

We have performed a detailed analysis of the magnetic (collinear and noncollinear) order and atomic and the electron structures of UO2, PuO2 and UN on the basis of density functional theory with the Hubbard electron correlation correction (DFT+U). We have shown that the 3-k magnetic structure of UO2 is the lowest in energy for the Hubbard parameter value of U=4.6 eV (and J=0.5 eV) consistent with experiments when Dudarev's formalism is used. In contrast to UO2, UN and PuO2 show no trend for a distortion towards rhombohedral structure and, thus, no complex 3-k magnetic structure is to be anticipated in these materials.Comment: 5 pages, 3 figures 1 table, submitted to Phys. Chem. Chem. Phy

Hidden magnetic order in plutonium dioxide nuclear fuel

A thorough understanding of the chemistry of PuO2 is critical to the design of next-generation nuclear fuels and the long-term storage of nuclear materials. Despite over 75 years of study, the ground-state magnetic structure of PuO2 remains a matter of much debate. Experimental studies loosely indicate a diamagnetic (DM) ground-state, whereas theoretical methods have proposed either a collinear ferromagnetic (FM) or anti-ferromagnetic (AFM) ground-state, both of which would be expected to cause a distortion from the reported Fm[3 with combining macron]m symmetry. In this work, we have used accurate calculations based on the density functional theory (DFT) to systematically investigate the magnetic structure of PuO2 to resolve this controversy. We have explicitly considered electron-correlation, spin–orbit interaction and noncollinear magnetic contributions to identify a hereto unknown longitudinal 3k AFM ground-state that retains Fm[3 with combining macron]m crystal symmetry. Given the broad interest in plutonium materials and the inherent experimental difficulties of handling this compound, the results presented in this paper have considerable implications for future computational studies relating to PuO2 and related actinide structures. As the crystal structure is coupled by spin–orbit interactions to the magnetic state, it is imperative to consider relativity when creating computational models

The "best language" stereotype threat : A pilot study

This work should be seen as a pilot study of the effect that the common-sense based statement that one understands and learns best in their best language has on pupils in Swedish schools. A number of students have been given a mathematical test in the language they use for their normal school activities, and that is not their mother tongue. Some of them (approximately 50%) were led to believe this particular test to be language fair. Their average score on the test has then been compared to the average score on the same test of the remaining 50% students who considered the test a normal one. The difference in performance between the two subgroups has been interpreted with the help of the concept of stereotype threat. This pilot study showed a trace of the hypothesised best language stereotype threat in a specific group of students and will hopefully serve as a guide for a larger work that could prove (or falsify) the existence of the best language stereo-type threat with statistical certainty, extend its range of applicability to a wider group of students and establish its size as compared to other related factors. A final caveat: This study is focussed on (and relevant only for those) students that perform their school activities in a language different from their mother tongue but that are fully operational in the teaching language

Theory of Crystal Fields and Magnetism of f-electron Systems

A parameter free approach for the calculation of the crystal field splitting of the lowest Russel-Saunders J-multiplet in f-electron systems has been developed and applied to selected compounds. The developed theory is applicable to general symmetries and is based on symmetry constrained density functional theory calculations in the local density or in the generalised gradient approximation. The magnetocrystalline anisotropy of Gd has been analysed. It has been shown that the peculiar orientation of the easy axis of magnetisation is consistent with an S-ground state. Further, the temperature dependence of the easy axis of magnetisation has been investigated and it has been shown that the temperature driven reduction of the effective magnetisation is the principal mechanism responsible for it. A new method has been developed that allows for theoretical studies of the electronic structure and total energy of elements and compounds in an intermediate valence regime. The method combines model and first-principles band structure calculations, therefore being accurate and computationally efficient. It has been applied to Yb metal under pressure obtaining a remarkable agreement with experimental observations for the equation of state and the x-ray absorption spectroscopy

Learned Primal Dual Reconstruction for PET

We have adapted, implemented and trained the Learned Primal Dual algorithm suggested by Adler and Öktem and evaluated its performance in reconstructing projection data from our PET scanner. Learned Primal Dual reconstructions are compared to Maximum Likelihood Expectation Maximisation (MLEM) reconstructions. Different strategies for training are also compared. Whenever the noise level of the data to reconstruct is sufficiently represented in the training set, the Learned Primal Dual algorithm performs well on the recovery of the activity concentrations and on noise reduction as compared to MLEM. The algorithm is also shown to be robust against the appearance of artefacts, even when the images that are to be reconstructed present features were not present in the training set. Once trained, the algorithm reconstructs images in few seconds or less

Alpha spectrometry with the inexpensive open-source detector Alphaino

A photodiode-based inexpensive detector working as counter and spectrometer for alpha particles, named Alphaino, is described and analyzed in depth. Prior to detector construction, Monte Carlo simulations by means of MCNPX ver. 2.7.0 code have been carried out to select the most suitable sensitive element for the intended applications. The detector has been tested for low-rate alpha-particle counting and spectroscopy, demonstrating a maximum achievable count rate of 4⋅103 s−1, with an energy resolution corresponding to a Full Width Half Maximum of 160keV over the entire energy range of measured alpha, namely 4−6.5 MeV, the intrinsic efficiency being 100%. Alphaino can be used for fast and qualitative analyses to radionuclides identification, and quantitative analyses when radionuclides monitored are characterized by well distinguished energy lines in spectra. Its applications will include 222Rn progeny monitoring by mechanical air sampling on a millipore filter, and disposable applications to qualitative measuring of potentially alpha-contaminated liquids (containing plutonium isotopes) by a droplet put on the photodiode itself, for a quick and cheap evaluation prior to initiate proper chemical treatments to classical alpha spectrometry by electrodeposition. Alphaino is completely open-source. The repository for the project is available on GITHUB at https://github.com/bemxgm/Radon-Monitor.QC 20190515</p

First-principles theory of intermediate-valence f-electron systems

Contains fulltext : 60441.pdf (publisher's version ) (Open Access

Spatio-Temporal Positron Emission Tomography Reconstruction with Attenuation and Motion Correction

: The detection of cancer lesions of a comparable size to that of the typical system resolution of modern scanners is a long-standing problem in Positron Emission Tomography. In this paper, the effect of composing an image-registering convolutional neural network with the modeling of the static data acquisition (i.e., the forward model) is investigated. Two algorithms for Positron Emission Tomography reconstruction with motion and attenuation correction are proposed and their performance is evaluated in the detectability of small pulmonary lesions. The evaluation is performed on synthetic data with respect to chosen figures of merit, visual inspection, and an ideal observer. The commonly used figures of merit-Peak Signal-to-Noise Ratio, Recovery Coefficient, and Signal Difference-to-Noise Ration-give inconclusive responses, whereas visual inspection and the Channelised Hotelling Observer suggest that the proposed algorithms outperform current clinical practice

Crystal Field Levels in Lanthanide Systems.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Crystal-Field Levels and Magnetic Susceptibility in PuO2.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe