Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

The emission of long-range charged particles in the slow neutron fission of heavy nuclei

Abstract: Light charged particles with ranges greater than 6 cm of air produced in the slow neutron fission of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{239}$ have been studied in detail by coincidence counting methods. In each case particles with a continuous range distribution extending to about 50 cm of air were observed, the distribution showing a broad maximum in the neighborhood of 20 cm. Direct measurement of the energies of the light particles from ${\mathrm{U}}^{235}$ showed a fairly symmetrical distribution about 15 Mev with a maximum energy of about 26 Mev. Comparison of the energy and range distributions shows that all the long-range particles are $\ensuremath{\alpha}$-particles. The frequency of emission of these $\ensuremath{\alpha}$-particles was found to be 1 in 405\ifmmode\pm\else\textpm\fi{}30 fissions for ${\mathrm{U}}^{233}$, 1 in 505\ifmmode\pm\else\textpm\fi{}50 fissions for ${\mathrm{U}}^{235}$, 1 in 445\ifmmode\pm\else\textpm\fi{}35 fissions for ${\mathrm{Pu}}^{239}$. No protons were observed, although the apparatus would have detected any with ranges lying between 10 and 100 cm of air. The energy distribution of fission fragments coincident with long-range $\ensuremath{\alpha}$-particles was also measured. The usual two peaks were observed indicating asymmetric division of mass, but each peak was shifted to a lower energy than is observed in binary fission. Quantitative comparison of the energies involved showed that, on the average, the total kinetic energy carried away in fission accompanied by $\ensuremath{\alpha}$-emission is about equal to that liberated in binary fission. Possible explanations for $\ensuremath{\alpha}$-emission in fission are discussed.

Neutron-scattering study of incommensurate magnetic order in the heavy-fermion superconductor UNi 2 Al 3

Abstract: Elastic neutron scattering from a single-crystal sample of the heavy-fermion superconductor UNi{sub 2}Al{sub 3} has revealed the onset of long-range magnetic order below T{sub N}=4.6 K. This order is characterized by an incommensurate (IC) ordering wave vector given by ( (1) /(2) {plus_minus}{tau},0, (1) /(2) ) with {tau}=0.11{plus_minus}0.003. The intensity of several magnetic satellite Bragg peaks within the (h,0,l) plane is well described by a model in which the spins lie within the basal plane and are modulated in amplitude from site to site. By applying a magnetic field to select from all the possible domains, we find that the moment is polarized along the {bold a} direction, with a maximum amplitude of 0.21{plus_minus}0.1{mu}{sub B} per uranium atom. The order-parameter exponent {beta} associated with this transition is 0.34{plus_minus}.03, which is typical of three-dimensional ordering transitions. Measurements down to {approximately}0.3 K show that the magnetic order coexists with superconductivity below T{sub C}{approximately}1.2 K, and that these states are coupled as shown by anomalous behavior of the magnetic order parameter around T{sub C}. Measurements were also made in magnetic fields of up to 8 T applied perpendicular to the (h,0,l) plane, along ({minus}1,1,0), a near-neighbor direction within the hexagonal basal plane. Whilemore » the field does not influence T{sub N}, it does increase the intensity of the magnetic Bragg peaks by a factor of {approximately}1.5, as well as increase the IC part of the ordering wave vector at low temperatures. {copyright} {ital 1997} {ital The American Physical Society}« less

Small-polaron hopping conduction in bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7

Abstract: We report anisotropic resistivity measurements on a ${\mathrm{La}}_{12}{\mathrm{Sr}}_{18}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T For $Tg~218\mathrm{K},$ the temperature dependence of the zero-field in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity ${\ensuremath{\rho}}_{c}(T)$ can be ascribed by an Arrhenius law with the same activation energy Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ${\ensuremath{\rho}}_{ab,c}(H,T)$ The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites

Frustrated magnetism in the double perovskite L a 2 LiOs O 6 : A comparison with L a 2 LiRu O 6

Abstract: The frustrated double perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, based on $\mathrm{O}{\mathrm{s}}^{5+}\phantom{\rule{0.16em}{0ex}}(5{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$ is studied using magnetization, elastic neutron scattering, heat capacity, and muon spin relaxation (\ensuremath{\mu}SR) techniques and compared with isostructural $(P{2}_{1}/n)\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6},\phantom{\rule{0.16em}{0ex}}\mathrm{R}{\mathrm{u}}^{5+}(4{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$. While previous studies of $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ showed a broad susceptibility maximum $({\ensuremath{\chi}}_{\mathrm{max}})$ near 40 K, heat capacity data indicate a sharp peak at 30 K, similar to $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ with ${\ensuremath{\chi}}_{\mathrm{max}}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a heat capacity peak at 24 K. Significant differences between the two materials are seen in powder neutron diffraction where the magnetic structure is described by $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, while $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ has been reported with $\mathbit{k}=(000)$, structure for face centered lattices. For the $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ structure, one has antiferromagnetic layers stacked antiferromagnetically, while for $\mathbit{k}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0)$ structure, ferromagnetic layers are stacked antiferromagnetically. In spite of these differences, both can be considered as type I fcc antiferromagnetic structures. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, the magnetic structure is best described in terms of linear combinations of basis vectors belonging to irreducible representations ${\mathrm{\ensuremath{\Gamma}}}_{2}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$. The combinations ${\mathrm{\ensuremath{\Gamma}}}_{2}--{\mathrm{\ensuremath{\Gamma}}}_{4}$ and ${\mathrm{\ensuremath{\Gamma}}}_{2}+{\mathrm{\ensuremath{\Gamma}}}_{4}$ could not be distinguished from refinement of the data. In all cases, the $\mathrm{O}{\mathrm{s}}^{5+}$ moments lie in the $yz$ plane with the largest component along $y$. The total moment is 1.81(4) ${\ensuremath{\mu}}_{\mathrm{B}}$. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the $\mathrm{R}{\mathrm{u}}^{5+}$ moments are reported to lie in the $xz$ plane. In addition, while neutron diffraction, \ensuremath{\mu}SR and NMR data indicate a unique ${T}_{\mathrm{N}}=24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the situation for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ is more complex, with heat capacity, neutron diffraction, and \ensuremath{\mu}SR indicating two ordering events at 30 and 37 K, similar to the cases of cubic $\mathrm{B}{\mathrm{a}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$ and monoclinic $\mathrm{S}{\mathrm{r}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$.