Spin-down Neutrons Research Articles

Superpolarizing neutron coatings: Theory and first experiments

A new method for improving polarizing neutron coatings is theoretically substantiated and experimentally verified. It is based on the use of layers with a negative potential of definite thickness to suppress reflection of neutrons with the undesired spin from the potential barriers formed by structural imperfections. Estimations showed that Ti, Co or Ti/Co interlayers at the interfaces and a protective Ti/TiO 2 surface bilayer may increase the flipping ratio for reflection from polarizing neutron coatings by orders of magnitude. It opens the possibility to build polarizers and analyzers of new generation. Superpolarizing coatings not only will improve the performance and thus widen the range of applications of the polarizing devices, but also may be the basis for designing novel neutron instrumentation. Even ultra-cold neutron beams can be efficiently polarized and analyzed with new polarizing neutron optics. A method for precise measurements of NSF and SF reflectivities of the spin-down neutrons for polarizing coatings with the flipping ratios up to 10 3–10 4 is suggested (method of two samples).

An ion-channel-containing model membrane: structural determination by magnetic contrast neutron reflectometry

To many biophysical characterisation techniques, biological membranes appear as two-dimensional structures with details of their third dimension hidden within a 5 nm profile. Probing this structure requires methods able to discriminate multiple layers a few Ångströms thick. Given sufficient resolution, neutron methods can provide the required discrimination between different biochemical components, especially when selective deuteration is employed. We have used state-of-the-art neutron reflection methods, with resolution enhancement via magnetic contrast variation to study an oriented model membrane system. The model is based on the Escherichia coli outer membrane protein OmpF fixed to a gold surface via an engineered cysteine residue. Below the gold is buried a magnetic metal layer which, in a magnetic field, displays different scattering strengths to spin-up and spin-down neutrons. This provides two independent datasets from a single biological sample. Simultaneous fitting of the two datasets significantly refines the resulting model. A β-mercaptoethanol (βME) passivating surface, applied to the gold to prevent protein denaturation, is resolved for the first time as an 8.2 ± 0.6 Å thick layer, demonstrating the improved resolution and confirming that this layer remains after OmpF assembly. The thiolipid monolayer (35.3 ± 0.5 Å), assembled around the OmpF is determined and finally a fluid DMPC layer is added (total lipid thickness 58.7 ± 0.9 Å). The dimensions of trimeric OmpF in isolation (53.6 ± 2.5 Å), after assembly of lipid monolayer (57.5 ± 0.9 Å) and lipid bilayer (58.7 ± 0.9 Å), are precisely determined and show little variation.

Orbital Decay of a Binary System Consisting of Spin-Down Neutron Stars

{W}e consider the gravitational radiation from two time variable mass stars, orbiting around each other under the influence of gravity. The total rates of the variation of the energy, angular momentum, semimajor axis, eccentricity and orbital period are obtained. The results could be important for the understanding of general relativistic effects in the case of the variation of the gravitational mass due to spinning down of the compact stars, which sensitively depends on the equations of state. The cases of the binary systems PSR 1913+16 and PSR 1534+12 are analyzed in detail, and, for different equations of state of nuclear matter, the corrections to the orbital decay due to gravitational radiation and to the spinning down of the pulsars are calculated. The results show that a future significant improvement in the observational techniques could lead to the observation of the specific general relativistic effect of mass variation of pulsars due to spinning down, via the study of orbital decay, even in slowly rotating binary systems.

Polarized neutron reflectometry study of thin Fe films prepared on V (1 0 0)

The magnetic properties of ultrathin Fe (1 0 0) films were investigated by polarized neutron reflectometry (PNR). Fe films with different thicknesses from 1 to 3 nm were prepared by Molecular Beam Epitaxy (MBE) on a V buffer layer, which was deposited on a MgO (1 0 0) single crystal. In order to avoid oxidation the Fe films were covered with V capping layers. During the preparation process the growth of the films was monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Combining the information obtained from PNR with X-ray reflectometry (XRR), the thicknesses of the Fe and V layers were determined precisely. From PNR measurements the absolute value of the magnetization of the films could be determined by fitting the spin-up and spin-down neutron reflectivities separately. The magnetic moments of the Fe films show a perfect linear dependence on the film thickness t Fe. The fitting line intersects the abscissa at t Fe=(0.1±0.01) nm. This means that the magnetic properties of the two V/Fe interfaces show up as a constant reduction of the magnetic moment equal to an Fe bulk layer of thickness t Fe=(0.1±0.01) nm.

Benefits of polarized small-angle neutron scattering on magnetic nanometer scale structure modeling

Recent use of polarized neutron technique in small-angle scattering (SANS) have led to impressive results in the case of magnetic nanometer-scale structure analysis. In some particular cases this method offers the possibility to survey structure models with the necessary accuracy for the first time. The different cross sections for spin-up and spin-down neutron scattering on magnetic precipitates can be combined with the method of chemical contrast variation. All data fitting using structure models will benefit of that kind of constraints. The analysis of the interference term of nuclear and magnetic scattering respectively enables the extraction of additional information about the composition and magnetization profiles of the samples. Here we place emphasis on the difference of spin-up and spin-down neutron scattering intensities to obtain this information. This technique profits by the clear distinction between magnetic and nonmagnetic scattering contributions and the strong auxiliary conditions for model fitting procedures. Depending on the relative orientations of the external magnetic field, the local magnetization of the precipitates and the scattering vector, significant scattering patterns can be scrutinized. Beside general formulas for some special cases of present experimental interest we exercise the approach to a nontrivial case of data obtained from polarised SANS experiments at the Berlin Neutron Scattering Center (BENSC).

A remanent Fe/Si supermirror transmission polarizer

In order to develop a neutron spin polarizer on the basis of magnetically remanent supermirrors, we produced a first Fe/Si supermirror (m=2, q=0.044 A-1) giving polarizations of 96% to 98%. The addition of the reactive gases O2 and N2 to Si reduced the stress in the supermirror and improved the matching of Si and Fe for spin-down neutrons.

Inversely polarizing CoCu neutron supermirrors

A new type of a neutron polarizing supermirror was developed which reflects spin-down neutrons, contrary to all hitherto produced polarizing supermirrors which reflect spin-up neutrons. Reflectivities of 80–90% and polarizations up to 96% were achieved in a scattering vector interval from 0.176 to 0.35 nm −1.

Test of parity-conserving time-reversal invariance using polarized neutrons and nuclear spin aligned holmium

A test of parity-conserving, time-reversal noninvariance (PC TRNI) has been performed in 5.9 MeV polarized neutron transmission through nuclear spin aligned holmium. The experiment searches for the $T$-violating fivefold correlation via a double modulation technique---flipping the neutron spin while rotating the alignment axis of the holmium. Relative cross sections for spin-up and spin-down neutrons are found to be equal to within $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ (80% confidence). This is a two orders of magnitude improvement compared to traditional detailed balance studies of time reversal, and represents the most precise test of PC TRNI in a dynamical process, to our knowledge.

Test of parity-conserving time-reversal invariance using polarized neutrons and nuclear spin aligned holmium.

A test of parity-conserving, time-reversal noninvariance (PC TRNI) has been performed in 5.9 MeV polarized neutron transmission through nuclear spin aligned holmium. The experiment searches for the T-violating fivefold correlation via a double modulation technique---flipping the neutron spin while rotating the alignment axis of the holmium. Relative cross sections for spin-up and spin-down neutrons are found to be equal to within $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ (80% confidence). This is a 2 orders of magnitude improvement compared to traditional detailed balance studies of time reversal, and represents the most precise test of PC TRNI in a dynamical process.

Nonstatic Spin-Isospin Order in Light Nuclei

A semimicroscopic model of the nucleus is presented in which spin-isospin order is realized as zero-point one-dimensional oscillations of spin-up protons and spin-down neutrons against spin-down protons and spin-up neutrons. This phase is favored by the one-pion-exchange potential in light deformed nuclei. The model is characterized by the lowering of the energy of the first excited state and enhancement of the $B(M2)$.

M3Suppression in Hartree-Fock Theory

With a zero-range interaction a valence spin-up neutron will not interact with core spin-up neutrons and hence will not deform them; it will deform spin-down neutrons. This introduces a spin-quadrupole correlation. An ideal operator for probing this correlation is the $M3$ operator, since this operator is crudely $E2\ifmmode\times\else\texttimes\fi{}M1$. The suppression of the $M3$ moment in $^{17}\mathrm{O}$ due to this effect, as well as due to the spin dependence of the interaction, is considered.