Larmor Labelling Research Articles

Simulations of foil-based spin-echo (modulated) small-angle neutron scattering with a sample using McStas.

For the further development of spin-echo techniques to label elastic scattering it is necessary to perform simulations of the Larmor precession of neutron spins in a magnetic field. The details of some of these techniques as implemented at the reactor in Delft are simulated. First, the workings of the magnetized foil flipper are simulated. A full virtual spin-echo small-angle neutron scattering instrument is built and tested without and with a realistic scattering sample. It is essential for these simulations to have a simulated sample that also describes the transmitted beam of unscattered neutrons, which usually is not implemented for the simulation of conventional small-angle neutron scattering (SANS) instruments. Finally, the workings of a spin-echo modulated small-angle neutron scattering (SEMSANS) instrument are simulated. The simulations are in good agreement with theory and experiments. This setup can be extended to include realistic magnetic field distributions to fully predict the features of future Larmor labelling elastic-scattering instruments. Configurations can now be simulated for more complicated combinations of SANS with SEMSANS.

RF neutron spin flippers in time of flight Spin-Echo Resolved Grazing Incidence Scattering (SERGIS)

RF coils have been routinely used for Larmor labelling on the Offspec instrument at ISIS. These coils encode directional information via the neutron polarization using a series of parallelogram-shaped pole shoes which can be tuned to different angles with an RF gradient flipper in the centre of each magnet. We report on measurements of the magnetic field integral through the coils in reflection geometry for a range of scattering angles and different pole shoe angles. Such information is mapped out by measuring the phase of the neutron polarisation and the measurements are discussed in light of data on patterned silicon gratings with a dewetted polymer and the visibility of in-plane structures to SERGIS.

Static and quasielastic properties of the spiral magnetBa2CuGe2O7studied by neutron resonance spin echo spectroscopy and neutron Larmor labeling

We provide a route to use elastic and quasielastic neutron spin echo spectroscopy (Larmor labeling) for a unified approach to study both the static and dynamic properties of incommensurate magnetic structures with small propagation vectors. The archetypal noncentrosymmetric spiral magnet ${\mathrm{Ba}}_{2}{\mathrm{CuGe}}_{2}{\mathrm{O}}_{7}$ serves as a model system for our study. We show how Larmor labeling can efficiently decouple instrumental wavelength resolution and effective-$\stackrel{P\vec}{Q}$ resolution and give high-precision access to the structural properties of the incommensurate domains of ${\mathrm{Ba}}_{2}{\mathrm{CuGe}}_{2}{\mathrm{O}}_{7}$.

Larmor labeling development

The first applications of polarized neutrons were to distinguish magnetic from nuclear scattering amplitudes. Much later the first set-ups for measuring the precession phase of the polarization passing a magnetic field were realized. This phase, determined by the field strength and interaction time, could label magnetic fields, wavelength of the neutrons and length over which the field is present. The latter could be used by proper shaping of the magnetic field to label also the direction of transmitted neutrons. The advantage of this labeling is that high precision measurements are possible without strong confinement of the beam by diaphragms. An overview of the use of Larmor labeling of polarized neutrons is given for applications in magnetism, in inelastic neutron scattering and small angle scattering.

Offspec, the ISIS spin-echo reflectometer

OffSpec has been purposely built as a low background, polarised neutron reflectometer with spin-echo for Larmor labeling. The instrument received first neutrons in December 2008 and five of the intended seven modes of operation have now been tested and operated with peer reviewed user experiments. The technical performance of the instrument will be reviewed in each of the modes and future scientific possibilities discussed. Particular attention will be paid to results from spin-echo SANS (SESANS) and spin-echo resolved grazing incidence scattering (SERGIS), where the largest number of measurements have been performed.

Larmor labeling by time-gradient magnetic fields

The Larmor labeling of neutrons, due to the Larmor precession of neutron spin in a magnetic field, opens the unique possibility for the development of neutron spin-echo (NSE) based on neutron scattering techniques, featuring an extremely high energy (momentum) resolution. Here, we present the experimental proof of a new method of the Larmor labeling using time-gradient magnetic fields.

Larmor labelling by thin spin flippers with rotating magnetic field: simulations of performance of neutron scattering instruments

The neutron spin-echo (NSE) method, that is the most powerful tool of high-resolution neutron spectroscopy, is known in two versions: with the long solenoids and short alternating fields. Here we consider an alternative version that makes use of spin flippers consisting of thin magnetic foils with an in-plane rotating magnetic field vector. MNCP simulations show the perfect performance of the NSE spectrometer build upon such flippers. Requirements to thin magnetic foils are estimated. Some possible applications of such technique are discussed. This approach can be considered as an alternative to the present-day NSE techniques.

Larmor labelling, polarization analysis and its applications

Polarized neutron provides exceptional possibilities for neutron scattering. Neutron polarimetry allows for measurements of vector properties of neutron polarization and provides a unique way of recovering significant directional and phase information lost when only neutron intensities are measured. On the other hand, the Larmor precession of neutron spin in magnetic field allows us to attach a “Larmor clock” to every neutron. Such a Larmor labelling opens the unique possibility for the development of “unusual” neutron scattering techniques, where the energy (momentum) resolution is decoupled monochromatization (collimation) of incident and scattered neutron beams. In this article we are reviewing some latest developments both in methods of three-dimensional neutron polarimetry and instrumentation based upon the Larmor labelling.

Larmor labelling by thin spin flippers with rotating magnetic field: simulations of performance of neutron scattering instruments

The neutron spin-echo (NSE) method, that is the most powerful tool of high-resolution neutron spectroscopy, is known in two versions: with the long solenoids and short alternating fields. Here we consider an alternative version that makes use of spin flippers consisting of thin magnetic foils with an in-plane rotating magnetic field vector. MNCP simulations show the perfect performance of the NSE spectrometer build upon such flippers. Requirements to thin magnetic foils are estimated. Some possible applications of such technique are discussed. This approach can be considered as an alternative to the present-day NSE techniques.

Larmor labelling, polarization analysis and its applications

Larmor labelling, polarization analysis and its applications