LENS Webinar #3, New Directions in Instrumentation: Polarized time-of-flight spectroscopy

Figure 1: Incoherent (left) and coherent (right) dynamical structure factors S(Q,hν) of D2O measured using polarized neutrons on the LET spectrometer. The former corresponds to single-particle dynamics, while the latter is (predominantly) due to collective motions. CREDIT: Gøran Nilsen

Gøran Nilsen: Polarized neutron time-of-flight spectroscopy

20 August 2020, 11:00am CEST

Chair: Christof Niedermayer, Paul Scherrer Institute

Join via Zoom: https://ill.zoom.us/j/99877481390

Abstract: Neutron time-of-flight spectroscopy is an exceptionally powerful tool to survey the structural and magnetic dynamics of materials over broad ranges in space and time. It has therefore been applied to a similarly broad range of scientific problems, from proton motions in biological matter to spin excitations in magnets and superconductors. In some cases, however, the signal of interest is hidden by other scattering contributions; for example, the weak scattering due to diffusion in transition metal battery cathode materials often coincides with much stronger inelastic signals of magnetic or coherent structural origin. This class of problem, and many others like it, can be solved using neutron polarization analysis, which exploits the distinct polarization dependences of the cross section components – nuclear coherent, spin incoherent, and magnetic – to separate them. Combining time-of-flight spectroscopy with polarization analysis has therefore been a long-standing goal at neutron scattering facilities around the world. On the other hand, the technical challenges presented by both the broad energy range of neutrons to polarize and the large detector solid angle to analyze have been significant, and have taken decades to surmount.

In this talk, I will introduce some concepts behind polarized time-of-flight spectroscopy, using its recent implementation on the LET spectrometer at ISIS as an example [1]. In particular, I will discuss the differences between polarization analysis on time-of-flight spectrometers versus other types of instrument, and hence the types of scientific problem the technique best lends itself to. Among these, perhaps the most promising is polarized quasi-elastic neutron scattering (QENS), as illustrated by the recent example of coherent-incoherent separation in D2O [2, Figure 1 (above)]. Although most of the technical obstacles to wide-angle polarization analysis have now been overcome, relatively little work has been done on how to analyze the resulting data, especially for magnetic samples. I will therefore continue by discussing some possible new approaches relevant to magnetic powders, before concluding with some future perspectives, including the possibility of installing polarization analysis on indirect geometry spectrometers.

[1] G. Cassella et al., J. Phys.: Conf. Series 1316, 012007 (2019)
[2] A. Arbe et al., Phys. Rev. Applied 2, 022015(R) (2020)

Gøran Nilsen is the polarized neutrons scientist at the ISIS facility in Oxfordshire, UK. In this capacity, he works on the 3He filling station FLYNN, as well as design of new polarized neutron instrumentation across the facility. The most recent completed project he participated in was the uniaxial polarization analysis option on LET (the subject of this talk). He is currently working on concepts for new polarized diffraction and indirect geometry instruments at ISIS, as well as on analysis of polarized neutron data. His science programme mainly focuses frustrated and low-dimensional magnetism, from the synthesis of novel materials to their detailed characterization using neutron diffraction and spectroscopy.

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