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Description
Neutron grating interferometry (nGI) is a relatively new neutron imaging technique which is the
adaption of a Talbot-Lau Interferometer for neutrons [1]. It simultaneously delivers information
about the transmission (TI), phase shift (DPC) and the scattering (DFI) inside a sample [1].
In particular the DFI has generated high interest, due to its ultra-small-angle neutron scattering
(USANS) contrast mechanism, allowing to indirectly resolve structures which cannot be directly
resolved by an imaging instrument [2],[3].
For instance, nGI is sensitive to magnetic domain walls and consequently allows to measure the
effect of induced stress in a sample onto the mobility of its magnetic domains [4]. Moreover, the
distribution of flux domains within type-I and type-II superconductors has recently been visualized
[5],[6]. Also there have been strong efforts to use nGI and particularly the DFI as tools for
quantitative measurements of microstructures in materials. A theory has been proposed, which
directly links the DFI contrast within the material to a Fourier back transform of its scattering
function evaluated at a correlation length ξGI~λ [7].
A prerequisite for such quantitative measurements is a high signal-to-noise-ratio (SNR). For DFI
measurements it has been shown that the main reasons for statistical uncertainties are (i) low DFI
signal and (ii) low visibility [8]. Here, the visibility is the quotient between the amplitude and the
mean value of the oscillation during an nGI scan and is an indicator for the performance of an nGI
setup.
While the DFI signal is, as mentioned above, connected to the correlation length which can be tuned
during the experiment, the visibility is strongly dependent on the quality of the gratings. Especially
the quality (absorptivity) of the analyzer grating (G2) is a great concern here, as it is generally the
grating with the smallest period (several μm). Current fabrication techniques cause the grating to
strongly deviate from an ideal binary absorption profile. As has been shown in [9] this strongly
degenerates the visibility. Furthermore, tuning the correlation length either lowers the achievable
real space resolution or results in a change in neutron wavelength which also causes a decrease in
visibility.
Hence a high visibility is an essential basis for quantitative measurements. In our contribution we
will present the upgraded nGI setup at the ANTARES beamline at FRM II. This nGI setup has been
heavily redesigned, compared to its precursor [10]. The redesign allowed to optimize the distances
between the gratings, as well as the grating periods. In particular, the source and analyzer gratings,
which are both absorption gratings, have been improved towards binary absorption profiles. With
these changes in the improved ANTARES nGI we have achieved a visibility of 75% over the whole
detector area (76mm x 76mm) at the design wavelength of 4 Å. It is worth noting that this visibility
is very close to the theoretical limit imposed by the spatial coherence generated by the used G0
grating.
[1] C. Grünzweig, PhD thesis (2009)
[2] C. Grünzweig et al., PRL 101, 025504 (2008)
[3] M. Strobl et al., 101, 123902 (2008)
[4] H. Weiss et al., pending (2018)
[5] T. Reimann et al., accepted at JLTP
[6] T. Reimann et al., Nat. Commun. 6:8813 (2015)
[7] M. Strobl, Sci. Rep. 4, 7243 (2014)
[8] R. Harti et al., Review of Scientific Instruments 88, 103704 (2017)
[9] R. Harti et al., Opt. Express 25, 1019-1029 (2017)
[10] T. Reimann et al., J. Appl. Cryst. 49, 1488-1500 (2016)
