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The primary purpose of this investigation is to determine the effectivecoherent extent of the neutron wave packet transverse to its mean propagationvector k, when it is prepared in a typical instrument used to study thestructure of materials in thin film form via specular reflection. There are twoprincipal reasons for doing so. One has to do with the fundamental physicalinterest in the characteristics of a free neutron as a quantum object while theother is of a more practical nature, relating to the understanding of how tointerpret elastic scattering data when the neutron is employed as a probe ofcondensed matter structure on an atomic or nanometer scale. Knowing such abasic physical characteristic as the neutron's effective transverse coherencecan dictate how to properly analyze specular reflectivity data obtained formaterial film structures possessing some amount of in-plane inhomogeneity. Inthis study we describe a means of measuring the effective transverse coherencelength of the neutron wave packet by specular reflection from a series ofdiffraction gratings of different spacings. Complementary non-specularmeasurements of the widths of grating reflections were also performed whichcorroborate the specular results. (Part I principally describes measurementsinterpreted according to the theoretical picture presented in Part II.) Eachgrating was fabricated by lift-off photo-lithography patterning of a nickelfilm (approximately 1000 Angstroms thick) formed by physical vapor depositionon a flat silicon crystal surface. The grating periods ranged from 10 microns(5 microns Ni stripe, 5 microns intervening space) to several hundred microns.The transverse coherence length, modeled as the width of the wave packet, wasdetermined from an analysis of the specular reflectivity curves of the set ofgratings.

physical review. a, atomic, molecular, and optical physics/physical review, a, atomic, molecular, and optical physics

Determination of the effective transverse coherence of the neutron wave  packet as employed in reflectivity investigations of condensed matter  structures Part I: Measurements