  The primary purpose of this investigation is to determine the effective
coherent extent of the neutron wave packet transverse to its mean propagation
vector k, when it is prepared in a typical instrument used to study the
structure of materials in thin film form via specular reflection. There are two
principal reasons for doing so. One has to do with the fundamental physical
interest in the characteristics of a free neutron as a quantum object while the
other is of a more practical nature, relating to the understanding of how to
interpret elastic scattering data when the neutron is employed as a probe of
condensed matter structure on an atomic or nanometer scale. Knowing such a
basic physical characteristic as the neutron's effective transverse coherence
can dictate how to properly analyze specular reflectivity data obtained for
material film structures possessing some amount of in-plane inhomogeneity. In
this study we describe a means of measuring the effective transverse coherence
length of the neutron wave packet by specular reflection from a series of
diffraction gratings of different spacings. Complementary non-specular
measurements of the widths of grating reflections were also performed which
corroborate the specular results. (Part I principally describes measurements
interpreted according to the theoretical picture presented in Part II.) Each
grating was fabricated by lift-off photo-lithography patterning of a nickel
film (approximately 1000 Angstroms thick) formed by physical vapor deposition
on 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, was
determined from an analysis of the specular reflectivity curves of the set of
gratings.
