Applications of neutron diffraction for small samples (<1mm<sup>3</sup>) or small fiducial areas are limited by the
available neutron flux density. Recent demonstrations of convergent beam electron and x-ray diffraction and focusing of cold (λ>1 Å) neutrons suggest the possibility to use convergent beam neutron diffraction for small sample crystallography. We have carried out a systematic study of diffraction of both monoenergetic and broad bandwidth
neutrons at the NIST Research Reactor and at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. Combining convergent beams with time-of-flight Laue diffraction is particularly attractive for high efficiency small sample diffraction studies. We have studied single crystal and powder diffraction of neutrons with convergence angles as large as 15° and have observed diffracted peak intensity gains greater than 20. The convergent beam method (CBM) shows promise for crystallography on small samples of small to medium size molecules (potentially even for proteins), ultra-high pressure samples, and for mapping of strain and texture distributions in larger samples.
The relatively low flux from neutron sources means that structural analysis using neutron diffraction requires large crystals that are often not available.We are exploring the possibilities of a polycapillary focusing optic to produce a small intense beam spot of size ⪅0.5 mm for small crystals.We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm,suitable for time-of-flight diffraction.Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength.We have performed similar measurements on a polychromatic beam on a pulsed neutron source,where the results are subject to background from short wavelength neutrons.The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of two.In a diffraction measurement from an alpha quartz crystal using a 2.1° convergent beam from a pulsed neutron source,we observed six diffraction peaks in the 1.5 Å -4 Å wavelength bandwidth transmitted by the optic.These diffraction spots show an intensity gain of 5.8 ±0.9 compared to a direct beam diffracting from the same sample volume as that illuminated by the convergent beam.
Capillary neutron optics improve the capabilities of neutron beam techniques such as neutron depth profiling and prompt gamma activation analysis. Millions of glass capillaries are configured to capture and guide low-energy neutrons by grazing total reflection from the smooth inner surface of the hollow channels. By precise orientation of the capillaries, beams of neutrons are readily collimated with good angular control or can be finely focused - as required by the application. In addition, the optics can improve the signal-to-noise ratio by diverting a neutron beam to a convenient off-axis direction, thereby circumventing interferences from gamma rays and fast neutrons characteristic of simple aperture collimation. The focused intensity of neutrons obtained in an area of 0.03 mm<SUP>2</SUP> may be increased up to a hundred times over that previously available for NDP or PGAA techniques. Furthermore, the spatial resolution can be improved by up to 100 times. Consequently, small samples, or small volumes within larger samples, may be better and more rapidly investigated with neutron probe techniques. We report on developments in the application of capillary neutron optics.
Neutron Depth Profiling (NDP) is a nondestructive analytical technique for measuring the concentration of certain light elements as a function of depth near the surface of a solid matrix. The concentration profile is determined by analyzing the energy spectrum of the charged particles emitted as a result of neutron capture by the elements. The measurement sensitivity is directly proportional to the neutron beam current density. A more intense neutron beam achieved by focusing improves sensitivity for specimens of small area. In addition, a narrowly focused beam adds lateral spatial resolution to the technique, which is advantageous compared with that obtained by collimating the beam size using apertures. Capillary neutron lenses have been shown to focus a neutron beam to sub-millimeter spot size. Preliminary tests have been performed in the NDP geometry using such a focusing device. A lateral resolution in the sub-millimeter range is demonstrated by a specimen of non-uniform lateral distribution composed of a row of borosilicate glass fibers.
A neutron lens constructed with glass capillary fibers has been used to focus neutron beams of various geometries and wavelengths. We give as an example measurements performed at an end position of a <SUP>58</SUP>Ni-coated cold neutron guide, where the exiting neutron beam with a cross section of 30 mm in diameter from the guide is incident on the lens and is focused to a 1 mm diameter spot, with a gain of 3.6 in neutron flux density. We have evaluated various features of the focusing performance of the lens, as well as some neutron properties of individual capillary fibers. The average reflectivity of the inner wall of the fibers has been determined to be greater than 0.99. The results of this study will be used for the design and construction of future lenses for neutron absorption experiments such as depth profiling and prompt gamma activation analysis.
The usually large cross section of the incident neutron beam is divided in the microguide into a large number of thin slices. This allows the neutron beam to be deviated by a large angle within a short distance and enables the design of an efficient neutron lens by giving the stack an appropriate shape. We present a novel approach for high quality microguides by using thin and thus flexible single crystal silicon wafers as the neutron transmitting medium, coated with optimized neutron reflecting thin-film materials. A crucial issue concerns the reflection coefficient R of the silicon thin-film interface. We have performed neutron transmission measurements through commercially available 200 micrometers thin wafers coated in both sides with nickel. Various rocking curves have been taken on assemblies of straight wafers with neutron pathways from 50 to 200 mm in silicon, and on curved wafers. A reflectivity value of 0.988 +/- 0.005 has been found for a neutron wavelength of 7 angstroms.