V. A. Sharov

Characterization of a cold neutron beam from a curved guide

Abstract A supermirror guide that includes both straight sections and curved channels has been installed at a cold neutron source facility. A description of the beam line and the beam characteristics are given. The transmitted neutron wavelength spectrum and the beam current density have been measured. Factors that affect the spectrum are discussed. Though the final straight guide section improves the spatial uniformity of the beam intensity at the guide exit, the spatial-angular correlations caused by the curved channels persist. These correlations are observed in the beam divergence measurements at various positions along the beam line using neutron imaging plate technology, and are explained by ray projections. The divergence of the transmitted beam is determined both by the critical angle of the guide and the collimation beyond the guide.

A polycapillary bending and focusing lens for neutrons

A glass polycapillary lens that both bends and focuses a cold neutron beam has been designed and constructed. The bender focuser guides part of the incident beam away from its line of sight and focuses it to a spot of width 0.65 mm at a distance 95 mm from the lens exit and 20 mm below the bottom edge of the beam path, with a gain of 20 in neutron current density. The neutron transmission characteristics of the lens have been determined with two types of position-sensitive detectors, a charge injection device, and an imaging plate. The lens has been tested with prompt gamma measurements on a gadolinium shard and titanium foil.

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).

Application of capillary optics to neutron radiography

We propose two neutron radiographic techniques that make use of the converging beam from a tapered monolithic lens. The first is a focusing technique that enables one to obtain an image of a given cross section within an object. The second technique is neutron radiography that uses a beam emanating from a near point source. Both methods provide an improvement in spatial resolution for neutron radiography.

An imaging-plate detector for small-angle neutron scattering

Small-angle neutron scattering (SANS) measurements have been performed on long-flight-path pinhole-collimation SANS instruments using, as a two-dimensional position-sensitive detector, both a neutron imaging plate, incorporating gadolinium, and a two-step transfer method, with dysprosium foil as the image transfer medium. The measurements are compared with corresponding data taken using conventional position-sensitive gas proportional counters on the SANS instruments in order to assess the viability of the imaging techniques. The imaging plates have pixel sizes of about two orders of magnitude smaller than those of the gas proportional counter. The reduced pixel size provides definite advantages over the gas counter in certain specific situations, namely when limited space necessitates a short sample-to-detector distance, when only small samples (comparable in size to the detector pixels) are available, or when used in conjunction with focusing beam optics.

Spatial distribution of neutrons guided through a monolithic tapered lens

The cold neutron transmission through a monolithic polycapillary lens is studied by scanning a pinhole beam across the entrance cross section of the lens. The relative transmission as a function of bending radius of a tapered channel is obtained. The results are explained in terms of a reduced critical angle for total reflection for the tapered channels. A technique for qualitative examination of monolithic lenses is proposed.

Imaging neutron beams with scintillating fiber faceplates

Abstract An imaging system for neutron beams based on scintillating fiber faceplates is described. The scintillating fibers are made from lithium silicate glass doped with Ce 3+ using isotopically enriched 6 Li. Several faceplates ranging in thickness from 1.3 to 3.2 mm have been tested using a position-sensitive photomultiplier or a Charge Injection Device camera as the photodetector. Advantages of scintillating fiber faceplates include high neutron capture efficiency, good position resolution, and the possibility of low-cost, large-area detectors.

Characterizing a neutron lens: study of capillary optics

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 58Ni-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.

Neutron transmission through tapered channels

The transmission of neutrons through a tapered channel is analyzed and the relationships among various parameters such as the distance between reflections and the polar and grazing angles of a trajectory at each reflection are established. These relationships are used in a simulation to study the transmission characteristics of a cylindrical tapered channel. The transmission, the transmitted beam angular divergence and the number of reflections are determined for different values of the taper angle and the incident beam divergence. The simulation results for the straight channel agree exactly with the derived analytical expressions. Those for the tapered channels can be described using the same expressions but with a lower effective critical angle. This reduced critical angle depends on the length of the taper and on the taper angle. For long gentle tapers, the reduction in critical angle is equal to the reduction in channel diameter.

The Number of Reflections for Neutron Transmission Through Cylindrical Channels

The fraction of trajectories that have a given number of reflections during the passage of a neutron beam through a long cylindrical channel is determined when the angular divergence of the incident beam is less than the critical angle of the inner surface of the channel. This is useful for estimating the reflectivity from neutron-transmission measurements. The results are shown on a universal curve.