A focusing neutron lens using glass polycapillary fibers has been introduced successfully into a prompt gamma activation analysis (PGAA) instrument placed at the exit of a cold neutron guide. The neutron current density gain of the lens is 80, averaged over the focused beam size of 0.53 mm diameter. PGAA measurements have been made on submillimeter particles of gadolinium and cadmium. The results indicate that elemental sensitivities of measurements are increased by ∼ 60, and that particles of sizes smaller than 0.5 mm can be discerned using the focusing lens. The measured gain in prompt gamma signals for these particles is less than anticipated, probably due to alignment difficulties. Gamma ray background associated with the lens is discussed and improvements are suggested.

Neutron guides using reflective optics are used to transport thermal neutron beams. Narrow channels enable the neutron beam to be bent more sharply than with wide guides. Recent developments in polycapillary fibers with thousands of channels having sizes of a few microns have allowed cold neutrons to be guided and focused. A neutron focusing lens can enable an increased lateral resolution of the measurement and an improvement in the detection limits for specific elements for neutron absorption techniques in analytical and materials research. Over an order of magnitude increase in neutron intensity within a submillimeter focal spot has been achieved. Beam benders can also provide more end-positions for experimental stations.

Using the principle of multiple mirror reflection from smooth surfaces at small grazing angles, polycapillary fibers of narrow inner channels (diameter of a few micrometers) have been used to transport and bend slow neutron beams. Neutrons in the cold and thermal range have been focused to a small spot to produce significant gains ( ~ × 10) in intensity. We report studies of neutron transmission properties of individual capillaries, using cold and thermal neutrons at the CNRF (Cold Neutron Research Facility) of the NBSR (the 20 MW research reactor at the National Institute of Standards and Technology, USA). Several aspects of these properties are investigated, namely, the transmission as a function of bending curvature, of length of the fibers, and of composition of fiber materials. From these studies we obtain information on the critical angle of reflection, the reflectivity, and the roughness of the inner surface. We also report the characterization of the first prototype neutron lens constructed at the Kurchatov Institute in Moscow and show that a focal point of 1 mm has been achieved. Finally we discuss the design of a lens suitable for neutron absorption experiments at the end of a neutron guide.

A neutron lens constructed with polycapillary glass fibers is used to focus a 50×45 mm2 beam exiting a cold neutron guide onto a spot of 0.53 mm (full width at half maximum) with a current density gain of 80. The characteristics of the lens are presented. This lens is designed to enhance the detection limit and lateral resolution for prompt gamma activation analysis using cold neutron beams.

A charge injection device (CID) camera and image processing system have been used as a position sensitive detector for energetic charged particles and low energy neutrons. This video radiation detector (VRD) is simple in design but highly effective for real-time radiography and dosimetry with many advantages characteristics. The VRD currently has a dynamic range of 65,000 intensity levels for a 755 X 484 pixel matrix, an active area of 7 mm X 9 mm, a spatial mapping resolution of about 14 micrometers for single detected events (7 micrometers for radiation from a point source), and is sufficiently radiation-hard to be operated in a neutron beam for extended periods of time. Radiation images are updated at a rate of thirty frames per second. The VRD is sensitive to fission fragments, alpha particles, and slow neutrons. Using commercially available image processing hardware and software and an off-the-shelf camera, the system is inexpensive, easy to use with simple interpretation of data, and is capable of performing radiography with only minimal adaptations. Applications in our laboratory include the characterization of focused cold neutron beams, the mapping of uranium and lithium distributions in samples by the detection of neutron absorption reaction products, and the mapping of spontaneous alpha radioactivity from environmental samples. Results provide information on x-y position, counts received, and energy deposited per count, each as a function of time.

The transmission efficiency and exit divergence of cold neutrons guided through straight and bent polycapillary fibers made of borosilicate glass have been measured. Experimental results have been compared with a ray-tracing simulation. Good agreement between the two has been obtained by considering in the simulation only reflectivity losses due to absorption. Therefore, we conclude that the influence of other loss mechanisms on reflectivity, such as surface roughness and waviness, are not significant compared with absorption loss for the borosilicate polycapillary fibers we have measured. Using the same simulation program, we have characterized the performance of a neutron focusing lens placed at the end of a 58Ni guide tube and optimized for a neutron spectrum from a cold source at a temperature of 65 K. An order of magnitude increase in neutron intensity within a submillimeter focal spot is predicted.

A METHOD for focusing X-rays by means of internal reflection inside hollow glass capillaries 1-3 has also been shown to be capable of guiding a neutron beam 4, and now a neutron lens using this principle has been demonstrated 5,6. Here we study the properties of multiple capillary fibre bundles by measuring the transmission efficiency as a function of bending radius. The fibres used are shown to bend a neutron beam through 20-degrees in a distance of 130 mm, with a transmission of 50%. A simple device made of a few fibres is used to demonstrate the focusing of cold neutrons (wavelength 0.2-0.9 nm) down to a 1-mm2 spot size. These investigations will form the basis for the design of a neutron lens for use in materials research, or a beam bender capable of dividing and directing a neutron beam towards several experimental stations.