Polycapillary Fibers Research Articles

Measurement and analysis of leaded glass polycapillary optic performance for hard x rays

The properties of borosilicate glass polycapillary x-ray optics have been extensively studied. Small-area scatter rejection borosilicate glass polycapillary optics have been demonstrated with good results. Many medical imaging and industrial radiographic applications for x rays would require large-area optics with good scatter rejection. Since shorter optics are easier to manufacture, optics with a shorter length would provide a faster route to bringing the benefit of polycapillary x-ray optics to these applications. Leaded glass would allow the optic to be much shorter and still give good contrast enhancement, because of the superior absorption of lead glass. In order to investigate the feasibility of using leaded glass polycapillary x-ray optics for these applications, measurements and simulations have been performed on the behavior of leaded glass polycapillary fibers in the 9–80 keV energy range. The transmission efficiencies of these fibers of different types and lengths were measured as a function of source location and x-ray energy. The measurements were analyzed using a geometrical optics simulation program, which included roughness, waviness, bending effects, and a leaded glass filter layer. Despite low transmission at low energies, leaded glass polycapillary x-ray optics with a length of 30–60 mm seem promising for many high-energy (>20 keV) x-ray applications. The longer fibers have transmission efficiency of up to 50% in the 35–40 keV, and very low scatter transmission of less than 0.06% up to 80 keV.

Measurements and analysis of radiation effects in polycapillary x-ray optics

Polycapillary x-ray optics are arrays of large numbers of small hollow glass tubes which deflect x rays by successive total external reflection. These optics have growing numbers of applications in areas ranging from medical imaging to microanalysis. An accelerated radiation effects study has been performed to understand the performance limitation of these optics for medium to high intensity radiation applications, to study x-radiation damage mechanisms, and to investigate possible ways to mitigate the radiation effects on x-ray transmission efficiency. Exposures have been done in white beam bending magnet radiation with peak energies at 5 and 11 keV and focused broad band radiation centered at 1.4 keV. In situ and ex situ measurements of loss of x-ray transport efficiency have been executed at doses up to 1.8 MJ/cm2. Thin polycapillary fibers displayed noticeable bending and experienced substantial degradation of x-ray transmission. Thicker polycapillary fibers showed a linear but much slower transmission loss as a function of total dose. Annealing effectively restored the low energy (∼8 keV) transmission efficiency of the fibers. Exposure of these fibers at slightly elevated temperatures prevented any measurable loss in the low energy transmission efficiency. A variety of analytical techniques has been used to understand these results. No significant change was observed in the chemical composition of the capillary surface. Profile measurements and high energy transmission efficiency spectra, along with computer simulation studies, suggest that radiation induced bending is the primary cause of transmission efficiency degradation of the fibers.

Performance study of polycapillary optics for hard x rays

In order to investigate the feasibility of using Kumakhov capillary x-ray optics for high energy x-ray applications, measurements have been performed on the behavior of capillary optics from 10 to 80 keV. Transmission efficiencies of straight polycapillary fibers of different types have been measured as a function of source location and x-ray energy. The measurements are analyzed using a geometrical optics simulation program, which includes roughness and waviness effects. Despite the low critical angle for total external reflection at high energies, capillary x-ray optics appear promising for many hard x-ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis.

Gain for neutron polycapillary lens as a function of wavelength

A focusing lens using glass polycapillary fibers has achieved a current density gain of 80 for a polychromatic beam from a cold-neutron guide. The intensity gain of a lens depends on both the wavelength and the divergence of the incoming beam, and a simulation is required to determine the trends in the gain as a function of wavelength. However, measurement has shown that the gain is relatively insensitive to wavelength.

Prompt gamma activation analysis enhanced by a neutron focusing capillary lens

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.

Cold neutron guiding and focusing

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.

The characterization of a polycapillary neutron lens

Materials-analysis measurements that employ neutron-absorption techniques may benefit from cold neutrons that may be focused using a lens consisting of polycapillary fibers. An expression is derived for the intensity gain and the size of the beam along the beam path, including that at the focus, and a criterion is developed for the depth of focus. The gain in intensity at the focus depends not only on the design parameters of the lens but also on the divergence and wavelength of the incident beam. This analysis is applied to the two neutron lenses that have previously been tested.

Use of capillary optics as a beam intensifier for a Compton x-ray source.

The use of Kumakhov capillary optics will significantly enhance the performance of near-monochromatic, Compton backscattered x-ray programs. The Vanderbilt University Medical Free-Electron Laser Center is developing the capability to create these tunable x rays for medical imaging. The present transport has only reflection optics, and the beam is quite large in diameter at the laboratory. Low loss collimation of this beam would allow higher x-ray intensities after transport. This article describes experimental and computer simulation results which predict the expected performance for a multifiber Kumakhov collimator for use in the x-ray beam transport. Estimates from our research are that a multifiber optic formed of individual polycapillary fibers could be used to capture the full 7 mrad of the Vanderbilt x-ray beam and collimate it to a 1-2 mrad divergence with approximately 40%-50% transmission efficiency. This optic should increase the x-ray intensity at the laboratory level by a factor of > or = 5 by decreasing the beam divergence and subsequent spot size. Additionally, analysis of monolithic optics of fused multicapillary fibers predicts an increase in the intensity of the x rays at the laboratory by a factor of 55. These optics can have tapered channels that greatly decrease their exit divergence. This will greatly enhance the capabilities of this unique x-ray source. This article reports the initial results from a collaboration between Vanderbilt, The Center for X-Ray Optics at University at Albany, SUNY, and X-Ray Optical Systems in Albany, NY.

Guiding hard X rays with glass polycapillary fiber

X rays can be guided through a polycapillary fiber by multiple total reflections from the smooth channel walls of the fiber. Using monochromatic synchrotron radiation at energies of 22 and 44 keV, we measured the efficiency of transmission of X rays through polycapillary fibers with channel diameters of about 13 μm. Efficiencies of 57.3 and 54.5% for 22 and 44 keV X rays, respectively, were obtained with a 120-mm-long straight polycapillary fiber aligned with the incident beam. These values are close to the open fraction of the fiber, which is about 60%. In addition, transmission efficiency was measured as a function of the tilt angle between the incident beam and the axis of the fiber. We also measured the transmission efficiency as a function of the deflection angle for a 114-mm-long curved polycapillary fiber. The measurements are compared with a ray-tracing simulation.

Polycapillary-based X-ray optics

Glass capillaries can guide a broad energy range of X rays by means of multiple total reflections. Arrays of curved polycapillary fibers have many potential functions, including beam collection and focusing, beam collimation and energy filtering. The transmission efficiency and exit divergence of X rays guided through straight and bent polycapillary fibers have been measured for CuK α radiation and bremsstrahlung radiation from a point X-ray source. Experimental results have been compared with ray-tracing simulations. The good agreement between the two indicates that simulations can reliably describe the propagation of X rays through smooth glass capillaries. The same simulation program has been used to characterize the performance of a capillary-based X-ray lens which collects and collimates X rays from an anode source for use in diffraction applications. A 250-fold enhancement of photon flux within a microsteradian is predicted.

Neutron focusing using capillary optics

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.

Neutron focusing lens using polycapillary fibers

Multiple mirror reflection at small grazing angles from the smooth surfaces of the narrow channels of polycapillary fibers can be used to transport, bend, and focus thermal neutron beams. We report the results of the focusing of a polychromatic cold neutron beam using a compact lens of borosilicate fibers and with a focal distance of 57 mm. The intensity profile of the beam at the focus is approximately conical in shape with a full width at half-maximum of 0.49 mm, and with an average gain in intensity of about 20. These experimental results agree well with those obtained by computer simulation.

A comparison of experiment and simulation for neutron guidance through glass polycapillary fibers

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.