D.G. de Kerckhove

Calcification provides mechanical reinforcement to whale baleen alpha-keratin

Hard α-keratins such as hair, nail, wool and horn are stiff epidermal appendages used by mammals in a variety of functions including thermoregulation, feeding and intraspecific competition. Hard α-keratins are fibre-reinforced structures consisting of cytoskeletal elements known as ‘intermediate filaments’ embedded in an amorphous protein matrix. Recent research has shown that intermediate filaments are soft and extensible in living keratinocytes but become far stiffer and less extensible in keratinized cells, and this stiffening may be mediated by air-drying. Baleen, the keratinous plates used by baleen whales during filter feeding, is an unusual mammalian keratin in that it never air dries, and in some species, it represents the most heavily calcified of all the hard α-keratins. We therefore tested the hypothesis that whale baleen is stiffened by calcification. Here, we provide, to our knowledge, the first comprehensive description of baleen material properties and show that calcification contributes to overcoming the shortcomings of stiffening this hard α-keratin without the benefit of air-drying. We also demonstrate striking interspecies differences in the calcification patterns among three species of baleen whales and provide novel insights into the function and evolution of this unusual biomaterial.

Micropatterning of Foturan photosensitive glass following exposure to MeV proton beams

A new proton lithography mechanism has been identified and investigated: the micropatterning of photosensitive etchable glass based on the crystallization of the glass after irradiation with MeV protons and heat treatment. The use of MeV protons results in a significantly reduced minimum feature size compared with reported results using ultraviolet (UV) irradiation and the threshold dose for etching is very low (4000 protons µm−2), offering the potential for creating complex microstructures by direct writing using very short exposures. The depth of the structures is determined only by the range of the protons in the glass, which allows structures with different depths to be fabricated. This technique appears to be very valuable for the rapid fabrication of high aspect ratio microstructures such as fluid networks and micro-optical devices in a material with useful optical and mechanical properties.

A beam rocking system for the Oxford nuclear microprobe: A new approach to channeling analysis

Abstract This paper describes the design and implementation of a beam rocking system for the Oxford nuclear microprobe. This system varies the angle of incidence of a focused MeV ion beam on a sample using two set of dipole scan coils of equal and opposite fields, allowing the sample to remain fixed while the beam is rocked in angle about a small area. This method of producing angle-resolved channeling images and scans does not require any modifications to existing data-acquisition software nor the use of a high-precision motorized goniometer. Beam optics calculation results are presented for a beam rocking system using three different standard microprobe focusing systems. The maximum angular range of this system is 3.15°. Channeling images and measurements of angular rotation in a strained Si 0.90 Ge 0.10 Si crystal are presented using this technique.

MASKLESS FABRICATION OF 3-DIMENSIONAL MICROSTRUCTURES IN PMMA USING A NUCLEAR MICROPROBE

Abstract This paper describes the use of a focused 3 MeV proton beam to produce 3-dimensional microstructures in polym-ethylmethacrylate (PMMA). Scanning electron micrographs of 125 μm thick structures in PMMA are presented, such as microgears. A four-axis goniometer is used to tilt the sample to produce structures with a range of inclination angles in conjunction with a computer-controlled scanning system. Two eight-blade microturbines, the first 500 μm wide with a 10° tilt angle on each blade, the second, 1 mm-wide with a 25° tilt on each blade, are presented to illustrate the potential of this method for producing complex 3-dimensional microstructures. This technique is compared to the LIGA process (deep-etch X-ray lithography) and to UV laser induced polymerization which are both capable of producing deep (hundreds of microns), high aspect ratio structures. We conclude that high energy proton lithography compares favourably with these techniques.

An optimised beam rocking system to produce angle-resolved information from small areas

Abstract Beam rocking is a new technique in nuclear microscopy which has been developed on the Oxford nuclear microprobe. Angle-resolved information is produced by rocking a focused MeV ion beam over several degrees using two sets of dipole scanning coils with opposite fields. This paper describes the modifications to the system which allow the beam to be rocked about areas a few microns wide without affecting the achievable angular range. Beam optics simulations of the beam displacement in the image plane are presented to show the effects of: rotational misalignment between two sets of dipole scan coils, imbalances in the field strengths of the scan coils, displacement of the sample along the beam ( z ) axis and adjustment of the quadrupole focusing lens strengths. Experimental grid patterns are presented showing that the beam displacement can be reduced to ±3 μm in the horizontal direction and ±6 μm in the vertical direction using these procedures.

Bending MeV proton beams in graded composition Si1−xGex/Si layers

Abstract This paper characterises the ability of graded composition Si 1− x Ge x /Si bilayers to bend MeV proton beams through small angles. The graded germanium epilayer composition results in off-normal lattice directions of the epilayer being gradually bent, so channeled protons are gently deflected away from their initial direction. The relationship between the incident beam energy, bilayer tilt angle, emergent angle and bending efficiency for 3, 5 and 10 MeV protons are simulated using a Monte Carlo channeling code and studied experimentally using the Utrecht magnetic spectrometer. A maximum bending efficiency of 35% of those protons, which are channeled at the front surface of the epilayer is measured with the bilayer in a reverse geometry, and good agreement with simulated results is demonstrated. These results are used to predict the bending behaviour at higher beam energies and the relevant optimum layer characteristics.

Confirmation of proton beam bending in graded Si1-xGex/Si layers using ion channeling

A graded composition Si1−xGex/Si [001] layer, which has recently been proposed as a method for bending and extracting protons from high-energy particle accelerators, has been studied by angle-resolved ion channeling analysis using focused MeV proton and He+ beams. Backscattering spectrometry confirms that the composition is linearly graded and a maximum Ge concentration of 0.16 was measured at the epilayer surface. Off-normal planes {111} are curved with respect to the substrate by a total angle of 0.332° and efficient bending of channeled particles along the curved planes and into the substrate is confirmed.

Characterisation of lens aberrations and parasitic fields using beam rocking

Abstract Beam rocking has been used on both the Oxford high-excitation triplet and the Melbourne Russian quadruplet to study lens aberrations such as multipole fields. A focused MeV proton beam is rocked over a fine mesh grid and the resulting beam displacement is mapped. Distorted patterns are produced in the presence of multipole fields originating either in the lenses or elsewhere due to stray fields. The minimisation of parasitic multipole fields is not only required in order to focus ion microprobes to small spot sizes, but is also important for the production of proper angular channelling scans using beam rocking since such fields distort an otherwise linear scan. In the absence of sextupole and higher order multipole fields, symmetric patterns are produced, exhibiting only spherical aberration. The patterns obtained experimentally are interpreted with the aid of simulations produced using an ion optics ray-tracing program.

Transmission ion microscopy using a quadrupole triplet as an objective lens

Abstract This letter describes the use of a triplet of magnetic quadrupoles as an objective lens to produce magnified, transmission images of free-standing grids using 3 MeV protons hitting a fluorescent screen. The maximum magnification achieved was approximately ×55, enabling 6 μm wide grid holes to be resolved. This work thus presents a simple working version of a transmission ion microscope.

Detection of small lattice strains using beam rocking on a nuclear microprobe

Beam rocking is a new technique in nuclear microscopy which has been developed at the Oxford nuclear microprobe to produce angle-resolved channelling information from small areas of crystalline material without any rotation or translation of the sample. This paper describes a different application of beam rocking whereby a focused 3 MeV proton beam has been used to detect and quantify small interface rotation angles in strained Si1−xGexSi samples with 0.015 < x < 0.175, where the sample has been selectively etched to expose the underlying substrate. By eliminating possible rotation errors due to translation of the sample stage or backlash in the gears of a goniometer, small rotation angles have been measured, and these are found to be in good agreement with electron back scatter diffraction (EBSD) results. It is also shown that, by minor modifications to the scanning or focusing system, the area over which the rocked beam moves on the sample surface can be reduced to < 6 μm for a ∼3° angle. In a further development of the Oxford nuclear microprobe scanning system, it is shown how the addition of a second set of scanning coils eliminates previous problems of dechannelling due to angular tilting in spatially resolved channelling scanning transmission ion microscopy (CSTIM) images.