We have developed a new type of X-Ray spectrometer intended for light element micro-analysis applications. This spectrometer typically has energy resolution better than 20 eV, very high count rates for low X-Ray energies characteristic of light elements, quasi-parallel data collection similar to Energy Dispersive Spectroscopy (EDS), weighs less than 20 lb., and is mechanically very simple. Fabrication of this spectrometer is possible due to our development of X-Ray collection/collimation optics for low X- Ray energy which collect large solid angles of sub-KeV x-rays and reflect them into a parallel beam. The spectrometer is intended for electron beam microanalysis, small spot XRF and other X-Ray micro-analysis methods. In this paper, we present the theory of operation, some details of collimator fabrication, spectrometer fabrication and some preliminary test data.
Frequently, EDS (Energy Dispersive Spectrometer) x-ray detectors cannot be placed very close to an x-ray source for microanalysis causing the detector to subtend a small solid angle thus reducing the available count rate. This is exacerbated by low count rates for the light elements and situations where low energy spectral lines are immersed in a background of higher energy x-rays from heavier elements. Larger detectors can be used at the expense of resolution but in many situations there is insufficient room for a large detector so a 10 mm<SUP>2</SUP> detector is used. Notch filters can sometimes be used to minimize higher energy counts but they do not allow a broad spectrum of low energy x-rays to pass and still allow passage of the highest energy x-rays to the detriment of light element detection. We have developed low energy x-ray optics which increase the solid angle seen by EDS detectors and can also act as low pass filters preferentially allowing passage of low energy x-rays. In a typical situation where 10 mm<SUP>2</SUP> detectors are used with a 35 mm distance between x-ray source and detector surface, our optics can provide a flux gain of about 22X for B (Boron) x-rays with gain decreasing toward unity at energy above O (Oxygen at 525 eV) with gain remaining at unity for higher energy. In other words, we greatly increase the performance at the lowest energies without affecting the higher energies.
Applications for laboratory soft-x-ray/VuV sources would benefit from the ability to collect a large energy bandwidth of radiation emanating from the very small source and redirect it into a well collimated beam without losing most of the incident radiation. Such optics would be beneficial to x-ray spectroscopy, x-ray lithography, diffractometry, and other applciations. We have been working to apply technology originally developed for astronomical x-ray telescopes to production of low cost replicated collimation optics for such x-ray/VuV instruments. Most of the steps in the production of these optics have previously accomplished with the larger astronomical optics but we want to reduce the size of these optics by at least an order of magnitude which introduces problems. In addition, very few copies of an x-ray telescope are made while we want to make hundreds of copies of our optics. This paper briefly discusses the design and fabrication of these small collimation optics and is a report on work in progress.
By stacking many very thin extraordinarily smooth sheets of material with a gap between each to form open channels, a new class of optical components for x-rays, ultraviolet light and neutrons can be made. Radiation propagates via grazing incidence reflections from the channel walls. If the stack is bent, radiation can be made to bend through large angles, and by properly forming the entrance or exit aperture, concentrators or collimators can be fabricated. This paper contains a discussion of the theory and fabrication of these laminar microchannel optics. Direct control over the reflecting surface and the stacking of the channels give these optics some unique advantages over other microchannel optics. Channel walls can be made as thin as 12 micrometers and coated with nearly any desired reflecting material and surface roughness below 2 angstroms can be readily achieved. Open spaces between the planar channels are as small as 8 micrometers or as large as desired. These optics provide the capability to bend, focus, or collimate broad energy bandwidths of radiation with optics that have apertures of up to several cm<SUP>2</SUP>, high throughput, and small size.
Physitron has constructed a grazing incidence nested optic designed to collect and collimate a broad energy bandwidth of diverging soft x-rays (nominally 1.5 keV) emanating from a point source. Although this optic was designed to collimate rather than focus x-rays, our optic is similar to the nested conical foil x-ray telescopes which have been constructed and successfully used by NASA. Key differences exist between our optic and NASA's telescopes. First, the aperture of our optic is 28 cm<SUP>2</SUP> which is considerably smaller than NASA's telescopes. Second, the reflectors in NASA's telescopes are contained in an annular ring, leaving the middle of the optic open. Our optic has a much smaller open area at its radial center. These differences required an innovative fabrication technique in which the reflective rings are formed as complete rings from lacquer-smoothed aluminum foil instead of forming the reflectors as quadrants as in NASA's technique. This paper will discuss the key design considerations and procedures for the collimator in addition to a description of the fabrication technique used.