Compact spectrometer focusing apparatus

A spectrometer focusing apparatus is provided that includes a hollow cylinder for x-rays to traverse a length thereof, a defracting element configured as a ring on an interior circumference of a portion of the hollow cylinder, at least one disk having an edge defining a circle aligned with the defracting element, and an aperture formed between the defracting element and the edge of the at least one disk.

BACKGROUND

1. Field of the Invention

The present invention relates generally to x-ray spectroscopy and, more particularly, to an apparatus for focusing a von Hamos type crystal x-ray spectrometer, and a method for operation of same.

2. Description of the Related Art

X-ray detectors fall into three general categories. Counting detectors do not discriminate different x-ray energy, i.e., wavelength. Energy-dispersive detectors, e.g., solid-state silicon or germanium, can discriminate x-ray energy with a limited resolution of approximately 150 eV. These two types of x-ray detectors are relatively simple and easy to use. Diffractive detection employs crystal spectrometers of various geometries, with energy resolution greater than 1 eV.

However, diffractive detection devices are generally large, complex and expensive.

An x-ray spectrometer is an optical device used to resolve and select different x-ray photon energies. The x-ray spectrometer operates by using a diffractive crystal which, for any given angle, diffracts x-rays of a specific energy according to the Bragg equation (a):
wavelength=2dsin Θ,  (1)
where d is crystallographic spacing of the diffracting crystal, and Θ is an angle that the x-ray is incident on the crystallographic plane.

A von Hamos x-ray spectrometer includes an x-ray source defined by a rectangular slit, a cylindrically bent crystal, and a position sensitive detector located on a crystal axis of curvature. The crystal is bent cylindrically around a horizontal axis, parallel to a direction of dispersion. The crystal provides focusing in a vertical direction. A front surface of the detector, the slit axis and the axis of curvature are all positioned along a same vertical plane. For a fixed position of the components, an incident x-ray location on the detector corresponds geometrically to a particular Bragg angle and therefore to a particular x-ray energy.

J. Hoszowska, et al., High resolution von Hamos Crystal x-ray Spectrometer, Nucl. Instr. Meth. A 376 (1996) 129, discloses a von Hamos spectrometer that uses a segment of cylindrically curved crystal to focus diffracted x-rays along a line, with different energies diffracted at different positions along the line. The different energies are selected by tuning an aperture along the line or using a detector capable of spatially resolving intensity along the line.

The von Hamos geometry permits the spectrometer to collect data over an energy bandwidth (30-300 eV), limited primarily by the detector length, for one position of the components thereof. Study of a greater energy interval is performed by adjusting a central Bragg angle by translation of the crystal and corresponding translation of the detector along their axes. Different crystals are used at different angular specifications to allow the spectrometer to observe x-rays in an expanded energy range from 0.547 to 16.8 keV.

U.S. Publ. No. 2009/0252294 of O'Hara discloses a scanning von Hamos type x-ray spectrometer in which a crystal is bent into a cylindrical surface, with the source and detector plane on an axis of the cylinder. O'Hara selects energy, i.e., wavelength, by scanning a set of apertures to limit the angle that x-rays pass to the diffractor. When the device of O'Hara scans, the detector position remains at a single point of focus.

However, conventional systems lack portability and are complex in the required operation and control of numerous components. Also, conventional systems diffract from a same place along the diffractor, eliminating any variation in efficiency that might arise from differences along the diffractor.

SUMMARY OF THE INVENTION

To resolve the drawbacks and disadvantages of conventional systems, the apparatus and method of the present disclosure provides a portable device that uses only an apparatus formed along a perimeter of a disk to limit an angle of incidence, and selects energy by scanning an entire detector/spectrometer assembly toward or away from a sample.

According to one aspect of the present invention, a medium resolution, i.e., 10-100 eV, spectrometer is provided that includes a hollow cylinder for x-rays to traverse a length thereof, a defracting element configured as a ring on an interior circumference of a portion of the hollow cylinder, at least one disk having an edge defining a circle aligned with the defracting element, and an aperture formed between the defracting element and the edge of the at least one disk.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described in detail with reference to the accompanying drawings. Detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring the subject matter of the present disclosure. Further, in the following description of the present disclosure, various specific definitions found in the following description are provided to provide a general understanding of the present disclosure, and it is apparent to those skilled in the art that the present disclosure can be implemented without such definitions.

FIG. 1is a profile view illustrating components of an embodiment of an apparatus of the present disclosure. As shown inFIG. 1, a spectrometer focusing apparatus is provided that includes a hollow cylinder configured for x-rays to traverse a length thereof. A defracting element120(FIG. 4), e.g., a crystal, defines an interior circumference on a portion of the hollow cylinder. At least one disk110is provided having an edge defining a circle aligned with the defracting element120. An aperture115(FIG. 4) is formed between the defracting element and the edge of the at least one disk110.

The spectrometer focusing apparatus100is provided for use with a medium resolution, i.e., 10-100 eV, spectrometer, and uses a defracting element120that is bent along an interior portion of a complete inner circumference of the hollow cylinder320(FIGS. 5-6), forming a ring. Energy is selected by translation of the apparatus along a line, i.e., an optic axis140, from a position S1, S2, S3of an x-ray source150to a detector160, with a position of the x-ray source150(FIG. 3) defining an angle of incidence of x-rays transmitted from the source through the aperture115to the detector160. The same ring is used for all energies transmitted through the apparatus.

In the apparatus of the present disclosure, the Bragg angle is not selected at a focus. Rather, the Bragg angle is selected at the crystal, using an opaque disk spaced apart from a surface of the crystal.

A distance D1, D2, D3between the disk110and a position S1, S2, S3of the source150defines an angle of incidence, and consequently defines a corresponding energy. Diffracted x-rays are measured by the detector160, which is positioned downstream from the disk110and the source150.

InFIG. 1, a distance between detector160and disk110is fixed. The distance between the detector160and the source (S1, S2, S3) allows for portability of the spectrometer focusing apparatus100and facilitates attachment of the spectrometer focusing apparatus100to various detector types, with the spectrometer focusing apparatus100being configured to directly mount onto the detector160. The cylinder320is configured to attach to one of a counting detector, an energy-dispersive detector, and a Canberra Ge solid-state detector. When attached to the detector, improved energy resolution of the spectrometer is realized of approximately 10 eV to 100 eV, without adding any moving parts.

As the detector160and spectrometer focusing apparatus100are moved toward a source150, the angle of incidence increases and the diffracted x-rays are focused short of the detector160, with the x-rays still being measured by the detector160. When the spectrometer focusing apparatus100and detector160are moved farther away from one of a plurality of positions (S1, S2, S3) of a source, the angle of incidence decreases and the diffracted x-rays are focused past the detector160, with the x-rays still being measured by the detector160.

The distance S1, S2, S3from the source150to the disk110is then calibrated to photon energy using Equations (2) and (3):
energy=12.4/2d*sin(angle)  (2)
tan(angle)=radius of crystal ring/source distance  (3)

For a crystal with known crystallographic spacing, a finite range of energy is identified for a given distance between the disk110and the detector160, based on an active area of the detector160. An accessible energy range is determined based on a range of angles that allow the diffracted x-rays to strike the active area of the detector. The range is shifted by changing a distance between the disk110and the detector150. Ranges can also be accessed by choosing crystals with different crystallographic spacings, with an accessible energy range being varied by changing the defracting element to a defracting element having different crystallographic spacings. Other thin crystal materials can be bent into the cylinder, and less flexible materials can be segmented into a number of flats oriented into the cylinder, with only a slight degradation in resolution and throughput.

FIG. 2is a graph showing energy resolution of an embodiment of the present disclosure.

InFIG. 2, energy and throughput are shown on the x and y axes, respectively. Energy resolution, or sharpness of the edges of a bandpass window, is a function of source size, owing to the slight spread of angles coming from different parts of the source however small, as illustrated at the far left ofFIG. 2.

FIG. 3is an expanded view ofFIG. 1that illustrates an angle of energy bandpass of the present disclosure. The angle of energy bandpass is a range of energies diffracted by the crystal120, as a function of the incident angle.FIG. 3shows an aperture115between the disk110and the crystal120, and a thickness of the disk110. The thickness of the disk110can vary, or the disk can be split into thinner pieces with adjustable spacing therebetween. Changing a thickness of the disk110changes energy broadcast to the detector160, and changes the angle of incidence, with the accessible energy range being determined based on a range of angles that are determined to allow diffracted x-rays to strike the active area of the detector160.

As shown inFIG. 4, which provides a profile view of an interior of an embodiment of the apparatus of the present disclosure, a plurality of disks110a,110bare provided. Each of the plurality of disks110a,110bhas an edge defining a respective circle that aligns with the defracting element120. One disk110ais movable with respect to the other disk110balong the optical axis140of the hollow cylinder320. The angle of incidence changes by varying the distance between the two disks110a,110b, i.e., separating the edge of a first disk110aaway from or closer to the edge of the second disk110b. Varying the distance between the two disks110a,110badjusts the angle or range of angles, and modifies the energy bypass to tune the bandpass to match specific applications and to enhance versatility and match additional applications.

FIG. 5is a perspective view of an apparatus of the present disclosure.FIG. 6is a front view of an apparatus of the present disclosure.

As shown inFIGS. 5 and 6, a hole310traverses a center of the apparatus. The hole310is preferably plugged when the spectrometer is in use. The spectrometer focusing apparatus100is mounted onto a detector160using the hole310by screw or similar attachment. The spectrometer focusing apparatus100has a length of several inches and a width of approximately half the length, and is approximately ten times smaller than conventional spectrometers.

The apparatus ofFIGS. 1 and 3-6includes a diffracting element, e.g., a crystal, that forms an inner circumference of a cylinder320of the spectrometer100. The diffracting element and an outer edge of the disk110are provided on the optic axis140to define an aperture115having a uniform circular shape. The diffracting element and the outer edge of the disk110are provided at a same position along a length of the cylinder320. The disk110fills a portion of an interior of the cylinder320, with the edge of the disk110positioned transverse to a lengthwise direction of the cylinder320that corresponds to the optic axis140.

The disk110is positioned adjacent to the diffracting element120, with an aperture115formed between the diffracting element120and the edge of the disk110, to permit electron passage from the source to the detector, as shown inFIGS. 1 and 3.

Energy is selected by adjusting a distance between the spectrometer and the sample, thereby determining the angle, while the detector can be stationary or can attach to and travel with the spectrometer.

Accordingly, a compact apparatus is provided that improves medium resolution energy resolution, i.e., approximately 10 to 100 eV, of conventional counting or energy-dispersive detectors, and provides a simple and cost-effective way to discriminate between x-ray fluorescence or emission energies that are less than 150 eV apart, for both synchrotron-based and laboratory sources.

The spectrometer focusing apparatus uses only one or more disks to limit the angle of incidence, and energy is selected by scanning an entire assembly of the detector and spectrometer focusing apparatus. The scanning is performed toward or away from the sample, providing an advantage of only having to control one motion, thereby simplifying operation. An additional advantage that is provided is only specifying a single point of diffraction along the diffractor, thereby eliminating variation in efficiency that can arise from inconsistencies along the diffractor. Another advantage is providing a simplified structure that can be readily affixed to various commercial detectors, in alignment with a sample position, to scan energy/wavelength simply by moving the detector toward or away from the sample, without adding moving parts to operability over energy ranges that do not depend on precisely equal diffraction at different positions along a defracting element, such as a single crystal ring that can be used for all energy ranges.

The spectrometer focusing apparatus operates with simple counting or solid state detectors, and does not require more expensive specialized strip or area detectors.

FIG. 7is a graph illustrating results of testing utilizing the spectrometer focusing apparatus100. The testing was conducted by attaching spectrometer focusing apparatus100to a Canberra Ge solid-state detector. A source was provided of Ni Ka (and Kb) fluorescence from a roughly 10 micron spot illuminated by higher-energy x-rays in an x-ray fluorescence microprobe. The spectrometer-to-sample distance was varied while monitoring the Ni Ka and Kb signal reaching the detector.

FIG. 7shows intensity in arbitrary units versus position of the spectrometer focusing apparatus, with energy decreasing to the right ofFIG. 7.

InFIG. 7, line420corresponds to Ni Ka fluorescence, and lines410correspond to Ni Kb fluorescence, which is about 800 eV higher energy, demonstrating that the spectrometer focusing apparatus can successfully isolate the two different fluorescence energies. The Ka peak has a width (full width at half maximum) of about 135 eV, which is significantly better than the energy resolution of a Ge detector (approximately 165 eV, FWHM). The structured intensity shown inFIG. 7just outside the main peak is due to imperfect test alignment. However, throughput, i.e., efficiency, is on the order of 0.2% of the total fluorescence that the 100 square mm detector area would see from the same source.