X-ray apparatus

Abstract

To provide X-ray diffraction, reflection, transmission and interference optical systems fabricated from lithium (Li), sodium (Na) and strontium (Sr) borate crystals.SOLUTION: An X-ray optical system incorporates a refractometer, interferometer, spectrometer, diffractometer or imaging device for analyzing a sample. The X-ray optical system is configured with a monochromator fabricated from low atomic mass metal borates MxByOz crystals, where M represents a low atomic mass metal, and x, y, and z represent respective atom numbers of metal, borate and oxygen in chemical formulas. The metal borates include borates of lithium (Li), sodium (Na) or strontium (Sr).SELECTED DRAWING: Figure 5A

Description

[Technical Field]

[0001] This disclosure relates to X-ray optics. In particular, this disclosure relates to X-ray diffraction, reflection, transmission, and interference optics fabricated from lithium (Li), sodium (Na), and strontium (Sr) borate crystals. [Background technology]

[0002] X-rays are electromagnetic radiation with properties similar to those of light, but at much shorter wavelengths. While visible light has a wavelength of around 6000 angstroms, X-ray wavelengths range from 0.1 to 300 angstroms. This ultra-short wavelength is what gives X-rays their ability to pass through materials that visible light cannot. For target materials commonly used in X-ray tubes, X-rays have well-known, experimentally determined characteristic wavelengths. In addition, a continuous X-ray spectrum is also produced.

[0003] X-rays are classified into two different directions: soft and hard X-rays. The former are characterized by relatively low energy, and those below 5 keV would be considered soft X-rays. Soft X-rays can be absorbed in air. X-rays with energies above 5 keV are typically referred to as hard X-rays. Hard X-rays have the ability and energy to pass through various types of materials, and therefore they are commonly used for industrial purposes to find internal defects in objects or parts.

[0004] X-ray technology has two main applications: medical applications and industrial applications. Medical applications fall into two categories: diagnostic procedures such as computed tomography (CT), fluoroscopy, and others, and therapeutic procedures such as cancer treatment. Industrial applications utilize X-rays as an invaluable source for non-destructive radiographic testing (RT) applications, which provide an outlet for internal component analysis in 2D or 3D techniques. For example, X-rays are a very common application of RT to access internal component analysis in 2D to identify faults or foreign bodies within components. Other X-ray industrial applications include spectroscopic, diffraction, reflection, interference, and transmission testing applications, which provide information about the composition and structure of materials and their bulk as well as their surface structure and topography.

[0005] X-ray optics include X-ray sources as well as X-ray diffractometers, X-ray topography tools, extended-range X-ray absorption fine structure (EXAFS) and wavelength X-ray fluorescence (XRF) systems, X-ray microscopes, and interferometers. All of these X-ray tools, with rare exceptions, are based on nearly perfect single crystals that function as diffracting, reflecting, transmitting, and interfering optical elements. A single crystal is a solid form of matter in which atoms and molecules are arranged with a high degree of order or regular geometric periodicity throughout the entire volume of the material. X-ray optics based on polycrystals are also known. Polycrystals consist of many individual single crystals of small size, commonly referred to as grains.

[0006] Generally, there are two measurement techniques that utilize X-ray optics: polychromatic and monochromatic. Monochromatic techniques are widely used in commercial applications and, obviously, require monochromatic radiation, which is usually produced by a single crystal monochromator. One common feature of all single crystal monochromators is a narrow curve of reflected intensity versus angle of incidence at the angular position that satisfies the Bragg diffraction condition for a given X-ray wavelength. This angular position is known as the Bragg angle. The curve is called a rocking curve. The width of the rocking curve is usually given as a full width at half maximum (FWHM) value, with the maximum intensity being the point where the Bragg diffraction condition is satisfied. For single crystal monochromators, FWHM values ​​typically do not exceed 10-20" arc seconds. The rocking curve is also characterized by the percentage of incident radiation that is reflected by the crystal; this characteristic is called reflectivity. Reflectivity is directly related to the absorption of radiation in the crystal, which is determined by the linear absorption coefficient, and to the crystal structure. The latter is in turn characterized by the so-called structure factor. Furthermore, a further distinguishing feature of single crystal X-ray monochromators is their structural perfection, i.e., the presence of a minimum amount of structural defects that contribute to rocking curve broadening and cause other undesirable effects.

[0007] Figure 1 shows the monochromatic Cu-K laser irradiating the wafer. α1 The following example illustrates a measured rocking curve for a 400 reflection from a nearly ideal silicon (Si) single crystal wafer, which indicates the quality of the crystal lattice, characterized by a small FWHM and a relatively large reflectivity value for the X-ray beam. The 400 label in the above reflection refers to the so-called hkl Miller indices that indicate the crystal lattice planes and X-ray reflections. Based on the above, for a given hkl reflection and incident X-ray wavelength, a smaller FWHM value and a larger reflectivity value in the rocking curve indicate higher quality of the single crystal X-ray monochromator. For an ideal single crystal of a specified structural type and chemical composition, the rocking curve with the lowest FWHM and highest reflectivity value can be calculated for a given hkl reflection and incident X-ray wavelength using X-ray diffraction theory. This curve is referred to as the intrinsic rocking curve.

[0008] The concept of rocking curves, both measured and inherent, can be better understood in the context of X-ray diffractometers, which are used in a variety of applications, including, for example, spectroscopy, diffraction, reflectometry, interferometry, and imaging, all well known to those skilled in the art of X-ray metrology. Each of these scientific measurement techniques uses a continuous or characteristic component of the X-ray spectrum to probe matter through its interaction with various components of the X-ray spectrum. Each technique measures the results of this interaction by detecting the intensity of various components of the X-ray spectrum scattered by the irradiated sample. For a given structure and chemical composition of a sample, factors that affect the measured intensity include the angle of incidence, scattering angle, and measurement time. These techniques are essential for X-ray analysis of biological tissues, thin film analysis, sample surface and texture structure evaluation, crystalline phase monitoring, crystalline structure and lattice defects, and the investigation of sample stress and strain.

[0009] Structurally, an X-ray diffractometer consists of a crystal monochromator that operates in the following manner: When an incident X-ray beam encounters the monochromator's crystal lattice at any angle of incidence, elastic and inelastic scattering of the X-ray beam occurs from the electrons of the crystal atoms. Most of the elastically scattered X-rays are rejected due to destructive interference, but when the angle of incidence equals a certain angle (i.e., the Bragg angle), diffraction then occurs. Some X-rays scattered in one direction from atomic planes are in phase with X-rays scattered from other atomic planes of the same type. The scattered in-phase X-rays constructively interfere to form a new, enhanced wavefront. The relationship by which diffraction occurs is known as Bragg's law or equation. Because each crystalline material has a characteristic atomic structure, it will diffract X-rays in a unique characteristic pattern.

[0010] 2 highly diagrammatically illustrates an exemplary optical schematic of an X-ray diffractometer 15, which includes a crystal monochromator 28 that diffracts X-ray radiation emitted by an X-ray source 22 and transmitted through a sample 16. The basic geometry of the X-ray diffractometer 15 involves a polychromatic radiation source 22 and an X-ray detector 24, a CCD camera shown in this figure, located downstream from the sample 16.

[0011] The crystal monochromator 28 is configured to ensure that the scattered or detected radiation is monochromatic. When the monochromator 28 is appropriately positioned before or after the sample 16, only the desired / selected wavelengths of the X-ray spectrum emitted by the X-ray source reach the sample 16 or detector 24 after being reflected by the monochromator 28 at specific angles of incidence and reflection. All other spectral wavelengths are diffracted at slightly different angles and thus avoid the detector 24. In other words, the monochromator 28 acts as a spectral filter or analyzer. The X-ray intensity scattered by or transmitted through the sample 16 reaches the detector 24, which collects X-ray photons in space and time and converts the collected photons into an electronic signal by well-known signal-shaping hardware and methods associated with the selected type of detector 24. The electronic signal is further processed in an electronic system known to those skilled in the art.

[0012] As mentioned above, for a selected X-ray wavelength, the requirements for a high-quality crystal monochromator include high reflectivity, small FWHM, and low linear absorption values. These values ​​are determined primarily by the structure, composition, and quality (i.e., defect concentration) of the crystal utilized, as well as the quality of the crystal's surface orientation and surface treatment. Additional requirements to be considered may be the available size and manufacturability of the crystal. Tuning a crystal monochromator for a particular analytical method is often based on a trade-off between the requirements listed above.

[0013] There are only a few crystals used in monochromatic X-ray optics that at least partially meet the above requirements. Among these crystals, silicon (Si) and germanium (Ge) crystals are of the highest quality (i.e., low defect concentration). Si crystals have low linear absorption comparable to Ge. However, Ge reflectivity is comparable to Si due to the high number of electrons that scatter the incident X-ray radiation. The rest of the known crystals utilized for monochromators, including particularly highly specific crystals with large interplanar distances, are far below Si and Ge crystals in quality and size scale.

[0014] In light of the ever-increasing industrial demand for high-power X-ray radiation, especially for recently introduced synchrotrons—particle accelerators capable of producing beams of X-rays several orders of magnitude more powerful than known conventional devices—this short list of monochromator crystals becomes particularly daunting. Furthermore, some currently used monochromatic X-ray crystals, such as acid phthalate crystals (e.g., KAP), are prone to rapid degradation even at relatively low powers. Even the highest-quality Si and Ge crystals are vulnerable to oxidation and tend to have useful lives not exceeding about three years. Note that Si and Ge crystal monochromators cost about $15,000–$20,000 each—not necessarily pocket change. Still other crystals, such as graphite, are known for their poor quality despite being stable and time-resistant. Summary of the Invention [Problem to be solved by the invention]

[0015] Therefore, a need exists to utilize optics fabricated from low atomic mass metal borates, such as those made from lithium (Li), sodium (Na), and strontium (Sr) borate crystals, for x-ray applications.

[0016] Another need exists for a method to monochromatize X-ray radiation by utilizing LBO crystals. [Means for solving the problem]

[0017] According to one aspect of the present disclosure, the X-ray optical system incorporates one of a refractometer, an interferometer, a spectrometer, a diffractometer, or an imaging device, and comprises an X-ray source that outputs broadband X-ray radiation in a wavelength range of 0.01 to 1 nm, and an LBO crystal-based monochromator that optically interacts with the received X-ray radiation.

[0018] According to another aspect of the present disclosure, a method for monochromatizing X-ray radiation includes utilizing an LBO crystal.

[0019] Various aspects of the present disclosure are described below with reference to the accompanying drawings, which are not intended to be drawn to scale. The drawings are included to provide illustration and further understanding of various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended as a definition of the scope of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain the principles and operation of the described and claimed aspects and embodiments. In the drawings, each identical or nearly identical part in the various views is represented by a like numeral. For purposes of clarity, not every part is labeled in every view. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 illustrates a measured rocking curve for 400 reflections from a nearly ideal silicon (Si) single crystal wafer. [Figure 2] 1 is an illustrative optical schematic diagram of a known prior art X-ray diffractometer. [Figure 3A] FIG. 1 illustrates the calculated intrinsic rocking (reflection) curve of LBO. [Figure 3B] FIG. 1 illustrates the calculated intrinsic rocking (reflection) curve of Si. [Figure 3C] FIG. 1 illustrates the calculated intrinsic rocking (reflection) curve of Ge. [Figure 4] FIG. 1 is an exemplary optical schematic diagram of a double crystal spectrometer with a single monochromator fabricated from an LBO crystal. [Figure 5A] FIG. 1 illustrates the measured (experimentally obtained) rocking curve of LBO. [Figure 5B] FIG. 1 illustrates a measured (experimentally obtained) rocking curve of Si. [Figure 5C] FIG. 1 illustrates a measured (experimentally obtained) rocking curve of Ge. DETAILED DESCRIPTION OF THE INVENTION

[0021] Described herein are optical schematics of X-ray diffractometers used in X-ray spectroscopy, diffraction, reflectometry, interferometry, and imaging. In particular, the illustrated schematics each include a monochromator constructed from an LBO crystal and operating in either reflection or transmission mode. LBO monochromators offer several advantages, including a narrow rocking curve, high reflectivity, and high mechanical integrity.

[0022] Figures 3A-3C illustrate the calculated intrinsic rocking (reflection) curves in relative units, i.e., intensity reflected from atomic planes versus the angle of incidence of the monochromatic X-ray beam. In particular, the curves are calculated for the strongest symmetric 111 reflection of CuKa1 X-rays in Bragg geometry for single crystal plates of LBO (Figure 3A), Si (Figure 3B), and Ge (Figure 3C), respectively. In the symmetric Bragg geometry, the reflective atomic plane, such as (111), is parallel to the upstream face of the monochromator or the sample to be tested. As can be seen, the intrinsic rocking curve of LBO has a FWHM that is nearly three times smaller than that of Si and nearly six times smaller than that of Ge. The theoretical peak reflectivity and linear absorption parameters of LBO are also better than those of Si and Ge, respectively, as summarized in the following table.

[0023] [Table 1]

[0024] 4 illustrates an exemplary optical schematic of a single crystal X-ray spectrometer 40. The spectrometer 40 includes an X-ray source 30 selected from a conventional tube, a rotating anode system, and a synchrotron. While the scope of the present invention includes all of the above types of X-ray source 30, preferably the source is a hard energy source emitting hard X-rays, although the latter does not exclude the possibility of working with soft X-rays. Polychromatic X-ray radiation is incident on a monochromator 32 at an angle of incidence θ.

[0025] In accordance with the principal concept of the present invention, the monochromator 32 is made from borate of lithium (LiB3O5) or strontium (SrB4O7) or sodium borate. The material for the monochromator can be selected single crystal or polycrystalline. For convenience, this description further refers to LBO single crystal, but the entire disclosure relates to a group of low atomic mass metal borates, including additional compounds each having a different chemical formula. For example, other LBOs besides LiB3O5 can include LiBO2 and Li2B4O7. Thus, for purposes of generality, metal borates encompassed in this disclosure are M x B y O z where M is Li, Na, and Sr, and x, y, and z are the number of atoms in the chemical formula of the compound.

[0026] The monochromator 32 is a reflector that selects a narrow spectral band of the broadband X-ray beam from the source 30 and reflects this intense monochromatic beam onto the single crystal sample 34. The angle of incidence is equal to the angle of reflection at the reflecting surface of the monochromator 32, resulting in a symmetrical diffraction diagram of the monochromator shown. The angle of incidence θ at the reflecting surface of the monochromator 32 is equal to or close to the angle of incidence Φ at the receiving / upstream reflecting surface of the single crystal sample 34, resulting in a diffraction diagram that is referred to as non-dispersive. However, because the sample 34 may represent not only single crystals but also polycrystalline materials, liquids, and even gases, a wide range of angles of incidence is utilized for the analysis of these samples. Thus, the monochromatic X-ray beam irradiates the single crystal sample 34 at an angle of incidence Φ, and the sample 34 reflects the incident beam at the same angle. A detector 38 is set at an angle 2Φ relative to the incident beam position to collect the X-ray photons reflected from the single crystal sample 34.

[0027] A variation of the optical schematic of FIG. 4 may include a triple crystal X-ray spectrometer in a symmetric diffraction configuration. Specifically, this configuration includes a monochromator, such as LBO or sodium (Na) or strontium (Sr) borate, that receives a polychromatic beam of X-rays from an X-ray source. The monochromator reflects the desired monochromatic beam, which is incident on the sample to be examined, similar to the schematic of FIG. 4. The monochromatic beam reflected from the sample is further incident on an analyzer crystal, which is identical to the monochromator. The analyzer reflects the received X-rays to a detector. The use of the analyzer provides background subtraction as well as improved resolution of the rocking curve collected for the sample.

[0028] Figures 5A-5C illustrate the respective rocking curves for the strongest 111 reflection measured in counts per second while varying the angle of incidence of monochromatic radiation. In particular, experiments were performed on flat LBO, Si, and Ge crystal plates approximately 0.7 mm thick in symmetric Bragg geometry with monochromatic Cu-Ka1 x-rays. The parameters of these rocking curves are shown in Table 2.

[0029] [Table 2]

[0030] The measured FWHM and peak integrated intensity values ​​vary between LBO, Si, and Ge, consistent with the corresponding changes in the values ​​calculated for the intrinsic reflectance curves shown in Table 1 and Figures 3A-3C. The observed absolute difference between the measured and calculated FWHM values ​​for each crystal is explained by the optical aberrations associated with the Ge220 monochromator and the minimum angular step limitations inherent in the utilized model of the X-ray diffractometer. However, the measured peak maximum intensity for LBO is lower than that of the calculated reflectance curve. This is also explained by the uncertainty in the calculation of the atomic scattering factors for Li and the temperature factors for Li, B, and O atoms in the LBO crystal, as well as the experimental nature of the measured LBO crystal plate, which has a (111) orientation, which is unusual for this material.

[0031] The peak maximum intensity of the LBO 111 reflection can be increased by a factor of 1.5 to 2.6 by asymmetric Bragg diffraction, i.e., reflection of X-rays from (111) atomic planes that are not parallel to the surface of the LBO crystal plate. To this end, as an example, the monochromator 32 in Figure 4 is intentionally cut from the LBO crystal so that its reflective (111) atomic planes make an angle with the surface of the monochromator plate, which is slightly smaller than the Bragg angle for the 111 reflection, thus minimizing the angle of incidence with respect to the crystal surface. This type of monochromator is called an asymmetric monochromator.

[0032] Having thus described several aspects of at least one example, it should be recognized that various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, the sample to be analyzed may be provided upstream from the monochromator. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the examples described herein. Accordingly, the foregoing description and drawings are by way of example only. [Explanation of symbols]

[0033] 15 X-ray diffractometer 16 samples 22 X-ray source, polychromatic radiation source 24 X-ray detector 28 Crystal Monochromator 30 X-ray source 32 Monochromator 34 Single crystal samples 38 detectors 40 Single Crystal X-ray Spectrometer

Claims

[Claim 1] 1. An X-ray optical system incorporating a refractometer, interferometer, spectrometer, diffractometer or imaging device for analyzing a sample, comprising a monochromator fabricated from a group of low atomic mass metal borates MxByOz, where M is the low atomic mass metal and x, y, z are the atomic numbers of the metal, borate and oxygen, respectively.

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