Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-04-20
2001-05-15
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S575000, C359S569000, C378S084000, C378S085000, C378S070000, C378S071000
Reexamination Certificate
active
06233096
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of electromagnetic radiation diffraction such as x-rays or neutrons. More particularly, it consists essentially of a diffractor for electromagnetic radiation based on a pseudo-spherical stepped geometry designed under the constant step width conditions.
BACKGROUND OF THE INVENTION
In many spectral devices characteristic radiation is generated by a small area of the sample surface. In such a case the source can be considered point like. In this case the conventional energy dispersive focusing technique is made either by cylindrical curved crystal (Advances in X-Ray Spectroscopy, Eds. C. Bonnelle, C. Mande, (Oxford, U.K., 1982)) or by doubly curved, on spherical or toroidal surfaces, crystal monochromators (U.S. Pat. No. 4,882,780; U.S. Pat. No. 4,807,268). These diffractors focus the monochromatic radiation onto the entrance detector slit. According to the Bragg's equation, the spectral resolution &Dgr;&lgr;/&lgr; depends both on &thgr; and &Dgr;&thgr;:
&Dgr;&lgr;/&lgr;=&Dgr;&thgr;/tan&thgr; (1)
The intensity of the monochromatic radiation is proportional to the area of the diffractor surface, that reflects x-rays under the given Bragg's angle &thgr; within the range ±&Dgr;&thgr;. However, increasing the reflecting area, a widening of the aperture ratio of the diffractor occurs associated to a simultaneous decrease of the spectral resolution.
In the last decade analytical investigations of the shape and size of the reflecting area of a crystal-monochromator surface, employing different focusing methods have been carried out (D. B. Wittry and S. Sun, J. Appl. Phys. 67, 1633 (1990); D. B. Wittry and S. Sun, J. Appl. Phys. 68, 387 (1990); D. B. Wittry and S. Sun, J. Appl. Phys. 69, 3886 (1991); D. B. Wittry and S. Sun, J. Appl. Phys. 71, 564 (1992); W. Z. Chand and D. B. Wittry, J. Appl. Phys. 74, 2999 (1993)). Indeed, x-ray diffractors with double curved crystal provide significantly greater aperture ratio compared to that based on the cylindrical Johann or Johannson geometries. For such devices assuming an incidence angle &thgr;>20° and a crystal height of L<0.1R, the reflecting surface projection on the XZ plane is rectangular and the projection on the focal circle plane (XY plane) is an arc of radius R=2r, where r is the focal circle radius. The knowledge of the shape of the reflecting surface allows an estimation of the parameters of a spherical diffractor designed with a stepped surface (D. B. Wittry and S. Sun, J. Appl. Phys. 69, 3886 (1991)) and in the case of constant step height, the aperture of this diffractor is larger than a spherical curved crystal.
SUMMARY OF THE INVENTION
An aim of the invention is to provide a diffractor specially dedicated to the x-rays range, based on a pseudo-spherical geometry directed to replace the plane or curved crystals in various apparatus (e.g., x-ray microanalyzer, x-ray photoelectron, spectrometer for chemical analysis, etc.).
For this purpose, the diffractor according to the present invention consists of a few small spherical curved dispersive elements (oriented crystal surfaces or gratings) that are located on a focal circle. The location on the focal circle of each element is made to guarantee the same Bragg angle for the incident radiation. Thus a diffractor according to the present invention is an array of diffracting elements (“steps”), each one contributing to the total solid angle of the diffractor, that increases the spectral output of the device without decreasing the resolution.
In the present invention a stepped diffractor is based on the physical condition that the single element of the diffractor subtends a constant angle width. With such design the efficiency of the diffractor is maximized and it is greater than both a spherical diffractor (U.S. Pat. No. 4,882,780; U.S. Pat. No. 4,807,268) and a stepped diffractor constructed under the constant step height condition (D. B. Wittry and S. Sun, J. Appl. Phys. 67, 1633 (1990)).
REFERENCES:
patent: 4807268 (1989-02-01), Wittry
patent: 4882780 (1989-11-01), Wittry
patent: 5027377 (1991-06-01), Thoe
patent: 5127028 (1992-06-01), Wittry
D.B. Wittry: “Properties of curved x-ray diffractors with stepped surfaces”, Journal of Applied Physics, vol. 69, No. 7, Apr. 1, 1991.
D.B. Wittry: “X-ray optics of doubly curved diffractors II”, Journal of Applied Physics, vol. 71, No. 2, Jan. 15, 1992.
D.B. Wittry: “Synthesis of x-ray intesity profiles for x-ray optical systems with curved diffractors”, Journal of Applied Physics, vol. 74, No. 5, Sep. 1, 1993.
V. Stojanoff et al.: “A high-resolution x-ray fluorescence spectrometer for near-edge absorption studies”, Review of Scientific Instruments, vol. 63, No. 1, Jan. 1992.
D.B. Wittry: “X-ray optics of doubly curved diffractors”, Journal of Applied Physics, vol. 67, No. 4, Feb. 15, 1990.
D.B. Wittry: “Focusing properties of curved x-ray diffractors”, Journal of Applied Physics, vol. 68, No. 2, Jul. 15, 1990.
Marcelli Augusto
Mazuritsky Mikhail I.
Soldatov Alexandre V.
Chang Audrey
Istituto Nazionale Di Fisica Nucleare
Lipton Robert S.
Lipton, Weinberger & Husick
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