X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
1999-05-07
2001-07-31
Kim, Robert H. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C250S505100
Reexamination Certificate
active
06269145
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
This invention relates to an apparatus that uses a plurality of thin lenses for the focusing, collection, collimation and general manipulation of x-rays for medical, industrial and scientific applications.
BACKGROUND—DESCRIPTION OF PRIOR ART
In the prior art the collection and focusing of x-rays has long been difficult to accomplish because x-ray reflection and refraction is limited to very small angles. Most x-ray optics use small-grazing-angle reflective surfaces that are limited to soft to moderate x-ray energies. Until recently, x-ray refractive lenses that are similar to ordinary visible-light refractive lenses, which collect, bend and focus visible photons, have not been considered to be feasible. Refraction of x-rays is difficult because the refractive index of all materials is slightly less than 1, (i.e. (n−1)<0 and |n−1|<<1) with the possible exception for photon energies near the photo-absorption shell edges of the lens substrate material, where n can be larger than 1.
Recently, renewed interest has been given to refractive x-ray lenses due to an important, but simple, idea as theorized by Toshihisa Tomie (U.S. Pat. No., 5,594,773) and demonstrated by A. Snigirev, V. Kohn, I. Snigireva and B. Lengeler, (“A compound refractive lens for focusing high-energy X-rays, Nature 384, 49 (1996)). It has long been known for optics in the visible spectrum that a series of N closely spaced lenses, each having a focal length of f
1
, has an overall focal length of f
1
/N (e.g. F. L. Pedrotti and L. Pedrotti, “Introduction to Optics,” Prentice Hall, Chapt. 3. p.60, 1987). Recently, Tomie and Snigirev et al. have shown that this can also be done in the x-ray region of the spectrum using a series of holes drilled in a common substrate that effectively mimics a linear series of lenses. This “compound refractive x-ray lens” (CRL) is manufactured using N number of unit lenses, each constituted by a series of hollow cylinders or holes that are embedded inside a material capable of transmitting x-rays. Two closely spaced holes form what appears to be a concave-concave (bi-concave) lens at their closest juncture. N holes result in N unit lenses. For x rays, the index of refraction of the material is less than 1; thus, unlike optical refraction optics, which will cause visible rays to diverge, the bi-concave lens performs in opposite fashion and focuses x-ray photon energies instead.
This embodiment of the prior art of Tomie and Snigirev et al. is shown in FIG.
1
A and
FIG. 1B. A
unit x-ray lens, shown in a top view in
FIG. 1A
, is made of a hollow cylinder
2
of radius R
h
has a focal distance, f
1
, represented by:
f
1
=
R
h
2
δ
(
1
)
where R
h
is the radius of the hole and the complex refractive index of material is expressed by
n=
1−&dgr;−
i&bgr;
(2)
As shown in
FIG. 1A
, a single hollow cylinder
2
represents two plano-concave lenses,
4
. Closely spacing a series of these holes as shown in
FIG. 1B
results in a focal length of:
f
=
f
1
N
=
R
h
2
N
δ
(
3
)
A series of hollow cylinders
2
approximates a series of bi-concave cylindrical lenses
6
. Comparing eqn. (1) and (3), the focal length, f, for the series of lenses is reduced by 1/N from that of a single lens. Thus, a single lens made of a hole in A
1
with radius R=100 &mgr;m, will have a focal length of 10 meters at 30 keV, whereas, a compound refractive lens composed of 100 holes will give a 0.1 meter focal length. This is a dramatic reduction in focal length, making such a refractive lens useful.
As stated previously, utilizing multiple lenses to reduce the focal length in other parts of the electromagnetic spectrum has been well know for years and is in a standard textbook for optics (Pedrotti and Pedrotti). The Tomie patent teaches particular fabrication techniques utilizing a single material substrate with holes or spheres for all the lens elements. In the prior art of Tomie, obtaining good focusing characteristics for a series of N lenses required that the machining of the holes be “conducted at a high precision capable of keeping the geometric error within a small fraction of the value obtained by dividing the wavelength of the x rays to be focused by &dgr; of the lens material (=&lgr;/&dgr;).”
Tomie suggests that arranging larger numbers of lenses in a cascading series of N individual unit lenses (not a single substrate for all lenses) stacked as shown in
FIG. 2
would work to reduce the focal distance f by f/N: however, “In this configuration . . . many unit X-ray lenses have to be arranged after fabricating the individual unit X-ray lenses. The thickness of each unit x-ray lens has to be very thin to avoid strong absorption of X-rays, making each unit X-ray lens very fragile and difficult to handle. Moreover, aligning the optical axis of all units along the X-ray lens axis with high precision would be extremely difficult. Hence, arranging many X-ray lenses in the configuration shown in FIG.
1
” (in the present patent: also
FIG. 2
) “is practically impossible.” (our underline, Tomie, U.S. Pat. No. 5,594,773, coll. 4, lines 19-28).
Note in
FIG. 2
, the thin lenses are in contact, which presents difficulties in both support and alignment. Indeed, there is no alignment or support structure shown. To solve this problem, Tomie utilizes a single common substrate with accurately machined holes or embedded spheres which act as quasi-lenses. He teaches that thin unit lenses that do not have such a common substrate cannot be utilized for CRLs since they would be difficult to stack and align (Their thinness and fragility prevent them from being stacked and aligned). The required thicknesses of between 1 to 100 microns make them difficult to stack without damage and difficult to align.
In the prior art, accuracy of the lenses' dimensions, alignment and spacing is achieved by utilizing a single substrate material with holes drilled by conventional means such as computer-driven machine drilling or laser drilling. Such drilling methods make it difficult to achieve lens thicknesses (e.g. spacing between holes, &Dgr;, as shown in
FIG. 1B
) of less than 25 microns, i.e. such spacing limits the minimum thickness of each individual lens component to 25 microns. Conventional machine drilling methods for hole spacing less than this will result in the drill breaking through the wall between holes. Conventional laser drilling techniques will result in tapered walls. Wall thicknesses of 25 microns or larger result in large absorption of x rays in a compound refractive lens of even a few single elements for x-ray energies below 4 keV. As stated by P. Elleaume, the Tomie lens design's “drawbacks are their limitation to high photon energies above 4 keV due to absorption, their strong chromatic aberrations and low aperture.” (P. Elleaume, “Two-Plane Focusing of 30 keV Undulator Radiation with a Refractive Lens.” pp. 33-35 in Research & Development, ESRF).
Tomie also pointed out that rather than cylindrical or spherical shapes, a material having a concave shape of a paraboloid of revolution is theoretically ideal as an x-ray lens. As stated in the above quote from Elleaume, it is well known that cylindrical and spherical surfaces will give strong chromatic and spherical aberrations. An ideal surface would be parabolic in shape. Such a shape is impossible to obtain using conventional machine drill techniques. In the prior art, only machine drill techniques have been utilized to achieve the Tomie design. (P. Elleaume, and Snigirev et al. papers cited above). He also points out in his invention that the extent to which the focal length can be shortened by reducing the radius of the cylinder or sphere has limits due to fabrication techniques, and absorption in the lens material. Hence, “the focal length f remains quite long even after maximum practical reduction.”
Another problem with the simple Tomie configuration, as stated by P. Elleaume in the above quote, is that that the ap
Beguiristain Hector R.
Cremer Jay T.
Pantell Richard H.
Piestrup Melvin A.
Adelphi Technology Inc.
Hobden Pamela R.
Kim Robert H.
Smith Joseph
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