US2999931A - X-ray interferometer - Google Patents

Description

Sept. 12, 1961 P. w. ZINGARO 2,999,931

X-RAY INTERFEROMETER Filed Dec. 4, 1958 2 Sheets-Sheet 2 INVEN TOR. .7? MLZZAM Zpvaweo.

BY M A.

United States Patent X-RAY INTERFEROMETER Placido William Zingaro, Hartsdale, N.Y., assi'gno'r to Philips Electronics, Inc. Filed Dec. 4, 1958, Ser. No. 778,175 15 Claims. (Cl. 250- 51.5)

My invention relates to an X-ray interferometer or device for the precision measurement of X-ray absorption edges and emission lines in the X-ray spectral region.

In many respects X-rays are similar to visible radiation. Thus, X-rays, like radiation in the visible spectrum, are a form of electromagnetic radiation and are transmitted, absorbed and reflected in many respects like visible radiation. However, because of their extremely short wave-length, X-rays are highly penetrating and are readily transmitted through most materials. Unlike visible radiation, the laws of optics do not apply to X-rays.

One of the principal difiiculties With X-rays is the measurement of their precise wave-length. Every element, and its isotopes, when suitably excited, e.g. when bombarded by high-energy electrons having a potential V in volts, emits X-rays the wave-length A of which is determined by the following relationship:

he W;

Where h is the Planck action constant and has the value 6.55 x 0 is the velocity of light which has the value 3X10 cm./sec. and e is the charge on the electron which has the value 4.8x 10- (1) thus reduces to:

.If the potential V is sufliciently high, the spectrum of the emitted beam will show sharp lines or peaks superimposed on the continuous background which are characteristic of each element. The characteristic emission lines appear in groups designated as K, L, M, N, 0 series, corresponding to the ejection of an electron from the corresponding electron shell of the atom. These same characteristic X-rays are also emitted as fluorescent X- rays if X-rays of suflicient energy fall upon the element.

Each of these series of emission lines contains several definite lines of difierent wave-lengths. Thus, the K series of all the elements except the lightest consists of four principal lines, the 7 (also designated 5 and actually a close doublet), ,8 (really a close doublet B and B and the doublet a and 0: in the order of increasing wave-lengths. Similar lines are found in the other series.

There are also absorption discontinuities observed in X-ray spectra whenever X-rays undergoing spectroscopic analysis pass through absorbing material; the wavelengths corresponding to these discontinuities, or edges, are also characteristic of each of the chemical elements. All rays with wave-lengths shorter than that of a given discontinuity, or edge, will be absorbed by the element to a markedly greater extent than rays with Wave-lengths longer than this critical value. Inother words, a material that is relatively opaque to X-rays of a range of wave-lengths up to a characteristic value is transparent to longer rays.

Heretofore, measurement of the wave-length of X-rays has been with diffraction by ruled gratings, diffraction by crystals, refraction in prisms, and measurement of absorption by known materials. Each of these methods has disadvantages in the precise determination of wavelength. Difiraction by crystals is probably the most widely used since a crystal is a natural grating in which parallel planes of regularly marshalled atoms are spaced Patented Sept. 12, 1961 from each other. at distances that are of the same order ofv magnitude as X-ray Wave-lengths. The determination of wave-length is governed by Braggs law which states:

nk= 2d Sin 9 where n is the order of reflection, A the wave-length, d the known distance between parallel planes in the crystal, and 0 is the spectrometrically measured angle of incidence of the ray upon these planes (or 20, the angle of diffraction or reflection). v

In order to measure A, the interplanar distance d and the angle of incidence 0 must be known with great precision. Since the interplanar spacing of certain crystals is known with considerable precision, the measurement of A essentially involves the measurement of the angle of incidence 9, or more conveniently, the angle of diffraction or reflection, 20. Absolute precision in the measurement of the angle 0 is almost unattainable. v

In accordance with the present invention, the wavelength of an X-ray can be measured with absolute precision since it obviates the requirement of measuring the angle 0. Since the velocity of X-rays, like that of light, is constant, and is related to frequency f:

it follows that if an X-ray beam can be divided into two parts and one part transmitted over a longer path than the other, and the two parts recombined before they are detected, an interference pattern will be created upon recombination of the divided beam. Consequently, if the length of the longer path is increased or decreased, thereby changing the phase of one of the beams with respect to the other beam at the point of recombination, the reunited beam will vary in intensity in a periodic manner from a low value to a relatively high value. The number of such alternations will be directly proportional to the change in length of the path, and inversely proportional to the wave-length of the X-rays.

The invention will be described in connection with the accompanying drawing in which:

FIG. 1 shows an embodiment of the invention;

FIG. 2 shows the curves of interference patterns between radiations of slightly difierent wave-lengths;

FIG. 3 shows a preferred embodiment of the invention;

FIGS. 4, 5 and 6 show different embodiments of the invention;

FIG. 7 shows a mechanism for changing the path length of one portion of the X-ray beam in any of FIGS. 3 to 6; and

FIG. 8 shows an embodiment of the invention in which an unknown is compared with a standard wave-length.

FIG. 1 is illustrative of the invention. An. X-ray beam 1, which may be generated by an X-ray tube (not shown) or an element exposed to X-rays having a wavelength shorter than the absorption edge of that element so that fluorescent characteristic X-rays are generated therefrom is incident upon a difiracting crystal 2 so positioned that a portion 3 of the rays are diffracted toward a crystal 4. Another portion 5 of the rays is transmitted through the diifracting crystal (which is thin enough so that some of the X-rays pass therethrough without being diffracted) toward a crystal 6. Crystal 2 being at an angle with respect to the beam will, in accordance with Braggs law, difiract or reflect rays of one wave-length 7\ toward crystal 4 which is positioned at right angles to the diffracted beam. Crystal 6 is also positioned at right angles to the transmitted beam.

If the interplanar spacings of crystals 4 and 6 are the same and if the wave-length diffracted by crystal 2 is numerically equal to 2d (d being the interplanar spacing in crystals 4 and 6) it will be diffracted back along the incident path and reenter crystal 2. Similarly, the transmitted portion of the beam incident upon crystal 6 will undergo diffraction of the part of the beam of wave length equal to the 2d spacing of the crystal 6 and be reflected back along the path of incidence toward crystal 2.

i The diflracted beam from crystal 4 incident upon crystal 2 is transmitted, in greater part, through the crystal while the difiracted beam from crystal 6 incident upon crystal 2 will be diflractcd by the latter crystal along the same direction as the diffracted beam from crystal 4 which is transmitted through crystal 2. Consequently, the diffracted beams from crystals 2 and 4, which are of the same wave-length, are recombined upon emerging from crystal 2 and impinge upon a detector 7 which can detect and record the intensity of X-rays, e.g. a Geiger-Muller Counter, proportional counter, scintillation counter, ionization gauge, film, etc.

If the length of the path between crystals 2 and 4 is the same as that between crystals 2 and 6, the separate beams will be in phase when they are reunited after emerging from crystal 2. If the path lengths differ by a fraction of a wave-length, the two beams will be out-of-phase when they are recombined and their combined intensity will be less than when in phase.

Therefore, if either crystal 4 or crystal 6 is moved relative to the diflracting crystal 2, thus changing the length of the path of one of the beams, a recombined beam of varying intensity will be produced. Since one wave-length, A will correspond exactly to the distance required to move one of the crystals to obtain precisely the same intensity, for determining the wave-length of the X-ray, it is necessary to measure the variation of the distance between one of the crystals 4 and 6 and diffracting crystal 2 whereby two succeeding intensities of the same value are obtained. Or, if the same intensity is measured as one of the crystals 4 or 6 is moved a fixed, precisely determined distance relatively to crystal 2, the wave-length will be that distance divided by the number of repetitions of the intensity pattern.

This interference pattern is comparable to the "Michelson Visibility Curves (cf. Monk, Light Principles and Experiments, p. 152) which are shown in FIG, 2, The curves show the minor periodic alternations in the Overall interference pattern when two radiations of slightly differing wave-lengths are used. These minor alternations form the envelope of the major alternations which are produced by the interferences due to the predominant radiation.

Since wave-lengths of X-rays are measured in angstroms cm and fractions of angstrom another embodi ment of my invention is shown in FIG. 3. In this em! bodiment, the beam is again divided by a crystal 8 but the divided beams travel over much greater path lengths to a detector 7.

As in FIG. 1, an X-ray bearnl is difiracted in part and transmitted in part. The diffracted portion of the beam is incident upon a crystal 9 so positioned that it difiracts once again the diffracted beam so that it is incident upon a second crystal 10. Y

The portion of the beam transmitted by crystal 8 is incident upon a crystal 11 so positioned that it difil'acts the transmitted portion of the beam parallel to the portion diffracted by crystal 8 so that it is incident upon crystal 12 which is so positioned that it diflracts the beam incident upon it parallel to the beam incident'upon crystal 9. The beam diffracted by crystal 12 is incident upon crystal 13 so positioned that the beam diffracted by crystal 9 and diffracted by crystal 10 passes through it while the beam difiracted by crystal 12 is diifracted by crystal 13. Consequently, the two beams are recombined after emerging from crystal 13.

H In order to measure the wave-length, crystals 9 and 10 are moved simultaneously in a vertical direction away from and toward the diflracting crystal 3 and detector 7, crystals 12 and 13 remaining stationary. A mechanism 14 for effecting this movement is shown by the dashed lines and is described below.

FIG. 4 shows an embodiment in which the path lengths between the difiracting crystal 8 and crystal 13 (where the rays are recombined) can be made identical.

This embodiment of the invention, as well as those shown in FIGS. 3, 5, and 6, has the advantage that it is suitable for any wave-length measurement.

In addition, the crystals may be arranged to difiract in the higher orders,'or in combinations of orders. In FIG. 4, which is the arrangement for equal path lengths, crystals 8 and 13 reflect in the first order and crystals 10 and 11 in the second order.

In FIG. 5, the arrangement is for first order reflection from the four crystals.

In FIG. 6, crystals 8 and 11 reflect in the second order and crystals 10 and 13 reflect in the first order.

The use of higher order reflection provides a means for greater instrumental resolution and therefore more accurate wave-length measurements.

A mechanism for moving crystals minute distances and at a slow rate is shown in FIG. 7. Crystals 9 and 10 (for simplicity this device, shown separately, is described in connection with FIGS. 3, 4, 5, and 6 but may be modified for use with FIG. 1) are mounted on supports 16 and 17, respectively, which are slidably mounted on guides 18 and 19 respectively. Supports 16 and 17 are connected by means of rigid connecting members 20 and 21, respectively to a moveable frame 22. Through a worm gear drive (not shown) frame 22 can be moved by shaft 23 driven by a motor 24 through a worm and gear set 25.

In FIG. 8, two interferometers, one of which .26 is a standard or comparison interferometer and the other 27 for measuring the unknown wavelength, are mounted on moveable members which are rigidly interconnected. In the standard 26, the crystals are preferably fixed for one wave-length which is known to a high degree of accuracy-such as the MoKoq line. In the other interferometer 27, the crystals have been adjusted to diifract the radiation whose wave-length is to be determined.

As the path lengths in both interferometers are being changed, the number of alternations of intensities in both instruments are recorded. The number of alternations noted for the standard will then serve as a measure of the change in path lengths.

By means of the devices herein described it is not only possible to measure wave-lengths of radiation as such but also to detect variations in distribution of the charge on th nucleus due to a change in the size of the nucleus as in the case of isotopes. Such change in the charge distribution in the nucleus will efiect a change in the energy levels of the electrons resulting in the production of X-radiation of changed wave-length.

Another application of this invention is the measure ment of wave-lengths of gamma radiations which are products of nuclear changes in the nucleus of the atom. Heretofore, the measurement of the wave-length of gamma radiation could only be determined with great difficulty because the wave-lengths are very much shorter than the shortest X-rays making precision measurements very difficult, if not impossible. Since these wave-length changes are so small, present methods are incapable of distinguishing between the wavelengths making such dc terminations impossible.

While I have described my invention in connection with specific embodiments and applications thereof, other modi fications and adaptations will be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims. 7

What I claim is:

1. An X ray interferometer comprising a detector, a first difiracting crystal positioned in the path of a beam of X-rays to divide said beam into two portions, a second difi'acting'c'rystal positioned to intercept and reflect one portionof the X-rays along a first given path toward said detector, a third difl'racting crystal positioned to intercept and reflect the other portion of the X-rays along a "second given path toward said detector, and means to move one of said second and third difiracting crystals relative to said detector to thereby vary the length of one of said paths and produce changes in intensity of the resultant beam of X-rays formed by combining said two portions of the X-ray beam in said detector.

2. An X-ray interferometer comprising a detector, a first diffracting crystal positioned in the path of a beam of 'X-rays to divide said beam into two portions, a second ditfracting crystal positioned to intercept and reflect one portion of the X-rays along a first given path toward said detector, a second diffracting crystal positioned to intercept and reflect the other portion of the X-rays along a second given path toward said detector, a fourth difiracting crystal positioned to intercept and combine said two portions into a single beam which is transmitted along a common path toward said detector, and means to move one of said second and third diffracting crystals relative to said detector to thereby vary the length of one of said given paths and produce changes in intensity of the resultant beam of X-rays formed by combining in said path the two portions of the X-ray beam incident upon said fourth difiracting crystal.

3. An X-ray interferometer comprising a detector, a first difiracting crystal positioned in the path of a beam of X-rays to ditfract a portion thereof and to transmit therethrough another undiffracted portion, a first re- ,flecting second diflracting crystal positioned to intercept and'reflect the diffracted portion of the X-rays along afirst given path toward said detector, a third diffracting crystal positioned to intercept and reflect the transmitted undiffracted portion of the X-rays along a second given path toward said detector, and means to move one of said second and third diffracting crystals relative to said detector to thereby vary the length of one of said given paths relative to the other and produce changes in intensity of the resultant beam of X-rays formed by combining in said detector the diffracted and undifiracted 'portions of the X-ray beam.

4. An X-ray interferometer comprising a detector, a :first diffracting crystal positioned in the path of a beam of X-rays to difiract a portion thereof and to transmit therethrough another undifiracted portion, a second difjtracting crystal positioned to intercept and reflect the diffracted portion of the X-rays along a given path ftoward said detector, a third diifracting crystal posi- ..tioned to intercept and reflect the transmitted undifiracted portion of the X-rays along a second given path toward said detector, a portion of said first and second given paths coinciding whereby said diifracted and undiflracted portions of said X-ray beam are recombined before entering the detector, and means to move one of said second and third difiracting crystals relative to said detector to -thereby vary the length of said path and produce changes :in intensity of the resultant beam of X-rays formed by "combining the diffracted and undifliracted portions of the X-ray beam.

5-. An X-ray interferometer comprising a detector, a first difliracting crystal positioned in the path of a beam of X-rays to diffract a portion thereof and to transmit therethrough another 'undiflracted portion, -a second diflracting crystal positioned to intercept and reflect the diffracted portion of the X-rays along said given path toward a detector, a third diffracting crystal positioned to intercept and reflect the transmitted undifiracted portion of the X-rays along a second given path toward said detector, a portion of said first and second given paths being coincident whereby said difiracted and undifiracted portions of said X-ray beam are recombined before entering the detector, means interposed in at least one of said given paths to equalize the total lengths of said first '6 l and second given paths, and means to move oneof said second and third diifracting crystals relative to said detector to thereby vary the length of one of said given paths and produce changes in intensity of the resultant beam of X-rays formed by combining the difiracted and undiffracted portions of the X-r-ay beam.

6. An X-ray interferometer comprising a detector, a first difiracting crystal positioned in the path of a beam of X-rays to 'diffract a portion thereof and to transmit therethrough another undifiracted portion, a second diffracting crystal positioned to intercept and reflect the diffracted portion of the X-rays along a given path toward said detector, a third ditfracting crystal positioned to intercept and reflect the transmitted undilfracted portion of the X-rays along a portion of said path toward said detector, and means to move one of said second and third difiracting crystals relative to said detector to thereby vary the length of at least a portion of said path and produce changes in intensity of the resultant beam of X-rays formed by combining in said path the diffracted and undiflracted portions of the X-ray beam.

7. An X-ray interferometer comprising a detector, a first difl'racting crystal positioned in the path of a beam of X-rays at an angle adapted to difiract a portion thereof and to transmit therethrough another undiflracted portion, a second diflracting crystal positioned normal to the diffracted portion of the X-ray beam to thereby reflect the same toward said first diifracting crystal, a third diifracting crystal positioned normal to the undifiracted portion of the X-rays to reflect the same toward said first diifracting crystal, said first difiracting crystal being positioned so that the diifracted portion of the X-ray beam is transmitted through the first ditfracting crystal and said reflected undiffracted portion of the X-ray beam is difiracted by the first diffracting crystal, the reflected diffracted and undifiracted portions of the X-ray beam after emerging from the first diifracting crystal being recombined, a detector positioned to intercept and detect the intensity of the recombined beam of X-rays, and means to move one of said second and third diffracting crystals relative to said detector to thereby vary the length of said path and produce changes in intensity of theresultant beam of X-rays formed by combining in said path the diffracted and undiflracted portions of the X-ray beam incident upon the diffracting crystal.

8. An X-ray interferometer comprising a detector, a first diflracting crystal positioned in the path of a beam of X-rays to difiract a portion thereof and to transmit therethrough another unditfracted portion, a second diffracting crystal positioned to intercept and reflect the difiiracted portion of the X-rays along a first given path toward said detector, a third diflracting crystal positioned to intercept and reflect the transmitted undiflraoted por tion of the X-rays along a fourth given path toward said detector, a second ditfracting crystal positioned to intercept and combine the ditfracted and unditfracted portions of the X-ray beam and transmit the recombined X-ray beam toward said detector, and means to move one of said second and third diffracting crystals relative to said detector to thereby vary the length of said path and produce changes in intensity of the resultant beam of X-rays formed by combining in said path the diffracted and undifiracted portions of the X-ray beam incident upon said fourth difiracting crystal.

9. An X-ray interferometer comprising a detector, a first diffracting crystal positioned in the path of a beam of X-rays to diffract a portion thereof and to transmit therethrough another undiffracted portion, a second diffracting crystal positioned to intercept and reflect flie diffracted portion of the X-rays along a first given path toward said detector, a third diffracting crystal positioned to intercept and reflect the transmitted undifiracted portion of the X-rays along a fourth given path toward said detector, a second diflracting crystal positioned to intercept and transmit therethrough one of said reflected porions of said X-ray beam while difiracting the other reflected portion along a path common to both portions whereby said portions are recombined before entering the detector, and means to move one of said second and third difiracting crystals relative to said detector to thereby vary the length of said path and produce changes in intensity or the resultant beam of X-rays formed by combining in said path the diflracted and unidifiracted portions of the X-ray beamincident upon said fourth diifracting crystal.

10. An X-ray interferometer comprising a detector, a first diffracting crystal positioned in the path of a beam of X-rays to difiract in the first order a portion thereof and to transmit thcrethrough another undiifracted portion, a second difiraoting crystal positioned to intercept and reflect in the second order the undiftracted portion of the X-rays along a first given path toward said detector, a third difiracting crystal positioned to intercept and refleet in the second order the undifiracted portion of the X-rays along a second given path toward said detector, a fourth difiracting crystal positioned to intercept and transmit therethrough one of said reflected portions of said X-ray beam while diffracting in the first order the other portion along a path common to both portions whereby said portions are recombined before entering the detector, and means to move one of said second and third diffracting crystals relative to said detector to thereby vary the length of said path and product changes in intensity of the resultant beam of X-rays formed by combining in said path the diffracted and undiifracted portions of the X-ray beam incident upon the fourth difiracting crystal.

11, An X-ray interferometer comprising a detector, a first diiiracting crystal positioned in the path of a beam of X-rays to diflract in the first order a portion thereof and to transmit therethrough another undifiracted portion, a second difiracting crystal positioned to intercept and reflect in the first order the undiffracted portion of the X-rays along a first given pathtoward said detector, a third difiracting crystal positioned to intercept and reflect in the first order the reflected'undiffracted portion of the X-rays along a second given path toward said detector, a fourth difiracting crystal positioned on said second path to intercept and transmit therethrough the twice reflected por tion of said X-ray beam while ditfracting in the first order the difiracted portion of said beam along path whereby said difiracted and undifiracted portions are recombined before entering the detector, and means to move said second and third difiracting crystals relative to said detector to thereby vary the length of said path and produce changes in intensity of the resultant beam of X-rays formed by combining in said path the diffracted and undifiracted portions of the 'X-ray beam incident upon the fourth diffracting crystal.

12 An X-ray interferometer comprising a detector, 21 first difiracting first crystal positioned in the path of a 'beam of X-rays to difiract in the second order a first portion thereof and to transmit therethrough a second undifiracted portion, a second difiracting crystal positioned to intercept and reflect in the second order the undifiracted second portion of the X-rays along a first given path toward said detector, a third diffracting crystal positioned -to intercept and reflect in the first order the reflected undifiracted second portion of the X-rays along a second given path toward said detector, a fourth diflracting crystal positioned to intercept both said diffracted first position and said reflected diffracted second position and to transmit therethrough one of said two last named portions of said X-ray beam whilediffracting in the first order the other of said two last named portions along a'path common to both portions whereby said portions are recom bined before entering the detector, and means to move one of said second and third crystals relative to said do tector to thereby vary the length of said path and produce changes in intensity of the resultant beam of X=rays formed by combining in said path the difiracted and on difiracted portions of the X-ray beam incident upon the fourth diflracting crystal.

13. X ray apparatus comprising a pair of X-ray interferometers each including a detector, a first difira ting crystal positioned in the path of a beam of X-rays to divide said beam into two portions, a second diftracting crystal positioned to intercept and reflect one portion of the X.- rays along a first given path toward said detector, and a third diffracting crystal positioned to intercept and refiect the other portion of the X-rays along a second given path toward the detector; one of said interferometers being adapted to receive radiation of known v -length and the other being adapted to receive radiation of unknown wave-length; and means to simultaneously move one of the second and third diifracting crystals in each of said interferometers to thereby obtain two intertcrence patterns whereby the unknown wavelength can be determined.

14. Xqay apparatus comprising a pair of X-ray inter ferometers each including a detector, a first difirac ng crystal positioned in the path of a beam of X rays to divide said beam into two portions. a second difiracting crystal positioned to intercept and. reflect one portion of the X-rays along a first given pa h toward said d tector, and a third difiracting cry tal positioned to intercept and reflect the other portion of, the X-rays along a second given path toward the detector; one of sa interf rometers being adapted to receive radiation of known wave length and the other being adapted to receive radiation of unknown wavelength; and means to simultaneously move like second and third difiractin ystals in each of said interferometers to thereby ob ain wo in erference patterns whereby the unknown wave-length can he dotemiined.

15. X-ray apparatus comprising a pair of ,Xn'ay interferomet rs each including a detector, 2. first diflrac ing crystal positioned in the path of a beam of X rays'ot ha known wave-length to divide said'beam into two pottions, a second difiracting crystal positioned to intercept and reflect one portion of the X-ray alonga first given path toward said detector, and a third ditfracting crystal positioned to intercep a d reflec the soother portion of the X-rays along a second given path toward the detector; one of said interferometers being adapted to receive radia 'tion of known w velength and the other being adapted to receive radiation of unknown wave-length; means to moveablysupport one of the second and third diffractigg crystals in each of said interferometers; and means to corinect said crystal support means together for jointly moving the same to thereby obtain two interference patterns whereby the unknown wave-length can be determined,

' References Cited in the file of this patent V UNITED STATES PATENTS 2,653,249 Harker ..r-, Sept. 22, 1,953

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patfint NO- 219991931 September 128, 1961 Placido William Zingaro It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, lines 31 and 32, strike out "first reflecting"; line 67, for "said" read a line 68 for "a", first occur== rence, read said column 6, line 54, for "fourth" read second line 55 for "second" read fourth line 73 for "fourth" read second line 74 for "second" read fourth column 7, line 45, after "along" insert said second line 55, strike out "first", second occurence; column 7, line 65, and column 8, line 1, for "position" read portion column 8, line 1, for "position" read portion same column 8, line 50, for "X-ray" read X-rays Signed and sealed this 17th day of April 1962.

(SEAL) Attest:

ESTON G; JOHNSON DAVID L LADD Attesting Officer Commissioner of Patents

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