DE7032595U - DEVICE FOR ELECTRON SPECTROSCOPY FOR CHEMICAL ANALYSIS - Google Patents

Description

···· ·. . * ·
• ·. J J. · ·. D-703rßöblingon PATENT ADVOCATE Girokwege DIPL-ING. KNUD SCHULTE Tulefon (0 70 31) 2 09 73
• 66 74 32

Patent attorney K. Schulte, D-703 Döbllngen, Gerokwcg β

? 20 43 323.2-52
Hewlett-Packard Comp.
Case 534

DEVICE FOR THE ELECTRON _{SPECTP 1} OSCOPY FOR CHEMICAL ANALYSIS) (ESCA)

The present invention relates to an apparatus for electron spectroscopy for chemical analysis (ESCA) with a source for generating an electromagnetic one Radiation along a first axis «a monochromator with a dispersion element that is on the first axis in the path of the electromagnetic radiation is arranged, wherein the monochromator is a part of this electromagnetic Radiation is focused on a receiver along a second axis or on the second axis and on the Rowland circle of the monochromator is arranged, whereby of the irradiated Electrons are emitted along a third axis, a detector, an electron spectrometer, which is arranged on the third axis in the path of the emitted electrons, the electron spectrometer electrons emitted by the irradiated collector are focused on the detector.

Volksbank Böbllngim AG. Account 8 458 (BLZ 60 390 220) Post check: Stuttgart 908 55-709

703259521.11.74

Usually the width of the characteristic X-ray beam line, which is used to excite the electron emission, and the width of the atomic energy level to be investigated make the main contributions to the width of an electron line in an ESCA spectrum, in order to obtain useful information about the atoms and the molecules through ESCA the width of the electron line should only reflect the width inherent to the atomic energy level to be investigated. An ESCA system in which the width of the X-ray line used to excite electron emission does not contribute to the width of the electron line in the ESCA spectrum is described in the applicant's unpublished US Pat. No. 3,567,926 described. In this system the dispersion of a crystal monochromator - used to focus a characteristic X-ray line on the receiver, is made equal to the dispersion of an electron spectrometer used to focus the photoelectrons emerging from the irradiated receiver and falling on a detector. The geometric arrangement of the ESCA system is such that is compensated by the dispersion of the electric spectrometer dispersion of Kristallmcnochromators and thereby prevents the width of the Röntganstrahllinie which is focussed on the collector is added to the width of the electron lines on focused on the detector. However, this can only be done for a relatively narrow electron

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energy range from about 10 to 20% of that optimal for dispersion compensation in such a system available e ...... ktron energy can be achieved.

Accordingly, the present invention is based on the object a device for the electron spectroscopy .i.e for chemical analysis (ESCA), in which a dispersion compensation in a much larger energy range can be performed to prevent the width of the X-ray line from becoming the width of the electron line added up.

According to the invention, this object is achieved in a device of the type mentioned at the outset in that an electron lens at least four focusing elements are provided, that along the third axis in the electron path between the Arranger and the electron spectrometer are arranged, and result in at least three independently adjustable electrical parameters and the electron lens emits an image of the irradiated interceptor and those emanating from this image Electrons pass through the electron spectrometer, with the total scattering of the electron lens and the electron spectrometer ' substantially balances the dispersion of the monochromator.

By looking at the electrical parameters of the electron lens suitably, the size and location of the image can be kept constant while the photoelectrons of

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the catcher in the energy frenzy: yes accelerated or decelerated to which the electron spectrometer is set. This makes the energy rich essential enlarged, in which a dispersion: lomperiSPiion carried out can be used to prevent the width of the X-ray line that focused on the receiver is added to the width of the electron line that is on the detector is focused *

In ESCA systems, the electrostatic electron spectrometer If used with hemispherical electrodes, a greater sensitivity for a given absolute resolving power can be achieved by increasing the solid angle under which the photoelectrons from the interceptor enter the electron spectrometer. This solid angle is referred to below as the capture angle. That angle is approximately proportional to the product of the angular dispersion range of the photoelectrons entering the electron-) spectrometer in the radial direction (the direction in which the photoelectrons are subsequently deflected and passed through the spectrometer can be dispersed) and from the angular dispersion range of the photoelectrons entering the electron spectrometer in the circumferential direction (the direction perpendicular to the radial direction) occur. Because of the aberrations in the electron spectrometer, the angular dispersion range must under the photoelectrons enter the electron spectrometer in radial direction, very small, i.e. in size

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Order of about 1 or 2 can be kept. The angle"

i ^ persion area, below which photoelectrons enter the Elekvfönspektcir.eter occur in the circumferential direction, in systems that consist of several electrodes electron lenses Use of the type described, limited by the diaphragm effect of the lens electrodes. Therefore, according to The present invention uses an improved ESCA system in which the angle of capture of the photoelectrons enlarged in the circumferential direction by about a factor of 10 ") which will considerably increase the sensitivity of the system without reducing its resolving power is affected.

According to a preferred embodiment of the invention, this is achieved in that the electron spectrometer has a generally conical entry end with an axis of symmetry passing through the center of the hemispherical Electrodes extends therethrough, the focusing elements four pairs of generally conical, fan-shaped electrodes include and these fan-shaped pairs of electrodes have the same axis of symmetry as the conical entry end of the Own spectrometer. These pairs of electrodes are arranged in such a way that their openings are narrowest in the radial direction and are furthest in the circumferential direction. The focusing effect of the electron lens therefore occurs mainly or entirely in the radial direction, whereby the capture angle is increased in the circumferential direction and for

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- 6 -

a given absolute resolving power one
greater sensitivity is achieved. Each of these electrode pairs is electrically isolated from the other electrode pairs, providing three independently variable electrical parameters for the size and location of the collector image and the kinetic energy of the photoelectrons
controls entering the electron spectrometer.

The invention is described in the following by exemplary embodiments: !! explained in connection with the accompanying drawing.

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• ■ it * «··· _(#

Fig. 1 shows a partly schematic and partly a section Elevation view of an improved uiöpersionsksir.punciGrter. ESCA systems according to a preferred embodiment of the present invention,

Fig. 2 shows an elevation view of the electron lens and the spectrometer in Fig. 1 _f viewed in a plane perpendicular to the plane of Fig. 1,

Fig. 3 shows an enlarged view of the catcher area in Fig. 1,

Fig. 4 shows a sectional elevation of an electron lens whose axis of symmetry is below a finite Angle is aligned with respect to the plane of the Rowland circle,

Fig. 5 shows a sectional elevation of an electron lens and a spectrometer used in accordance with another embodiment The present invention images an improved dispersion compensated ESCA system and

FIG. 6 shows an illustration of the electron lens and the spectrometer of FIG. 5, viewed in a plane perpendicular to the plane of FIG. 5.

In Fig. 1 is an ESCA system with dispersion compensation which can be used to study the chemical composition of a selected collector IO. This The system includes a fixed radiation source 12 that emits an x-ray beam 14 along a first axis 16.

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The radiation source 12 can, for example, be designed as it is on pages 178-179 of the book "Elektronenspektoskop fürchemischenverbindungen" by Kai Siegbahn and others, published in December 1967 by Almqvist and Wicksells Boktryckeri (hereinafter referred to as the ESCA book should be designated) is described. A diffraction crystal 18 of a crystal monochromator is fixedly arranged on the first axis 16 in the path of the X-ray radiation 14. The diffraction crystal 18 has a curved surface so you .. 'atomic layers have a radius which is equal to the diameter of the Rowland circle 20th The dispersion of the crystal monochromator (which is caused by Bragg reflection on the diffraction crystal) generates a spectrum of the X-ray radiation 14. A characteristic line 22 of this X-ray spectrum is focused by the crystal nonochromator along a second axis 24 which forms an angle β of approximately 22 ° with the first axis 16. The collector 10 is removably arranged on the second axis 24 in the beam away from this characteristic X-ray line 22. Both the collector 10 and the radiation source 12 are arranged on the Rowland circle 20 of the crystal monochromator.

By irradiating the collector 10 with the characteristic X-ray line 22 are emitted from the interceptor photoelectrons along a third axis 26, the forms an angle θ + γ - φ (see Fig. 3) of approximately 70 ° with the second axis 24. Because of the dispersion of the crystal monochromator the characteristic X-ray line 22 has a finite line width, which has the effect of

that photoelectrons emerge from the same energy level but from different parts of the interceptor with different energies. For example, as shown in FIG. 1, a higher-energy photon hits the right (viewed in the beam direction of the characteristic X-ray line) from a photon with lower energy and generates photoelectrons that have a higher energy (E _fc + ΔΕ) than the photoelectrons of energy (E. - ΔΕ), which are generated by the photon with lower energy. )

As shown in Figs. 1 and 2, an electron spectrometer is used 28, which is fixed with its entry end on the third axis 26, for examining the photoelectrons related to the irradiated receiver 10. For example, the electron spectrometer 28 may consist of an electrostatic There are spectrometers with hemispherical electrodes 32 and 34, of the kind shown in FIGS. 1 and 2 is, or it can consist of a semicircular magnetic spectrometer, as about in the section VIII: 3 described on pages 182 and following of the ESCA book is. In either case, the electron spectrometer 28 is adjusted by having its electrodes 32 and 34 be brought to selected potentials V. or V_ to a relatively narrow energy range of photoelectrons (e.g. from about 1OeV out of a spectrum of about 150OeV) on a detector 36 located at the Exit end of the electron spectrometer 28 is arranged.

The detector 36 can consist of a fixed photomultiplier tube or consist of a removable photographic plate as shown schematically in FIG is.

On the third axis 26, between the collector 10 and the entry end of the electron spectrometer 28, an electron lens 38 is fixedly arranged, ura to sweep or slow down the photoelectrons coming from the collector in the energy range to which the electron spectrometer 28 is set. The electron lens 38 comprises four conical, ring-shaped electrodes 40, 42, 44 and 46, which are arranged symmetrically around the third axis 26 and at a distance from one another along this third axis 26 so that the smallest opening in the vicinity of the collector 10 and one larger opening is located in the vicinity of the entry end of the electron spectrometer 28. These four electrodes 40, 42, 44 and 46 are electrically isolated from one another, and they are kept at such independently controllable potentials V ₃ , V, V ₅ and V _ß that the potential difference V. - V, between the electrodes 40 and 42, the potential difference V ₅ - V ₀ between the electrodes 40 and 44 and the potential difference V _g - V ₃ between the electrodes 40 and 46 can be changed independently. The potentials V 1 and V _ of the spectrometer electrodes 32 and 34 should be chosen so that the electron beam which passes through the spectrometer follows an equipotential surface which has the same potential V as the lens electrode.

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As a result of this condition, ν .n becomes a positive value for the potential difference V _n - V between the inner spectrometer electrode 32 and the closest lens electrode 4 ^ and a negative value for the potential difference V ~ - V _fi between the outer spectrometer electrode 34 and the am next lying lens electrode 46 is determined. Due to the potential difference V, - V_ between the electrode 40, which is closest to the collector 10, <? Ar is at the same potential V ₃ as the electrode 40, and the electrode 46, which is the entrance egg. ^{: The} closest to the electron spectrometer 28, the ratio of the final to the initial kinetic energy of the photoelectrons exiting the interceptor 10 and passing through the electron spectrometer 28 is controlled. By taking this potential difference V ₄ . Is scatter - (30OeV or about 300 to about 150 150OeV example), accelerated in the region or are braked einge- to which the electron spectrometer 28 provides - V_ einstellt- suitable barrel photoelectrons, whose kinetic energy over a .weiten initial area! .

The focusing effect of the electron lens 38 generates an image 48 of the interceptor 10 at the entry end of the electron spectrometer 28. The sensitivity and resolution of the system are affected by the size and location of the image 48. The size and the position of the image 48 can be adjusted by adjusting the potential difference V ₄ - V ₃ between the lens electrodes 40 and 42 and the potential difference

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'' ItIt ι>

• ti

- 12 -

V - V, selected between the lens electrodes -.0 and 44 and furthermore kept constant, so that one can decelerate regardless of the initial kinetic energy of the photoelectrons, which are accelerated or decelerated in the energy range to which the spectrometer 28 is set, a system with maximum sensitivity and maximum resolution is obtained.

In order to prevent the width of the characteristic X-ray line 22 from becoming the width of the electron line which is focused on the detector 36 is added, the total dispersion of the electron lens 38 and the electron spectrometer 28 chosen so that the dispersion of the crystal monochromator is rendered ineffective. This is achieved by the geometrical dimensions of the system and the electrical parameters are chosen so that the following equation is satisfied:

E _ μ sin6 sin 0 s tan9

- ^ - ~ ^M sin (Θ + γ) _E

In this equation (which can be seen in detail from FIGS. 1 and 3):

ρ is the mean radius of the spectrometer 28; R is the radius of Rowland's circle 20; M is the magnification of the electron lens 38; θ is the Bragg angle (the angle between the second axis 24 and a tangent to the Rowland circle at the intersection with this second axis);

ο. λ angle between the third axis 26 and the

. ahlten surface of the catcher 10; γ is the angle between the irradiated surface of the Interceptor 10 and a tangent to the Rowland circle 20 at the point of intersection with both the second axis 24 and the irradiated surface of the collector 10; E_ is the kinetic energy of the mean electron beam

in the spectrometer 28; and ~ _x

E is the mean energy of the photons in the characteristic X-ray steal line 22.

From the above equation it follows that for a given geometry and a given ratio of E to E it is necessary for a dispersion co-compensation that the magnification M of the electron lens 38 and thus the size of the receiver image 48 be kept constant when the Range of the initial photoelectron energies to be examined is changed. As already described above, this takes place in that the potential differences V ₄ - V_ and Vr - V _{3 are} each set or readjusted when the potential difference V ₁ . - V_ is adjusted in order to change the range of the initial photoelectron energies to be examined.

A dispersion-compensated ESCA system of the type described above was built with the dimensions given in the table below (the letters af and r.-r _{5 representing} the longitudinal dimensions or inner diameter of the electron lens 38 ^s ^ T ^ ³ JOi 9 ^ O ^ fi Vl ^Pig * ^{4 an} 9 ^e 9 ^even

ίο

P = 15.55 cm a = 39.4 cm (P ₁ = 12.7 cm) b = 1.27 cm (Po = 19 cm) c = 4.3 cm R = 15.2 cm d = 5.0 cm θ = 78.5 ° e = 10.1 cm 0 = 38.5 ° f = 10.1 cm γ = 30 ° ^E s ⁼ 17OeV ^Ε Ρ ⁼ 150OeV M = 3

1 ~ 1 /O cm 2 ⁼ 2 ' ⁵ cm 3 ^β 5 , 0 cm 4 ⁼ 1 0.1 cm 5 ⁼ 5 , 0 cm

r. =

A dispersion compensation is included in this system Deceleration ratios E / E. from e.g.! / 3 and 1/6 thereby obtained that the electrodes of the electron spectrometer and the lens were kept at the values as described in the next The table below contains (in the E. the mean kinetic energy of such phctoelectrons at the exit from the catcher running through the spectrometer):

^E s

17OeV 17OeV

The electron lens 38 used in the system shown in FIG. 1 may or may be entirely electrostatic made of both electrostatic and magnetic components

V ^E t ^V 1 ^V 2 ^V 3 ^V 4 ^V 5 ^V 6 ^E t 1/3
1/6 -2G4V
-774V -4O2V
-912V OV
-7 68OV
1000V -355V
-77OV -34OV
-85OV 51OeV
102OeV

be constructed. In addition, the electron lens 38 can be arranged so that its axis of symmetry is in the plane of the Rcwland circle 20 or, as indicated by the dashed lines in FIG. 4, at a finite angle with respect to the plane of the Rowland circle 20 lies. The latter embodiment can be used with advantage in systems in which the electron lens 38 would otherwise absorb a large proportion of the X-ray radiation 14 before it reaches the receiver 10.

To achieve the greatest possible sensitivity, the The collecting arm of the electronic spectrometer 28 is centered around the axis along which the highest photoelectron emission per solid angle unit with the lowest loss of monochrome is achieved. This axis runs for a gaseous collector, in which photoelectrons can be generated over the entire area along the arc of Rowland's circle, over which the characteristic X-ray line 22 extends, perpendicular to the plane of the Rowland circle. With one of these gaseous interceptor, it is therefore particularly advantageous to arrange the electron lens 38 and the spectrometer 28 so that that the central beam of photoelectrons emitted by the spectrometer is caught, perpendicular to the plane of the Rowland circle 20 runs as shown by the solid lines in FIG. The same considerations also apply, when an electron spectrometer 28 is used in an ESCA system without an electron lens 38.

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5 and 6, another electron spectrometer 50 and an electron lens 52 of the type that can be used in the ESCA system shown in FIG adversely affect the resolving power of the system. The electron spectrometer 50 comprises two hemispherical electrodes 54 and 56 which are held at different potentials V and V ₂ . These hemispherical electrodes are modified so that the J spectrometer has a conical entry end 58 with an axis of symmetry 60 passing through the center C of the hemispherical electrodes.

The electron lens 52 has four pairs of conical, fan-shaped electrodes 62 , 64, 66 and 68, which have the same axis of symmetry 60 as the conical entry end 58 of the electron spectrometer 50. These electrode pairs 62,64,66 and 68 are electrically isolated from one another and they are kept at mutually independently adjustable potentials ^V 3 » ^V 4» ^v c ^{un <} * Vg in order to create three independently variable potential differences, namely V. - V ₃ between the first and second pairs 62 and 64, V ₅ - V- between the first and third pairs 62 and 66 and V- - V, between the first and

ο ο

the fourth pair 62 and 68. By adjusting these three independently mutually variable potential differences can be the size and the position of the collector image and the kinetic energy the photoelectrons entering the electron spectrometer 50, be controlled in such a way that one extends for one

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Energy range, as explained above in connection with FIG. 1, a dispersion compensation is achieved.

The electron lens 52 is arranged so firmly that its smallest conical opening is in the vicinity of the interceptor 10 and its largest conical opening in the vicinity of the conical entry end 58 of the electron spectrometer 5oj. The tapered openings of the electron lens 52 are narrowest in the radial direction and widely in the circumferential direction, so that the Λ focusing effect of the electron lens, if any, occurs mainly or entirely in the radial direction and only slightly in the circumferential direction. This has the effect that the collection angle is increased in the circumferential direction, whereby a greater sensitivity of the spectrometer for a given absolute resolving power is obtained.

Ideally, the catcher 10 should lie on a conical surface that forms the adjacent surfaces of the pair of electrodes 62 intersects at right angles and through the axis of symmetry 60 of the electron lens 52 or in the vicinity of this Axis of symmetry runs. The closer the catcher 10 to the axis of symmetry 60 of the electron lens 52, the more the aberrations can be reduced. All photoelectron beams, that are captured by the electron lens 52 are nearly perpendicular to the conical surfaces that form the edges the lens openings and entrance end 58 of the electron spectrometer 50 form. This also helps to reduce the aberrations. The electron lens 52 creates an image of the captured Rays in a cone 70, which runs through the center C of the hemispherical Εΐεφ§φφ§ $ § JbJw ^ S ^.

Claims (14)

P 21> 43 323.2-52 Protection claims Device for electron spectroscopy for chemical Analysis (ESCA) with a source for generating electromagnetic radiation along a first axis, a Monochromator with a dispersive element that is on the first axis in the path of de: electromagnetic radiation is arranged, wherein the monochromator has a small wavelength range of the radiation along a second axis focused on a receiver, which is arranged on the second axis and on the Rowland circle of the monochromator, whereby electrons are emitted from the irradiated receiver along a third axis, a detector, a Electron spectrometer, which is arranged on the third axis in the path of the anitized electrons, where the Electron spectrometer from the irradiated receiver focused electrons emitted on the detector, thereby characterized in that an electron lens (38) with at least four focusing elements (40 - 46, 62 to 68) is provided along the third axis (26) in the electron path between the collector (10) and the Electron spectrometer (28) are arranged and at least three independently adjustable electrical parameters result and the electron lens emits an image of the irradiated receiver and the emanating from this image Electrons pass through the electron spectrometer, with the total scattering of the electron lens and the Electron spectrometer the dispersion of the monochromator (18) essentially offsets. 70325952i.ti.74 A · • - mm 2. Apparatus according to claim 1, characterized in that the radiation source (12) on the Rcwland circle (20) of the monochromator (18) is arranged is. 3. Apparatus according to claim 1 and 2, characterized in that the radiation source (12) is an X-ray source is. 4. Device according to one of claims 1 to 3, characterized marked. that the electron lens (38,52) has an axis of symmetry (26,60) in the 3 plane of the Rowland circle (20) of the monochromator (18). 5. Apparatus according to claim 1 to 3, characterized in that g e k e η η, that the electrons. ' inse (38,52) has an axis of symmetry (26, 60) which coincides with the plane of the Rowland circle (20) of the monochromator (18) forms a finite angle. 6. Apparatus according to claim 5, characterized in that that the axis of symmetry of the electron lens is perpendicular to the plane of the Rowland circle of the monochromator, 7. Apparatus according to claim 6, characterized in that that the third axis (26) is perpendicular to the plane of the Rcwland circle (20) of the monochromator (18) and the electron spectrometer (28) is arranged on the third axis that the central beam of the electron spectrometer The electron emission captured is perpendicular to the plane of the Rowland circle of the monochromator. 8. Apparatus according to claim 1 to 7 _f, characterized in that the potential differences (V ₄ - V ₃ ; Vc "* V ₃ ; Vg - V_) between a first (40) and a second (42), the first (40) and a third (44) and the 76825952ItLTi first and fourth (46) of the focusing elements are adjustable to control the size and location of the image of the anyeiahlten AuffSnfsrs (IG) and the relationship between the final and the initial kinetic energy of the emitted by the irradiated interceptor and passing through the electron spectrometer Electrons. 9. Apparatus according to claim 8, characterized in that the potential differences of Focusing elements are such that the size and position of the irradiated receiver is kept constant, accelerated during the irradiated interceptor in the selected energy range of the electron spectrometer or be braked. 10. Apparatus according to claim 1 to 9, characterized in that the focusing elements from generally conical, ring-shaped electrodes (40 to 46) are made and symmetrical about the third axis (26) and are arranged at a distance from one another along this third axis. 11. The device according to claim 1 to 10, characterized in that the electron spectrometer is a Has pair of hemispherical electrodes with independently adjustable potentials for focusing are operated by electrons within the selected energy range on the detector. 12. The apparatus according to claim 11, characterized in that the electron spectrometer is a generally conical entry end (58) with an axis of symmetry (6O) passing through the center (C) of the hemispherical Electrodes (54, 56) extends therethrough, the focusing elements four pairs of generally conical, 79325S521.11.74 • · t include fan-shaped electrodes (62 to 68) and these fan-shaped ^ lelectrode pairs the same axis of symmetry like the conical entry end of the spectrometer own. 13. Apparatus according to claim 12, characterized in that the image of the irradiated receiver (10) through the electron lens (52) at the conical entry end (58) of the electron spectrometer (50) is formed in a cone (70) whose apex is in the center (C) of the hemispherical electrodes. 14. The device according to claim 13, characterized in that g e k e η η ζ e i c h-n e t that the hemispherical electrodes, the radiation source., the monochromator, the catcher, that Electron spectrometer and the electron lens so arranged are that _u sin 0 · sin «p . E _c . tan9 P sin (Θ + γ) E _n κ ρ where ρ is the mean radius of the hemispherical electrodes of the electron spectrometer, R the radius of the Rowland circle of the monochromator, M the magnification of the electron lens, 0d ^ r angle between the second axis and a tangent to the Rowland circle at the intersection of the second axis and the irradiated surface of the interceptor, ψ the angle between the third axis and the irradiated surface of the interceptor, γ the angle between the irradiated surface of the interceptor and a tangent to the Rowland circle of the monochromator at the intersection of the second axis and the irradiated surface of the Catcher, E is the kinetic energy of the central electron beam in the electron spectrometer and E is the mean energy of the photons in the in the characteristic line of the X-rays. 793251521, *. »