RONTGEN RAYS, W. K. Röntgen discovered in 1895 (Wied. Ann. 64, p. 1) that when the electric discharge passes through a tube exhausted so that the glass of the tube is brightly phosphorescent, phosphorescent substances such as potassium platinocyanide became luminous when brought near to the tube. He found that if a thick piece of metal, a coin for example, were placed between the tube and a plate covered with the phosphorescent substance a sharp shadow of the metal was cast upon the plate; pieces of wood or thin plates of aluminium cast, however, only partial shadows, thus showing that the agent which produced the phosphorescence could traverse with considerable freedom bodies opaque to ordinary light. He found that as a general rule the greater the density of the substance the greater its opacity to this agent. Thus while this effect could pass through the flesh it was stopped by the bones, so that if the hand were held between the discharge tube and a phosphorescent screen the outline of the bones was distinctly visible as a shadow cast upon the screen, or if a purse containing coins were placed between the tube and the screen the purse itself cast but little shadow while the coins cast a very dark one. Röntgen showed that the cause of the phosphorescence, now called Röntgen rays, is propagated in straight lines starting from places where the cathode rays strike against a solid obstacle, and the direction of propagation is not bent when the rays pass from one medium to another, i.e. there is no refraction of the rays. These rays, unlike cathode rays or Canalstrahlen, are not deflected by magnetic force; Röntgen could not detect any deflection with the strongest magnets at his disposal, and later experiments made with stronger magnetic fields have failed to reveal any effect of the magnet on the rays. The rays affect a photographic plate as well as a phosphorescent screen, and shadow photographs can be readily taken. The time of exposure required depends upon the intensity of the rays, and this depends upon the state of the tube, and the electric current going through it, as well as upon the substances traversed by the rays on their journey to the photographic plate. In some cases an exposure of a few seconds is sufficient, in others hours may be required. The rays coming from different discharge tubes have very different powers of penetration. If the pressure in the tube is fairly high, so that the potential difference between its electrodes is small, and the velocity of the cathode rays in consequence small, the Röntgen rays coming from the tube will be very easily absorbed; such rays are called "soft rays." If the exhaustion of the tube is carried further, so that there is a considerable increase in the potential differences between the cathode and the anode in the tube and therefore in the velocity of the cathode rays, the Röntgen rays have much greater penetrating power and ale called "hard rays." With a highly exhausted tube and a powerful induction coil it is possible to get appreciable effects from rays which have passed through sheets of brass or iron several millimetres thick. The penetrating power of the rays thus varies with the pressure in the tube; as this pressure gradually diminishes when the discharge is kept running through the tube, the type of Röntgen ray coming from the tube is continually changing. The lowering of pressure due to the current through the tube finally leads to such a high degree of exhaustion that the discharge has great difficulty in passing, and the emission of the rays becomes very irregular. Heating the walls of the tube causes some gas to come off the sides, and by thus increasing the pressure creates a temporary improvement. A thin-walled platinum tube is sometimes fused on to the discharge tube to remedy this defect; red-hot platinum allows hydrogen to pass through it, so that if the platinum tube is heated, hydrogen from the flame will pass into the discharge tube and increase the pressure. In this way hydrogen may be introduced into the tube when the pressure gets too low. When liquid air is available the pressure in the tube may be kept constant by fusing on to the discharge tube a tube containing charcoal; this dips into a vessel containing liquid air, and the charcoal is saturated with air at the pressure which it is desired to maintain in the tube. Not only do bulbs emit different types of rays at different times, but the same bulb emits at the same time rays of different kinds. The property by which it is most convenient to identify a ray is the absorption it suffers when it passes through a given thickness of aluminium or tin-foil. Experiments made by McClelland and Sir J. J. Thomson on the absorption of the rays produced by sheets of tin-foil showed that the absorption by the first sheets of tin-foil traversed by the rays was much greater than that by the same number of sheets when the rays had already passed through several sheets of the foil. The effect is just what would occur if some of the rays were much more readily absorbed by the tin-foil than others, for the first few layers would stop all the easily absorbable rays while the ones left would be those that were but little absorbed by tin-foil.
The fact that the rays when they pass through a gas ionize it and make it a conductor of electricity furnishes the best means of measuring their intensity, as the measurement of the amount of conductivity they produce in a gas is both more accurate and more convenient than measurements of photographic or phosphorescent effects. Röntgen rays when they pass through matter produce - as Perrin (Comptes rendus, 124, p. 455), Sagnac (Jour. de Phys., 1899, (3), 8, and J. Townsend (Proc. Camb. Phil. Soc., 1899, Jo, p. 217, have shown - secondary Röntgen rays as well as cathodic rays. A very complete investigation of this subject has been made by Barkla and Sadler (Barkla, Phil. Mag., June 1906, pp. 812-828; Barkla and Sadler, Phil. Mag., October 1908, pp. 55 0 -5 8 4; Sadler, Phil. Mag., July 1909, p. 107; Sadler, Phil. Mag., March 1910, p. 337). They have shown that the secondary Röntgen rays are of two kinds: one kind is of the same type as the primary incident ray and may be regarded as scattered primary rays, the other kind depends only on the matter struck by the rays - their quality is independent of that of the incident ray. When the atomic weight of the element exposed to the primary rays was less than that of calcium, Barkla and Sadler could only detect the first type of ray, i.e. the secondary radiation consisted entirely of scattered primary radiation; elements with atomic weights greater than that of calcium gave out, in addition to the scattered primary radiation, Röntgen rays characteristic of the element and independent of the quality of the primary rays. The higher the atomic weight of the metal the more penetrating are the characteristic rays it gives out. This is shown in the table, which gives for the different elements the reciprocal of the distance, measured in centimetres, through which the rays from the element can pass through aluminium before their energy sinks to 1/2.7 of the value it had when entering the aluminium; this quantity is denoted in the table by X.
. 58.7? (61.3)
The radiation from chromium cannot pass through more than a few centimetres of air without being absorbed, while that from tin is as penetrating as that given out by a fairly efficient Röntgen tube. Barkla and Sadler found that the radiation characteristic of the metal is not excited unless the primary radiation is more penetrating than the characteristic radiation. Thus the characteristic radiation from silver can excite the characteristic radiation from iron, but the characteristic radiation from iron cannot excite that from silver. We may compare this result with Stokes's rule for phosphorescence, that the phosphorescent light is of longer wave-length than the light which excites it.
The discovery that each element gives out a characteristic radiation (or, as still more recent work indicates, a line spectrum of characteristic radiation) is one of the utmost importance. It gives us, for example, the means of getting homogeneous Röntgen radiation of a perfectly definite type: it is also of fundamental importance in connexion with any theory of the Röntgen rays. We have seen that there is no evidence of refraction of the Röntgen rays; it would be interesting to try if this were the case when the rays passing through the refracting substance are those characteristic of the substance.
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The incidence of Röntgen rays on matter causes the matter to emit cathodic rays. The velocity of these rays is independent of the intensity of the primary Röntgen rays, but depends upon the "hardness" of the rays; it seems also to be independent of the nature of the matter exposed to the primary rays. The velocity of the cathodic rays increases as the hardness of the primary Röntgen rays increases. Innes (Proc. Roy. Soc. 79, p. 442) measured the velocity of the cathodic radiation excited by the rays from Röntgen tubes, and found velocities varying from 6.2 X 109cm./sec. to 8.3 X Io 9 cm./sec. according to the hardness of the rays given out by the tube. The cathodic rays given out under the action of the homogeneous secondary Röntgen radiation characteristic of the different elements have been studied by Sadler (Phil. Mag., March 1910) and Beatty (Phil. Mag., August 1910). The following table giving the properties of the cathode rays excited by the radiation from various elements is taken from Beatty's paper; t 1 is the thickness of air at atmospheric pressure and temperature required to absorb one-half of the energy of the cathode particles, t 2 is the corresponding quantity for hydrogen.
Tin. .. .
The properties of the cathode rays excited by the radiation from tin correspond very closely with those produced in a discharge tube when the potential difference between the anode and cathode is about 30,000 volts. When Röntgen rays pass through a thin plate the cathodic radiation on the side the rays emerge is more intense than on the side they enter. Kaye (Phil. Trans. 209, p. 123) has shown that when cathode rays fall upon a metal two kinds of Röntgen rays are excited, one being the characteristic radiation of the metal and the other a kind independent of the nature of the metal and dependent only upon the velocity of the cathode rays. The faster the cathode rays the harder the Röntgen rays they produce. It would be interesting to see if there is any connexion between the velocity of the cathode rays required to excite Röntgen rays as hard as those given out say by tin and the velocity of the cathode rays which the radiation from tin produces when it falls upon any metal. Sadler has shown that metals can give off cathodic radiation even when the incident Röntgen rays are too soft to excite the characteristic Röntgen radiation of the metal, but that there is a large increase in the cathodic radiation as soon as the characteristic Röntgen radiation is excited. It is possible that the shock produced by the emission of these cathode particles starts the vibrations which give rise to the characteristic rays; the cathode particles emitted when the incident rays are too soft to excite the characteristic radiation coming from a different source from those tapped by the hard rays.
The wide variations in the penetrating power of Röntgen rays from different sources is shown by the above table of the penetrating power of the characteristic rays of the different elements. Many experiments have been made on the penetration of the same rays for different substances. It is a rule to which there is no wellestablished exception that the greater the density of the substance the greater is its power of absorbing the rays. The connexion, however, between the absorption and the density of the substance is not in general a simple one, though there is evidence that for exceedingly hard rays the absorption is proportional to the density.
The power of any material to absorb rays is usually measured by a coefficient A, the definition of which is that a plate 1 /A centimetres thick reduces the energy of the rays when they pass through it normally to 1/e of their original value, where e is the base of the Napierian logarithms and equal to 2.7128. It has been shown that however the physical state of a substance may alter, - if, for example, it changes from the liquid to the gaseous, - A/D, where D is the density of the substance, remains constant. It has also been shown that if we have a mass M made up of masses M i, of substances having coefficients of absorption Ai, A 2, and densities Di, then if AID for the mixture is given by the equation Maid = MiXi/Di+M2A2/D2+M3A3/D3-F this equation is true whether the substances are chemically combined or chemically mixed. From this equation, when we know AID for a binary compound and for one of its constituents, we can find the value of AID for the other constituent. By the use of this principle we can find the value of AID for the elements which cannot be obtained in a free state. Benoist (Jour. de Phys. (7), 28, p. 289) has shown that if the values of AID are plotted against the atomic weight we get a smooth curve; if we draw this curve it is evident that we have the means of determining the atomic weight of an element by measuring its transparency to Röntgen rays when in combination with elements whose transparency is known. Benoist has applied this method to determine the atomic weight of indium.
The value of A/D for any one substance depends upon the type of ray used, and the ratio of the values of A/D for two substances may vary very greatly with the type of ray; this is especially the case when one of the substances is hydrogen. Thus Crowther (Proc. Roy. Soc., March 1909) has shown that the ratio of A for air to A for hydrogen varied from Poo for rays given out by a Röntgen tube at a comparatively high pressure when the rays were very soft to 5.56 when the pressure in the bulb was very low and the rays very hard. Beatty (Phil. Mag., August 1910) found that this ratio was as large as 175 for the characteristic rays given out by iron, copper, zinc and arsenic, but fell to 25o for the rays from tin.
A great deal of attention has been paid to a phenomenon called the polarization of the Röntgen rays. The nature of this effect may be illustrated by fig. i. Suppose that AB is a stream of cathode rays striking against a solid obstacle B and P giving rise to Röntgen rays, let these rays impinge on a small body P, P under these conditions will emit secondary rays in all directions. Barkla (Phil. Trans., ig05, A, 204, p. 467; Proc. Roy. Soc. 77, p. 247) found that the intensity of the secondary rays, tested by the ionization they produced in air, was less intense in the plane ABP than in a plane through PB at right angles to this plane, the distances from P being the same in the two cases; the difference in the intensities amounting to about 15%. Haga (Ann. d. Phys. 28, p. 439), who tried a similar experiment but used a photographic method to measure the intensity of the secondary rays, could not detect any difference of intensity in the two planes, but experiments by Bassler (Ann. der Phys. 28, p. 808) and Vegard (Proc. Roy. Soc. 83, p. ` .379) have confirmed Barkla's original observations.
The "polarization" is much more marked if instead of exciting the secondary radiation in P by the Röntgen rays from a discharge tube we do so by means of secondary rays. If, for example, in the case illustrated by fig. I we allow a beam of Röntgen rays to fall upon B instead of the cathode rays, the difference between the intensities in the plane ABP and in the plane at right angles to it are very much increased. It is only the scattered secondary radiation which shows this "polarization"; the characteristic secondary radiation emitted by the body at P is quite unpolarized. The existence of this effect has a very important bearing on the nature of Röntgen rays. Whether Röntgen rays are or are not a form of light, i.e. are some form of electromagnetic disturbance propagated through the aether, is a question on which opinion is not unanimous. They resemble light in their rectilinear propagation; they affect a photographic plate and, Brandes and Dorn have shown, they produce an effect, though a small one, on the retina, giving rise to a very faint illumination of the whole field of view. They resemble light in not being deflected by either electric or magnetic forces, while the characteristic secondary radiation may be compared with the phosphorescence produced by ultra-violet light, and the cathodic secondary rays with the photo-electric effect. The absence of refraction is not an argument against the rays being a kind of light, for all theories of refraction make this property depend upon the relation between the natural time of vibration T of the refracting substance and the period t of the light vibrations, the refraction vanishing when t/T is very small. Thus there would be no refraction for light of a very small period, and this would also be true if instead of regular periodic undulations we had a pulse of electromagnetic disturbance, provided the time taken by the light to travel over the thickness of the pulse is small compared with the periods of vibration of the molecules of the refracting substance. Experiments on the diffraction of Röntgen rays are very difficult, for, in addition to the difficulties caused by the smallness of the wavelength or the thinness of the pulse, the secondary radiation produced when the rays strike against a photographic plate or pass through air might give rise to what might easily be mistaken for diffraction effects. Röntgen has never succeeded in observing effects which prove the existence of diffraction. Fomm (Wied. Ann. 59, p. 50) observed in the photograph of a narrow slit light and dark bands which looked like diffraction bands; but observation with slits of different sizes showed that they were not of this nature, and Haga and Wind (Wied. Ann. 68, p. 884) have explained them as contrast effects. These observers, however, noticed with a very narrow wedge-shaped slit a broadening of the image of the narrow part which they are satisfied could not be explained by the causes. Walter and Pohl (Ann. der Phys. 2 9, p. 33 1) could not observe any diffraction effects, though their arrangement would have enabled them to do so if the wave-length had not been smaller than I. 5 X io-° cm. Sir George Stokes (Proc. Manchester Lit. and Phil. Soc., 1898) put forward the view that the disturbances which constitute the rays are not regular periodic undulations but very thin pulses. Thomson (Phil. Mag. 45, p. 172) has shown that when charged particles are suddenly stopped, pulses of very intense electric and magnetic disturbances are started. As the cathode rays consist of negatively electrified particles, the impact of these on a solid would give rise to these intense pulses. The electromagnetic theory therefore shows that effects resembling light, inasmuch as they are electromagnetic disturbances propagated through the aether, must be produced when the cathode rays strike against an obstacle. Since under these circumstances Röntgen rays are produced, it seems natural, unless direct evidence to the contrary is obtained, to connect the Röntgen rays with these pulses. This view explains very simply the "polarization" of the rays; for, suppose the cathode particle moving from A to B were stopped at its first impact with the plate B (fig. I), the electric force transmitted along BP would be in the plane ABP at right angles to BP. When this electric force reached the body at P it would accelerate any electrified particles in that body, the acceleration being parallel to AB. Each of these accelerated particles would start electric waves. The theory of such waves shows that their intensity vanishes along a line through the particle parallel to the direction of acceleration, while it is a maximum at right angles to this line; thus the intensity of the rays along a horizontal line through P would vanish, while it would be a maximum in the plane at right angles to this line. In this case there would be complete polarization. In reality the cathode particle is not stopped at its first encounter, but makes many collisions, changing its direction between each; and these collisions will send out electric disturbances which when they fall on P are able to excite waves which send some energy along PC. The polarization will therefore be only partial and will be of the kind found by Barkla.
The velocity with which the waves travel has not yet been. definitely settled. Marx (Ann. der Phys. 20, p. 677) by an ingenious but elaborate method came to the conclusion that they travelled with the velocity of light; his interpretation of his experiments. has, however, been criticized by Franck and Pohl (Verh. d. D. Ph ysi k Ges. 10, p. 489) .
Another view of the nature of Röntgen rays has been advocated by Bragg (Phil. Mag. 14, p. 4 2 9); he regards them as neutral electric doublets consisting of a negative and a positive charge of electricity which are usually held together by the attraction between them, but which may be knocked asunder when the rays strike against matter and turned into cathodic rays. On this view when the rays pass through a gas only a few of the molecules of the gas. are struck by the rays and so we can easily understand why so few of the molecules are ionized. On the ordinary view of an electric wave all the molecules would be affected by the wave when it passed through a gas, and to explain the small fraction ionized we must either suppose that systems sensitive to the Röntgen rays are at any time present only in a very small fraction of the molecule or else that the front of an electric or light wave is not continuous but that the energy is concentrated in patches which only occupy a. fraction of the wave front.
The tube now used most frequently for producing Röntgen rays is of the kind introduced by Porter and known as a focus tube (fig. 2). The cathode is a portion of a hollow sphere, and the cathode rays come to a point on or near a metal plate A, called the anti-cathode, connected with the anode; this plate is the source of the rays. This ought to be made of a very unfusible metal such as platinum or, still better, tantalum, and kept FIG. 2.
cool by a water-cooling arrangement. The anti-cathode is generally set at an angle of 45° to the rays; ! it is probable that the action of the tube would be improved by putting the anti-cathode at right angles. to the cathode rays. The walls of the tube get strongly electrified. This electrification affects the working of the tube, and the production of rays can often be improved by having an earth-connected piece of tin-foil on the outside of the bulb, and moving it about until the best position is attained. To produce the discharge an induction coil is generally employed with a mercury interrupter. Excellent results have been obtained by using an electrostatic induction machine to produce the current, the emission of rays is more uniform than when an induction coil is used. The rays are emitted pretty uniformly in all directions until the plane of the anti-cathode is approached; in the neighbourhood of this. plane there is a rapid falling off in the intensity of the rays. After long use the glass of the bulb often becomes distinctly purple. This is believed to be due to the presence of manganese compounds. in the glass. (J. J. T.)
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