Positive Rays

Rays with Multiple Charges.—If during ionization more than one electron is split off, the resulting positive ray will have a double or multiple charge. Taking the case of a doubly charged particle it may give rise to two distinct effects. In the first place if it retains its double charge while passing through the analysing fields its behaviour will be quite indistinguishable from that of a normal ray of half its mass. Thus the effective mass of a doubly charged oxygen atom, written 0++, will be 8. Parabolas due to C -+ and 0 ++ may be seen in Plate, fig. 2. In the second place, the ing intensity )3,7, etc., indicate the atoms which have retained two, three or more charges.

Dempster's Method of Analysis.

This method makes use of the principle first used by Classen and now generally adopted for beta-ray analysis, that a homogeneous beam of charged par ticles diverging from a narrow slit may be focussed by bending them through 180° in a uniform magnetic field. One form of Dempster's apparatus is shown in fig. 3. The element, the rays of which are to be analysed, is va porized in a small electric furnace G and its vapour ionized by bom bardment with electrons from a coated platinum strip F near the mouth of the furnace. The posi tive ions so produced pass into an accelerating field through the aperture in the iron plate P. The energy of the rays at this point is regarded as negligible so that all reach the slit with the same energy which can be controlled by varying the potential between P and usually 800---1,000 volts. The rays passing down in a nar row beam through SI are bent into a semicircle by the magnetic field, and so focussed upon a second slit 52. Beyond this they are collected on an insulated plate and their charge measured. This is done by connecting the plate to an electrometer W and balancing the current brought up by the rays against a known and controlled leak produced in the ionization chamber L by means of a constant radioactive source.

If

V is the accelerating potential and r the radius of curvature it can easily be shown that By keeping the mag netic field constant and plotting current against accelerating po tential a curve is obtained having a peak for each value of mass present in the beam of rays. The ratios of the masses can be cal culated from the voltages corresponding to the peaks. In this way Dempster was able to perform the first analyses of magne sium, calcium and zinc into their isotopes. (See ISOTOPES.) Fig. 4

shows a curve obtained from zinc.

The method is unfortunately limited in its application but it has the notable advantage that the relative abundance of the different isotopes can be directly deduced from the heights of their respective peaks.

Aston's Method of Analysis, the Mass-spectrograph.—This instrument was primarily designed to determine the constitution particle may retain its double charge through the whole potential fall of the discharge but capture an electron in the fine tube. It will then constitute a ray of normal ratio of mass to charge but with double the normal energy, so that the normal parabola will show an extension towards the axis OY to a point half way between that axis and the line pq. The extension of the oxygen parabola due to this cause is clearly shown on the photograph.

Most elements are capable of losing two electrons, some, such as krypton, three or more, while mercury can lose no less than eight at a time. The results of the multiple charge on atoms of mercury is beautifully illustrated in Plate, fig. 3. The parabola a corresponding to normal single charge will be seen extended almost to the origin itself, while above a series of parabolas of diminish of neon, a problem which required considerably greater accuracy than that given by the parabola method by which this element had been previously examined. In it electric and magnetic fields are employed to deflect the rays but they are so arranged that their deflections are at 18o° instead of at 90° as used in the parab ola method. The principle is indicated in fig. 5. The rays are collimated into an extremely thin ribbon by passing them through the two parallel narrow slits They are then deflected by the electric field between the plates P1,P2. This spreads them out into an electric spectrum in which the deflection of any particular particle is After emerging from the electric field the rays may be taken, to a first degree of approximation, as radiating from a virtual source Z half way through the field. A group of these rays is now selected by means of the diaphragm D, and allowed to pass between the poles of a powerful electromagnet. For simplicity the poles may be taken as circular, the field between them uniform and of such a sign as to bend the rays in the op posite direction to the foregoing electric field.