Radioactivity

Nature and Properties of the

the from active substances are of small penetrating power compared with the (3- or 7-rays, they are responsible for most of the energy evolved by radioactive substances and contribute most of the ionization. Rutherford showed in 1903 that the a-rays were de flected in a powerful magnetic and electric field and consisted of positively charged atoms of matter projected with high velocity. From the first it seemed probable that the a-particle was an atom of helium carrying two positive charges, and this was subsequently confirmed in a number of ways. The value of ratio of the charge on the particle to its mass—and the velocity can be determined from observations on the deflection of the pencil of rays by a magnetic field and electric fields. In this way Ruther ford and Robinson in 1914 showed that the a-particle, whether from radon, radium A or C, gave a value of e/m= 4,82o e.m. units, agreeing within the limit of error with the electrochemical value of e/m =4,826, assuming that the mass of the helium atom is 4.00 and that it carries two unit positive charges. The magnitude of the charge carried by each particle was measured by Regener and Rutherford and Geiger, and found to be twice that carried by the electron. The velocity of the a-particles expelled from radium C (of range 7.06 cm.) was found to be 1.92 X cm. per sec., or about the velocity of light. From this result the velocity of expulsion of all a-particles can be calculated from the relation found by Geiger, that = KR where V is the velocity of the particle and R its range in air. The evidence indicates that the particles from active products are in all cases atoms of helium. The a-particles from a given product are all emitted with constant velocity, which is characteristic for that product. We have already mentioned that the velocity of expulsion appears to be connected with the period of transformation of the element. The laws of absorption of the a-particle were first worked out by Bragg and Kleeman. On account of its great energy of motion, the a-particle travels in nearly a straight line through the gas, producing intense ionization along its track. The effects produced by the a-particle, whether measured by ionization, phosphorescence or photographic action, vanish suddenly after the a-particle has traversed a definite amount of matter. The definiteness of the end of the range of the a-particle of given velocity is remarkable. It is found that the ionization per centimetre of path due to a narrow pencil of a-rays increases with the distance from the active matter, at first slowly, then more rapidly, near the end of the range. After passing through a maximum value the ionization falls off rapidly to zero corresponding to the end of the range. If a uniform screen of matter is placed in the path of the pencil of rays the range is re duced by a definite amount proportional to the thickness of the screen. All the a-particles have their velocity reduced by the same amount in their passage through the screen. The ranges in air of the a-rays from the various products of the radio-elements have been measured and are usually impressed in terms of cm. of air traversed at 15° C and 76o mm. pressure. The ranges for the different products vary between 2.8 cm. and i1.3 cm.

Bragg has shown that the range of an a-particle in different elements is nearly proportional to the square roots of their atomic weights. This approximate empirical relation has proved very use ful in estimating the reduction in range of the a-particles in traversing uniform sheets of different kinds of matter.

Rutherford and Geiger in 1908 devised an electrical method of counting the a-particles expelled from radioactive matter. The a particle enters through a small opening into a metal tube contain ing a gas at a reduced pressure. The ionization produced by the a particle in its passage through the gas is magnified several thou sand times by the movement of the ions in a strong electric field.

In this way, the entrance of an a-particle into the detecting vessel is shown by a sudden and large deflection of an electrometer.

A still more sensitive detector has been devised by Geiger con sisting of a fine needle point opposite an opening in a metal tube, charged to a suitable voltage. This counter is equally efficient for (3- as well as for a-particles. By the use of a string electrometer, photographic registration of the a-particles can be obtained for several thousand a minute. By magnifying the current by means of suitable amplifiers, the entrance of an a-particle into the detecting vessel can be made to operate a signal.

On account of its great energy of motion the effect due to a single a-particle can be detected in a variety of ways. Sir William Crookes first noted that the a-rays produce scintillations when they fall on a screen of phosphorescent zinc sulphide. It is now known that each of these scintillations is due to the impact of a single a-particle. The number of scintillations can be counted with the aid of a suitable microscope, and this method has proved of great utility in many investigations. Scintillations due to a-rays are observed in certain diamonds, but they are usually not so bright as in zinc sulphide. Kinoshita has shown that a single a particle produces a detectable effect on a photographic plate.

When the a-rays fall on a plate nearly horizontally the track of the a-particle is clearly visible under a high-power microscope. By the expansion method developed by C. T. R. Wilson, the track of the a-particle through the gas is made visible by the condensation of the water on each of the ions produced. In a similar way the track of a 0-particle can be easily shown. The photographs of these trails bring out in a striking and concrete way not only the individual existence of and 0-particles, but the main effects produced in their passage through matter.

When the a-rays fall on a sheet of matter, a cloud of slow speed electrons is emitted. These were first studied by Sir J. J Thomson and were called by him the delta (6) rays. Most of the B rays have an energy corresponding to only a few volts but a few particles are present which have much higher speeds, some reach ing to twice the velocity of the a -particle. These 6-rays are a secondary phenomenon and are liberated from the atoms of mat ter by the action of the a-particle. The 6-rays can be best studied by photographing the tracks of a-particles in a Wilson chamber containing a gas at low pressure. The swifter 3-particles are able themselves to ionize the gas and their tracks are easily visible.

Chadwick and Emeleus and also Auger have shown that the num ber and velocity of the swifter 3-rays agree excellently with the view that they are produced by the collision of the a-particles with the electrons in the atom. The primary process of ionization by the a-particle is the removal of the electron and the secondary ionization is produced by the swifter 5-particles. In general the primary ionization is about one-half of the total ionization observed.

The a-particle at the moment of its expulsion from a radioactive atom carries two charges and it was at first supposed that it re tained this charge until very near the end of its range where the charge was neutralized by capture of two electrons. Later work, however, by Henderson and Rutherford showed that the a particle changes its charge several thousand times in its passage through matter. When the a-rays pass through a sheet of matter in a good vacuum, the issuing rays consist of doubly charged, singly charged and neutral helium atoms. For high speeds, the doubly charged particles predominate; at low speeds most of the particles are singly charged or neutral. It seems clear that the a particle in its passage through matter occasionally captures an electron and that this may be removed in a subsequent collision. This process of capture and loss of electrons is repeated many thousand times before the a-particle is brought to rest.