Although most of the a-particles travel in nearly a rectilinear path through absorbing material there is in general a small scatter ing or deflection of the a-particles in passing through matter, amounting on the average to a few degrees. This scattering in creases with the atomic weight of the absorber and with reduction of velocity of the a-particle. In addition to this small angle scatter ing, a small fraction of the a-particles is deflected by collisions with heavy atoms through angles greater than a right angle. This large angle scattering is in general due to a close collision of an a particle with a single atom of matter. A close study of the laws of single scattering of a-particles has been made by Geiger and Marsden. The observations on this large angle scattering first disclosed the nuclear structure of the atom and has given us most definite information on the laws of force inside the atom close to the nucleus and on the dimensions of atomic nuclei. (See Nu cLEus.) Blackett has obtained a number of expansion photo graphs showing close collisions of a-particles with the nuclei of light atoms like hydrogen, helium, nitrogen and oxygen. In gen eral a forked track is obtained, one branch due to the scattered a-particle and the other to the recoiling nucleus.
Properties of 13- and 7-Rays.—We have seen that the (3 particles, which are emitted by a number of radioactive products, consist of swift negative electrons spontaneously liberated during the transformation of active matter. The velocity of expulsion and the penetrating power of 0-rays vary widely for different products. For example, the rays from radium B are much more easily absorbed by matter than the swift (3-rays from radium C. At first sight, the laws of absorption of the (3-particle by matter appear very different from those observed with a-particles. This is mainly due to the fact that the average (3-particle has much less energy than the a-particle and is consequently much more scattered in passing through matter. This scattering is so marked that in general more than half the (3-particles falling on a thin sheet of matter are reflected or rather scattered in the backward direction. The 0-particles on the average have a very tortuous path before they are finally stopped. In the early days, it was found that the absorption of (3-rays appeared to follow an ex ponential law and the different groups of 0-rays were charac terized by a definite absorption coefficient. We now know that a group of homogeneous 0-rays are not absorbed according to an exponential law at all and that the 0-particles arising from dis integration of a single product are expelled over a wide range of velocity. In this respect a 13-ray transformation is entirely dif ferent from an a-ray transformation where the a-particles are all expelled with the same speed. It has however been found that in general one disintegration electron comes from each trans formed atom but the speed of the 0-particles from different atoms may differ widely. It is difficult to account for the striking
difference in the a- and (3-modes of transformation. Meitner has suggested that the 0-particles are initially liberated from the nucleus with identical speeds but that some lose part of their energy by collisions with electrons in escaping from the atom. If this were the case, we should expect on the average more than one 0-particle to escape from each disintegrating atom. Ellis, however, has shown that the heating effect of the (3-rays from radium E is a measure of the actual energy of the escaping g rays, supposing, as Emeleus observed, that one (3-particle is ex pelled in each transformation. On the Meitner hypothesis, the heating effect should have been greater corresponding to that due to (3-particles all liberated with the maximum speed.
Gray has shown that (3-rays in passing through matter give rise to 7-rays, and that these in some cases correspond to the char acteristic X radiations observed by Barkla. The absorption of the 7-rays has been determined by the electrical method. Radium B has been found to emit several groups of 7-rays which differ in penetrating power. The greater part of the rays from radium C consist of penetrating y-rays which are nearly exponentially absorbed by matter. The ionization in an electroscope falls off according to the equation where d is the thickness of matter traversed and ,u the coefficient of absorption. When lead is used as an absorbing material the value of /I. = 0.5 for the most penetrating 7-rays from radium C. The absorption coefficient for different kinds of matter is proportional to the density, indicating that absorption depends on the mass of matter traversed.
There is undoubtedly a close connection between and T rays, and swift 0-rays are usually accompanied by penetrating 7-rays. For example, radium C, which emits very swift 0-rays, some of which reach a velocity more than 0.98 of the velocity of light, gives rise to the most penetrating y-rays observed in the uranium-radium series. There is one very notable exception, viz., radium E which emits swift 0-particles but very little 7-radiations.
Jacobsen has recently found some evidence that in the disin tegration of radium C, the 7-radiation is not emitted until about second after the expulsion of the 0-particle. It may be that, in general, the 7-ray does not accompany the expulsion of the g -particle but is liberated later due to a rearrangement of the constituents of the nucleus. If this be the case, the liberation of a 7-ray represents a new type of transformation without change of the charge on the nucleus. On this view, it is to be supposed that in the case of radium E there is no subsequent 7-ray trans formation.