Radioactivity

Since the particle is an atom of helium, all radioactive matter which emits a-particles must produce helium. This has been found to be the case for every a-ray product that has been examined. The rate of production of helium by radium in equilibrium was measured with accuracy by Dewar, Boltwood, and Ruther ford. In terms of the international radium standard, the rate of production of helium by one gram of radium in equilibrium with its three a-ray products has been found to be 164 cu.mm. per year. This value is in excellent accord with that calculated from the rate of emission of a-particles. The rate of production of helium by the radon, ionium and polonium has been found by Boltwood to be in fair agreement with calculation. Soddy has observed the production of helium by purified uranium, while Strutt showed that the rate of production of helium in uranium and thorium minerals was in good accord with calculation.

Strutt has made a systematic examination of the amount of helium present in many minerals and rocks which contain minute quantities of radium, and has utilised the results to estimate the age of the geological deposits. On account of the tendency of the helium to escape from minerals in the course of geologic ages, this method gives only a minimum estimate of the age of the mineral, except in the case of very dense and compact specimens. The measurement of the lead content should ultimately prove a more reliable method of estimating the age.

Heat Emission of Radioactive Matter.

In 1903 it was shown by Curie and Laborde that a radium compound was always hotter than the surrounding medium, and radiated heat at a con stant rate of about ioo gram-calories per hour per gram of radium. The rate of evolution of heat by radium has been measured sub sequently by a number of observers. There is no doubt that the evolution of heat by radium and other radioactive matter is mainly a secondary phenomenon, resulting largely from the energy of the absorbed radiation. Since the particles have a large kinetic energy and are easily absorbed by matter, all of these particles are stopped by the radium itself or by the envelope surrounding it, and their energy of motion is transformed into heat. The evolution of heat from any type of radioactive matter is thus proportional to the energy of the expelled a-particles, together with the energy of the 3- and 7-rays absorbed in the envelope. The energy supplied by the recoil of the radioactive atom after the expulsion of an a-parti cle is about 2% of the energy of the a-particle.

These conclusions have been confirmed by the measurements of Rutherford and Robinson, who found that each of the a-ray prod ucts gave a heating effect proportional to the energy of the a particle and absorbed (3- and 7-rays. Radon and its products when removed from radium were responsible for three-quarters of the heating effect of radium in equilibrium. The heating effect of radon, radium A and radium C decayed at the same rate as their activity. From their measurements they found that the total heating effect of radium in equilibrium surrounded by sufficient material to absorb all the radiations was 134.7 gram-calories per hour per gram. Of this, 123.6 gram-calories were due to the a particles, 4.7 to the 9-rays and 6.4 to the 7-rays. The energy of the f3- and 7-rays comes from radium B and radium C, but on account of their great penetrating power it is difficult to measure the 7-energy with accuracy. The results, however, show that the energy of the 7-rays is even greater than that of the 0-rays, and the two together are equal to about 28% of the energy of the a particles from radium C.

Measurements have been made of the heating effect of radium, uranium and thorium, and of uranium and thorium minerals. In each case the evolution of heat is of about the magnitude to be expected from the energy of the radiations.

Experiments on the evolution of heat from radium and its emanation have brought to light the enormous amount of energy accompanying the transformation of radioactive matter where a-particles are emitted. For example, the radon from one gram of radium in equilibrium with its products emits heat initially at the rate of about 109 gram-calories per hour. The total heat emitted during its transformation is about 14,500 gram-calories. Now the initial volume of radon from one gram of radium is .6 cubic millimetres. Consequently one cubic centimetre of radon during its life emits 2.4X gram-calories. Taking the atomic weight of the emanation as 222, one gram of the emanation emits during its life 2.4X gram-calories of heat. This evolution of heat is enormous compared with that emitted in any known chemi cal reaction. There is every reason to believe that the total emis sion of energy from any type of radioactive matter during its transformation is of the same order of magnitude as for radon. The atoms of matter must consequently be regarded as containing enormous stores of energy which are only released by the dis integration of the atom.