100 YEARS AGO

Concerning the recently discovered heat emission from radium, it is perhaps worth noting that it appears to be connected with, and is probably an immediate consequence of, the remarkable observation by Rutherford that radium emits massive positively-charged particles, which are probably atoms, with a velocity comparable to one-tenth of the speed of light... Because it is easy to reckon that the emission of a million heavy atoms per second, which is a small quantity barely weighable in a moderate time such as a few weeks (being about the twentieth part of a milligramme per century), with a speed equal to one-tenth that of light, would represent an amount of energy equal to one thousand ergs per second; that is to say, would correspond to heat enough to melt a milligramme of ice every hour. And inasmuch as these atoms are not at all of a penetrating kind, but are easily stopped by obstacles, they would most of them be stopped by a small thickness of air, and their energy would be thus chiefly expended in the immediate proximity of the source, which source would thereby tend to be kept warm.

From Nature 2 April 1903.

50 YEARS AGO

Before the War one could make materials artificially radioactive by bombardment in big machines like cyclotrons, or by using relatively weak neutron sources. In the cyclotron, one can generally only use one target at a time and the irradiation is therefore costly. The weak neutron sources induce only weak activities. Therefore only a few research workers profited from the radioisotopes which one could produce in these ways. The situation changed suddenly with the discovery of fission of uranium in 1939. This discovery showed that chain reactions were possible in which more neutrons are created than used. Accelerated by war research, the first chain-reacting atomic pile was working on December 2, 1942, in Chicago... New radioisotopes were quickly discovered and the chart of radioactive isotopes started to expand. To-day, there are more than six hundred radioactive isotopes, of which, however, only some hundred can be made conveniently in an atomic pile. For most elements there is at least one usable radioactive isotope. The only notable exceptions are the two elements nitrogen and oxygen for which no convenient radioactive isotope exists.

From Nature 4 April 1953.