Was it just a chance that polonium and radium were the first to be discovered among radioactive elements? The answer is apparently no. Owing to its long half–life radium can be accumulated in uranium ores. Polonium has a short half–life (138 days) but it emits characteristic high–intensity alpha radiation. Though the discovery of polonium gave rise to a controversy it soon died off.
The third success of the young science of radioactivity was the discovery of actinium. Soon after they had discovered radium the Curies suggested that uranium ore could contain other, still unknown radioactive elements. They entrusted their collaborator A. Debierne with verification of this idea.
Debierne started his work with a few hundred kilograms of uranium ore extracting the “active principle” from it. After he had extracted uranium, radium, and polonium he was left with a small amount of a substance whose activity was much higher than the activity of uranium (approximately, by a factor of 100 000). At first, Debierne assumed that this highly radioactive substance was similar to titanium in its chemical properties. Then he corrected himself and suggested a similarity with thorium. Later, in spring of 1899 he announced the discovery of a new element and called it actinium (from the Greek for radiation).
Any textbook, reference book or encyclopedia gives 1899 as the date of the discovery of actinium. But in fact, to say that in 1899 Debierne discovered a new radioactive element–actinium–means to ignore very significant evidence to the contrary.
The real actinium has little in common with thorium but we did not mean this chemical difference as evidence against the discovery of actinium by Debierne. The main argument is as follows. Debierne believed that actinium was alpha–active and its activity was 100 000 times that of uranium. Now we know that actinium is a mild beta–emitter, that is, it emits beta rays of a fairly low energy which are hot that easy to detect. Of course, the primitive radiometric apparatus of Debierne was not capable of doing it.
Then what did Debierne discover? It was a complex mixture of radioactive substance including actinium. But the weak beta radioactive of actinium was quite indistinguishable against the background of the alpha rays emitted by the products of actinium decay. It took several years to extract the real actinium from this mixture of radioactive products.
In 1911 the outstanding British radiochemist F. Soddy published a book entitled chemistry of Radioactive Elements where he described actinium as an almost unknown element. He wrote that its atomic weight was unknown, the mean life time was also unknown, it did not emit rays (this shows how difficult it was to detect the beta radiation of actinium), and its parent substance was unknown. In a word, much about actinium was still vague.
The evidence presented by Debierne for his discovery of actinium did not seem convincing to his contemporaries. It is no wonder that soon another scientist–the German chemist F. Giesel–claimed a discovery of a new radioactive element. He also extracted a certain radioactive substance whose properties were similar to those of the rare–earth element. This fact is closer to the truth in the light of our current knowledge. Giesel named the new element emanium because it evolved a radioactive gas–emanation–which made a zinc sulphide screen to glow. Along with the radiotellurium vs. polonium controversy there appeared a similar controversy between the supporters of actinium and emanium. The first controversy ended by establishing identity between the elements in question. The second controversy proved to be more complicated and could not be speedily resolved since the behaviour of the third new radioactive element was too wayward. The name of Debierne went into the historical records as the name of the discoverer of actinium. However, the substance extracted by Giesel contained a significant proportion of pure actinium as was shown later. Giesel also succeeded in observing the spectrum of emanium. Many scientists believed that they proved identity of actinium and emanium. Gradually, the controversy lost its edge.
The British radiochemist A. Cameron was the first (1909) to place the symbol Ac into the third group of the periodic system (actually, he was the first to put forward the name radiochemistry for the relevant science). But only in 1913 was the position of actinium in the periodic system established reliably. As increasingly pure actinium preparations were obtained the scientist encountered an amazing situation–the radiation emitted by actinium proved to be so weak that some scientists even doubted if it emits at all. It has even been suggested that actinium undergoes an entirely new, radiation less, transformation. It was only in 1935 that beta rays emitted by actinium were reliably detected. The half–life of actinium was found to be 21.6 years.
For a long time extraction of metallic actinium was just out of question. Indeed, one ton of pitchblende contains only 0.15 mg of actinium while the content of radium is as high as 400 mg. A few milligrams of metallic actinium were obtained only in 1953 after reduction of AcCl3 with potassium vapour.