Actinium preparation

The actual discovery was made by Mile. Marguerite Perey at the Curie Institute in Paris. In 1939 she purified an actinium preparation by removing all the known decay products of this element. In her preparation she observed a rapid rise in beta activity which could not be due to any known substance. She was able to show that, while most of the actinium formed radioactinium, an isotope of thorium, by beta emission, 1.2 0.1 per cent of the disintegration of actinium occurred by alpha emission and gave rise to a new element, which she provisionally called actinium K, symbol AcK (35, 36). This decayed rapidly by beta emission to produce AcX, an isotope of radium, which was also formed by alpha emission from radioactinium. Thus AcK, with its short half-life, had been missed previously because its disintegration gave the same product as that from the more plentiful radioactinium. 

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 scientists 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, radiationless, 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. 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

The actual situation with regard to the purity of most of the actinide metals is far from ideal. Only thorixun (99), uranium 11,17), neptunium 20), and plutonium 60) have been produced at a purity > 99.9 at %. Due to the many grams required for preparation and for accurate analysis, it is probable that these abundant and relatively inexpensive elements (Table I) are the only ones whose metals can be prepared and refined to give such high purities, and whose purity can be verified by accurate analysis. The purity levels achieved for some of the actinide metals are listed in Table II. For actinium (Ac), berkelium (Bk), californium (Cf),... 

This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

Americium, californium, and einsteinium oxides have been reduced by lanthanum metal, whereas thorium has been used as the reductant metal to prepare actinium, plutonium, and curium metals from their respective oxides. Berkelimn metal could also be prepared by Th reduction of Bk02 or Bk203, but the quantity of berkelium oxide available for reduction at one time has not been large enough to produce other than thin foils by this technique. Such a form of product metal can be very difficult to handle in subsequent experimentation. The rate and yield of Am from the reduction at 1525 K of americium dioxide with lanthanum metal are given in Fig. 2. 

In principle, a promising method for the preparation of Ac metal is the tantalothermic reduction of AcC, as described generally in Section II,C. This method has not been tried as yet, however, so the metallothermic reduction of an actinium halide or oxide remains the only proved method. 

Actinium-227 occurs in uranium ore and is a decay product of uranium-235. It is found in equilibrium with its decay products. It is prepared hy homhard-ing radium atoms with neutrons. Chemically, the metal is produced hy reducing actinium fluoride with lithium vapor at 1,100°C to 1,300°C. 

Ac, actinium, was initially identified in 1899 by Andr6-Louis Debierne, a French chemist, who separated it from pitchblende. He dissolved the mineral in acid, then added NH4OH, and found that a radioactive species was carried down with the rare earth hydroxides. He named the element actinium after the Greek aktinos which means ray. Because of its low abundance in U, the element is usually not obtained by isolation from U. It can be obtained in mlligram amounts by irradiation of Ra-226 in a nuclear reactor. The preparation of Ac metal involves reduction of AcFs by Li at high temperature. 

The failure to discover francium earlier is easy to understand when it is remembered that the half-life of the longest lived isotope is only 21 minutes. This gives the element the distinction of being the most unstable to radioactive disintegration of all elements up to number 98 (38). It is also noteworthy that this is the only element in the group discussed in this chapter which was not discovered by artificial preparation in the laboratory. Nevertheless, the rarity of actinium in nature is so great that this element is best prepared artificially when its properties or those of its daughter elements are to be studied. 

Metallic actinium cannot be obtained by electrolytic means because it is too electropositive, II has been prepared on a milligram-scale through the reduction of actinium fluoride in a vacuum with lithium vapor at about 350 °C The metal is silvery white, faintly emits a blne-rinted light which is visible in darkness because of its radioactivity, The metal takes the form of a face-centered cubic lattice and has a melting point of 1050 50°C. By extrapolation, it is estimated that the metal boils at about 3300 0. An amalgam of metallic actinium may be prepared by electrolysis on a mercury cathode, or by the action of a lithium amalgam on an actinium citrate solution (pTT — 1.7 lo 6.8). 

Up till a few years ago four elements were still missing from the roll-call, in fact 43, 61, 85, and 87, although their discovery had been announced repeatedly but always incorrectly. These elements have now all been prepared artificially and they have been found to be radioactive, while there are also grounds for assuming that stable isotopes of these elements cannot exist. These elements, if they have ever existed, have, at least as far as the first two are concerned, very probably died out on earth long ago, just as radium (half-life 1590 years) would also have remained unknown to us if it had not continually originated afresh from the extremely slowly decaying uranium (half-life 4.49 io9 years). The presence of 87 in extremely small quantities in the decomposition products of actinium was first dis-... 

Actinium and protactinium are present in uranium minerals but recovery is not generally practiced. Neptunium (237Np and 239Np) and plutonium (M9Pu) are present in minute amounts in uranium ores because they result from reaction of neutrons with uranium isotopes, not due to survival from primordial formation. All other actinides are entirely synthetic. Methods of preparation for those up to Cf are given in Table 20-2 syntheses of the other elements are noted in Section 20-18. 

There are no practical commercial uses of actinium. Actinium of 98 percent purity is prepared for research studies. 

Little need be said about the halides of actinium. All of the binary halides of this element have been prepared by conventional procedures, and the straightforward, uncomplicated chemical behavior that actinium exhibits generally is also evident here. Thorium, however, presents quite a different problem. Thorium gives no indication of existence in aqueous solution in an oxidation state lower than +4. From considerations of the... 

Freeman and Smith (32) have prepared the anhydrous chlorides of a number of lanthanides and of thorium by dehydrating the hydrated chlorides with thionyl chloride. Although efforts to obtain anhydrous plutonium trichloride in this way were unsuccessful, it is believed that this may be a useful procedure for actinide elements such as actinium, americium, and curium that have a particularly stable (III) oxidation stage. In general, aqueous methods for preparing tetrachlorides are of little value but anhydrous trichlorides, particularly of the transuranium elements, can be obtained readily from the hydrated trichlorides by dehydration in an atmosphere of hydrogen chloride. 

They discussed actinium then, distinguished their work from that of Curie and Savitch and pointed out that all so-called transuranics would have to be reexamined. Not quite prepared to usurp the prerogative of the physicists, they closed on a tentative note ... 

---