DISCOVERY OF RADIUM
In the year 1896 H. Becquerel discovered that uranium emits spontaneously a radiation that produces an impression on a photographic plate through a sheet of black paper, and ionises the air. Mme. P. Curie proved that this property, later called radioactivity, is characteristic of the atom of uranium and is possessed also by thorium. But she found that uranium minerals were much more active than could be predicted from their uranium content. By the hypothesis of the existence of a very radioactive unknown substance present in very small quantity, she undertook, with Pierre Curie, research for this substance in the uranium mineral called pitchblende.
The method they used in that work was entirely new; the result of the separations made by the ordinary process of chemical analysis was controlled by tests of the activity of every fraction; the activity was measured quantitatively by the current produced by the substance when placed in a special “ionisation chamber.” Thus concentration of the radioactive property was traced in two fractions of the treatment, the fraction containing bismuth and the fraction containing barium.
In July 1898, P. Curie and Mme. Curie published the discovery of polonium, the element accompanying bismuth; in Dec. 1898, P. Curie, Mme. Curie and G. Bémont published the discovery of radium. Though the existence of these new substances was certain, they were present only in a very small proportion in the products obtained at that time; yet Demarçay was able to detect in the barium-radium mixture three new lines belonging to radium.
Only in 1902 did Mme. Curie succeed in preparing the first decigram of pure radium salt and made a determination of its atomic weight. The separation of barium was made by a process of fractional crystallisation. The work proved exceedingly difficult in practice on account of the great quantities of material that had to be treated. Later Mme. Curie made a new determination of its atomic weight and prepared metallic radium.
The new method used by P. Curie and Mme. Curie for the discovery of polonium and radium—chemical analysis controlled by measurements of radioactivity—has become fundamental for the chemistry of radioelements; it has served since for the discovery of many other radioactive substances. The discovery of radium and the preparation of the pure element has had very great importance in laying the basis of the new science of radioactivity. The identification of its spectrum and the determination of its atomic weight have been decisive facts for convincing chemists of the reality of the new elements.
INDUSTRIAL PRODUCTION OF RADIUM
Radium has been manufactured in several countries. The first factory was started in France in 1904, not six years after the discovery of radium.
Minerals.—Radium is to be found in all uranium ores; however only those that have been mined in sufficient quantity for the extraction will be here mentioned.
Pitchblende or Uraninite.—Uranium oxide more or less impure. Mines in Bohemia and Belgian Congo.
Autunite.—Double phosphate of uranyle (UO2) and calcium. Mines in Portugal, United States and elsewhere.
Carnotite.—Vanadate of uranyle and potassium. Mines in Colorado, Australia and elsewhere.
Betafite.—Niobo-titanate of uranium and calcium, with rare earths. Mines in Madagascar.
The first radium was prepared from pitchblende from Bohemia. Later the principal exploitation was that of carnotite in Colorado and of autunite in Portugal. At the present time the most important supply is extracted in Belgium from the pitchblende of Belgian Congo. A mineral containing more than one decigram of radium per ton is considered as very rich. Minerals were treated down to a few milligrams per ton.
Industrial Treatment.—The method of industrial extraction of radium, in its essential points, is still the original method that was used and described by Mme. P. Curie. The operation can be divided in three parts: dissolution of the mineral, purification of a barium-radium salt, separation of radium from barium by fractional crystallisation.
The treatment for dissolving the mineral differs from one mineral to another. Autunite and certain carnotites are soluble in hydrochloric acid, but nearly all other minerals must be attacked by more energetic agents, for instance with the aid of sodium carbonate.
When the mineral does not contain much barium, a certain quantity of barium salt is added in order to carry away the radium. The barium-radium mixture is separated. With some variations in the mode of separation of uranium and lead (always present in the mineral) or eventually vanadium, niobium, etc., the operation consists in separating barium-radium by precipitation as sulphates and redissolving these sulphates by ebullition with sodium carbonate followed by a hydrochloric attack. Generally the radium-barium mixtures pass more than once through the state of sulphates.
After the purification of the barium-radium chloride, radium is concentrated by a process of fractional crystallisation, radium chloride, less soluble than barium chloride, being concentrated in the crystals. After this first enrichment the active salt is again purified particularly by the elimination of a residue of lead, and is transformed into bromide for the continuation of fractional crystallisation (the use of bromide was suggested by Giesel). The final crystallisations are made on small quantities of salt in very acid solutions. All the operations are controlled by the ionisation method, to avoid the loss of radium. At the end of the purification, great care must be taken to protect the chemist from the action of the radiations, especially at the moment of the filling of the tubes or apparatus with the radium salt. The radon liberated in the room during the fractionation must be eliminated by a constant aeration.
Mesothorium.—Some minerals of uranium contain also thorium. In these minerals radium is mixed with another radioelement, mesothorium I, isotope of radium. Mesothorium I is much more active than radium, but has less commercial value for the same activity, because its life is much shorter (6.7 years). Mesothorium can be used instead of radium in certain cases.
MEASUREMENTS
Measurement of the quantities of radium is very important for scientific as well as for commercial purposes. The quantities to be measured vary greatly. For instance, the tubes of radium prepared for medical use contain in general from 1 to 100 milligrammes, but radium has often to be dosed in natural waters or minerals, where the quantities are of the order of 10-7 gr. and even down to 10-12 gr. per litre of water or per gram of mineral.
For very small quantities, only an ionisation method of measurement can prove successful. For relatively great quantities, radium could be weighed but it would require a careful purification of the radium salt and the precision would in general be very unsatisfactory, considering the high price (about £10 the milligramme of this element). Here also, therefore, ionisation measurements are used for the sake of accuracy.
For quantities ranging from about one-tenth of a mg. to the greatest quantities available the radium must be sealed in a tube where radon and the active deposit accumulate. Measurements are made by comparing the ionisation produced in an ionisation chamber by the penetrating rays from the tube containing the unknown amount of radium and from a standard tube placed in the same conditions. A correction is made for differences of form or of absorbing power of the two containers.
The sale of radium is made according to the certificates delivered by national technical laboratories: National Physical Laboratory (London), Radium Institute (Paris), Radium Institute (Vienna), Physikalische-technische Reichsanstalt (Berlin), U.S. Bureau of Standards (Washington). Discrimination between radium and mesothorium I in sealed tubes is rather difficult. For this purpose, of commercial interest, scientific methods have been devised, based on the different penetrating power of the γ-rays, or on the different production of heat.
An international radium standard was prepared in 1911 by Mme. P. Curie. A quantity (about 22 milligrammes) of very pure radium chloride was exactly weighed and sealed into a thin tube of glass. This standard is kept at the International Bureau at Sèvres, and secondary standards, carefully compared with this one, have been prepared for different countries.
Determination of the very small quantities of radium contained in a few grams of a mineral, or a few litres of mineral water, is made by measuring the quantity of radon it produces in a definite amount of time. The mineral is dissolved and radon removed by passing a slow current of air through the solution, then after a few days, the radon accumulated in the solution contained in a close vessel is conveyed to a special ionisation chamber, where the current produced by the α-rays of radon and radium A, B, C, is measured. This allows calculation of the amount of radium in the solution, if the ionisation chamber has been standardised by a similar operation made with a known quantity of radium. For the standardisation a very dilute solution is prepared by taking a small definite fraction of α solution containing a quantity of radium directly measurable by the penetrating rays. To facilitate the standardisation of apparatus, technical laboratories deliver standardised solutions or samples of radium-barium salt containing a known proportion of radium. The method of quantitative determination is exceedingly sensitive and can be used for amounts of radium from 10-6 gr. to 10-10 grammes.
