MEASUREMENT OF THE HALF-LIFE OF UX,' A Lecture Demonstration A. H. BOOTH Atomic Energy Project, National Research Council of Canada, Chalk River, Ontario
FuNnAMmmL techniques of radiochemistry can be ml. beaker. The solution, which should have a b H of demonstrated using only the naturally occurring r a d i ~ not less than 5.0, is heated nearly t o boiling for one isotopes normally available in uranyl salts and a simple hour. A sizable fraction (40-60 per cent) of the UX, Geiger counter. I n the purification of uranyl salt adsorbs on the wall of the beaker, leaving the lanthanum reagents the decay products of uranium are removed, in solution. The beaker can be washed with water hut the first two products of the decay, UX1 (thorium), without any loss of the UX,, which is then stripped from and UXe (protactinium), quickly grow in again. They the walls with nitric acid. A portion of the nitric reach equilibrium in a few months. The next member acid solution is evaporated on a platinum tray (crucible. of the series, UI1, is very long lived, so that the series is, lid) and used as the source in the following experiment. in effect, arrested a t this point. The uranyl salt ob- The residue on the tray should be invisible, or nearly tained from a chemical supplier will contain the equilih- so. rium amount of UX1 and of UX2 (and its isomer UZ), Determination of the Half-life of UX,. The I%, hut no appreciable amounts of the other members of which has a half-life of 1.14 minute, is in equilibrium the decay series. with the parent UXI, whose half-life is 24.5 days. It is quite easy to separate the UX1 from the uranium The observed activity of the equilibrium mixture de-. by means of a carrier precipitate. But to perform the cays with the half-life of the parent. But half of t h e experiment described later the UX, must first be iso- beta particles really come from the UX2, as can be lated in a weightless (carrier-free) form. The following shown by placing a series of aluminum absorber strips, procedure has been found to give a cleaner separation between the source and the Geiger tube. There is. than other proposed methods, and to he more con- seen to be a soft component and a hard component, veniently carried out. A quantitative yield is not ob- for the UX2 beta rays are more penetrating than those tained but this is not required. The new point in the of UXI. To demonstrate the decay of UX2 i t must b e method is that use is made of the fact that UX, forms separat,ed from the UX,, collected in a suitable form, a so-called "radiocolloid" in nearly neutral solutions and inserted in the counter in a very short time. No. that do not contain too high a concentration of electro- very rapid method for doing this has been reported and lytes. This radiocolloid, which is probably thorium it has hitherto been necessary to start with a rather hydroxide formed by hydrolysis, adsorbs strongly on large quantity of material in order to have enough activity remaining at the start of counting. glassware, especially when the solution is heated. The method now described is so simple to carry out. Preparation of a Carrier-free UXI Source. About that it can be done very readily in a classroom. The. 10 to 50 g. of uranyl nitrate hexahydrate crystals are dissolved in ether. A stoppered flask is used since separation is clean and rapid and made directly on t h e there is danger of separation of an insoluble solvated counting planchet. It does not give a completely salt by evaporation of the etherial solution. The quantitative recovery hut this is not necessary. It small aqueous layer formed from the hydrate water gives a 50 per cent recovery of the UX2 originally contains almost all of the UX, and can be separated by present and takes less than one minute to carry out, so centrifuging the solution and decanting the ether. that one need start with only four times as much UXr The aqueous residue is evaporated with a few drops of as one wishes to have remaining at the start of counting. concentrated nitric acid until crystals start to separate, The method was suggested by an observation of J. W. is difficult to remove. and then the ether extraction is repeated on a smaller Knowles2 who found that Paz3% scale. To separate the UX1 from the small amount of from platinum ware after an acid solution has been uranium remaining, the residue is diluted with 2 per evaporated. Thorium so treated is readily soluble. cent nitric acid, 5 mg. of lanthanum nitrate carrier Protactinium oxide is known t o be but slightly soluble. added, and the lanthanum and UX, precipitated with in acid, but it is perhaps rather surprising that this oxalic acid. The lanthanum precipitate is separated fact can be exploited when dealing with amounts of the. and ignited t o oxide, then dissolved in nitric acid and order of 10-lo g. The UXl/UX2 source, on a platinum tray, is covered evaporated to dryness on the steam bath. The residue is dissolved in 75 ml. of water in a 100- with one or two drops of concentrated nitric acid and' warmed gently to dissolve the UX,. The UX2 also 1 Prrsrmtd sr the Annual \lrrtmgof rhc ('l~tvuiwl Instituteof (hnada ((:hrmic:!l Eduvntion M + c t Dividion) June 21, 1050.
2
144
KXNOWLE~, J. W., Ph.D. thesis (McGill) 1948.
MARCH, 1951
dissolves, for that which is present was formed from UX, in the dry residue and so had no chance to adhere to the platinum. The following operations should now be carried out in the minimum possible time. The nitric acid is evaporated to dryness on a hot plate (about 30 seconds). The tray is immediately dipped into concentrated nitric acid for one second, then plunged into a beaker of distilled water to rinse off the acid. It is quickly withdrawn, blotted with filter paper, and placed in the counter. Almost all the UX, is dissolved off by the acid but about 50 to 70 per cent of the UXI remains on the tray. The decay of this UX2is readily apparent and can be plotted by students if the scaling circuit used gives audible clicks or has indicator lights. The curve should be plotted on semilog paper. After complete decay of the UXI a relatively small activity due to undissolved UX1 will remain. This amounts to less than 2 per cent
of the original. When it is subtracted from the other activity readings, the resultant curve is a straight line from which the 1.14-minute half-life of UX, can be read off. The separation yield can be determined by projecting the decay curve back to zero time; i. e., the time a t which the separation was effected. This can he taken as the time a t which the solution was just completely evaporated. This extrapolated value is then compared with the original UXI activity, which is the value given by the UXl/UX2 source when the soft beta raw of UX, are screened out with aluminum foil. The experiment can be used to illustrate some fundamental points of radiochemical technique, including the analysis of mixtures of activities by the resolution of absorber curves and decay curves, and it serves as a good introduction to a discussion of decay rates and secular equilibrium in radioactive chains of decay.
