W. E. Lawson Cockcroft Slough College of Technology Wellington Street Slough SLl IYG, England
A Praseodymium-144 Cow
One of the problems in teaching the basic features of radioactive decay is t h e difficulty of providing a clean and reasonably active source of a nuclide with a half-life which is short enough t o allow t h e decay process t o be followed for a period longer than one half-life, a n d yet t o be accomplished within the two or three hours of the customary laboratory session. In the absence of a neutron source, such desirable short-lived nuclides have t o be extracted from some nuclide system in which they exist i n equilihriu m with a very much longer lived parent nuclide. Among the systems of genetically related nuclides, the cerium-144/pr&eodymiu&44 system has a number of advantages which merit special consideration. The parent nuclide is a beta-emitter bf half-life 284 da. Its decay results in the production of praseodymium-M4, another beta-emitter b u t with a very much shorter half-life-approximately 17 min. T h e heta-particles emitted by t h e daughter nuclide have a maximum energy of 4.8 x 10-13 J (2.98 MeV) a n d are easily detected by t h e glass-walled Geiger-Mueller tubes (such a s the Mullard M X 124101) commonly used for the measurement of activity in radioactiveliquids. Although the neodymium-144 produced by the decay of ~raseodymium-144is also radioactive, its very weak a1pha:activiti and long half-life (2.4 x 10IJ yr) present n o additional problems. Methods of separating these nuclides, based on t h e differences in chemical properties between cerium and the remaining lanthanoids, have been described by Overman and Clark,' Bradley and Adamowicz,Z and by Peet,3 the last-named author also included separations based on the use of ion-exchange resins and solvent extraction. T h e major drawback of the methods using the precipitation of cerium(N) iodate to separate the two nuclides is t h a t they require a n appreciable degree of skill and, particularly, speed of operation t h a t may not always be within t h e capacity of a student chemist. This difficulty can b e overcome by employing t h e selective adsorptive properties of manganese dioxide to separate the two nuclides. Exnerimental 8
T h e adsorptive properties of manganese dioxide vary considerably with the method of preparation. T h e following method, due t o Bigliocca4 and coworken, gives a tetragonal product with satisfactory adsorption characteristics which are reproducible from one batch t o the next. A solution of manganesefll) culfate is prepared from 36 g of the hydrated salt diiwlwd m 5W ml of distilled watcr and main. tained at 363°K ((KI'C',. '1'0 this solution .i added, sluulv and with continuous stirring, a solution of 11 g of potassium per manganate dissolved in 500 ml of distilled water. The hydrated manganese dioxide is filtered off and washed with 600 ml of distilled water. After drying at 333-336°K (60-63'C) for several hours (preferably overnight), the coarse, dry product is lightly ground and sieved, 'Overman, R. T., and Clark, H. M., "Radio-isotope Techniques." McGraw Hill, New Yark, 1960, pp. 333-57. 2Bradley, A., and Adamowicz, M., J. CHEM. EDUC., 36, 136 (1959). Peet, J. H. J., Sch. Sci.Reu., 53,353 (1971). *Bigliocca, C., Girardi, F., Pauly, J., and Sabbioni, E., A w l . Chem , 39,1634 (1967).
the fraction between 72-120 mesh being retained. A thin slurry of this fraction of the sieved oxide is prepared with dilute nitric acid (0.1 mole I-') and poured into a small buret, lightly plugged above the tap with glass wool, to produce a column appraximately 2 cm in height. The column is then washed with dilute nitric aeid (0.1 mole I-') until the eluate is free from fine suspended particles of manganese dioxide. Once prepared, the column should not be allowed todry out. The column is charged with a solution of carrier-free cerium144 in dilute nitric acid (0.1 mole 1-1). The amount of activity added will he determined by the amount of praseodymium likely to be required by the student group. As a rough guide, 2 ml of a solution with an activity of 12,000 min-I ml-' (measured in a Mullard MX 124/01 liquid counting G/M tube), applied to the column and left for one hour to equilibrate, provided enough of the daughter nuclide to give 20,000 min-' on elution with 6 ml of dilute nitric acid (0.1 mole I-'). If this quantity of activity is used in a single determination of the half-life, it is necessary to take into account the lasses caused hy the paralysis time of the G/M tube by applying the correetion formula
where Ct,,. is the true number of counts per minute; Cobs is the observed number of counts per minute; and t is the paralysis time of G/M tube or counting equipment. When the paralysis time of the counting equipment is not known, it is advisable to commence with approximately 10,000 min-I in order to reduce the relative effect of the paralysis time losses. Further quantities of praseodymium-144 may be eluted from the column as the daughter activity grows toward its equilibrium level-satisfactory quantities may be eluted after approximately three half-life periods (50 m i d . Values of the decav constant and half-life obtained by students agree very well with literature values, even when the observations have extended over several half-life periods, and indicate that a clean separation has been achieved. The genetic relation between cerium-144 and praseodymium144 may be studied by adsorbine the equilibrium mixture of nuclides & to a small sample of t i e manganese dioxide and, after equilibration, displacing the praseodymium-144 with a solution of lanthanum nitrate. A 50 mg sample of the sieved a i d e is equilibrated far 1hr with 1 ml of the stock cerium-144 solution. The solution is then removed, using a Pasteur pipet, and discarded to the active liquid waste bottle. A small volume (2 ml) of lanthanum nitrate solution (0.1 mole I-') is added to the oxide and the mixture gently shaken for a period of 2 min. Subsequent counting operations are timed from the moment of addition of lanthanum nitrate to the manganese dioxide. After 2 min, the lanthanum nitrate solution is removed and discarded to the active waste bottle, and the manganese dioxide rapidly washed with two portions of 2 ml dilute nitric acid (0.1 mole I-'). The manganese dioxide is then transferred, as a thin slurry in the dilute nitric aeid, to a clean planchet, and the excess liquid removed by small pieces of absorbent tissue, which are discarded to the active solid waste container. One minute counts of the source are made, initially at 2-min intervals and then at intervals of 3 or 4 min over a total period of approximately 70 min. This source, once prepared, may be used several times if reasonable care is taken not to lase manganese dioxide during the displacement and washing operations. Conclusion Since this method has been adopted, students a t all levels have been able t o study the decay of a short-lived nuclide more accurately, more conveniently, and more economically than before. Volume 50, Number 9. September 1973 / 633
