E. M. Morimoto and Milton Kahn University
of
N e w Mexico Albuquerque
Preparation of
Carrier-Free Lead-212 (Thorium B)
The 10.6-hour lead-212 (thorium B), a member of the naturally occurring thorium series, can be separated in carrier-free form by collection of the active deposit produced by decay of the 52-second radon-220 (thoron).' A solid sample of thorium hydroxide or other suitable compoundz is placed in an electric field. Thoron escapes from the solid into the gas phase where it decays to 0.158-second polonium-216 (thorium A) which, in turn, decays to positively charged ions of thorium B; these ions are collected on the negatively charged plate. Examination of the thorium series reveals that the separated thorium B will be contaminated with its decay products (i.e., thorium C, etc.). However, if enough time (-five hours) is allowed for transient equilibrium to he attained, determination of the total beta and gamma activity of the sample will serve as a measure of the quantity of thorium B present. This paper describes a simple apparatus for the collection of carrier-free thorium B based on the aforementioned procedure. The amount of radioactivity obtained is more than sufficient to carry out experiments in the undergraduate physical chemistry laboratory. The apparatus is shown in the figure. In operation, the rubber sponge is saturated with water in order to maintain the humidity a t a high level. The platinum coil is adjusted to a height of about inch above the surface of the emanating source and a potential of 1200 to 1500 volts is applied across the plates. After a suitable collection period (-ten hours), the stopper assembly is removed and the activity is dissolved off the coil in 3 N nitric acid. The yield of thorium B is approximately 20%. Effects of variations in voltage, plate separation, and humidity mere investigated. The efficiency of collection increased from about 10yo at 300 and 600 volts to 20% at 1200 volts; increasing the voltage to 1500 volts produced negligible gains. Varymg the plate This oommunieation is based on work done under the auspieen of the Los Alamos Scientific Labocttory and the Atomic Energv Commission. I HFIYESY, G . , AND PANETH,F., "A Manual of Radioactivity," Oxford University Press, London, 1926, p. 90. WAHL.A. C.. AND BONNER.N. A,. Editors. "Radioaetivitv Applied tb chemistry," John wiley & Sons, k c . , New YO&, 1951, p. 293.
296
/
Journal of Chemical Educofion
separation between inch and inch resulted in no significant difference in the amount of activity collected as long as the voltage was over 1200 volts. The effect of the humidity of the atmosphere within the apparatus was pronounced Increasing the relative humidity from 30 to 50% and 80% a t 25°C produced, respectively, threefold and sevenfold increases in the efficiency of collection. Above 80% humidity, gains were not significant. The thorium hydroxide used as the emanating source was precipitated from 400 ml of 0.036 M thorium nitrate. The precipitate was oven dried at 100°C for six hours and powdered with mortar and pestle. Because the 3.64day radium-224 (thorium X) is not coprecipitated with thorium hydroxide it is necessary to allow time (-two weeks) for its growth in order to realize the 20% yield. During this time the emanating source is allowed to equilibrate with the moisture in the atmosphere in the apparatus. The authors wish to express their gratitude to Dr. J. D. Knight of the Los Alamos Scientific Laboratory for valuable discussions.
stopper
,Rubber
, One-liter wide-mouth spiralo or bottle
,Platinum coil, 1 -in. diam. (negative phtel
,Aluminum plat* 3-in. diom. (positive platel
-
Sponge
Apparatus for mllecting iead-212
