Manuel Navarrete University of Mexico Mexico City, Mexico
Method to Determine the Half-life of 40K
This method has the aim of determining the half-life of the natural radionuclide 401C by means of a simple experiment which it is possible to carry out in any radiochemical laboratory. The only interest in this experiment lies in the use of detect,ion efficiency to obt,ain n decay constant,. The easiness in the determination assumes that the exact,nessin the value obtained is not very precise. However, the margin of error has been +lo%, which anyway is surpassed by the several values registered in the literature. The method may be useful either as a practice for teaching purposes, or t o obtain detection efficiencies in similar conditions. Experimental
A concentrated solution of IC2C08was count,ed in a liquid Geiger-3luller for about 15 hr, in order to evduate the activit,y due to O ' K present,. Taking the value of the concentration of the solution, the number of O ' K atoms present in the sample is calculated. If the absolute activity were known, it should be possible to obtain the value of the decay const,ant from t,he radioactive decay equat,ion and hence its half-life. Therefore, the problem is reduced to finding out the detection efficiency in order to establish the disintegration rate taking the experimental value of the count rate. T o solve it,, a solution wit,h known concentration of uranyl nitrate was made and thorium-234 (UXd was taken out of t,he radioactive series of uranium by solvent ext.raction. This ext.raction is described a t the end of t,his paper and its efficiency has been established previously as 80% for both steps in which it takes place. By means of the uranyl nitrate concentration the number of 238Uatoms present. in a given volume of the solut,ion is obtained. From it, the number of Z34Th atoms and the corresponding activity can be determined
' F.~IRES, R. A,, AND PARKS, B. H., "ltadioisotope Laboratory Techniques," George Newnes Lt,d., London, 1964, p. 121.
368 / Journal of Chemical Education
theoretically because 238Uand 234Thare in equilibrium and the extraction efficiency is known. The maximum energy of the beta particles emit,ted by 234Th(UX1) is 0.19 MeV, and therefore they are stopped by the wall of the tube in a liquid Geiger counter. Nevertheless, this activity can be detected by the beta particles emitted by its daughter 234Pa (UXP) whose maximum energy is 2.29 MeV and which is in equilibrium with the parent. 234Pa(UZ) is not present in any considerable extent, and besides it emits weak betas as well. So, by comparing t,he activity calculated theoretically and the count rate, the detect,ion efficiency of beta particles from 234Pa(UX*) is obtained. The maximum energy of beta particles emitted by "Ii is 1.32 MeV. Therefore, they have a comparable spectrum of those emitted by 2341'h(UXz), whose maximum energy is 2.29 RleV. The difference in transmission of both can be linown from the diagram empirically made plotting absorber thickness against maximum energy of beta particles and indicat,ing the percentage of transmission for each couple of the preceding values.' This difference is about 16% for an absorber thickness from Detection Efficie(lcy.
Volume of 284Thz8'-Pos~lutionpresent in the mixtures versus efficiency in its detection.
20 to 40 mg/cm2, the range in which the glass wall of the liquid Geigers are. As the other factor affecting detection efficiency is different density of the solutions (since geometry and wall of the tube are identical in both detections) the following method was adopted to know the detection efficiency of beta particles emitted by "I
