W. T. Smith and J. H. Wood University of Tennessee
Knoxville
I
I
A Half-life Experiment for General Chemistry Students
The need for including nuclear chemistry experiments in the first year course, especially for the nonscience major, is well recognized, and several good experiments have been described in the chemical literature. Among these, the measurement of the half life of an unstable isotope has received considerable attention.' This paper is to describe our experiences over the past two years with the measurement of the half life of bismuth-210 by large numbers of first year students. The choice of this isotopeZ is predicated upon several favorable factors. (1) The bismuth is continually supplied from cheap radiolead ('i,OPb, t,,, = 22 years) obtained from uranium minerals so that once a supply is obtained, it lasts for several years. Further, after the solutions have been prepared for the first time and the apparatus accumulated, little additional work is required on the part of the storeroom personnel to ready the exercise for the next quarter's or the next year's classes. ( 2 ) The half life is short (5.0 days) so that the rapid CHARLES A., 'BROWN,
AND
ROCHOW, E. G., J. CHEM.EDUC.,
fall in activity permits the use of cheap rate-meters, yet it is not so short as to require undue haste in handling; moreover, since each student counts his sample only once during a laboratory period for three successive laboratoly meetings, the number of counters needed to serve a given number of students is at a minimum. We find one counter is adequate for a section of 40 students. (3) The stock solutions are not hot enough to require shielding, yet the 1.17-Mev beta radiation is sufficiently energetic to require no special counting techniques. The daughter isotope, ';:Po, is relatively long-lived and emits alpha radiation which is stopped by a scotch tape, the granddaughter is stable lead-206. Consequently, only the beta radiation emanating from bismuth-210 is counted. (4) The fact that the starting material and the decay products represent a segment of the well-known uranium-238 series of radioactive elements adds interest to the work and serves to better correlate the laboratory work with lecture work.
28, 521 (1951); DILLAED, CLYDER.,AND MORTON, LYNWOOD, The Experimenl J. CHEM.EDUC., 35, 238 (1958). The student removes a 10-ml sample of radiobismuth We are indebted to Dr. G. K. Sohweiteer of the University of Tennessee for recommending this isotope. chloride from the stock solution and uses this sample to
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plate out on a '/, X '/*in. piece of nickel-foil enough bismuth to give the desired initial count, about 1200 counts per minute. Since bismuth is below nickel in activity, the plating occurs spontaneously: 3Ni
+ 2BiCla
-
3NiCL
+ 2Bi
The parent isotope, lead-210, does not plate on the nickel foil. About 25 minutes a t room temperature or
1
7 1 4 0 0 counts per min.
TIME idoys) Figure 1.
Typisol student graph.
3 minutes a t 10O0C are required for the plating. The solution is then returned to the stock bottle. The nickel foil with the radiobismuth is now fastened to a copper planchet by means of scotch tape. The sample is then counted by the studeut a t each successive laboratory meeting for a period of 14 days. These data are plotted on semi-log graph paper and the half life is read from the graph. Figure 1 shows a typical student graph. Table 1 shows the results obtained by 849 students during the spring of 1959. These are average students from the colleges of home economics, engineering, agriculture, business administration, education, law, and liberal arts (we do not permit our B.S. in chemistry majors to do this experiment since they will do similar ones later on a t a more advanced level). The students were mainly under the supervision of teaching assistante, of whom ouly about two-thirds had had previous experience with nuclear chemistry experiments. Also, the staff was unaware of our intentions to collect the results. Excluding the cost of the counter, we estimate that the experiment is costing from 2 4 b per student. This low cost is due to the fact that the radioactive material, copper planchet, and nickel foil are reusable for several years. The cost of the couuters is hard to estimate for we do not know how long they will last. Our three counters have survived two years of use (a total of 144 working hours for each) without a breakdown. Since Table 1.
Observed Half-life" of 'k;Bi
No. of students
Half-life range 3 . 5 days or less 3 . 6 4 . 5 dnvs 4.6-5.5 days 5 . G 6 . 5 days 6 . 6 dnvs or loneer
Accepted half-lii-5.0
days
about 2000 students have now done this experiment, the counter cost per student to date is 30 cents. The radiolead nitrate is added with stirring to 0.5 N HC1 (one-fourth pound to 4 liter^).^ Most of the lead accumulates in the bottom of the bottle as PbCI*. I n our laboratories we place the supernatant liquid at convenient locations along with a 10-ml syringe pipet standing in a large graduated cylinder (to collect drainage). Each student removes his 10-ml sample and places it in a small beaker. After the plating has been accomplished, he returns the solution to the original stock bottle. These "spent" solutions are collected and returned to the bottle containing the PbCl,. Since 'i,"Pb disintegrates to form '::Bi, the concentration of the bismuth will increase until "equilibrium" is again reestablished in the stock solution, a process which requires a few weeks. Hence, the stock solutions may be used several years if the radiolead is carefully husbanded. Since the RiCL tends to become entrapped in PbCL crystals where it forms, we have found it desirable to warm the stock mixture until the solid phase is dissolved and then cool to reprecipitate the PbC12. This is done the day before the supernatant liquid is to be removed for dispensing. Several commercial makes of cheaper rate meters are
Figure 2.
Counting chamber
satisfactory for the counting. (We use Model 1013A Classmaster, Nuclear-Chicago.) Since these instruments usually are not equipped with counting chambers, it is necessary to construct a chamber such as the one seen in Figure 2. The Geiger tube is securely strapped to the frame so that it will not be moved with respect to the counting slots during the 2-wk counting period. The frame is constructed of wood, plastic, or metal. Also, the frame is built so that the sliding tray containing the sample may be placed very close to the Geiger tube. One or two of the metal guards protecting the Geiger tube are clipped out so that ouly air will be between the sample and the tube. This reduces the amount of bismuth that must he present to give a desired count. The sliding tray has a l-in. circular depression in the center for positioning the copper planchet immediately underneath the window. The student, of course, must mark the planchet so that he can orient his sample with respect to the wiudow in the same manner for each reading. Canadian Radium and Uranium Corp., 630 Fifth Avenue, New York. Minimum purchase, onefourth pound ($12.50). Vclume 36, Number 10, October 1959
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