Gregory R. Choppin and Carson L. Nealy Florida State University Tallahassee
Demonstration of a Parent-Daughter Radioactive Equilibrium Using 137CS-137111 Ba
M a n y teachers use the neutron activation of silver with the subsequent decay of lo8Ag (tllz = 2.4 m) and "OAg (tl12 = 24 S) to demonstrate radioactive decay. However, it is not possible to show to the students by this experiment the relationship between radioactive half life and both the rate of decay and of growth of a radioactive daughter. The 13?Cs13'"'Ba system offers an opportunity to do this and, also, has the advantage of a short-lived daughter separable by simple techniques without requirement of an activation source. Furthermore, since gamma ray emission is associated only with the lmmBa,scintillation counting with a NaI(T1) crystal makes it possible to follow the behavior directly of the 137mBain the presence of I3'Cs. The 137mBa(tl12= 2.6 m) in equilibrium with its parent, 13'Cs (t112= 30 y) is separated by precipitation of BaSO, and its decay followed in a well-type This experiment has been developed as part of the program by the NAS-NRC Suboommittee on Radiochemistry for stimulating publication of new radioehemical experiments. SEE THIS JOURNAL, 40,618 (1963). Supported in part by the U S . Atomic Energy Commission.
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scintillation couuter. The separation is repeated on the same lnCs solution and the supernatant liquid counted. This fraction shows the growth of I3'"Ba. Hayes and Butler described an experiment in which separation of the 137Cs-'S"Ba was achieved hy ion exchange elution.' However, it would seem necessary from the description to use higher levels of radioactivity than in our experiment to obtain the same accuracy. The Experiment
Part A. Approximately 2 X lo4 cpm of 13'Cs is added to 2 ml of 1 N HzS04 in a test tube which fits in the well of a scintillation counter. This solution is warmed in a beaker of hot water on a hot plate and approximately 0.5 ml of Ba2+ carrier solution [40 mg/ in1 of Ba(NO&] is added. The mixture is digested in the water bath for 30 sec and centrifuged for 30 sec. The supernatant solution is decanted into a second test tube and the first tube with the BaSOl precipitate placed in the counter well. Readings are taken every 15 sec without stopping the counter (it is well to have two people cooperating on this) for 6-7 min. The decay curve is constructed by plotting on semilog paper the
diierence between successive 15-sec readimgs as a function of time (Fig. 1). Part B. Approximately 10 min after the first precipitation, another 0.5 ml of Ba2+ carrier solution is added to the warm supernatant solution. The mixture is digested, centrifuged, and decanted as before and the supernatant solution containing the la7Csplaced in the
Figure 1.
daughter atoms, Nz, present a t any time t for a number of parent atoms, NIO, present a t t = 0 (i.e., time of separation of parent and daughter) is:
Decoy of LaWo in the 8 ~ 5 precipitate. 0 ~
well counter. Again, readings are taken every 15 sec with no interruption in the counting for 7-8 min. The growth cuwe is constructed by plotting on semilog paper the difference between successive 15-see readings. The relationship of this growth rate and the half life is shown by plotting the difference between the observed count rate and the saturation count rate (taken on the supernatant solut,ion 15 rnin after the BaSOl precipitation) versus time as shown in Figure 2.
For the la'Cs-lnnBa system, i2 >> hl and N,O = N , (i.e., no significant decrease in the number of parent atoms during time t). Also, for tlip = 30 y, e - h t = 1 for times t of a few minutes. Therefore, NI
A = 'N,(l
A2
=
- e+t)
I Since the activity, A, is equal to XN,
(2)
A, (1 - e - M )
(3)
Discussion
The decay scheme for the '37Cs-13'nBa system is shown in Figure 3.' Since the NaI(T1) well crystal has a metal covering which allows gainma rays but not beta particles to be detected, the la7"Ba but not the ' W s can be counted. This is a statistical advantage in the growth experiment (Part B). If the count rates are too low in either (Part A) or (Part B), it may be necessary to subtract the background count rate (for 15 sec) from each reading. This experiment illustrates the parent-daughter relationship for the case of secular equilibrium. The longlived 13'Cs parent shows no change in decay rate within statistics during the period of the experiment and, consequently, in (Part B) the Ia7'Ba grows to a constant value. The rate a t which it grows to this constant value is a function of the half life of la'"Ba and not of 'Ws. The exact equation expressing the number of 'HAYES,R. L., 37,500 (1960).
AND
BUTLER, W. R.,JR., J. C m . EDUC., Figure 3.
Dosay &ems
for lnCr'nmBo.
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If we take logarithms of equation (3), In A, - In AP =
- Xd
(4)
Thus, a semilog plot of the difference between A1 (the saturation value) and A%,the measured decay rate a t time t as a function of t should give a straight line with a negative slope equal to x%, the decay constant for laimBa. I n both Figures 1and 2 , the tl,* determined by the procedure in this experiment agrees satisfactorily STROMINOER, D., HOLLANDER, J. M., Rev. Mod. Phys., 30, 585 (1958).
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AND
SEABORG, G. T.,
with the literature value.2 We have performed this experiment very satisfactorily by using the well scintillation counter in connection with a count rate meter whose output drove a recorder. When a linear count rate meter is used, it is necessary to replot the curve on semi-log paper to obtain linear graphs. If a log count rate meter is used, the recorder curve is the final curve desired. For the affluent, the combination of log count rate meter and recorder is the most desirable method of demonstrating the 13?CslS7"Ba parenbdaughter relation.
