P. P. Parekh a"o M. Sankar Das
Atomic Energy Establishment Trombay, Bombay, India
I
I
Genetically. Related Radionuclides Growth-decay
O n e of the elemeutary experiments in a first course in radiochemistry at the graduate level is the study of the growth of a short lived radionuclide from its relatively long lived parent, thereby illustrating their genetic relationship. Many pairs of radionuclides have been recommended (1-5) for this purpose. Of these, one which has convenient half-lives for the purpose in view is ThB (Pb212)-ThC(BiZ12)with the following decay scheme:
studies of
Th8-ThC
Our purpose in this paper is to describe an experimental demonstration of the different relationships that exist in the activities of a pair of genetically related radionuclides which will help the student to grasp the basic concepts. The Experiment
ThB is isolated iu a radiochemically pure form from thorium nitrate by solvent extraction (6). If the 0- activity of the isolated ThB is measured, it will be found to increase gradually with time because of the growth of ThC (and ThC") from ThB. From this measurement the student determines: (1) The growth of 21ZBifrom its initially pure pareut flzI'h. - .. (2) The time of maximum activitv of the mixture
Dillard and Mortou ( 5 ) , who recommended this pair before, have suggested the isolation of carrierfree 2'2Ph-212Biand following the decay of the same. From this data, the growth curve of the daughter from the parent and the independent decay of the pure daughter are computed. The more direct aspect of following the growth of a daughter from an initially pure parent is not included.
(T,& and the daughter (T',,,) a d compares them with the theoretical values ( 1 ) . (3) The half-life value with which the system (as well as the daughter) decays after reaching equilibrium. (4) The half life of the pure daughter from its rate of decay. Special reagents: thorium nitrate; lead carriers: 10 mg Ph/ml and 100 pg Pb/ml; bismuth carriers: 10 mg Bi/ml and 100 pg Bi/ml; 0.02% w/v dithizone in CClr.
Procedure Dissolve 2 g thorium nitrate, 100 pg each of lead and bismuth, 5 g citric acid, and 2 g tartaric acid in 20 ml distilled water. Adjust the pH t o 9.5 using phenolphthalein. Extract twice with 20 ml of 0.02% dithizone in CCI,. Organic layer: Scrub with dilute ammonia. (VH9.5)
Aqueous layer: Discard.
I
Organic layer: Extract twice with 0.001M HCI (note time). Organic layer (contain8 Bi): Extract with 1M HCI, and discard the organic layer. The aqueous layer contains the Bi. Add 10 m e of Bi: oot. with HIS: filter: drv: mount and count fb; 2 hrs a t 15 k i n intkrvdi L
1 1
Aqueous l a w (contains Pb): Add 10 mg of Pb; ppt. with H S ; filter; dry; mount and count for 4 hrs a t 15 min intervals: and then far 4 hrsl a t 1 hr intervils.
If there is insuEeient time, count the following day.
354 / Journal o f Chemical Education
ilyueous layer: Discard.
/
1
1
Measured activities were corrected for coincidence and background.
(5) and (6) T',, is very short which calls for a very quick chemical separation so as to observe any noticeable growth. Again pairs (2), (3), (4), (5), and (6) are the cases of secular equilibria so that it is not practically possible to demonstrate the rate of decay of the system after it attains equilibrium. With normal facilities students can gain maximum information from this single experiment performed in about 8 working hours.
Typical experimental growth-decay curves are shown in Figures 1 and 2. In Figure 1, curve a is the plot of the observed gross activity of the system as a function of time on a semilog paper. From this plot T,., is determined. The growth portion of curve a is extrapolated to zero time (as indicated by the dotted line). Through this activity a t zero time curve b is drawn parallel to the decay portion of curve a. This represents the decay of the pure parent. Subtraction of curve b from curve a gives curve d which represents the activity of 2'2Bi from an initially pure z'ZPh. From this plot T,.,' is determined. Extrapolation of the
1
2
Figure 1. Letten on the groph represent: la1 total activity of on initiolly pvre Pb-212; lbl activity due to Pb-212; ( 4 decoy of pvre 81-212 fraction; Id) 81.212 activity growmg from a n initially pvre Pb-212 fraction; (el total Bi-212 aclivity in Pb-212 81-212 froctionr
+
4
3
TIME (N WIURS)
Figure
2.
Decoy of freshly irolated 81-212
The authors wish to express their sincere thanks to Dr. V. T. Athavale, Head, Analytical Division, AEET for his keen interest in the work.
decay portion of curve d to zero time gives curve e. Subtraction of curve d from curve e gives curve c, which represents the decay curve of pure 212Biin absence of its growth from 212Pb. From curve c the half life of zLZBiis found aud compared with an independent measurement on the pure 2'2Bifraction giveu in Figure 2. In Table 1 a comparison is made between the genetically related pairs of radionuclides used for teaching and the recommended ThB-ThC system. From the table, it is clear that for the pairs (I), (2), (3), and (4), T,.,' is very long, ranging from a few days to a few months. The duration of the experiment is naturally long. 011the other hand, for the pairs Table I.
2. =0Sr - 'OY 3. RaD RaE (PIOPb- "OBi) 4. UI - UXI (WJ - *"Th)
1 4 4 C-~ 14
