R. 1. Hayes and W. R. Butler, Jr.
Medical Division l s i t of N
e Studies1 Oak Ridge, Tennessee
I1 (
Growth and Decay of Radionuclides A demonshafjon
Laboratory exercises in radioactive decay can be conveniently carried out with many different radionuclides. If multiple laboratory sessions are available, radionuclides having half lives of several days or more can he used ( 1 ) . On the other hand, if the exercise is to be completed in one session, it is desirable to use nuclides having half lives in the range of hours (3,S), or better still, minutes. Use of the latter provides a dramatic demonstration and leads to a more accurate half-life determination by the student, since be can easily follow the decay process through a number of half lives during the laboratory session. The purchase and transport of individual radioisotopes having extremely short half lives is obviously impractical. Activation of various stable isotopes will lead to a number of short-lived radionuclides (4, 5), hut neutron sources for activation often are not available. Another approach to the supply problem is the use of a parent-daughter pair. The half life of the parent should he long, and the short-lived daughter should decay to either a stable or long-lived isotope. If some rapid method is available for the separation of the short-lived daughter, an apparatus can then be set up for laboratory production of the daughter as required. The daughter is "milked" from the apparatus containing the parent, and hence the setup is referred to as a "cow." Of the very short-lived parent-daughter pairs that have been described as laboratory aids for demonstrating radioactive decay, the pair, Bi207 (1112 = 8 yr) - Pb2" (tllz = 0.8 sec), suggested by Jones (6),is probably not usable for other than an auditory demonstration of decay, since the half life of the daughter is so short. Special equipment would he required for quantitative measurements. Booth (7) and recently Carswell and Lawrance (8) have suggested the use of the parent-daughter pair, Thz34 (t1i2 = 24.1 days) - Pa2S4(tlls = 1.18 rnin). The half life of the parent in this pair is unfortunately rather short. Carswell and Lawrance separate the thorium234 from natural uranium; protactinium234is then separated from thoriumZ39ysolvent extraction techniques. The parentdaughter pair, Ce144(tm = 285 days) Pr"' (tllZ= 17 min), suggested by Bradley and Adamowicz (9),is a better pair for use in lahoratory exercises. Even so, the method suggested for daughter separation (precipitation of cerium as the iodate, followed by filtration) is objectionable from the viewpoint of time consumed and potential laboratory contamination, unless techniques in handling radioisotopes are to be stressed. 1 Under contract with the United States Atomic Energy Commission.
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Another parent-daughter pair, CsL3'(tllz = 30 yr) Ba13"" (tliz = 2.63 min), has a number of advantages over other pairs suggested for laboratory exercises in radioactive decay. The half life of ce~ium'~'is quite long. Cesium1" is available and inexpensive in the amounts required for laboratory exercises. I n addition, an ion exchange technique can be used for the separation of barium from cesium, and the inconvenience of solvent extraction or precipitation is thus avoided. Newacheck, et al., (10) have reported the design of an ion exchange device that can be used to separate barium13'" from cesiumla'; however, they give no details on the elution procedure used with their device.
I Figure 1.
Diogrem of exchange column used for production of ila'a7'm.
A simple ion exchange technique has been developed a t the Medical Division for the separation of Ba137m from Cs13' in approximately 30 seconds and in a yield of 80 % of the theoretical. The method as used in our laboratory exercises has been highly successful. Because of the high yield and short separation time, it is also possible for the student to study radionuclide growth, in addition to decay, by measuring the buildup
of barium18'" on the column after elution. This demonstration of growth to secular equilibrium can then he related to neutron activation in reactor production of radioisotopes or other processes involving a (1 - e-') relationship. The following is a description of the apparatus and the technique developed for its use in laboratory exercises. Apparatus
The ion exchange apparatus is shown in Figure 1. It is constructed completely of transparent plastic except for the stopcock plug, which is made of Teflon. The plug indicated is a type that is commercially available under the trade name ultram ma^."^ The all-plastic construction was an attempt to make the column as "student proof" as possible. The column can, of course, be fabricated from a buret, but under t,hose circumstances there is always the possibility of accidentally shattering the column and contaminating the working area. If only a few microcuries of cesium'37 are involved, there would be no immediate physical danger; however, spillage does involve the potential hazard of human ingestion as well as instrument contamination, if the area is not properly cleaned. I n the plastic column a glass wool plug and a stainless steel retainer are inserted above the ion exchange resin to prevent loss of active resin should the column be tipped over. The resin height indicated in Figure 1 was dictated by the height of the assembled column, which in turn was the result of an emphasis on stability and storage convenience. Another resin height can, of course, be used; the data given in the discussion of the method, however, apply only to the dimensions shown in Figure 1. The column is constructed of commercially available plastic tubing. With the diameter and height of the resin column indicated in Figure 1, a 10-ml portion of eluant is sufficient for stripping the column of the available bari~m'~'"activity. If the column is filled with resin (wet mesh 20-50) by gravity, no vacuum is required, and a 1 0 4 portion of eluant will flow through the column in approximately 30 seconds. When the column is not in use, the funnel and stopcock tip can be replaced by the caps indicated in Figure 1. The apparatus may then be conveniently stored in a horizontal position. Laboratory Method
Barium1b7m can he separated from its cesium13' parent by elution from cation-exchange resins of the sulfonic acid type with ethylenediamine tetraacetic acid (EDTA). In arriving at a technique for a laboratory exercise, emphasis was necessarily placed on speed of separation as well as on yield. Another important consideration was the potential cesium13' contamination of the preparation, since ce~iurn'~' does gradually descend the column. In initial trials, it was found that the permeability of the exchange resin was an important factor in the yield of barium137". Of the various exchange resins tried, Dowex 50W-XI (wet mesh 20-50) gave the highest yield. Of the various forms of Dowex 50W-X1 tried the potassium form gave the 3
Fisher and Porter Company, Hittboro, Pennsylvania.
best bariumla'" yield. The difference between various alkali metal forms was not great, however. The EDTA elution yield of barium1a7mfrom the potassium form of Dowex 50W-X1 is constant from a pH of 13 down to a pH of 10. Below pH 10 the yield ' to spread drops. Above pH 11.5 the ~ e s i u m ' ~tends rapidly down the column with repeated elutions. A compromise pH of 11 was consetluently adopted for the cofumn. The yield of ba~iurn'~'"from the potassium form of Dowex 50W-XI with EDTA eluants at wH 11 is constant at about 80% of theoretical for EDTA concentrations from 1% down to 0.05%. Below 0.05% the yield drops rapidly. On the other hand, although the barium'3"" yield is independent of EDTA concentration in this range, the higher the EDTA concentration used the greater is the mobilization of the ~esium'~' on the exchange column. This behavior is shown in Figure 2. An EDTA concentration of 0.05y0 was consequently adopted for experiments requiring a maximum elution of hariumla'".
Figure2. Crla' contominmtion of B 0 1 3 ' ~ preporation as o fundionof number of elutions for voriour EDTA concentr~ti~nr.Resin, Dowex SOW-XI in potassium form. Elu.ntpH, 1 1 .
With an EDTA concentration of 0.05% and a resin column of the height and diameter shown in Figure 1, approximately 30 elutions can be made before the ce~iurn'~' contamination reaches a level of O.O1yo of the bari~rn'~'"yield. Even after the hari~m'~""preparation has decayed through eight half lives, this level of cesiumL3' contamination wiU introdure an error of only 2.5y0 in the net counting rate. The useful life of the cow can of course be greatly extended by increasing the height of the resin column. A larger volume of eluant will then be required. If the yield of barium13'" is not important, the useful life can be increased even further hy decreasing the EDTA concentration (Fig. 2). The loss in bari~rn'~""yield will, however, dictate a limit on the increase in column life by this method. By using such methods the useful life of the cow-can be extended t o hundreds of elutions. Our procedure for setting up the cesium'37 row is to convert the Dowex 50W-X1 resin to the potassium form, fill the column by gravity, and charge the top of the resin column with cesium13' in dilute hydrochloric acid.8 The column is next flushed with distilled water and then with three 10-ml portions of EDTA eluant. "Processed, carrier-free" from Oak Ridge National Lsbarai tory. Volume 37, Number I I , November
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The free acid is used in the preparation of the EDTA eluant; the pH is adjusted with potassium hydroxide. I n laboratory exercises with this demonstrator, it is best to use a scintillation detector, if one is available. Wide ranges in counting rates can then be handled without the coincident loss that would occur with a G-M detector. Also, the amount of cesium'37 required will be a t a minimum (a fraction of a microcurie for decay exerrises) if a scintillation detector is used. A counting time of 10 seconds has been adopted for our laboratory exercises. This short interval is used so that the students can accumulate a large number of measurements. The counting period can be of any reasonable length so long as the same period is adhered t o during the course of the exercise. I n an experiment t o demonstrate radionuclide growth to secular equilibrium, it would be desirable t o have as complete a separation of the daughter from the parent as possible. This is the reason emphasis was placed on the maximum yield of bariumla"" in the development of a laboratory technique for the separation of barium137" from cesium13'. It is impossible under the circumstances to realize a complete separation, since a finite time is involved in carrying out the operation. However, a t the separation levels that have been achieved, the growth process is easily observed by taking a count on the column immediately after the elution and using this count to correct further counts for the initial incompleteness of the separation. A count of the column before elution is assumed to be the secular equilibrium count. Figure 3 shows a typical buildup curve obtained by this procedure.
covering over the detector. If an end-window GMtube is used as the detector in an exercise to demonstrate radionuclide growth, it may be necessary to add additional shielding on the column or over the detector to avoid an excessive counting rate for the zero time correction. A typical plot of bariurnl3'" decay is shown in Figure 4. I n our experience students have usually obtained a half-life value within 0.1 min of the reported half life, 2.60 and 2.63 min (11). This is quite gratifying for both student and instructor.
Figure 4.
Typical plot of Ba1a7mdecay.
Summary
A simple and rapid laboratory method for the separation of barium137" from cesium137has been developed. The technique may be used for laboratory exercises or for lecture demonstrations to show radionuclide growth to secular equilibrium as well as radionuclide decay. S i c e the half life of barium13'" (tliz = 2.63 min) is conveniently short, both processes can be easily demonstrated in one laboratory or lecture period. Literature Cited
Figure 3.
Growth of
on exchange column after elution.
Cesium187decays t o barium13"" by beta minus emission while barium13'" decays to stable barium's7 through a gamma-ray isomeric transition that is partially converted. Most of the electrons associated with these decay processes are stopped by in. of plastic, the wall thickness used in our plastic exchange column; some particles are, however, energetic enough to traverse this thickness. I n our laboratory exercise, these energetic electrons are absorbed by the
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(1) SMITH,W. T.,AND WOOD,J. H., J. CHEM.EDUC.,36, 492 (1959). (2) DILLARD, C. R., AND MORTON, L., J. CHEM. EDUC.,35, 238 (1958). EDUC.,36,296 (3) M o n r ~ o mE. , M., AND KAHN,M., J. CHEM. (1959). (4) OVERMAN, R. T., COFPEY,D. L., AND MUSE, L. A,, J. CHEM.EDUC.,35,296 (1958). E. G., J. CHEM.EDUC., 28,521 (5) BROWN, C. A,, AND ROCHOW, ll!XlL ~~~~~ ,~ (6) JONES,W. H., J. CHEM.EDUC.,34,406 (1957). (7) BOOTH, A. H., 3. CHEM. EDUC.,28,144 (1951). D. J., AND LAWRANCE, J. J., J. CHEM.EDUC.,36, (8) CARSWELL, 499 (1959). (9) BRADLEY, A., AND ADAMOWICZ, M., J. CHEM.EDUC.,36,136 (1959). R. L.,BEAUPAIT, L. J., AND ANDEB~ON, E. E., (10) NEWACHECK, Nueleonies, 15, No. 5, p. 122 (1957). D., HOLLANDER, J. M., AND SEABORG, G . T., (11) STROMINGER, Re". Mod. Physics, 30,585 (1958).
