A Double Deoay Experiment Half-Life Determination in a Mixture of Two Independently Decaying Radionuclides I. Houdaverdls and S. S. Kontis' Institute of Physical Chemistry, National Research Center for Physical Sciences "DEMOKRITOS", Athens GR-15310, Greece
103.8 min-a factor of about 1:8), which are obviously suitable for the purpose. Thus, the whole experiment should not exceed 3 h. A d it can be convenientlv started 20 min after the end of the irradiation. (4) A very s i o r t irradiation time is enoueh for the nurnose of this exneriment. (5) No s ~ e c i a l c h e m k or phy&ai manipulationof radioactive material is reauired. thus decreasing safetv hazards. (6) The same samplea can be reused a numier of times, afte;areasonable cooldown period. Natural neodymium consists of seven isotopes, with mass numbers 142, 143, 144, 145, 146, 148, and 150. Only the isotopes with mass numbers 146, 148, and 150 can yield radioactive products hy thermal neutron capture. The pertinent nuclear data are presented in the table. As can be seen, after the radiative neutron capture there will be more than two radioisotopes present in the sample. The daughter nuclides of 149Ndand 151Nd are also radioactive, as well as 14?Nd.which is also formed. The latter also eives a radioactivedaughter nurlide,"'Prn, but the half-livesofparent and dauahter are so large (10.98 davs and 2.62 -vears..res~ectively) that the activi& due t o the daughter will not aiter the shape of the resulting curve in the experiment. Moreover. the; rays emitted by i47~mhave suchlow energies that the& measurement can easilv be excluded by the suitable use of an integral discriminator. With a discriminator setting of 1 MeV, the corresponding relative detection coefficients for l47Nd, 149Nd,149Pm,151Nd, and lSIPm are 0.2, 1.6, 0.7, 2.1, and 1.5, respectively. I t can be seen that the isotopes of interest (149Ndand lSINd)have the greatest detection coefficients.
Many articles have appeared in this Journal describing experiments for the study of radioactive decay. Most of them, however, deal with secular equilibrium (e.g., 1 3 ) ,that is, the case of a long-lived mother radionuclide producing a short-lived daughter, or with the decay of one single nuclide (e.g., 4), or involve a-emitting nuclides, extremely harmful if inhaled or swallowed (e.g., 2, 4). We decided that another good way of acquainting the students with the laws and other asoects of radioactive decav would be through the study of'the simultaneous decay of two independently deravine radionurlides in a mixture. With that goal in mind we have iesigned a suitable experiment for thehalf-life determination in such a mixture. The experiment, as described here, i s suitable for undergraduate o r beginning graduate students as one of their first experiments involving radioactivity. Baslc Conslderatlons For this experiment we chose to use the isotopes ""d and t"1Nd. obtained bv the thermal neutron irradiation of natural neodymium. In this process a neutron is incorporated into the nucleus with the practically simultaneous emission (within about 10-l4 s) of y radiation. In this experiment the two pertinent nuclear reaction equations are:
The product nuclei are 8-emitters, and each decays with its characteristic half-life. In each case gamma rays are also emitted. Detection of these y's with a NaI(T1) crystal provides the simplest means of monitoring the decay and constructing the decay curve. The use of natural neodymium has several advantages: (1) Only one element has to he irradiated. This saves the trouble of preparing a mixture of elements. (2) The natural abundances and thermal neutron cross sections of the pertinent stable isotopes bave values that give product activities suitable for observation of the decay curve. (3) The radionuclides ~ r o d u c e dbave very convenient half-lives (12.4 and
Experimental
A sample of 0.5 g of Nd(N0&5HzO was sealed in a small (4.0 X 0.8 cm) polyethylene tube. This was enclosed in a larger tube and placed in a rubber cylinder. The whole was placed in a lead cylinder, dipped in the reactor pool, and irradiated for 1min in a thermal neutron flux of 2.7 X 10'1 n/ s/cmz. After a ~ ~ r o x i m a t eal v2-min cool-down neriod. the rubher cylindeiaas removed from the lead cover;wipeddry, and transported to the laboratow. There the cvlinder was opened, the innermost tube was rinsed with EDTA solution,
' Author to whom correspondence should be addressed.
Nuclear Data lor the Neodymium Iw1ope.s Involved In the Experiment (9) Stable Isotope '%d
"%d lSoNd
Abundancs
(n.7) crass section
17.22% 5.73% 5.64%
2.5 b 1.2 b
'Abbreviatbnr 4: y =years.
1.4 b
r
Prlmry
Product nuclide
decay T112
daughter nuclide
wNd
10.98 d 1.73 h 12.4 mln
"'Pm '"Pm 'S'Pm
,a,, ?I+,
@decay TI,,
Secondary daughter nuclide
2.62 y 53.1 h 28.4 h
"*Sm lslSm
Tqm
lo" Y
stable 90 Y
d = days, h = hwrs, min = minutes, b = barns.
Volume 68 Number 2 February 1991
171
after 10 min, which is when the sample is handled by the students. Once inside the crvstal. the radiation dose escaoing the 2-in. lead shielding iipractically nil. After the endhf the experiment, the calculated activity at 240 minis 1.2 uCi. As far as radioactivity considerations for the sample are concerned, the experiment is harmless, provided that (1) the tuhe containing the sample remains well sealed and (2) the general safety rules for radioactive material handling- and disposal are applied. Comments As can be seen from the nuclear data, the decay schemes involved in the neodymium thermal neutron capture are rather complicated, and a t first sight they might seem discourapina. as far as their use for educational Durnoses is concemea. But, in fact, the two half-lives are reidi& determined, because the side products have small detection coefficients and large half-l&es. The information that needs to he given to the students is: (1) The theory of half-life determination in a mixture of two nuclides. (2) Instructions for the use of counting apparatus. (3) Suggestions as ta the points to be used for each half-life determination.
+
Time (min)
Example of a plot resulting trm one experlmntal run. The dots owrespond to the experimental polnts (total activity): dashes indicate the actlvity due to "9Nd. while dots and dashes remesent the shorter lived I5'Nd isotme actlvitv. The dotted llnes illushate the graphical evaluation of the half-life as the tlme necessary for each i~otopeactivity l o drop to half Its inltial value.
dried, and then placed inside a 3-in. well-twe NaI(T1) scintillation crystal for counting. In all the exp&iments, the first count was taken 12-15 rnin after the irradiation. After auite a few trials, we chose to measure the activity i s counts 6s at the following intervals: every 1rnin for the first 20 min, then everv 3 rnin for the next 15 min. and then everv 5 rnin for the remainder of the experiment. Measurements taken in the first 30-40 min (after the end of the irradiation) are used for the half-life determination of the shorter lived isotope, whereas measurements between 105 and 160 min are used for the longer lived isotope half-life determination. The data analysis procedure is described in most radiochemistrv textbooks (e.g., 5-8) and is illustrated in the figure. FO; three replicate determinations the average half-life values and the co~respondingstandard deviations were found to he: for 149Nd104 1rnin and for lSINd12.9 0.3 min. Note on safety: The calculated total initial activity of the sample prepared by the above procedure is 21.2 pCi. This value, which is well inside the safety limits, drops to 13.8 ~
~
~
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~
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~
*
172
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Journal of Chemical Education
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In case no reactor facilitv is available. other neutron sources, such as S b B e or spontaneous fission sources, can be used. T o counteract the smaller activities resulting from the lower neutron flux, it is ponnible to (1) increase the sample size and irradiation time, (2) decrease the time interval between theendof irradiation and start of theexperiment, and ( 3 ) measure the activity as counts per 20 or 30 s, instead of the orooosed 6. It is advisable. however. to keeo the irradiationtime low, because the shorter the irradiation time, the more enriched the mixture is to the shorter lived isotope. When such modifications are made, i t may he necessary to modify the specified time intervals. Acknowledgment The authors wish to thankR. Nissiotouand I. Zerlentisfor their collaboration in the preparation and carrying out of the experiments. They also wish to thank P. Katrivanos and S. Archimandritis for their help with the instruments and the Reactor Staff of "DEMOKRITOS" for their cooneration in the irradiation of the samples. I.H. wishes to'thank the Phvsical Chemistrv Institute of "D" for the oo~ortunitvto do iesearch using the existing facilities. The &t appe&ing in this article was done usina the CERN HBOOK and HPLOT graphics packages, a n d with the friendly assistance of the "D"Computer Staff. Literature CRed 1. 2. 3. 4. 5. 6. 7. 8. 9.
Williama,K.R.;Lipfod,L. C. J. Chem.Educ. 1985,62,89&898. Wai, C. M.;Lo, J. M . J Chsm.Educ. 1984.61.257-258, Choppin, G.R.;Nes1y.C. L. J. Chom.Educ. 1964.41.598-800. Pinncl, R. P. J. Chsm. Educ. 1970,47,459-460. Friedlander,G.; Kennedy, J. W.; Miller, J. M. Nucleor and Rodiochemistry. 2nd ed.; Wiiey: New York,1964 pp 69-71. Ouerrnsn. R. T. Bosic Concents 01 Nudeor Chamisfry;Chapman & Hall: New York, 1965;pp 85-86. Harvey, B. 0.Introduction to Nuclear Phyaica and Chemisrry; Prrnfie~-Hall:En& ciim, NJ, 1962;pp 31-33. Taylor, D. The Meosuremsnf ofRodio Isotopes; Methuen& Co: London, 1957: pp 7-9. Reus. U.;Weatmeier. W. AfomicDota ondNucleor Dofa Tobles 1983,29,193-406.
