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Potassium in Apricots-A
Nuclear chemistry Experiment
J a c k C. Norman College of Environmental Sciences, University of Wisconsin-Green Dehydrated fruits, especially dried apricots, are often recommended as a good source of dietary potassium. Various nutrition handbooks (1,2) give the potassium content of dried apricots as approximately 1%. This concentration of potassium may easily he semiquantitatively determined by measurine the eamma radiation from naturallv occurrine 4"K usin&impleinstruments normally available undergra&ate teachine lahoratories. The followine ex~erimentis an interesting and relevant way to introduce the importance of background radiation measurements, "counting statistics," and propagation of errors in radiochemistry, radiohiology, and nuclear physics lahoratories. Natural potassium contains 0.0118% 40K, a radionuclide that decays with a 1.3 X lo9-yhalf-life. It decays by two modes: 89% by 8- emission to the ground state of 40Ca and 11% by electron capture to an excited state of 4%r. The electron capture is followed hy the emission o I n 1.48;-MeV gamma ray ( 3 ) .4"K accountri for A I U UBOri ~ of the internal irradiation and 20% of the u m l natural harkgrtmd radiation dosr received by people (4). The detector background and count rates for a hag of dried apricots and a bottle of analytical reagent grade potassium chloride may he determined using a 3 X 3-in. (7.6 X 7.6-cm) NaI(T1) scintillation detector and a scaler. A single-channel analyzer or multichannel analyzer system might also be used. A short counting time (10-15 min) may he used for the KC1, hut two or more loneer ( 3 0 4 0 min) counts must he used for the background an(]apricots. The tahle rites thr rounts obtained with a iinrle-channel analyzer set t o count only the 1.46-MeV gammaray of 40K. The apricots and background were counted alternately for 30 min each. The standard errors (standard derivations) given are those due to "counting statistics" only, i.e., calculated from the square root of the total count. The standard error in the net count rates was calculated using the usual rule: the square root of the sum of the squares of the errors in the sample and background count rates (5).
in
Natural Radloactlvnv in Aoricots Count Rate
Time
i Std. Dev.
Sample
Caunt
(min)
(cpm)
Back~round
14233
60
237.2 i 2.0
Net Specific Std. Dev.
Activity
*
(cpm/g)
...
These data yield a value of 0.66% K in the apricots. Since the count rate with the apricots is only slightly above hackground, the standard error due to the statistical nature of
Bay, Green Bay, WI 54301 radioactive decay in the net count rate is substantial. The standard error in the percent potassium is fWi6 [(0.0121 (0.01/3.1C~'1' = 10 38% K,nearly 60% ot the value. 0.021"J Although the uncert&ty is large, (0.66 f 0.381% K in apricots is in reasonable agreement with the handbook values, which are based on chemical analysis. Dehydrated hananas, which also have an exceptionally high potassium content (I), may he used instead of apricots. Brazil nuts, which may have a naturally occurring radium content ) .will uo to more than 1000 times greater than most foods (,4,. give a count rate slightly above background, The gamma rays from radium are more difficult to detect with the eauinment found in most undergraduate lahoratories, because the principal gamma ray (0.187 MeV) occurs in only 4% of the decays (3). Two 30-min counts each of hackeround and of 227 e of dehydrated hananas give a net count rLte of 0.034 f O.O1fcpmlg hananas. This corres~ondsto a ~ o t a s s i u mcontent of (1.1 0.4)% K. The net specific activity from 285 g of shelled Brazil nuts using a 3 X %in. NaI(T1) crystal and single-channel analyzer set to detect gamma rays with energies between 0.01 MeV and 2.5 MeV was found t o he 0.17 f 0.05 cpmlg Brazil nuts. Comparison with a nominal 0.5 wCi radium source indicates that the radium content of these brazil nuts is about 50 pCi/ 100 g. Since most students are surprised t o learn that there are measurable levels of radioactivity in foods, this makes an interesting exercise for an early lahoratory in any nuclear science course. The foods used may easily he obtained locally (most health food stores have dehydrated hananas). Using them in the original packaging in which they were purchased is effective in convincine students that thev are "real" food. not samples that might contain added radiGactivity. The importance of "counting statistics" and the methods of propagation of errors, as well as the importance of background radiation, may be discussed. The exercise may he arraneed to fit conveiiently into a 3-h laboratory peribd. The idre time during the long counts may he used for other exercises, e.g., sample preparation or the use of other detector systems.
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Literature Cited Ill Enslminger, A., Enaiminger, M., Konlands, J.. and Robon. .L"Focds and Nutrition
Eocyelopedia; Peg= Preas, Clovis, CA, 1983. 12) Tucr. D., and R u d , P.. "Nutrition and Hpslth Eneyelopedis,.lVan Nostrand Reinhold Co., New York, 1981. 1%)LFderer, C. M., Hollander, J. M.,andPerlman.I.,"TablcofIsotop."6Bed..John Wiley &Sons. Inc,Now York, 1967. (41 Eiaenbud. M.. "Environmental Radioactivity: 2nd ed., Academic PIB, New Yark, 1973. I51 Choppin, G. R., and Rydhrg, J., "Nuclear Chemistry; Pergamon Press. Oxford, 1980.
Volume 62 Number 5
May 1985
439
