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Application Note: Low Level Liquid Scintillation Counting of Radioactivity in Food Products

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Low level liquid scintillation spectroscopy can be successfully used for counting alpha and beta activity in food products to monitor the natural radioactivity, contamination by nuclear fallouts, …

Low level liquid scintillation spectroscopy can be successfully used for counting alpha and beta activity in food products to monitor the natural radioactivity, contamination by nuclear fallouts, contaminants from nuclear power stations or fuel reprocessing plants. It also provides means for surveying the synthetic additives in food on the basis of the natural radioactivity to be found in the products.

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  • 1. a p p l i c at i o n N o t e Liquid Scintillation Counting Low Level Liquid Author Scintillation Counting Lauri Kaihola of Radioactivity in Wallac Oy, P.O.Box 10, SF-20101 Turku, FINLAND Food ProductsLow level liquid scintillation spectroscopy can be successfully used for counting alpha and beta activity in food productsto monitor the natural radioactivity, contamination by nuclear fallouts, contaminants from nuclear power stations orfuel reprocessing plants. It also provides means for surveying the synthetic additives in food on the basis of the naturalradioactivity to be found in the products.Cosmogenic carbon isotope is first discussed as an example. This isotope is a natural ingredient of biological food productsand acts as an important tracer, revealing any deviations from the origin of the food.C-14 IN NATURECosmic radiation produces C-14 (radiocarbon) in stratosphere by neutron bombardment of nitrogen 14 1 14 1 N + n --> C + p 7 0 6 1The production rate is 7.5 kg isotope C-14 yearly. The C-14 concentration stays approximately constant due to rapid mixingof the atmosphere, although the cosmic intensity is higher at the poles due to the deflection of charged cosmic particlesalong the magnetic field lines of the earth (corresponding to neutron intensities in the ratio 5:1 at the poles and the equator,respectively). Consequently, C-14 atoms combine to form heavy CO2, which, except in the radioactive decay (and isotopicfractionation effects), is indistinguishable from the ordinary carbon dioxide. The total amount of C-14 on earth in equilibriumis 62 tons, which is 10exp(-10) per cent of all carbon in biosphere, atmosphere and oceans.C-14 circulates through the reservoirs in the same pattern as the ordinary carbon.
  • 2. Decay of C-14 is through beta emission, where simultaneously emitted neutrino takes part of the decay energyand therefore beta particle is not monoenergetic but has a long tailed energy spectrum with maximum energy150 keV and the mean energy 30 keV: 14 14 - C --> N + b + y 6 7Half life in the above decay is 5730 +- 40 a.PRINCIPLE OF C-14 DATINGAll the living matter becomes marked with C-14, which decays exponentially after the death of the object. Oneis then able to calculate the age t of the object by taking a sample and comparing its present activity C with theactivity at its death Co 5730 Co t = ----- ln (-----). ln2 CObviously one has to make the assumption that the C-14 concentration in the exchange reservoir has beenconstant or is known at the time of death of the object.This principle of radiocarbon dating was proposed by W.F. Libby in 1946 and he was conferred a Nobel prize inchemistry for it in 1960. The same principle has then been applied using other cosmogenic isotopes as tracers(K-Ar dating etc.).MAN-MADE FLUCTUATIONS IN C-14 CONCENTRATIONThe constancy of the C-14 content is quite a reasonable assumption to make and from measurements on knownage samples it appears that within a few percent the assumption is valid back to 1500 B.C. There have beenfluctuations caused by climate as well as short and long term variations due to the sunspot activity and changesof the geomagnetic moment. There are, however, also man-made fluctuations in the C-14 content:1) Fossil effect or the Suess effectThe combustion of coal and oil releases into the atmosphere large quantities of CO2 in which the C14 hasdecayed long ago. This dead carbon dilutes the C-14 concentration in the air. Therefore, the activity of woodsamples, grown say in 1950 (prior to hydrogen bomb testing) is in fact lower than in samples grown in 1850prior to the industrial revolution despite the decay that has occurred in the latter.2) Effect of nuclear weapon testsIt has been estimated that neutrons released in fission and fusion explosions until 1962 have caused theformation of about 2 tons of C-14. If this were distributed uniformly, there would be an excess of 3 % of C-14.However, there is a hold-up in the atmosphere and so the effect is much greater and the concentration wastwice the prebomb one in 1963 (see fig. 2) and has since decreased to 30 % excess. (The bomb effect hasbeen useful in studying the carbon cycle because of the prohibition of the atmospheric and ocean tests in 1963by an agreement between the superpowers.) 2
  • 3. COUNTING OF C-14 BY LOW LEVEL LIQUID SCINTILLATION SPECTROMETRY In most cases for radiocarbon dating the organic samples are converted into benzene (C6H6) for liquid scintillation counting by first combusting into CO2, which is further, synthesized into lithium carbide with lithium above 750C 2CO2 + 10Li ---> Li2C2 + 4Li2O. This is then hydrolized into acetylene in reaction Li2C2 + 2H2O ---> C2H2 + 2 LiOH. Acetylene is converted in the presence of a catalyst into benzene. Counting of C-14 activity is accomplished by adding scintillation agent, e.g. butyl-PBD in the benzene sample. LIQUID SCINTILLATION COUNTING OF FOOD A) Disclosing adulteration of food There are legislative requirements that synthetic food or alcohol can not be distributed for use. In most cases standard chemical methods are satisfactory to monitor unauthorized use of synthetic substituents. In some other cases they are not identifying synthetic materials of identical composition. Synthetic food may be made out of petroleum derived raw material having minimal content of C-14 activity. In those cases it is possible to enforce the law by analysing food for the content of C-14 (the same principle can be used for dating of wines). Normal C-14 activity for fresh food will be around 18 dpm/g carbon (= modern carbon). Lower activities decline due to dilution by synthetic raw materials. Counting can be accomplished on CO2 produced by combustion and dissolved in the scintillation liquid or by synthesizing the food into benzene like in radiocarbon dating. The former method does not allow large concentrations of carbon to be introduced in the cocktail and the latter one means quite elaborate synthesis. Another way is to ferment the product, starch, sugar; etc. into alcohol with can be counted directly in a cocktail. Measurement of the C-14 content of ethanol (or gasohol in some countries, which must originate from renewable sources, i.e. contains only modern carbon) can be done directly mixing with the scintillation liquid. The mixing ratios may be up to 3:2 in ethanol:scintillation liquid ratio. It is advantageous to use alcohol with less than 10 % water in it. Lower limit of detection in Quantulus is 0.1 dpm/g carbon for 100 min counting time and 3 sigma resolution criterion (Schönhofer). This means that less than a 1 per cent dilution of modern alcohol by synthetic one is revealed at 99.5 % probability in 100 min counting. Direct counting of caffeine and cinnamic aldehyde has been carried out by Noakes and Hoffman to evaluate the presence and degree of adulteration. B) Counting of radioactive contamination of food Recent nuclear fallout has clearly shown the need to be able to monitor the radioactivity of food by reasonably fast methods. In most instances gamma spectrometry is the appropriate method to apply. The semiconductor detectors have excellent energy resolution enabling efficient identification of the often numerous low activity isotopes. Low level liquid scintillation spectrometry has its areas of application as is seen in the following examples.3
  • 4. Cs-137Cs-137 can be monitored on the basis of its 662 keV gamma emission, which is actually from the metastabledaughter Ba-137m. Actually Cs-137 first decays by beta emission, with 82 % probability through 500 keV betaemission into Ba-137m and with 8 % probability directly to Ba-137. Beta emission can be very well measured byliquid scintillation spectrometry. Identification of this isotope is further determined by the monoenergetic conversionelectron peak at 625 keV.Sr-90/Y-90These isotopes have only beta decay mode and therefore liquid scintillation method is a very suitable one to use.Alpha countingPulse shape analysis provides a means for alpha/beta separation on the basis of their different pulse lengths (shapes).Fluorescent decay of scintillation light is composed of prompt and delayed components; most of the light isproduced in the prompt component. The amount of the light in the delayed component has long been knownto be dependent on the particle or decay type: electrons originating from beta decay, gamma and X-rays are thepredominant sources of prompt fluorescence, whereas alpha decay contributes more to the delayed componentgiving longer pulses than those produced by electrons. Beta particles are about ten times more effective in producinglight than alpha particles. This is why the beta spectrum covers the alpha spectrum range in a scintillation counteralthough alpha particle energies are higher, typically 4 to 9 MeV.The Pulse Shape Analyser works by producing a PSA value, which relates the pulse length to the pulse amplitude.Thus the amplitude dependence of the pulse length is minimized. Counting of radon in water is a good example ofthe application of the pulse shape analyser. Direct mixing of the sample is also in this case a sufficient method ofsample preparation.Rn-222 is a daughter of radium-226 in the uranium decay series. In Finland the bedrock is radioactive to some extentto introduce alpha activity in waters from drilled wells. (Problematic is radon from soil in some areas penetrating intohouses and representing the major radioactive load.)The alpha spectrum of Rn-222 shows four peaks, Rn-222 and Po-214 intermixed and Po-218 very well separated ata higher energy. In the equilibrium we then have three times of the alpha activity of radon present in the sample.Radium and its predecessors do not very much dissolve in water to be seen in the spectrum. The beta emissions inthe series are by Pb-214 and Bi-214 and by the next isotope in the series after Po-214 is Pb-210 with 22 a half-life,thus being of low activity.Very low backgrounds are achieved in alpha counting, typically 0.05 cpm in teflon counting vial and 0.3 in glass vials.At Am-241 alpha energy (5.5 MeV) these values lead to LLD = 0.1 and 0.3 mBq/sample. When one measures higheralpha energies like Po-214 (7.6 MeV), the background is even less, 0.005 cpm in a teflon vial and LLD = 0.03 mBq. 4
  • 5. REFERENCES Baxter, M.S. and Walton, A., Carbon-14 Concentrations in Recent Wines and Spirits. J. Food Sci. 36, 540 (1971). Resmini, P., Volonterio, G. and Cecchi, L., Detection of the synthetic labelled (C-14) ethanol added to wines and other alcoholic products (in Italian). Rivista di Viticoltura ed Enologia di Conegliano, N. 8, Agosto 1976, Instituto di Industrie Agrarie DellUniversita di Milano. Kaneko, T., Ohmori, S. and Masai, H., An Improved Method for the Discrimination between Biogenic and Synthetic Acetic Acid with a Liquid Scintillation Counter. J. Food Sci., 38, 350 (1973). Osina, P., Berk, H. and Moghissi, A.A., Determination of the origin and age of alcoholic beverages by liquid scintillation counting. In Liquid Scintillation Counting, Recent Applications and Development Vol. II, Sample Preparation and Applications, ed. Chin-Tzu Peng, Donald L. Horrocks, Edward L. Alpen, Academic Press, New York, 1980, p. 469. Kostadinov, K.N. and Yanev, Y.L., Liquid Scintillation Measurement of C-14 in Ethanol Extracted from Bulgarian Wines. Nucl. Instr. Methods in Phys. Res. B17, 511 (1986). Schönhofer, F. and Weisz, J., Measurement by Ultra Low Level Liquid Scintillation Counting Following the Chernobyl Accident. J. Radioanal. Nucl. Chem. 115 (1), 125 (1987). Schönhofer, F. and Henrich, E., Recent Progress and Application of Low Level Liquid Scintillation Counting. J. Radioanal. Nucl. Chem. 115 (2), 317 (1987). Schönhofer F., C-14 in Austrian Wine and Vinegar. Radiocarbon 34(3), 768-771 (1992). Noakes, J.E. and Hoffman, P.G., Determination of natural product purity by radiocarbon measurement. In Liquid Scintillation Counting, Recent Applications and Development Vol. II, Sample Preparation and Applications, ed. Chin-Tzu Peng, Donald L. Horrocks, Edward L. Alpen, Academic Press, New York, 1980, p. 457. Noakes, J.E., Applications of Low-Level Liquid Scintillation Counting, in Advances in Scintillation Counting, eds. S.A.McQuarrie, C. Ediss and L.I. Wiebe, University of Alberta, Edmonton, 1983, p. 407. Salonen, L., Determination of Sr-90 and Sr-89 in environmental samples by liquid scintillation counting. In Liquid Scintillation Counting, Vol. 5., 15-31, 1978. Salonen, L. and Kaihola, L., Low-Level Liquid Scintillation Counting of Sr-89 and Sr-90. 1988. Juznic, K. and Fedina, S., Radiochemical determination of Sr-90 and Sr-89 in soil. Fresenius Z. Anal. Chem. 323, 261 (1986). Suomela, J., F”renklad metod f”r analys av strontium-90 i mjölk (A simplified method for Sr-90 analysis in milk). SSI- rapport 87-22. Swedish Radiation Protection Institute, 1987, 7 pp. Melin, J. and Suomela, J., Rapid determination of Sr-89 and Sr-90 in food and environmental samples by Cerenkov counting. 2nd Research and Coordination Meeting of Coordinated Research Programme on Rapid Instrumentation And Separation Methods For Monitoring Radionuclides In Food And Environmental Samples. IAEA, Vienna, 12-16 August, 1991. 9 pp.5
  • 6. Buzinny, M.G., Zelensky, A.V. and Los, I.P., Beta-Spectrometric Determination of Sr-90 in Water, Milk and Other Sampleswith Ultra-Low-Level Liquid Scintillation Counter. In Liquid Scintillation Spectrometry 1992, Proc. of the Int. Conf. on Advancesin LSC, LSC 92, Vienna, Austria, Sept 14-18, 1992. Eds. J.E. Noakes, F. Schönhofer and H.A. Polach. Radiocarbon, Tucson1993., pp. 439-446.Bjornstad, H.E., Lien, H.N., Yu-Fu, Y. and Salbu, B., Determination of Sr-90 in environmental and biological materials withcombined HDEHP solvent extraction - low level liquid scintillation counting technique. J. Radioanal. Nucl. Chem., Letters 156(5), 165-173 (1992).Suschny, O., Determination of Environmental Radioactivity at Two Different Concentration Levels. Results of Two Recent IAEAInter-Comparisons. Nucl.Instr.Meth.Phys.Res. 223, 477 (1984).Lopes, J.S., Pinto, R.E., Almendra, M.E. and Machado, J.A., Variation of C-14 Activity of Portugese Wines from 1940 to 1974.In Proc. Int. Conf. on Low-Activity Measurements and Applications, 6-10 Oct 1975, Bratislava, 1977 p. 261.Asikainen, M., Natural Radioactivity of Ground Water and Drinking Water in Finland. Ph.D. Thesis. STL-A39. Institute ofRadiation Protection, Helsinki, Finland, 1982.Salonen, L. and Hukkanen, H., Advantages of low-background liquid scintillation alpha-spectrometry and pulse shape analysisin measuring Rn-222, uranium and Ra-226 in groundwater samples. J. Radional. Nucl. Chem. 226 (1-2), 67-74 (1997).Muck, K., Sinojmeri, M. and Steger, F., Long-term Availability of Sr-90 in Foodstuff after Nuclear Fallout. P-11-246 inhttp://www.irpa.net/pub/pr/index.htmlPerkinElmer, Inc.940 Winter StreetWaltham, MA 02451 USA P: (800) 762-4000 or(+1) 203-925-4602www.perkinelmer.comFor a complete listing of our global offices, visit www.perkinelmer.com/ContactUsCopyright ©2011, PerkinElmer, Inc. All rights reserved. PerkinElmer® is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners.009647_01 Printed in USA