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Quantitative Determination of Tritium in Natural Waters

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Quantitative Determination of Tritium in Natural Waters Quantitative Determination of Tritium In Natural Waters GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-D Quantitative Determination of Tritium In Natural Waters By C. M. HOFFMAN and G. L. STEW ART RADIOCHEMICAL ANALYSIS OF WATER GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-D UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 15 cents (paper cover) CONTENTS Page Abstract. _ _____________________________________________________ Dl Introduction. ____________________________________________________ 1 Electrolytic enrichment,__________________________________________ 2 Factors influencing separation efficiency _________________________ 4 Multistage enrichments.______________________________________ 5 Counting procedures-_____________________________________________ 6 Gas phase technique._-_---______-_--_---___-____---__-___--_- 6 Liquid scintillation technique._________________________________ 10 Enrichment calculations.__________________________________________ 11 Theory and calculations.__-_---_-_-_____-__---__-____--___-_-_-___ 13 Analytical errors.________________________________________________ 14 Comparative sensitivities of counting techniques.--___-_-_-----_.- 15 Counting statistics.__--____________-__________-___-_____-_--_- 15 Summary_ _____________________________________________________ 17 References_ ____________________________________________________ 17 ILLUSTRATIONS FIGURES 1-7. Diagrams of: Page 1. Nickel-steel electrode for electrolytic enrichment process__ _________________________________ D3 2. Vacuum distillation apparatus for the quantitative recovery of sample._________________________ 6 3. Gas production apparatus used to prepare H2 from water. __ ________________-____-____________ 7 4. Internal sample Geiger-Mueller detector.________ 8 5. Counting array...._-_-_--____-____-__---__-_. 0 6. Block diagram of electronic circuitry.___________ 9 7. Falling drop apparatus for measuring of D2O in water.___________________________________. ]2 8. Graph of falling time of sample droplet as a function of the deuterium content-_________________________________ 12 9. Graph of percent error, sample activity and counting time relationships for the liquid scintillation system. ------ 16 TABLES Page TABLE 1. Effect of temperature and current on tritium recovery.------- D5 2. Analytical results of tritium concentration measurements. ____.. 15 m RADIOCHEMICAL ANALYSIS OF WATER QUANTITATIVE DETERMINATION OF TRITIUM IN NATURAL WATERS By C. M. HOFFMAN and G. L. STEWART ABSTRACT This report describes the methods and procedures used by the U.S. Geological Survey Tritium Laboratory to analyze natural waters for tritium activity. Samples having low activity are electrolytically enriched in cells with nickel and steel electrodes, and their tritium concentration is subsequently determined by liquid scintillation and gas phase counting techniques. Samples having high tritium concentrations can be counted directly, and no enrichment is required. Theory, calculations, equipment, and procedures are described in some detail. INTRODUCTION In recent years, as the Nation's water supply problem has caused more concern, the hydrologist has turned to radioisotopes to aid him in his studies of water residence times and movement patterns. Tritium is superior to other water tracers in some kinds of hydrologic studies because it is incorporated directly into the water molecule. Tritium, a naturally occurring radioactive isotope of hydrogen, has a mass number of 3. It is a soft beta emitter and has a maximum energy of 0.018 Mev (million electron volts) and a half life of 12.26 years. The discovery of tritium is attributed to Sir Ernest Rutherford and his coworkers at the Cavendish Laboratory in England; they produced tritium by deuteron-deuteron interaction: .H*(d, p)JB*. (1) In this reaction, He3 is also produced, and Rutherford was unable to ascertain which isobar was radioactive. Later, Alvarez and Cornog (1939) at Berkeley, Calif., determined tritium to be the radioactive component and He3 to be stable and naturally occurring. Dl D2 RADKXTHEtMHOAiL AmLYiSIS OF WATER Rutherford also attempted to detect tritium in natural waters. He attempted to concentrate tritium by electrolyzing large quantities of natural water but was unable to detect tritium in the enriched water by mass spectrometry. In 1951, Dr. Willard F. Libby, at the University of Chicago, measured the enriched water produced by Rutherford with a Geiger- Mueller detector instead of the less sensitive mass spectrometer and found the sample contained radioactive tritium. Tritium is produced naturally in the atmosphere by cosmic radi­ ation, and the following mechanism is believed to be mainly respon­ sible for its production: 6C12. (2) It is also produced naturally in geologic formations by the following reaction: 3Li6 M iH3. (3) In addition, tritium is produced synthetically in thermonuclear explo­ sions, and large concentrations are trapped in rain and snow. The usefulness of tritium in dating or tracer studies depends on the accuracy of its analysis. Tritium levels for some natural waters are extremely low, and the analysis may require special enrichment techniques and other critical steps, all of which must be performed with great care to produce accurate and meaningful results. The following discus­ sion presents methods employed by the U.S. Geological Survey Trit­ ium Laboratory to determine quantitatively the tritium concentra­ tions in natural waters. Acknowledgments. The authors are grateful to Mr . T. A. Wyerman and Mr. R. K. Reaves for the assistance they rendered in the Labora­ tory while the authors were working out the analytical procedures re­ ported here. ELECTROLYTIC ENRICHMENT Pre-hydrogen bomb tritium concentrations in water are low, and it was calculated by Faltings and Harteck (1950) that there is only one mole of natural tritium in the entire atmosphere, or one tritium atom for every 10 cc (cubic centimeters) of air. As a convenience, tritium concentrations are normally expressed in tritium units, 1 TU being de­ fined as one tritium atom for every 1018 hydrogen atoms. This ratio is approximately equal to 3.2 picocuries per liter. Low tritium concentrations in natural waters are not directly meas­ urable and therefore the water samples must be enriched. Four meth­ ods of enrichment are currently employed: electrolysis, thermal diffusion, distillation, and gas chromatography. Fractional electrol- DETEEiMIN'ATiaN" OF TETTIUM D3 ysis is the method most commonly used and is the method used in the tritium laboratory. The electrolysis is performed in cells of the type shown in figure 1. At present, cells of 1-liter and 250-, 40-, and 10-milliliter capacity are used, the size of the cell being determined by the degree of enrichment required. As a general proposition, samples are enriched according to their estimated original tritium concentrations as follows: Volume reduction Concentration (tritium units) (mittillters) > 1,000 __ _ __--____-____-___-____ None measurable directly. 100-1,000.__. _____________________ 40-4 10-100. ---_----_-_--___----__-___ 250-4 -_-___---_________-_____.__ l,OpO-4 These ranges are flexible and may be adjusted according to the amount of time available for counting a sample. For example, a sample containing 400 TU may be measured directly but this requires a long counting time and monopolizes the counting equipment unnecessarily. The electrodes are constructed from uniform steel and nickel tubing, and the two segments are separated by a Teflon spacer of uniform /Steel Nickel las vent Spacer FIGURE 1. Nickel-steel electrode for electrolytic enrichment process. D4 KADKJCHEIMIGAL ANlALYiSIS OF WATER thickness. The cells are fitted with stoppers to eliminate cross con­ tamination and a gas outlet to allow the H2 and O2 produced during electrolysis to escape. The reactions that occur at the electrodes are Nickel anode: 2H2O-*4H++O2+4e-, (4) Steel cathode: 2H++2e--*H2, (5) where H may be protium (H1), deuterium (H2), or tritium (H3). Nickel and steel are used as electrode materials because of the good separation efficiency obtained (that is, the reduction of protium in reaction (5) is more rapid than the reduction of tritium), their corrosion resistant character, and their ready availability. FACTORS INFLUENCING SEPARATION EFFICIENCY Factors that adversely affect the separation efficiency during the electrolysis are: 1. Variation in interelectrode spacing and associated current density differences. 2. High temperatures in the electrolysis cell which produces water vapor losses. 3. Irregularities on electrode surface. 4. High salt concentrations which react with the electrodes. During electrolysis, protium is preferentially reduced and discharged at the cathode, and the solution surrounding the cathode increases in tritium concentration relative to the rest of the solution. This con­ dition is undesirable since a larger proportion of tritium is available for reduction at the cathode, and the separation efficiency of the process is reduced. This concentration gradient is eliminated to some extent by diffusion, convection caused by thermal differentials, and mixing through gas evolution (Urey, 1933). If irregularities are present on the electrode surface, electrolysis takes place hi the elec­ trode cavities since gas bubbles tend
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