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A liquid scintillation counter for low specific Beta activities Terentiuk, Fred 1953

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A LIQUID SCINTILLATION COUNTER FOR LOW SPECIFIC BETA ACTIVITIES by FRED TERENTIUK A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n PHYSICS We accept t h i s thesis as conforming to the standard required from candidates for the degree of DOCTOR OF PHILOSOPHY. Members of the Department of Physics. THE UNIVERSITY OF BRITISH COLUMBIA Augus-t, 1953. ABSTRACT. A s c i n t i l l a t i o n counter for the measurement of ex-tremely low s p e c i f i c beta a c t i v i t i e s has been developed which achieves i t s high s e n s i t i v i t y through the application of large amounts of radioactive substance, and the use of an e f f i c i e n t l i q u i d s c i n t i l l a t i o n phosphor. The counter can V be used i n two d i f f e r e n t ways; as a volume counter and as a raultiple-sainple counter. In the f i r s t type of counter the s o l i d radioactive sample i s mixed with the l i q u i d phosphor to form a transparent paste; i n the second type a smaller amount of s o l i d sample i s spread out on a large surface of cellophane and immersed i n the s c i n t i l l a t i n g f l u i d . 40 1& 3 5 Experiments have been made with K , C , and S to determine the range of application, s e n s i t i v i t y , and e f f i c i e n c y of the counters. The volume counter can be used to assay a wide variety of material and i s e s p e c i a l l y suited for b i o l o g i c a l problems. Its s e n s i t i v i t y was found to be be-—11 t t e r than 10 curies/gm., while that of the multiple-sample counter i s of the order of 10 curies/gm.. These sensi-t i v i t i e s can be measured with a good degree of r e p r o d u c i b i l i and are superior to the conventional methods, with the ex-ception of the elaborate screen-wall counter method. THE UNIVERSITY OF BRITISH COLUMBIA Faculty of Graduate Studies PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY THURSDAY, AUGUST 20th, 1953 at 11:30 A.M. IN ROOM 301, PHYSICS BUILDING External Examiner - J.W. Spinks of FRED TERENTIUK B.Sc. (Alberta) 1948 M.A. (Brit. Col.) 1949 COMMITTEE IN CHARGE H.F. Angus, Chairman 0. Bluh A.M. Crooker F. A. Kaempffer G. M. Shrum J. Allardyce W.C. Gibson G.N. Tucker S.H. Zbarsky LIST OF PUBLICATIONS The Electric Conductivity of Weak Electrolytes and Ampholytes at High Electric Field Strengths. 0. Bluh and F. Terentiuk. The Journal of Chemical Physics 18, 1664, 1950. Liquid S c i n t i l l a t i o n Beta Counter for Radio-active Solids. 0. Bluh and F. Terentiuk... Nucleonics 10, No. 9, 48, 1952. T H E S I S A LIQUID SCINTILLATION COUNTER FOR LOW: SPECIFIC BETA ACTIVITIES A s c i n t i l l a t i o n counter for the measurement of extremely low specific beta activities has been developed which achieves i t s high sensiti-vity through the application of large amounts of radioactive substance, and the use of an efficient liquid s c i n t i l l a t i o n phosphor, The counter can be used i n two different ways; as a volume counter and as a multiple-sample counter. In the f i r s t type of counter the solid radioactive sample is mixed with the liquid phosphor to form a trans-parent paste; i n the second type a smaller amount of solid sample i s spread out on a large surface of cellophane and immersed i n the s c i n t i l l a t i o n f l u i d . Experiments have been made with K 4 0 , C'14, and to determine the range of application, sensitivity, and efficiency of the counters. The volume counter can be used to assay a wide variety of material and is especially suited for biological problems. Its sensitivity was found to be less than 10 _H curies/gm., while that of the multiple-sample counter is of the order of 10-'10 curies/gm.. These sensitivities can be measured with a good degree of reproducibility and are.superior to the conventional methods, with the exception of the elaborate screen-wall counter method; GRADUATE STUDIES' Field of Study: Physics Biophysics — 0 . Bluh Electromagnetic Theory and Special . Relativity — W. Opechowski Electron Optics — K.R. More Chemical Physics — A.J. Dekker Geophysics — A.R. Clark Spectroscopy and Theory of Measurements — A.M. Crooker Quantum Mechanics — G.M. Volkoff Electronics — A. Van der Z l e l Nuclear Physics — K.C. Mann Other Studies: Recent Advances i n Physiology — J. Allardyce Physical Organic Chemistry — H.M. Daggett Electrochemistry — L.W. Shemilt Colloid Chemistry — M. Kirsch ACKNOWLEDGMENTS I am greatly indebted to Professor Blfjfh for his valuable a i d and permanent interest i n the progress of this work. Through his many enlightening discussions I have gained a f u l l e r appreciation of the problems of bio-physics. I should l i k e to express my thanks to Mr. G. W. Williams and Dr. E. K. Darby for their assistance i n the design and construction of the apparatus. I also wish to acknowledge the assistance of the members of the shop s t a f f of the Physics Department. This work was made possible by research grants to Dr. Bltfh and Scholarships to the author (1950-51, 1951-52) from the National Research Council of Canada. A Scholar-ship from the B r i t i s h Columbia Telephone Company for the teriri 1952-1953 allowed me to complete this work. A l l these grants are g r a t e f u l l y acknowledged. TABLE OF CONTENTS. Page ACKNOWLEDGMENTS. ABSTRACT. I. INTRODUCTION. • 1 1. Gas Counting . .... 1 2. S o l i d Sample Counting 2 3. S c i n t i l l a t i o n Counting . . 4 I I . LIQUID PHOSPHORS. 7 1. Theory . .. 8 2. Solutions.. 11 3. Response to Radiation 12 I I I . APPARATUS. . 1 4 IV. BACKGROUND COUNT. 22 V. VOLUME COUNTER. 26 1. Experiments and Results . 27 2. Counting E f f i c i e n c y Losses.. 41 3 . Sensitivity.- 43 4. Reproducibility 44 VI. MULTIPLE-SAMPLE COUNTER. 46 1. Holders 47 2. Experiments and results 49 3. Counting E f f i c i e n c y Losses 52 4. S e n s i t i v i t y 53 5. Reproducibility 53 SUMMARY.' 54 REFERENCES. LIST OF ILLUSTRATIONS. Facing Page Fig. 1. Variation of Maximum S c i n t i l l a t i o n Inten-s i t y with Concentration of Terphenyl i n Toluene.- 12 Fig . 2. Block Diagram ' --^  F i g . 3. Container for Radioactive Sample 15 Fig . 4. Schematic Showing Arrangement of Counter i n L i g h t - t i g h t Box 16 F i g . 5. Photograph of Counter Assembled i n Light-tight Box with Shielding Removed .16 F i g . 6. C i r c u i t s of Preamplifier for Photo-multi-p l i e r and Linear Amplifier IS F i g . 7 . C i r c u i t s of Coincidence Mixer and Low Voltage Power Supply 17 F i g . 8. C i r c u i t of Anticoincidence Mixer 18 Fig . 9. C i r c u i t of Regulated Photo-multiplier Supply.. 19 F i g . 10. Holder for Supporting the Cellophane Sheets Perpendicular to the Photocathodes of the-M u l t i p l i e r Tubes 47 F i g . 11. Holder for Supporting the Cellophane Sheets P a r a l l e l to the Photocathodes of the M u l t i -p l i e r Tubes 48 I LIST OF TABLES Page 40 Table 1. Assay of Naturally Occurring K i n Samples of Potassium Salts 28 Table 2. Assay of Naturally Occurring K-^ i n Mixtures of Potassium Salts with Other Substances . • 30 Table 3. Assay of Radioactive Carbon C 1^ i n Urea Samples 32 Table 4. Assay of Radioactive Carbon 14 i n Various Substances 34 Table 5. Assay of Sulphur 35 i n Various Samples 36 Table 6. Multinle-Sample Counter Assay of - • K40, Cl4, and S35 i n Various Samples 50 I* INTRODUCTION. The determination of very low s p e c i f i c beta-ray a c t i v i t i e s i s a d i f f i c u l t problem at best, and p a r t i c u l a r l y so where low energy p a r t i c l e s are concerned. There has been an increased use of radioactive isotopes i n b i o l o g i c a l tracer studies, and the assay problems that occur when b i o l o g i c a l l y important isotopes such as carbon 14, sulphur 35, and t r i t - i ium are used cannot always be adequately dealt with by con-ventional counting methods, esp e c i a l l y when only samples of low s p e c i f i c a c t i v i t y (that i s , r a d i o a c t i v i t y per gram of sample) are avai l a b l e . Measurements of the r a d i o a c t i v i t y of geological and archeological samples, to determine the age of the specimens, involve low s p e c i f i c a c t i v i t y determinations The investigation of the d i s t r i b u t i o n of natural radioactive isotopes i n plants and animals, as described by Healy (1952), again requires low s p e c i f i c a c t i v i t y measurements with t h e i r concurrent d i f f i c u l t i e s . There are three conventional assay methods i n use and they are b r i e f l y the following: 1_. GAS COUNTING The radioactive isotope i n the sample being assayed i s converted to a gas and the r a d i o a c t i v i t y of the gas deter-mined with an io n i z a t i o n chamber, as described by Calvin et a l (1949), or with a Geiger tube as has been done by Brown and M i l l e r (1947), and, Bernstein and Ballentine (1950), to 2 mention a few. The radioactive isotopes that can be assayed by th i s method are lim i t e d to those that form gases, and i n the case of the Geiger tube the gas must be suitable for the operation of the Geiger tube. Radioactive carbon 14 has been assayed i n the form of carbon dioxide and methane, and t r i t i u m , i n the form of methane, has also been measured. This method has a high geometric e f f i c i e n c y and no losses due to self-absorp-t i o n of the radioactive p a r t i c l e s , but only small amounts of counting substance can be used and the s p e c i f i c a c t i v i t y of the sample must be high. Since the absolute counting e f f i -ciency of t h i s method i s nearly 100%, the absolute counting rate from a gaseous sample can be determined with a suitable Geiger tube, and the method i s used i n the preparation of standard samples. However, the d i f f i c u l t y of maintaining re-producible operating conditions of the Geiger tube and the ever-present danger of contamination make thi s method undes-ir a b l e for routine low s p e c i f i c a c t i v i t y measurements. Kummer (1948), and Rosen and Davis (1953), have used a thin window Geiger tube with the radioactive gas outside the tube. A lower counting e f f i c i e n c y i s obtained with t h i s method but absolute counting rates can be determined. 2. SOLID SAMPLE COUNTING This i s the most common method that i s used. The r a d i o a c t i v i t y of a s o l i d sample i s measured with a th i n window Geiger tube, windowless flow counter, or a demount-able Geiger tube. The thin window Geiger tube, such as described by Kamen (1948), has a low geometric e f f i c i e n c y due to the small s o l i d angle that the window of the tube can accept. The absolute counting e f f i c i e n c y from a s o l i d source i s reduced by the loss of beta p a r t i c l e s due to self-absorp-ti o n of the p a r t i c l e s by the source material, and absorption of the beta p a r t i c l e s i n the window of the tube, and i n the a i r space between the tube and source. A windowless flow counter of the type described by Tait and Haggis (1949), has almost 2 TT geometry and the main losses r e s u l t from s e l f - a b -sorption by the sample. It i s e s s e n t i a l l y a bell-type Geiger tube with the source placed on the inside of the window', how-ever, the tube operates at atmospheric pressure and i s sup-p l i e d continuously with f i l l i n g gas. The demountable Geiger tube, such as described by Labaw (1948), and T a i t and Haggis (1949), allows the s o l i d radioactive sample to be placed d i r e c t l y within the sensitive volume of the Geiger tube there-by insuring a high geometric e f f i c i e n c y . Self-absorption i n the sample i s again a l i m i t i n g factor, e s p e c i a l l y when low energy beta-particles are being measured. A l l three coun-ters u t i l i z e only the surface layers of the sample as the beta-particles from the lower layers are absorbed by the sample i t s e l f , therefore the s e n s i t i v i t y of these counters depends on the surface area of the sample that can be succes-s f u l l y employed. The most e f f i c i e n t counter of this type i s the screen wall counter, developed by Libby (1934), which allows a sample area of 400 square centimeters to be used. In contrast to th i s the sample areas that are accepted by conventional coun-ters are about 10 square centimeters so that only a small amount of sample i s actually used, and the s p e c i f i c a c t i v i t y of the sample must again be high. 3_. SCINTILLATION COUNTING a) . S o l i d Phosphor. Although s o l i d phosphors, used i n conjunction with a photomultiplier tube, have been applied i n the detection of electrons i n f i e l d s such as beta-ray spec-troscopy, they have not proven to be suitable for beta-ray assay work. Anger (1951), has used a s o l i d phosphor for assay-ing Iron 59 and cobalt 60 samples by counting the emitted gamma-: rad i a t i o n . The sample i s placed into a well i n the phosphor to provide good geometric e f f i c i e n c y and res u l t s i n a high gamma-radiation counting e f f i c i e n c y . Damon and Hyde (1952), have used z. s c i n t i l l a t i o n coun-ter to measure the r a d i o a c t i v i t y of gases. A container, coated with a phosphor, i s sealed to a photomultiplier tube and f i l l e d with the radioactive gas. They have used t h i s method to determine the radium and thorium contents of rocks. Only a small amount of sample can be u t i l i z e d . b) . Liquid Phosphor. The general construction and operation of counters using l i q u i d phosphors has been des-cribed by Falk and Poss (1952). In order to get an increased e f f i c i e n c y with low energy beta-particles Raben and Bloemberger 5 (1951), and Farmer and Berstein (1952), have dissolved the radioactive sample d i r e c t l y i n the l i q u i d phosphor. This pro-vides a high counting e f f i c i e n c y but only a small number of sample J.substances can be dissolved i n the l i q u i d phosphor without destroying i t s counting properties, and the small amount of solute that can be used l i m i t s the method to samples with a high s p e c i f i c a c t i v i t y . In the three methods just described the common factor l i m i t i n g the s e n s i t i v i t y i s the small amount of sample mater-i a l that can be e f f i c i e n t l y used. This means that the speci-f i c a c t i v i t y of the radioactive sample must be high. In order to assay the r a d i o a c t i v i t y of a sample of low s p e c i f i c a c t i -v i t y by these methods the isotope must be concentrated, a d i f f i c u l t problem i n most cases and very often impractical. If isotope concentration i s to be avoided only two other a l t e r -natives remain. 1) The counter must be made to u t i l i z e a larger amount of sample material; that i s , self-absorption losses due to sample thickness must be:'minimised, allowing the beta-particles from a larger volume of sample to be detected by the counter. 2) The s e n s i t i v i t y of the counter must be increased by increasing the geometric e f f i c i e n c y and reducing absorp-ti o n losses. A l i q u i d s c i n t i l l a t i o n counter which f u l f i l l s both 6. these alternatives and has a wide range of application has been developed, and a preliminary report published, (Bltfb. and Terentiuk 1952). The method u t i l i z e s s o l i d sample counting, the sample being mixed with a l i q u i d phosphor and put between two photomultiplier tubes (see the block diagram i n F i g . 2). The s c i n t i l l a t i o n s produced i n the phosphor by the radioactive p a r t i c l e s , reach the m u l t i p l i e r tubes and the p a r t i c l e s recor-ded by a coincidence counting system. The counter can take two d i f f e r e n t forms depending on the amount of radioactive sample material available. It can operate as a -wolume counter when a large amount of sample i s available or as a multiple-sample counter when only a small amount of sample material i s a v a i l -able. Both forms of the counter are discussed i n t h i s t h esis, and a series of experiments to determine the p r a c t i c a l l i m i t s of s e n s i t i v i t y and e f f i c i e n c y are described. I_I. LIQUID PHOSPHORS. The advent of the photomultiplier tube, e s p e c i a l l y the head-on type RCA-5319, and the development of s c i n t i l l a -t i o n materials, (substances that produce l i g h t flashes on absorption of energy from radioactive p a r t i c l e s ) has resulted in an increased use of the s c i n t i l l a t i o n counter v/hich i n many cases has replaced the Geiger counter. S c i n t i l l a t i o n counters are superior to Geiger counters i n e f f i c i e n c y and speed of response. The e f f i c i e n c y of the s c i n t i l l a t i o n coun-ter depends to a large extent on the development of large clear phosphors, while the speed of l-esponse i s a property of the substance forming the phosphor. Early s c i n t i l l a t i o n counters employed s o l i d phosphors, preferably large, single, chemically pure, organic and inor-ganic c r y s t a l s such as anthracene, and thallium activated sodium iodide. Crystals conforming to these s p e c i f i c a t i o n s are d i f f i c u l t to prepare and i n some cases the large c r y s t a l s required cannot be made. In an e f f o r t to overcome the d i f f i -c u l t i e s of preparing and handling large c r y s t a l s a search was made for suitable transparent l i q u i d phosphors. It was found that small amounts of organic compounds, that are by themselves good s c i n t i l l a t o r s , when dissolved i n solvents, which by them-selves s c i n t i l l a t e d only very weakly, gave good r e s u l t s . The duration of the l i g h t f l a s h and i t s spectrum i s c h a r a c t e r i s t i c of the dissolved organic material. In general,the i n t e n s i t y of the fluorescent l i g h t from a l i q u i d phosphor i s less than that from a c r y s t a l phosphor. 8 1. THEORY Pure l i q u i d s when excited by radioactive p a r t i c l e s emit fluorescent l i g h t of very small i n t e n s i t y . However, i f suitable solutions are excited they fluoresce strongly, the l i g h t having the spectral c h a r a c t e r i s t i c s of the solute mole-cules. Experiments show'that the d i r e c t e x c i t a t i o n of the same amount of solute as i s present i n the solution produces much smaller l i g h t emission than i s observed with the s o l u t i o n and therefore cannot be responsible for the large amount of l i g h t emitted. This fact has been explained on the basis of energy transfer from the excited solvent molecules to the so-lute molecules. The energy transfer to the solute molecules i s described as a competition between two processes. 1) the migration of the e x c i t a t i o n energy through the solvent and i t s trapping by the solute molecule, and 2) the process of quenching the e x c i t a t i o n energy i n the solvent. If n g = number of excited solvent molecules produced/unit time P n = p r o b a b i l i t y of non-radiative quenching of the excited solvent molecules by unexcited molecules P^ = p r o b a b i l i t y of e x c i t a t i o n energy transfer from the solvent to the solute, molecule. n e - number of excited solute molecules produced /unit time then ii = p t n_ e = -5— s n = s 1 + P / D However, since the energy of an excited solute molecule can be quenched lay i n t e r a c t i o n with unexcited solute molecules ) ; ( i . e . by self-quenching),not a l l excited solute molecules w i l l contribute to the emitted radiation i n t e n s i t y . If P Q = p r o b a b i l i t y of l i g h t radiation by the solute molecule P s = p r o b a b i l i t y of self-quenching of the solute molecule Pi = p r o b a b i l i t y of quenching of the excited solute mole-cule by the solvent then the emitted l i g h t i n t e n s i t y i s .given by P 1 = n e • e P e + ps + p i _ n^ 1 4- (Ps + Pj.) n s Since only P^ and P^ are proportional to the concen-t r a t i o n C of the solute molecules, l e t P q = k C P i = k» C P~ e and 1 + P s = k" P e 10 Substituting i n the above equation you get *8(c + k)(k* C + k" ) This expression describes the f i n a l l i g h t emission as a function of the solute concentration, taking into con-sideration the competition of the separate reactions which are necessary for the understanding of experimental r e s u l t s . From the equation i t i s seen that the increase i n l i g h t i n -tensity i s proportional to the concentration for small concen-t r a t i o n s . Here the transport of e x c i t a t i o n energy to the fluorescent solute molecules i s most e f f e c t i v e and l i t t l e s e l f -quenching between these molecules occurs. For higher concen-trations the Intensity approaches saturation as a r e s u l t of the second term of the equation as long as k C, At s t i l l higher concentrations the i n t e n s i t y reaches a maximum and then gradually decreases as the denominator of the second factor becomes more dominant. Over t h i s middle region of the curve the self-quenching processes compete with the increasing trans-port of energy. On the decreasing portion of the curve the transport of energy to the fluorescent molecules, which i s proportional to C , no longer increases proportionately Tim with the concentration but does so more slowly, since only a l i m i t e d amount of energy^ can be drawn from the solvent. Both self-quenching of the fluorescent molecules and saturation i n the transport of energy to these molecules are now e f f e c t i v e . This behavior of l i g h t i n t e n s i t y as a function of solute con-centration i s v e r i f i e d by experimental r e s u l t s . (See f i g . 1). 11 2_. SOLUTIONS The present theory does not indicate what compounds w i l l give a good s c i n t i l l a t i o n solution but the re s u l t s of investigation of a large number of solutions has been publish-ed. (Kallmann and Furst 1950 A, 1950 B, and 1951). An attempt was made to f i n d a good l i q u i d phosphor with a solvent, containing a high percentage of carbon, that could e a s i l y be made from carbon or carbondioxide. Such a phosphor would present a very e f f i c i e n t method for assaying the low l e v e l natural carbon 14 content of organic specimens, since the l i q u i d phosphor would contain i t s own source. In-corporation of the carbon into the solvent rather than the solute, (as described i n section I, 3b), i s more desirable since i t would allow a larger amount of carbon to be used. Solvents such as carbon disulphide, carbon tetr a c h l o r i d e , methyl alcohol, ethyl alcohol, and acetone a l l prove to be very poor solvents for l i q u i d phosphors. Also, small amounts of these substances when added to a suitable l i q u i d phosphor destroyed i t s s c i n t i l l a t i o n properties. It appears that a more complex solvent such as toluene or xylene would have to be used, and the d i f f i c u l t y of making such solvents would pre-vent t h i s method of assaying from being•of much general \alue. However, i n the problem of age-dating carbon samples by measur-ing t h e i r carbon 14 content, assay by t h i s e f f i c i e n t method could extend the range of the present screen-wall Geiger tube 120 P o CD T3 P CfQ CD to 100 o > I-x UJ X UJ CO _l ID CL 15 X < 80 60 40 2 0 h 0 3 0 C-C. S A M P L E S OF PHOSPHOR A C T I V A T E D WITH CO 6 0 F i g , 1 Variation of Maximum S c i n t i l l a t i o n Intensity With Concentration of Terphenyl i n Toluene _L 0 1 2 3 4 5 6 7 CONCENTRATION OF T E R P H E N Y L IN T O L U E N E - GM / LITRE 12 method s u f f i c i e n t l y to warrant the added technical d i f f i c u l t i e s of the chemical preparation. The l i q u i d phosphor used i s a solution of para-ter-phenyl i n toluene. Toluene was chosen as a solvent because i t i s easy to obtain i n a pure form and i s quite economical. Solvents such as xylene and phenolcyclohexane are reported to give a s l i g h t l y higher i n t e n s i t y l i g h t output but are consider-ably more expensive. Commercial xylene consists of a mixture of the ortho, meta, and para forms of xylene, which by them-selves form s c i n t i l l a t i o n solutions of varying e f f i c i e n c y , and the concentration of each form may d i f f e r considerably from sample to sample. This problem does not ari s e when to-luene i s used, but the presence of very small amounts of impuri-t i e s , i n both the para-terphenyl and the toluene, requires that the optimum concentration be experimentally determined when a new solution i s made. F i g . 1 shows graphically the v a r i a t i o n of the output l i g h t i ntensity with concentration of para-terphenyl i n toluene. In general the optimum concentra-ti o n has been found to be 4 grams of para-terphenyl per l i t r e of toluene. Such a solution has i t s maximum l i g h t i n t e n s i t y output at 3470A0 as measured by Ravilious (1952). This i s lower than the maximum spectral s e n s i t i v i t y of the RCA type 5819 photomultiplier tube which i s at 4800 A°. 3. RESPONSE TO RADIATION A charged p a r t i c l e w i l l be counted as long as i t has s u f f i c i e n t energy to give r i s e to a minimum number of photons. This minimum number depends on the l i g h t c o l l e c t i o n e f f i c i e n c y and the noise l e v e l of the detecting system. Experimentally i t i s found that l i q u i d s c i n t i l l a t o r s are s i m i l a r to organic c r y s t a l s i n that for a given energy loss i n the phosphor a l i g h t l y i o n i z i n g p a r t i c l e , such as an electron, w i l l give r i s e to a larger pulse than w i l l a more densely i o n i z i n g p a r t i c l e , such as a proton or alpha p a r t i c l e . A t h e o r e t i c a l explanation of t h i s e f f e c t i n organic c r y s t a l s i s given by Bowen et a l . (1949). Falk, Poss and Yuan (1951), have shown that for a given type of e x c i t i n g p a r t i c l e the response of a l i q u i d s c i n t i l l a -tor i s roughly proportional to the energy, e s p e c i a l l y at lower energies. Most radioactive radiations can be measured, but, as i s the case when Geiger tubes are used, some radiations are detected by i n d i r e c t means. Gamma-radiation i s counted by the electrons r e s u l t i n g from the photoelectric, Compton, and pair production processes, while neutrons are detected by the r e -c o i l protons they produce. However, the e f f i c i e n c y of detec-ti o n of both these radiations with a l i q u i d s c i n t i l l a t i o n coun-ter i s greater than that obtained with Geiger counters, a l -though inorganic c r y s t a l s having a high atomic number are pre-ferred i n gamma-radiation measurements. Anticoinc Discriminator Photomultiplier I Photomultiplier Linear Amplifier Discriminator H O H5 P o CD T3 P B L O C K DIAGRAM F i g . 2 . Anticoinc Mixer Scaler 14 I I I . A P P A R A T U S . The apparatus used i s shown in the block diagram, f i g . 2, and consists of the following. The radioactive sample together with the l i q u i d phosphor i s placed i n a container be-tween two RCA-5819 photomultiplier tubes. The photomultiplier tubes are attached to preamplifiers and are surrounded by iron and lead sh i e l d i n g . This part of the apparatus i s enclosed in a l i g h t - t i g h t box which i s cooled by a stream of a i r , and there i s additional iron and lead shielding above and below the box. The voltage pulses from the preamplifiers are put into l i n e a r amplifiers, then discrimatOrs, and f i n a l l y into a coincidence mixer. Coincidence counting must be used to re-duce the high background count that arises from the noise pulses of the photomultiplier tubes which are due to thermal electrons emitted by the photo-cathode. These noise pulses have a maximum value of 0.2 v o l t s when the photomultiplier tubes are operated at an o v e r a l l potential of 1000 v o l t s , and cannot be separated from the pulses produced by low energy beta p a r t i c l e s by electronic discrimination. The .coincident pulses are put into an anticoincidence mixer and the r e s u l t -ing pulses recorded with a s c a l e r . Only large background pulses are allowed to pass the anticoincidence discriminator and reach the anticoincidence mixer where the coincidences due to these large background pulses are supressed and do not reach the s c a l e r . R C A 5819 Photomultiplier RCA 5819 Photomultiplier — Collar V -Well For Phosphor — Container Sample Sauereisen Waterglass Seal i g . 3. Container For lladioactive Sample. To face page 15 1. CELL FOR RADIOACTIVE SAMPLE The container for the radioactive sample, f i g . 3, consists of a c l o s e l y f i t t i n g glass c o l l a r cemented to a RCA -5819 head-on type, photomultiplier tube. The RCA-5819 photo-m u l t i p l i e r tube has a 1.5 inch diameter photocathode on the inner glass surface at the end of the bulb, and t h i s large, s l i g h t l y convex photosensitive surface forms the bottom of the container. The container was made i n t h i s way i n order to keep the number of glass surfaces between the l i q u i d phos-phor and the photosensitive surface at a minimum, and thereby reduce l i g h t losses due to absorption and r e f l e c t i o n . The cement that can be used to attach the container to the tube : . must not be soluble i n the l i q u i d phosphor, and as the photo-m u l t i p l i e r tube cannot be heated to more than a maximum of 75°C, only cold s e t t i n g cements can be used. Sauereisen cement meets these requirements but i t i s porous and w i l l not hold organic solvents, and i n contact with moisture i t w i l l slowly soften. However, by covering the Sauereisen cement with a few layers of water glass (sodium s i l i c a t e ) these d i f f i c u l -t i e s are eliminated and a s a t i s f a c t o r y seal i s achieved. A second RCA-5819 photomultiplier tube, f i g . 3, dips into the container from above. A glass cylinder, sealed to th i s upper photomultiplier tube, f i t s into a well attached to the container and the well f i l l e d with the l i q u i d phosphor. This seals o f f the container and prevents evaporation of s o l -vent from the l i q u i d phosphor i n the sample, esp e c i a l l y when To Amplifier Light-tight Box Preamplifier Photomultiplier Tubes Container Preamplifier Air Lead Shielding Iron Shielding ••ig. 4. Schematic Showing 'Arrangement of Counter In Light-tight Box. To face page To face page 16 16 long counting times are necessary. Both glass cylinders are si l v e r e d to reduce l i g h t losses. Since both photocathodes are i n di r e c t contact with the l i q u i d phosphor good o p t i c a l coupling.is assured with a reduction i n l i g h t losses. The photomultiplier tubes may be separated by any distance up to 2.5 centimeters. 2. PREAMPLIFIERS The preamplifiers and photomultiplier tube housings are constructed as single units and are mounted i n a v e r t i c a l p o s ition ( f i g . 4 and 5). The c i r c u i t of the preamplifier i s shown i n f i g . 6. It has a single stage of amplification with a gain of 3, and a low impedance cathode follower output stage to prevent lengthening of the rise-time of the pulses from the r photomultipliei" tubes. 3. LINEAR AMPLIFIERS The pulses from the preamplifier are introduced to the l i n e a r amplifier, ( c i r c u i t shown i n f i g . 6), through an attenuator which can reduce the input signal by factors of 2, 4. 3, or 16. One, type 1N34, c r y s t a l diode i s used to l i m i t the maximum input signal and prevent overloading of the ampli-f i e r , while a second c r y s t a l diode eliminates any undershoot i n the input s i g n a l . The li m i t e d pulses are put into a "ring of three" type amplifier with a cathode follower output stage. The amplifier has a gain of 100 with a maximum pulse output of 100 v o l t s and a pulse rise-time of less than 0.2 microseconds. j6AL5 6AC7 6AL5 6AC7 DISCRIMINATOR-^-Input To M.V. plate Resolving time - 0.17/C/s. Dead t ime - 3 ps. Input range- +1-^ +100 v. C O I N C I D E N C E . M I X E R A N D P O W E R S U P P L Y 17 The rise-time was measured with a Tektronix oscilloscope, tjrpe 51 IA. 4. DISCRIMINATORS AND PULSE SHAPERS The amplitude discriminator used i s a modification of the Schmitt trigger c i r c u i t and the c i r c u i t i s shown i n f i g . 7. It has an e a s i l y adjustable, stable, dicrimination voltage and i s capable of, accepting the narrow pulses from the amplifier. Very large signals are not distorted and do not af f e c t the operating c h a r a c t e r i s t i c s of the discriminator. The output pulses are shaped by a tmivibrator c i r c u i t and are negative pulses of 120 v o l t s . 5. COINCIDENCE MIXER The coincidence mixer c i r c u i t i s also shown i n f i g . 7, and consists e s s e n t i a l l y of a Rossi p a i r . The resolving time of the coincidence mixer i s 0.19 microseconds while the dead time i s 3 microseconds. The resolving time was measured with a pulse generator designed to put out double pulses whose time separation can be varied. These pulses are put into the coincidence mixer and t h e i r time separation lengthened u n t i l no coincident output pulses are obtained from the mixer. This time separation was measured with a Tektronix oscilloscope and i s the resolving time of the coincidence mixer. The resolving time was also determined by recording the number of accidental coincidence counts obtained with a random counting rate i n each separate discriminator channel. Anticoincidence 1 Discriminator O-l/iF1 Coincidence Discriminator 20 K |0 01 fiF - !5o ^>-AAA/ V o ! * s 5 0 0 K 18 If C i s the accidental coincident counting rate, and N-^  and N 2 the counting rates of each separate discriminator channel, then the resolving time T i s given by the equation T = C These measurements gave an average resolving time of 0.185 microseconds which checks well with the value obtained from the oscilloscope measurements. 6. ANTICOINCIDENCE MIXER The anticoincidence c i r c u i t ( f i g . 8) consists of an anticoincidence discriminator, whose c i r c u i t i s the same as that shown i n f i g . 7, coupled with a RCA SAS6 pentode. A large negative voltage On the screen gr i d of th i s tube w i l l block any signal output that would aris e from a posit i v e pulse on the control g r i d . The large negative pulses are obtained from the anticoincidence discriminator and are used to sup-press the coincident pulses from the coincidence mixer which are due to background ra d i a t i o n . The action of the a n t i -coincidence mixer w i l l be explained i n more d e t a i l i n section IV. 7. HIGH VOLTAGE POWER SUPPLY Since the gain of a photomultiplier tube varies con-siderably with the voltage on the tube a regulated power supply must be used. The power supply used, ( c i r c u i t shown i n f i g . 9), has a regulated voltage range of 900 - 1300 v o l t s with a maxi-mum output current of 10 milliamperes, and a regulation factor I i Hammond 215-60 «3 2 X 2 6.3v. Z k v . i r » 5 . •-3 O Hi P o CD T5 P cm CD Hammond 167-E Y 65JT-GT -1500 v. I35v. ^ •I'llh ^ ||.0^f 3000 V. 1 1.0/, f 3000^ 100 K 2W. 6V6 R E G U L A T E D PHOTO-MULT IPL IER S U P P L Y of 1 v/hen used in combination with a constant voltage 10,000 transformer. The power supply has a grounded posi t i v e t e r -minal which allows the c o l l e c t i n g anode of the photomultiplier tube to be operated at ground p o t e n t i a l . With negatively grounded power supplies the c o l l e c t i n g anode of the photomul-t i p l i e r tube operates at a high pos i t i v e p o t e n t i a l , and a good qual i t y coupling condenser i s then required to pass the signal without introducing any spurious signals. As the pulses are generally small a low loss condenser i s necessary i n order that a l l of the signal voltage w i l l appear across the load r e s i s -tance. This can be more e a s i l y achieved when operating at ground p o t e n t i a l . 8. LOW VOLTAGE POWER SUPPLIES Two low voltage power supplies are used. The f i r s t , shov/n i n f i g . 7, has a regulated, 300 v o l t , 200 milliamperes, posi t i v e output, and a 150 v o l t , 100 milliamperes, negative output, s t a b i l i z e d only with gas diodes. The posit i v e supply has a high regulation factor since a very steady voltage i s required for the discriminators. This power supply provides the operating voltages for the preamplifiers, discriminators, coincidence and anticoincidence mixers. The second power supply i s a model 25, 300 v o l t , 100 milliamperes regulated, pos i t i v e supply manufactured by the Lambda Electronics Corporation. It provides the voltage for the l i n e a r amplifiers. 20 A l l power supplies are operated from a Sola, model B-52, constant voltage transformer to reduce short-time voltage flu c t u a t i o n s . 9. SCALERS The coincident pulses are recorded with a scale of 64 Atomic scaler, model 101-A, and a mechanical r e g i s t e r . The pulses from the separate photomultiplier tubes are recorded by a Berkley Decimal Scaler, model 100. 10. PHOTOMULTIPLIER TUBES The photomultiplier tubes used are RCA type 5819. The advantage i n th i s tube l i e s i n i t s large, r e l a t i v e l y f l a t , end-on, photocathode. The large photosensitive area allows a bigger phosphor to be used, while the f l a t surface permits easier coupling between the phosphor and the photocathode. The spectral response of the type 5819 tube covers a range from about 3000 A° to about 6400 A°, with a sharp s e n s i t i v i t y maximum at 4800 A°. At 3470 A°, which i s the wavelength of maximum fluorescence from a terphenyl-toluene phosphor, the s e n s i t i v i t y i s 50% of maximum. The voltages on the dynodes of the photomultiplier tube are taken from taps on a voltage divider connected across the high voltage power supply. One megohm dynode r e s i s t o r s , as shown i n f i g . 6, are used, and the l a s t 3 dynodes are by-passed v/ith 0.01 microfarad condensers i n order to maintain t h e i r potentials when large pulses are produced. The tubes are operated at an o v e r - a l l potential of 1000 v o l t s , that i s 21 about 90 v o l t s per stage, and are matched with regard to noise pulses, due to thermal electrons from the photocathode, and the signal to noise r a t i o . 22 IV. BACKGROUND COUNT. Accurate measurements of the r a d i o a c t i v i t y of a sample cannot be made unless the count obtained from the sample i s at least equal to that of the background count. The smaller the background count the lower the s p e c i f i c a c t i v i t y that can be detected. The background count obtained with the s c i n t i l l a t i o n counter has 4 main sources. 1) There i s a coincident count idue to the f i n i t e resolving time of the coincidence mixer. Since the voltage pulses r e s u l t i n g from low energy beta-radiation .are small, and of the same order of magnitude as the noise pulses due to thermal electrons emitted from the photocathodes, they cannot be separated from the noise pulses by elec t r o n i c discrimina-t i o n . The discriminators of each separate channel must be ad-justed to pass these small pulses, with the r e s u l t that a random counting rate, due to thermal noise, i s observed at a l l times and i s fed into the coincidence mixer. The accidental coincidence counting rate depends on the individual channel counting rates, and the resolving time of the coincidence mixer, and can be calculated from the.formula C = 2NX N 2 T where and N 2 are the individual channel counting rates and T i s the resolving time of the mixer. Only a small contribution to the background count .is 23 from th i s source and i s found to be 0.04 counts per second. If a very active source i s being assayed by the coin-cidence method the counting rate i n each channel w i l l be greatly increased above the background counting rate. Thus the obser-ved coincidence count must be corrected for any additional accidental coincidence counts that would be expected from the increased channel counting rates. 2) Local gamma-radiation from the laboratory walls and surroundings, and the soft component of the cosmic r a d i a -tion introduces a large background count because of the r e l a -t i v e l y high e f f i c i e n c y of the s c i n t i l l a t i o n counter for gamma-radi a t i o n . The count from th i s source i s reduced by surround-ing the sample c e l l with a lead c a s t l e three inches thick, with an additional three inch layer of lead above and below' the l i g h t - t i g h t box which houses the sample c e l l (see f i g . 4 and 5). This lead i s s u f f i c i e n t to remove the e f f e c t s of the gamma-radiation, but the lead i t s e l f contributes to the back-ground count because i t contains natural gamma-emitting impuri-t i e s . Anderson, Arnold, and Libby (1951), have estimated t h i s contribution to be about 30 counts per minute when the screen wall counter i s used, but the higher e f f i c i e n c y of the s c i n t i l l a -t i o n counter for gamma-radiation re s u l t s i n a much higher count from th i s source. As the best compromise among price, radio-chemical purity, and high atomic number they have used a large amount of iron s h i e l d i n g . A further reduction i n background 24 count from the l o c a l gamma-radiation was achieved by Kulp and Tyron (1952), by using one inch of radiochemically pure mer-cury i n addition to the iron s h i e l d i n g . An additional s h i e l d of 1/2 inch iron reduced the background count from the gamma-impurities of the lead by a factor of 4. Another 1/4 inch of iron shielding produced no further s i g n i f i c a n t reduction i n the background count, i n d i -cating that the remaining gamma-radiation has quite a h/igh energy, and could be removed only with considerably more iron s h i e l d i n g . 3) The hard component of the cosmic ra d i a t i o n , that i s the meson component, cannot be eliminated by s h i e l d i n g . An anti-coincidence device i s used to reduce the background counts due to thi s source. The voltage pulses a r i s i n g from one of the photomultiplier tubes are directed also to a se-cond amplitude discriminator, which i s marked anti-coincidence discriminator i n f i g . 1. This discriminator i s adjusted to pass those pulses greater than some maximum value, and allow them to reach the anti-coincidence mixer. I f , for example, a sample containing carbon 14 i s to be assayed the size of the voltage pulses,- that r e s u l t from the beta p a r t i c l e s of the carbon 14, w i l l vary continuously up to some maximum value which depends on the maximum energy of the emitted beta par-t i c l e s . Any pulses larger than th i s maximum value w i l l be due to the meson and radio-impurity sources. If the a n t i - c o i n -cidence discriminator i s set to pass a l l pulses greater than 25 the maximum size expected from the carbon 14, then the pulses reaching the anti-coincidence mixer w i l l be those from the background sources. Coincident counts due to these sources are quenched in the anti-coincidence mixer and do not reach the s c a l e r . With radioactive samples emitting high energy par-t i c l e s the effectiveness of t h i s anti-coincidence device i s reduced, since the anti-coincidence discriminator must be set high enough to prevent pulses from these high energy par-t i c l e s from reaching the anti-coincidence mixer. V. VOLUME COUNTER. The name volume counter i s given to t h e ^ l i q u i d s c i n -t i l l a t i o n counter Iwhen i t i s used to assay large amounts of radioactive sample material. The s o l i d sample whose radio-a c t i v i t y i s to be measured, mixed together with the l i q u i d phosphor to form a paste, i s placed into the beaker formed by the glass c o l l a r cemented to the lower photomultiplier tube. A l l a i r pockets are c a r e f u l l y removed from the mixture. Up to 30 grams of sample material may be ".used i n the present arrangement with a maximum separation of the photomultiplier tubes of about 2 cm.. This represents a volume of 45 cc. i n the c e l l and requires about 30 cc. of l i q u i d phosphor.' The s o l i d sample, to be assayed by thi s method, must f u l f i l l two requirements. 1) The sample material used must not be soluble i n the l i q u i d phosphor, since t h i s would in the majority of cases destroy the counting property of the phosphor. 2) The mixture of sample and phosphor must be of s u f f i c i e n t transparency to allow the l i g h t produced by a beta p a r t i c l e i n the l i q u i d phosphor to reach both photomultiplier tubes. Both these conditions are f u l f i l l e d by most inorganic s a l t s , amino acids, proteins, and divers b i o l o g i c a l material and they would therefore be suitable for assay by t h i s method. However, fats i n general could not be assayed i n thi s way. 27 1_. EXPERIMENTS AND RESULTS In order to determine the useful range of application and the s e n s i t i v i t y of the method, a series of experiments were ca r r i e d out using the radioactive isotopes potassium 40, carbon 14, and sulfu r 35. a). Potassium 40. Potassium 40 i s a naturally occ-urring radioactive isotope of potassium and i s present i n a l l potassium s a l t s . Its abundance i s 0.012% and i t has a h a l f -S 40 l i f e of 2.4 x 10 years. K emits c h i e f l y beta p a r t i c l e s of 1.35 and 0.4 Mev., and some 1.4 Mev. gamma-radiation. The absolute beta- and gamma-ray disintegration rates of K 4 ° have been determined by Faust (1950). The mean gamma-ray a c t i v i t y found from recent nfeasurements i s 3.4 + 0.2 gamma-rays/sec./gm. of potassium, while the s p e c i f i c beta-ray a c t i -v i t y i s 31.8 +_ 1.0 beta rays/sec./gm. of potassium. This means a t o t a l absolute s p e c i f i c a c t i v i t y of 35.2 +1.2 counts/ sec./gm. of natural potassium. Measurements were made on KC1 and the res u l t s obtained are shown i n Table 1. One- gram of KC1 i s expected to y i e l d an absolute counting rate of 18.4 counts/sec. (taking the atomic weight of K=39.09 and CI.=35.46). The measurements made on the KC1 show that the l i q u i d s c i n t i l l a t i o n counter method has an absolute counting e f f i c i e n c y of 55.2% for the 40 K i n a 28 gm. sample. E f f i c i e n c i e s of th i s order are also obtained for samples of KC1 up to 40 grams i n weight. oo CM TABLE I. ASSAY OF NATURALLY OCCURRING- K 4 ° HT SAMPLES OF POTASSIUM SALTS Run Sample Counts/sec. Run Sample Counts/sec. Absolute Efficiency • Back-ground With Co 6 0 K 4 0 With Co 6 0 Above Bckgd. Absolute 1 2 3 Mean NaCl 15 gm. 1.88 1.90 1.86 1.88 72.3 72.0 71.8 72.1 1 2 3 Mean KC1 14 gm. 141' 142 143 142 206 210 208 208 140.1 257.6 54.4% 1 2 3 Mean NaCl 30 gm. 1.57 1.58 1.55 1.57 66.1 67.0 66.7 66.5 : 1 2 3 Mean KC1 28 gm. 287 285 286 286 349 348 351 349 284.4 515.2 55.2$ 1 2 3 Mean NaCl 45 gm. 1.41 1.39 1.41 1.40 63.7 62.9 63.0 63.3 1 2 Mean KC1 40 gm. 394 397 395 456 460 458 393.6 736.0 53.4% 1 2 Mean Na gS0 4 35 gm. 2.01 1.99 2.00 46.1 45.8 45.9 1 2 Mean KN03 50 gm. 137 138 137.5 177 175 176 135.5 679.0 20.0% 2,9 The background count was measured with a NaCl - phos-phor mixture whose transparency i s comparable to that of the KC1-phosphor mixture. The transparency of the mixture was mea-sured with the help of a small cobalt 60 source. The counting rate above background was measured separately i n NaCl and KC1 60 mixtures, which were i r r a d i a t e d by the Co source, and was found to be approximately the same'in both cases. The count-60 ing rate above background which results when the Co source i s vised gives a measure of the transparency of the mixture. Samples of KC1 d i l u t e d with NaCl gave the same counting rate per gram of KC1 as a sample of' KC1 by i t s e l f , as can be seen from the measurements shown i n Table 2. To check the s e n s i t i v i t y of the method, measurements were made on a small s o l i d sample of KC1 d i s t r i b u t e d throughout a large amount of s o l i d NaCl, (Table 2). The absolute count-ing rate expected from the 250 mgm. of KC1 i n a 30 gm. sample of NaCl i s 4.60 counts per sec. The observed counting rate ,of 2.25 counts per sec. represents an absolute counting e f f i c i e n c y of 48.9%. This counting rate i s almost twice the background counting rate so that good counting s t a t i s t i c s can be obtained with r e l a t i v e l y short counting times. The t o t a l a c t i v i t y of -10 the sample i s s l i g h t l y greater than 10 curies, (1 curie equals 3.7 x l O ^ d i s i n t e g r a t i o n s / s e c ) , and t h i s means that -11 the s p e c i f i c a c t i v i t y of the 30 gm. sample i s less than 10 curies/gm. of NaCl. The number of counts per sec. i s small TABLE II. ASSAY OF NATURALLY OCCURING K^O IN MECTURES OF POTASSIUM SALTS WITH OTHER SUBSTANCES Run Sample Counts/sec. Run Sample Counts/sec. Absolute Back-ground YJitti Co 6 0 With Co 6 0 Above Bckgd. Expected Eff i c i e n -cy'.r •;. - , 1 2 tfaCl 30 gm. 1 2 KC1 15 gm. + NaCl 15 gm. 144 144 204 206 Vlean 1.57 66.5 Mean 144 205 142.4 376.0 51.6$ 1 2 NaCl • 15- gm. 1 2 KC1 10 gm. + NaCl 10 gm. 95.1 94.6 157 154 Mean 1.88 72.1 Mean 94.8 155.5 92.9 184.0 50.6$ 1 2 NaCl 3Q. gm. 1 2 SCI 0.250 gm. 4 ;:' NaCl 30 gm. 3.79 3.84 67.V 67.1 Mean 1.57 66.5 Mean 3.82 67.4 2.25 4.60 48.9$ 1 2 S i l i c a gel 30 gm. 1.41 1.45 33.7 33.0 1 2 . KC1 10 gm. + S i l i c a gel 20 gm. 76.8 74.3 120 122 Mean 1.43 33.4 Mean 75.5 121 74.1 184 40.5$ 1 2 S i l i c a gel 20 gm. 1.53 1.57 39.8 39.3 1 2 KC1 10 gm. 4 S i l i c a gel 10 gm. 96.0 95.8 120 . 122 Mean. 1.55 39.6 Mean 95.9 121 94.3 184 51.2$ 1 2 Mean Na gS0 4 35 gm. 2.00 45.9 1 2 Mean KC1 5 gm. 4 Na2S04 30 gm. 42.2 41.8 42.0 85.3 86.0 85.7 40.0 92.0 43.4$ 31 and since, the counts come from a large volume they represent an extremely low s p e c i f i c a c t i v i t y which could not be measur-ed otherwise. The measurements made on samples of KNO3 (Table I) y i e l d an absolute counting e f f i c i e n c y for the naturally occur ing K^O i n the samples of only 20.0%. The smaller counting-e f f i c i e n c y for thi s s a l t can be accounted for by the lower transparency of the KNO3 -phosphor paste, as i s seen from the 60 Co measurements. The increase i n the counting rate above 60 background, when the Co source i s used, i s only half as great for the KNO3 mixture as i t i s for K C 1 . The lower trans parency i s largely due to the greater opacity of the KNO3 c r y s t a l s . KMO3 c r y s t a l s are also smaller than those of KC1 and as a re s u l t produce closer packing i n the phosphor mix-ture. K 40 was used i n these experiments because i t i s an ea s i l y available isotope whose radioactive emission i s s i m i l a to that of some of the isotopes used i n b i o l o g i c a l tracer work. Its natural d i s t r i b u t i o n assures equal a c t i v i t y from a l l potassium s a l t s . The following isotopes, to mention a few, also emit radiations s i m i l a r to that from K^O, namely P 3 2, emitting 1.71 Mev. beta p a r t i c l e s ; C 1 3 S , emitting 1 . 1 , 2.8, and 5.0 Mev. beta p a r t i c l e s , and 1.65 and 215 Mev. gamma-radiation; Ca emitting 2.3 Mev. beta par-t i c l e s and 0.8"Mev. gamma-radiation. A l l these isotopes are important i n b i o l o g i c a l tracer studies and could be TABLE III. ASSAY OF RADIOACTIVE CARBON C 1 4 IN UREA SAMPLES Run Sample Counts/sec ' Run Sample Counts /.sec. Absolute With Co 6 0. c 1 4 60 With.Co Above Bckgd. itepec-ted Efficiency 1 2 3 4 Urea 30 gm. 0.681 0.709 0.660 0.692 41.8 41.5 42.7 42.1 1 2 3 4 Urea A 30 gm. 159 157 160 158 201 204 200 195 Mean 0.691 41.8 Mean 159 200 158.3 614.4 28.6$ • 1 2 Urea 10 gm. + Urea± 20 gm. 107 105 147 144 Mean Urea 30 gm. 0.691 41.8 Mean 106 146 105.3 409.6 25.7$ 1 2 Urea 20 gm. + Ureafc 10 gm. 53.7 54.9 94.4 95.1 Mean Urea 30 gm. 0.691 41.8 Mean 54.3 94.8 53.6 204.8 26.3$ 1 2 Urea 25 gm. + Ureaft 5 gm. 25.3 24.1 64.9 66.1 ' Mean Urea 30 gm. 0.691 41.8 Mean 24.9 64.5'. 24.2 102.4 23.9$ Urea Jr. - containing C14 assayed by t h i s method with s i m i l a r e f f i c i e n c i e s and se n s i -t i v i t i e s . b). Carbon 14. Carbon 14 i s a pure beta p a r t i c l e emitter with a haLf-life of 5000 years. The beta p a r t i c l e s emitted from C^- have a maximum energy of 0.160 Mev. and a mean energy of 0.050 Mev.. These lower energy beta p a r t i c l e s present a more d i f f i c v i l t assay problem than i s encountered 40 with an isotope such as K. which emits higher energy par-t i c l e s . 14 The C was used i n the form of urea, the sample having a r e l a t i v e l y high s p e c i f i c a c t i v i t y . Part of t h i s source was di l u t e d by d i s t r i b u t i o n ..into a larger amount of non-active urea and measurements were made on samples of low s p e c i f i c a c t i v i t y ( T a b l e 3). The absolute counting e f f i -1 4 ciency of the C beta p a r t i c l e s from a 30 gm. ..urea sample was 28.6%. The considerably lower e f f i c i e n c y , as compared with KC1, i s i n the main due to the self-absorption of the low energy beta p a r t i c l e s by the sample i t s e l f . However, 60 the Co measurements show that there i s a r e l a t i v e l y low transparency of urea which a f f e c t s the counting e f f i c i e n c y . The background count, measured with an non-active urea sample, was only 0.691 counts/sec. The maximum pulses 40 obtained for the K isotope are of the order of 100 vo l t s (after amplification), whereas those from the C ^ are only 20 v o l t s . By se t t i n g the anti-coincidence discriminator to CO TABLE IV. ASSAY OF RADIOACTIVE CARBON 14 IN VARIOUS SUBSTANCES Run "r: Sample Counts/sec. Run Sample Counts/sec. Absolute Ef f i c i e n -cy 60 With Co C1 4 With Co 6 0 Above Bckgd. JiScpecT ted • 1 2 lean NH CI 30 gm. 0.615 0.633 0.624 41.5 42.1 41.8 1 2 Mean NH4C1 25 gm. + UreaA 5 gm. 24.8 24.6 24.5 66.7 64.0 66.4 23.9 102.4 23.4$ 1 2 Mean (NH4)2C0.3.H20 30 gm. 0.745 0,i720 0.732 72.5 73.2 72.8 1 2 Mean (NIL ) CO .HO 25 gm. f UreaA 5 gm. 27.8 27.4 27.6 90.1 91.8 90.9 26.9 102.4 25.3$ Mean (NH4) CO .H 0 ^ O ^ ? 0.732 72.8 "1 2 Mean (NH 4) gC0 3.H 20 20 gm.. 4 Urea k 10 gm. 56; 1 54.7 55.4 123.5 125.0 124.2 54.7 204.8 26.8$ 1 2 Mean Dextrose 35 gm. 0.814 0.792 0.803 53.6 54.7 54.2 1 2 Mean Dextrose 30 gm. 4 Ureaft 5 gnu 28.5 27.1 27.8 80.4 81.1 80.8 27.0 102.4 26.4$ 1 2 Mean Starch 40 gm.- 0.781 0.769 0.775 22.4 21.3 21.9 Urea & - containing c!4 pass a l l pulses greater than 20 v o l t s the background count-ing rate i s reduced by a factor of 2. The lower background count p a r t i a l l y compensates for the decreased counting e f f i -ciency and a t o t a l a c t i v i t y of less than 10~ 9 curies can e a s i l y be detected. Thus the s p e c i f i c a c t i v i t y of a 30 gm. sample of C 1 4 active urea may be less than 10 1 0 curies/gm. of sample. As can be seen from Table 3, the 'counting e f f i c i e n c y decreases s l i g h t l y as the a c t i v i t y of the sample i s lowered. In order to observe the range of e f f i c i e n c y and sen-s i t i v i t y obtainable with other sample materials, they were made radioactive with small amounts of the radioactive urea. These samples are l i s t e d i n Table 4 and include ammonium car-bonate, ammonium chloride and dextrose. A l l these compounds mentioned are s i m i l a r with regard to color and c r y s t a l form, and show the same counting e f f i c i e n c i e s and s e n s i t i v i t i e s as the urea. Starch v/ould have a considerably lower counting e f f i c i e n c y due to i t s poor transparency, as seen from the Co®^ measurements. This i s to be expected since the starch has the form of a f i n e white powder and shows dense packing i n a phos-phor mixture. c ) . Sulfur 35. Sulfur 35 i s also~~a pure beta par-t i c l e emitter with a h a l f - l i f e of 87.1 days. The emitted beta p a r t i c l e s have a maximum energy of only 0.17 Mev. and so present the same assay problems as C^ 4. The measurements 35 made with the S are given i n Table 5. Samples of Na2S04 TABLE! V. A S S A Y O F S U L P H U R 35 I N V A R I O U S S A M P L E S Run Sample Counts/sec. Run Sample Count s/s.ec. _ Absolute EfffiClan-cy nth Co 6 0 s35 ^ lith Co 6° Bckg. jiApec-ted 1 2 Mean Na gS0 4 35 gm. 0.833 0.861 0.852 46.1 45.8 45.9 1 2 Mean Na 2S0 4 35 gm. +• S 3 5 91.8 92.0 91.9 134.5 133.9 134.2 91.0 677 13.3% Mean Urea 30 gm. 0.691 41.8 1 2 Mean Urea 30 gm. + S^5 122.7 123.4 123.1 164.1 .164.8 164.5 122.4 677 18.1% Mean NH4G1 30 gm. 0.624 41.8 1 g Mean' 35 NH CI 30 gm. + S 4 ^ 117.8 118.3 118,1 158.2 157.6 157.9 117.5 677 17.4% Mean (NB^JgCOg.BgO 30 gm. 0.732 72.8 1 2 Mean (NH 4) 2C0 3.E 20 303gm. + . 08 126.1 126.7 126.4 198.9 198.1 198.5 125.7 677 18.5% Mean S i l i c a gel 30 gm. 1.43 33.4 1 2 Mean Si l i c a gel 30 gm. + S35 47.2 47.2 47.2 82.1 81.5 81.8 45.8 677 6.75% 37 enriched with a known a c t i v i t y of S , i n the form of sodium su l f a t e , y i e l d an absolute counting e f f i c i e n c y of only 13.3%. The background count of 0.852 counts/sec. and the 13.3% count-—9 ing e f f i c i e n c y permit a t o t a l r a d i o a c t i v i t y of 10 curies to be e a s i l y detected. This means that a 3.5 gm. sample having a s p e c i f i c a c t i v i t y of less than l O " 1 ^ curies/gm. of sample can be assayed. In order to check the e f f i c i e n c i e s and s e n s i t i v i t i e s 14 obtained with the C enriched samples, the experiments were 35 repeated using S enrichment. The e f f i c i e n c i e s obtained with 35 14 the S are lower than those obtained with the C , and are also shown i n Table 5. d). Adsorption Counter. The volume-counting method can be applied also to.radioactive solutes, vapours, and gases that can be adsorbed or absorbed by materials such as s i l i c a g e l . The sorbent, with the radioactive material bound to i t , i s placed i n the s c i n t i l l a t i n g l i q u i d and i t s r a d i o a c t i v i t y measured. A few experiments have been made with s i l i c a gel as a sorbent. It forms a very transparent mixture with the l i -quid phosphor, but care must be taken to remove a i r bubbles which tend to form very e a s i l y i n t h i s type of paste. S o l i d KC1 added to s i l i c a gel phosphor mixtures allows the naturally 40 occuring K to be assayed with an absolute e f f i c i e n c y which depends on the amounts of KC1 and s i l i c a l gel present. Table 2 38 shows the resu l t s obtained with two such mixtures. When con-siderably more s i l i c a gel i s present i n the sample, the abso-lute counting e f f i c i e n c y i s only of the order of 40%, the de-crease i n e f f i c i e n c y being due to the eff e c t of absorption of 40 the K beta p a r t i c l e s by the s i l i c a g e l . Experiments were also performed using, s i l i c a gel as. an adsorbent and KC1 i n solution as the adsorptive. S i l i c a gel has the property of adsorbing ions from an al k a l i n e s o l u -t i o n . 30 gram samples of s i l i c a gel were s t i r r e d for periods of up to 24 hours with 500 cc. of d i l u t e KC1 solutions, made s l i g h t l y a l k a l i n e with NaOH, and the uptake of the potassium 40 measured by assaying the K content of the sample. The ad-s o r p t i v i t y of s i l i c a gel for ions i n solution i s very low, espe c i a l l y i n solutions with low concentrations of sample mat-e r i a l . 35 In another experiment an aqueous solution of S ac-ti v e s u l f u r i c acid was absorbed by s i l i c a gel and the beta p a r t i c l e a c t i v i t y measured. The results are shown i n Table 5. The counting e f f i c i e n c y i s very much reduced by the strong absorption of the low energy beta p a r t i c l e s i n the s i l i c a g e l . With a sample of thi s type a large number of the beta p a r t i c -l e s originate inside the s i l i c a gel grains and do not reach the l i q u i d phosphor. The s i l i c a gel.can be obtained i n a variety of grain s i z e s , but the smaller the c r y s t a l s are, the lower the transparency of the mixture. The counting e f f i e i e n c y 3 9 35 for the low energy beta p a r t i c l e s from the S i s only 6.75%. The use of more e f f e c t i v e adsorbents depends on th e i r s t a b i l i t y i n organic solvents, since disso l v i n g would probably destroy the counting properties of the phosphor. The trans-parency of the adsorbent-phosphor mixture must be s u f f i c i e n t l y high to allow the s c i n t i l l a t i o n s to reach both m u l t i p l i e r tubes. Fine powders, such as c e l i t e have a very low transparency be-cause of packing and are i n general unsatisfactory. The, trans-parency can be improved somewhat by mixing the opaqtie material with a l i g h t guide, which can be some substance such as s i l i c a gel or quartz. Recently, organic exchange resins have been developed which are very insoluble i n organic solvents. They have high ion-exchange e f f i c i e n c i e s and ion-binding ca p a c i t i e s , and can be obtained as -. anion or cation exchangers.. However, the present ion exchange resins available are colored, usually yellow or brown, and a large f r a c t i o n of the l i g h t emitted by the phosphor i s l o s t through absorption i n the r e s i n . This eff e c t could be reduced considerably by the use of a l i q u i d phosphor which emits l i g h t with a maximum inte n s i t y i n the yellow region, provided that photomultiplier tubes with suitable response c h a r a c t e r i s t i c s are also used. A c o l o r l e s s trans-parent r e s i n would of course be more advantageous. Such ion exchange resins can be used i n monitoring the p o l l u t i o n of water supplies and r a i n i n which the presence of soluble radio-active substances i s suspected. They have the advantage of 40 permitting low isotope concentrations to be detected since the ions from a large volume of water can be concentrated into a small sample. The assay of such samples by the volume counter method allows the detection of alpha p a r t i c l e and low energy beta p a r t i c l e emitters. The use of an adsorbent, such as s i l i c a gel,- for moni-toring the r a d i o a c t i v i t y , o f vapours and gases also seems fea-s i b l e . Modifications i n technique, and i n the container for the radioactive sample would be required, but experiments of th i s type have not been c a r r i e d out. e). Investigation of B i o l o g i c a l Material. The two properties required by the sample (Section V) i n order that i t may be assayed by the volume counter: method are met by a large variety of b i o l o g i c a l substances. Material, such as bone, muscle, plant material, molds, and b a c t e r i a l cultures, dried and powdered or shredded,, could be dispersed d i r e c t l y i n the l i q u i d phosphor. F a t - l i k e substances, soluble in. the solvent of the phosphor, may f i r s t have to be extracted, since the toluene-soluble f a t aff e c t s the counting properties of the s c i n t i l l a t o r . In some cases the ash of the sample material could be used. In many instances t h i s procedure, would remove the necessity of.complicated chemical recovery of.the radio-active, substance. Very low concentrations of radioactive tracers could be assayed immediately from large amounts of the tissue or other b i o l o g i c a l substances (for example red 41 blood c e l l s ) . A few materials of b i o l o g i c a l o r i g i n have been i n -vestigated. .These include a technical gelatine, a proteose, casein, and agar. As a l l are yellow i n color the counting e f -f i c i e n c y of these substances i s decreased by t h e i r absorption of the blue l i g h t emitted by the phosphor. Here again a l i q u i d phosphor giving s c i n t i l l a t i o n s with a maximum inte n s i t y i n the yellow, i n combination with yellow-sensitive photomultiplier tubes, would be u s e f u l . 2. COUNTING.EFFICIENCY LOSSES The losses i n counting e f f i c i e n c y experienced' with the volume counter ari s e from 3 sources. .1) Self-absorption i n the sample. This factor' accounts for a major-portion of the disintegrations that are not counted.. In the case of isotopes emitting alpha or beta p a r t i c l e s the radiation must f i r s t leave the s o l i d c r y s t a l p a r t i c l e s and enter the l i q u i d phosphor. Many of the alpha p a r t i c l e s and low energy beta p a r t i c l e s are absorbed i n the c r y s t a l s and do not reach the l i q u i d phosphor. Having reached the phosphor,, such a p a r t i c l e must lose s u f f i c i e n t energy to the phosphor to produce a s c i n t i l l a t i o n before i t s t r i k e s ano-ther c r y s t a l of the sample. Whether or not the. r e s u l t i n g s c i n -t i l l a t i o n w i l l reach the photomultiplier tubes depends on the transparency of the sample-phosphor paste. The decreased counting e f f i c i e n c y observed for C 1- r e l a t i v e to that for K^O \ 42 i s due to t h i s factor. 2) I n s u f f i c i e n t sample-phosphor mixture transpar-ency. A l l mixtures w i l l absorb some of the l i g h t emitted by the phosphor, and the degree of absorption w i l l depend on the transparency of the mixture. This transparency i s governed by the color and size of the sample material p a r t i c l e s . A l -though small sample p a r t i c l e s reduce the self-absorption of the emitted radiation, they permit a closer packing of the sample and produce i n most cases a drastic reduction i n trans-parency. A high counting e f f i c i e n c y for colored samples can be maintained only i f suitable phosphors and photomultiplier tubes are used. The transparency of the sample-phosphor mixture de-termines the amount of sample material that can be e f f i c i e n t l y used. Since a coincidence counting system i s applied, a l i g h t f l a s h o r i g i n a t i n g near one photomultiplier tube must pass through the mixture, and reach the second m u l t i p l i e r tube with s u f f i c i e n t i n t e n s i t y to produce a pulse that can be registered. Photomultiplier tubes "with very large photosensitive areas, such as the RCA type-5037 described by Greenblatt et a l . (1952), could be used to advantage i n the volume counter since i t would allow, for eqtial volumes between the photocathodes, a reduction i n the distance between the cathodes. This would r lead to. an improved transparency of the sample-phosphor paste 43 and a better- u t i l i z a t i o n of nearly a l l of the s c i n t i l l a t i o n flashes. 3) Geometric E f f i c i e n c y . Since the sample p a r t i c l e s are completely surrounded by l i q u i d phosphor the d i r e c t i o n i n which the beta r a d i a t i o n i s emitted from a s o l i d p a r t i c l e does not matter, and i n t h i s respect i t can be said that 4tr geometry i s attained. However, incomplete r e f l e c t i o n of the l i g h t from the photocathode faces, and absorption of the l i g h t i n the glass of the container and sample-phosphor paste, combine to reduce the over a l l geometric e f f i c i e n c y of the system. O p t i c a l l y the geometry depends on the distance be-tween the m u l t i p l i e r tubes and the diameter of the photosen-. s i t i v e cathodes. By keeping the distance between the tubes small and using large diameter tubes, a high o p t i c a l e f f i -ciency can be maintained. 3. SENSITIVITY The lowest s p e c i f i c a c t i v i t y that can be measured depends on the amount of sample material that can be success-f u l l y u t i l i z e d , o n the counting e f f i c i e n c y of the system, and on the background count. The amount of sample can be increa-sed without lowering the counting e f f i c i e n c y only by using photomultiplier tubes with larger photosensitive cathodes. A further separation of the present m u l t i p l i e r tubes to acco-modate the larger volume, would increase the l i g h t absorption and reduce the counting e f f i c i e n c y . 44 The background could be reduced by the use of con - t siderably more iron s h i e l d i n g . At l e a s t S inches of iron shielding would be required to eliminate the background due to l o c a l gamma-radiation. With the'present apparatus the back-ground count i s dependent on the amount of samp-le-phosphor paste between the tubes. In general, sample-phosphor mixtures having a good transparency show an increase i n background count with increasing amounts of mixture, while mixtures with poor transparency usually have a lower background count as a re s u l t of l i g h t losses.' 4. REPRODUCIBILITY Although the volume counter method makes use of a two-phase source the r e p r o d u c i b i l i t y of the system i s good,as can be seen from the tables. C r y s t a l l i n e samples such as KC1, NaCl, s i l i c a gel, and urea give re s u l t s that are reproducible with an accuracy of 2%. The v a r i a t i o n i n si z e of the c r y s t a l -l i n e material i n a sample i s not important, since other samples of the same material w i l l exhibit s i m i l a r variations i n cry-s t a l s i z e , i f given the same preparatory treatment, and there-fore w i l l have the same counting c h a r a c t e r i s t i c s . The v a r i a -t i o n i n c r y s t a l s i z e i n samples of d i f f e r e n t material w i l l i n general res u l t i n d i f f e r e n t counting e f f i c i e n c i e s . For example, in the case of KC1. and KNO3 the smaller KNO3 c r y s t a l s produce a le s s transparent mixture than the KC1 as a r e s u l t of packing. The great number of c r y s t a l s i n a large volume sample leads 45 s t a t i s t i c a l l y to a more uniform average grain size than i s obtained i n the precipitated samples used v/ith Geiger counters. The --reproducibility of the system for b i o l o g i c a l materials such as proteose and agar v/as found to be not as good, and was due to the d i f f i c u l t y of preparing samples with the same average p a r t i c l e s i z e . With careful sample prepara-ti o n the r e p r o d u c i b i l i t y can be maintained to within an ac-curacy of better 'than ,-5%. In a l l cases care must be taken to remove a i r bub-bles since t h e i r presence can cause changes i n transparencjr. The sample-phosphor mixture should be well s t i r r e d to free any trapped a i r and thus prevent formation of bubbles while the measurements are being made. This precaution must be p a r t i c u l a r l y observed when.using samples containing s i l i c a g e l , and the mixture allowed to stand for about 15 minutes before measvtrements are begun. 46 VI_. MULTIPLE-SAMPLE COUNTER. The counter described can also be used for a c t i v i t y measurements i n cases where only small amounts of sample ma-t e r i a l are available; i n t h i s form i t i s referred to as a multiple-sample counter. The radioactive sample i s deposited on sheets of cellophane stacked together with the help of a special holder; the holder with the l i q u i d phosphor i s placed i n the container between the m u l t i p l i e r tubes. Geiger counter methods are l i m i t e d by the area of the sample that can be exposed to the counter, and i t i s only the radiation from the surface layers of the samples that can be detected. The sample area i s small for thin window Geiger tubes and most windowless flow counters, the two me-thods generally used i n assay work. Self-absorption of the radiati o n by thick samples, esp e c i a l l y i n the case of low energy beta p a r t i c l e s from isotopes such as C 1 4 and S , re-duces the number of beta p a r t i c l e s entering the Geiger coun-ter to a small f r a c t i o n of the number present i n the sample. For example, with a barium carbonate sample, containing C^ 4, a thickness of 10 mgms./cm^ w i l l reduce, through self-absorp-t i o n , the f r a c t i o n of p a r t i c l e s available for counting to 65% of the t o t a l s p e c i f i c a c t i v i t y present. By mounting the samples on sheets of cellophane a counting area of 50 cm2 i s e a s i l y obtained. This i s 10X the area that can be used by the Geiger tube methods. The -Tines Supporting Cellophane F i g . 1 0 . Holder For Supporting the Cellophane Sheets Perpendicular to the Photocathodes of the Mu l t i p l i e r tubes. To face page 47 47 beta p a r t i c l e s can bombard the phosphor on both faces of the thin cellophane support; on one side they enter the l i q u i d phosphor d i r e c t l y from the sample material, on the other side they pass through the cellophane with small energy losses. As a r e s u l t of t h i s the e f f e c t i v e surface area i s doubled and the sample thickness reduced. 1_. HOLDERS Two types of sample holders have been used. 1) A sketch of the simple holder used i s shown in/ fig.10 and consists of a copper ri n g to which are attached a set of tines for supporting the cellophane s t r i p s . The c e l l o -phane s t r i p s are cemented to the tines with a small amount of water glass and the sample mounted on the sheets. The holder, with the v e r t i c a l l y mounted cellophane, f i t s into the con-tainer with the cellophane sheets perpendicular to the photo-cathodes of the m u l t i p l i e r tubes. The tubes are separated by a distance of 2 cm., and contact of the cathodes v/ith the phosphor assures good o p t i c a l coupling. o A t o t a l counting area of 40 cm i s available and up to 1 gm of transparent or opaque sample material can be conven-i e n t l y used. The material i s bound to the cellophane with a small amount of water glass. Beta p a r t i c l e s also enter the phosphor through the thin cellophane, and t h i s means that very favourable counting conditions are obtained for such a source. The cellophane and water glass binder together have Holder for Supporting the Cellophane Sheets P a r a l l e l to the Photocathodes of the M u l t i p l i e r Tubes. 48 2 a thickness o i less than 4mgms./cm.. The e f f e c t i v e thickness of a given mass of sample i s reduced to 1/10 of that presen-ted to a thin window Geiger tube because of the increased counting area and high e f f i c i e n c y . 2) The second type of holder, shown i n f i g . 11, allows a s t r i p of cellophane 12 inches long and 1 inch wide to be folded and stretched over the tines of the holder i n such a way as to give 10 p a r a l l e l l ayers. The cellophane holder i s placed i n the beaker between the m u l t i p l i e r tubes, with the sheets, p a r a l l e l to the photocathodes, and the beaker f i l l e d with the l i q u i d phosphor. Again, since both photocathodes are i n contact with the phosphor, good o p t i c a l coupling i s achieved. The l i g h t absorption i n cellophane i s very small and, as the r e f r a c t i v e indexes of the cellophane and l i q u i d phosphor are nearly the same, the l i g h t losses through mul-t i p l e r e f l e c t i o n are also small. The distance between the cellophane sheets i s 1 mm. so that a stack of 10 sheets can be placed between the photo-cathodes when they are 12 to 15 mm. apart. A sample area of p 60 cm. i s e a s i l y available even i f the sample i s mounted on 7 only one side of the cellophane sheet. The sample material that can be assayed with t h i s holder must be transparent but i t has a larger counting area than the holder with the ver-t i c a l l y mounted sheets. This permits a larger amount of sample to be measured with the same s e n s i t i v i t y and e f f i -ciency obtained with the v e r t i c a l holder. . 49 A holder of t h i s type would be more suitable for use with a large-surface photomultiplier tube such as the RCA H-5Q37. "With t h i s tube i t would be possible to maintain a large counting area while,reducing the number of sheets be-tween the tubes.. Transparency and geometry would then be im-proved by the smaller distance between the tubes. 2. EXPERIMENTS AND RESULTS. a). Potassium 40. The a c t i v i t y of K^® i n a sample of KC1 was measured by t h i s method. 250 mgm. of KC1 were dis t r i b u t e d on cellophane sheets over an area of 40 cm2, and the t o t a l a c t i v i t y of the source determined with an absolute e f f i c i e n c y of 62.6%. The re s u l t s of these measurements are given i n Table 6. The background count, measured with a non-active sample of NaCl, i s 1.26 counts/sec. so that a t o t a l a c t i v i t y of 10"^^ curies can e a s i l y be measured. However, since a smaller amount of sample material i s used the sp e c i -f i c a c t i v i t y of the sample must be higher than i n the case of the volume counter. Isotopes emitting beta p a r t i c l e s with 40 energies s i m i l a r to that of K could be assayed i n concen-trations of 10 curies/gm. of sample. The transparency of the sample i s not too c r i t i c a l since the l i g h t can travel through the phosphor between the cellophane sheets to reach the m u l t i p l i e r tubes. A sample made up of 250 mgm. of BaC03, which has the form of a fine opaque- powder and consequently has a poor transparency, gave the same background count as TABLE VI. MJLTIPLE-SAMELE COUNTER ASSAY OF K 4 0, C AND S IN VARIOUS SAMPLES Run Sample Counts/sec. Run Sample Counts/sec. Absolute Bckgd-. With Co 6 0 Isotope 60 With Co Above Backgd. inspec-ted E f f i c -iency 1 2 NaCl 0.250 gm. 1.14 1.12 81.9 82.4 1 2 KC1 "0.250 gm. 4.01 4.00 85.1 85.3 Mean 1.13 82.2 Mean 4.01 85.2 2.88 4.60 62.6% 1 '• 2 • 3 4 Urea 0.250 gm. 0.680 0.681 0.684 0.678 81.3 81.3 81.4 81.9 1 2 3 4 Urea& 0.250 gm. 2.85 2.86 2.83 2.80 83.8 84.0 84.2 83.6 Mean 0.681 81.5 Mean 2.84 83.7 2.16 5.12 42.2$ 1 2 Na SO 0.250 gm. 2 4 0.690 0.685 80.7 81.1 1 ' 2 Na SO 0.250 gm. 2 4 + S35 & 13.5 13.85 94.3 94.9 Mean 0.687 80.9 Mean 13.67 94.6 13.0 338 38.7$ 1 2 . 3 BaC03 0.200 gm. 0.650 0.634 0.637 76.5 76.1 76.9 Mean. 0.640 ' 76.8 Urea & - urea containing C-the transparent NaCl sample, (see Table 6). b) . Carbon 14. The beta p a r t i c l e s from C 1 4 i n 250 mgm. of activated urea were detected with an absolute count-ing e f f i c i e n c y of 42.2%, (Table 6). The background count i s reduced,to 0.681 counts/sec. by use of the anti-coincidence -9 system. A sample with a s p e c i f i c a c t i v i t y of 10 curies/gm. can be assayed without the necessity of a long counting time. 3 5 c) . Sulfur 35. S beta p a r t i c l e s were measured from a similarly mounted source of activated NagSO^; the re-su l t s are shown i n Table 6. An absolute counting e f f i c i e n c y of 38.7% was obtained which i s close to that obtained for C^^ i n the urea samples. The background count i s again reduced to 0.687 counts/sec. with the help of the anti-coincidence mixer, and that means that samples with a s p e c i f i c a c t i v i t y -9 of 10 curies/gm. can be e a s i l y assayed. Cellophane sheets 3 5 impregnated with S i n the form of d i l u t e s u l f u r i c acid, were also mounted i n the holder; these measurements gave a higher absolute counting e f f i c i e n c y . The increase i n count-ing e f f i c i e n c y i s due to an almost complete lack of s e l f - a b -sorption i n the source, only a small f r a c t i o n of the beta p a r t i c l e s being absorbed by the cellophane support. Strips of paper with adsorbed radioactive compounds can be mounted i n t h i s holder and the a c t i v i t y measured. Ac-t i v i t y measurements of t h i s type arise i n chromatographic se-parations of radioactive organic compounds which are adsorbed 52 on a s t r i p , o f f i l t e r paper. The d i s t r i b u t i o n of an isotope can be determined by cutting such a paper-chrOmatogram into s t r i p s and measuring the r a d i o a c t i v i t y . The amount of radio-a c t i v i t y that i s to be detected i s generally very small and 14 35 v/ith isotopes such as G and S absorption of the low energy beta p a r t i c l e s makes assaying d i f f i c u l t . By mounting the s t r i p s of f i l t e r paper v e r t i c a l l y i n a holder nearly 4 T geometry from the paper source i s obtained and an o v e r a l l high e f f i c i e n c y maintained.. 3. .COUNTING-EFFICIENCY LOSSES The results of the KC1 measurements indicate that a rather high e f f i c i e n c y loss occurs as a re s u l t of l i g h t losses. Self-absorption losses i n such a thin sample of KC1 would be very small because of the r e l a t i v e l y high energy beta-radia-40 t i o n from.K , so that most of the. betas lose t h e i r energy to the phosphor and produce s c i n t i l l a t i o n s . The i n t e n s i t y of the s c i n t i l l a t i o n s i s diminished by absorption i n the phosphor and incomplete r e f l e c t i o n from the container walls and multiple l a y e r s . Losses of t h i s type could be reduced by decreasing the distance between the m u l t i p l i e r tubes. Greater l i g h t losses are experienced by the second type of holder d e s c r i - . bed since the s c i n t i l l a t i o n s must also pass through the sample material. However, p a r t i a l compensation for t h i s ef-fect i s attained since, for equal areas, the second type of holder allows a shorter distance between the tubes to be maintained. In both cases decreasing the distance between the tubes to reduce l i g h t losses r e s u l t s i n a decrease i n sample area. 4. SENSITIVITY The conditions a f f e c t i n g the s e n s i t i v i t y of the volume counter discussed i n Section V apply i n t h i s case also. Lower absorption losses lead to an increase i n the counting e f f i c i e n c y over that obtained with the volume coun-ter, but nevertheless the multiple-sample method shares some of the l i m i t a t i o n s of the conventional methods. The small amount of sample material that can be used allows only samples with a higher s p e c i f i c a c t i v i t y to be investigated, but by increasing the counting area the amount of sample ma-t e r i a l can be increased. 5. REPRODUCIBILITY The multiple-layer samples, mounted v e r t i c a l l y , can be reproduced with an accuracy of 3.0% (see Table 6) pro-vided that s u f f i c i e n t care i s taken i n , t h e i r preparation. Similar d i f f i c u l t i e s of preparation are encountered here as v/ith the. p r e c i p i t a t i o n of sample material to be assayed by Geiger counter methods. Tlie r e p r o d u c i b i l i t y of r e s u l t s with the second type of holder i s not as good because of the losses of source material that can occur when the cellophane s t r i p ' i s fixed to the mount. However, v/ith practice, samples of t h i s type can be easily, reproduced with an accuracy of 5%, and l i k e l y better. 54 S U M M A R Y . The measurement of low s p e c i f i c beta a c t i v i t i e s i s an important problem of radioactive assay. Such investiga-tions include observations on the natural abundance of radio-active isotopes, age-dating measurements, and isotopic physio-l o g i c a l tracer studies. The measurement of small a c t i v i t i e s presents the phy s i c i s t and biophysicist with the question of devising me-thods which are highly s e n s i t i v e , e f f i c i e n t , and accurate. Some of the methods used so far obtain these properties through time consuming concentration processes, elaborate counting techniques, and long counting times. In the present investigation a counter for extremely low a c t i v i t i e s has been developed which achieves i t s high s e n s i t i v i t y through the application of large amounts of radio-active substance, and the use of an e f f i c i e n t l i q u i d s c i n -t i l l a t i o n phosphor. The counter can be used i n two d i f f e r e n t ways; as a volume counter and as a multiple-sample counter. In the f i r s t type of counter the s o l i d radioactive sample i s mixed with the l i q u i d phosphor to form a transparent paste; in the se-cond type a smaller amount of s o l i d sample i s spread on a large surface of cellophane and immersed i n the s c i n t i l l a t i n g f l u i d . 55 A v a r i a t i o n of the volume counter i s the adsorption  counter which uses a large amount of transparent adsorptive to which radioactive material i s bound. Another form of the counter uses as sample material a non-active s a l t l i k e sodium chloride into which more or less opaque b i o l o g i c a l radioactive materials,' l i k e proteins, c e l l material, and bone powder'are dispersed, the non-active s a l t acting as a l i g h t guide. Ra d i o a c t i v i t i e s of the following beta-emitting i s o -topes were measured: potassium 40,carbon 14, and s u l f u r 35. The isotopes were incorporated i n s a l t s and organic materials, namely K C 1 , KN0 3, Na 2 S 0 4, (NILj^COs.H2O, urea, and dextrose. The a c t i v i t y of the potassium i s due to the naturally occur-ing isotope K"^; i n the other cases the a c t i v i t y was pro-duced by d i l u t i n g small amounts of C*~^ and into the samples. By using the volume-type counter the s e n s i t i v i t y obtained was less than 1 0 " "^ curies/gm. , i n samples contain-ing 30 gm. of material, and with counting times of the order of 30 minutes. For the multiple-sample counter the -10 available s e n s i t i v i t y i s of the order of 10 curies/gm. with -"samples up to 1 gm. and s i m i l a r counting times. These sensi-t i v i t i e s can be measured v/ith a good degree of r e p r o d u c i b i l i t y and are superior to the conventional methods, v/ith the excep-tion of the elaborate screen-wall counter method. The high absolute counting e f f i c i e n c i e s obtained with both forms of the counter are also considerably better than most counting systems. Various applications of t h i s counter can be found in the assay of material of low s p e c i f i c a c t i v i t y and i n par-t i c u l a r i n physiological and c l i n i c a l tracer studies where the use of low a c t i v i t i e s i s desirable to avoid the e f f e c t s of r a d i a t i o n damage. REFERENCES. * 7 1. Anderson, E.C., Arnold, J.R., Libby, W.F. 2. Anger, H . O . 3. Bernstein, W. and Ballentine,R, 4. Bltfh, 0. and Terentiuk, F. 5. Bowen, E.J., Mikiewics, E. and Smith, F . W . 6. Brown, S.C. and M i l l e r , W.W. 7. Calvin M, Heidelberger, C., Reid, J.C., Tolbert, B.M., and Yankwich, P.F. 8. Damon, P.E. and Hyde, H.I. 9. Falk, C.E. and Poss, H.L. 10. Falk, C.E., Poss, H.L. and Yaun, L.C.L. 11. Farmer, E.C. and Berstein,I.A. 12. Faust, W.R. 13. Greenblatt, M.H., Green, M.W., Davison, P.W. and Morton, G.A. 14. Healy, J.W. 15. Kallmann, H. and Furst, M. 16. Kallmann, H. and Furst, M. 17. Kallmann, H. and Furst, M. Rev. Sci.Instruments 22, 225, (1951). Rev. Sci.Instruments 22, 912, (1951). Rev. Sci.Instruments 21, 158, (1950). Nucleonics 10, 48, (1952). Proc.Phys.Soc.(London) A62, 26, (1949). Rev.Sci.Instruments 18, 496, (1947). Isotopic Carbon. John Wiley and Sons Inc. 1949, Chpts. 4 and 5. Rev.Sci.Instruments 23, 766, (1952). Araer.Jrn. of Phys. 20, 429, (1952). Phys.Rev. 83, 176, (1951). Science 115, 460, (1952). Phys.Rev. 78, 624, (1950). Nucleonics 1£, 44, (1952). Nucleonics 1£, 14, (1952). Nucleonics 7, 69, (1950 A). Phys.Rev. 79, 857, (1950 B). Phys.Rev. 81, 853, (1951). 18. Kamen, M.D. 19. Kulp, J.L. and Tyron, L.E. 20. Kummer, J.T. 21. Labaw, L.W. 22. Libby, W.F. 23. Raben, M./S. and Bloemberger,N. 24. Ravilious, C.F. 25. Rosen, F.D. and Davis, W. 26. T a i t , J.F. and Haggis, sG.H. Radioactive Tracers i n Biology. Academic Press, N.Y., 1948. Chapter IV. Rev.Sci.Instruments 23, 296, (1952). Nucleonics 3, 27, (1948). Rev.Sci.Instruments 19, 390, (1948). Phys.Rev. 46, 196, (1934). Science 114A, 363, (1951). Rev.Sci.Instruments 23, 760, (1952). Rev.Sci.Instruments 24, 349, (1953). J.of Sci.Instruments and of Phys. i n Industry 26, 269, (1949). 

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