Measurement of Radioactive Isotopes LEON
F. CURTISS, Chief, Radioactivity Section, National Bureau of Standards, Washington 25, D. C.
T h e N a t i o n a l B u r e a u o f S t a n d a r d s is c u r r e n t l y e n g a g e d i n ob t a i n i n g a n d s u p p l y i n g i n f o r m a t i o n t o users o f i t s s t a n d a r d r a d i o a c t i v e s a m p l e s w h i c h will a s s u r e reliable r e s u l t s . . . C o r r e c t m e t h o d s a n d s t a n d a r d s o f a c c u r a c y have b e e n p o s t u l a t e d RADIOACTIVE isotopes, now articles of
commerce, are finding increasing uses in biological and medical experiments and even in clinical treatment of diseases. For these reasons the determination of the number of radioactive atoms in a sample of radioactive isotope has become a matter of great importance. Uniform quantita tive results in all laboratories can be ob tained only by using procedures that will yield absolute measurements or by the use of uniform standards of comparison that will, under proper conditions of measure ment, give the same result whenever the determination is made. Establishment of standards for radioisotope measure ments—as for quantitative measurements In all fields of physical science—is the responsibility of the National Bureau of Standards. Detailed and accurate infor mation must therefore be made available 80 that standard radioactive samples being furnished by the bureau to biological, medical, educational, and industrial labo ratories can be used in a way to assure cor rect and reliable results. Two factors are involved in the prepara tion and use of these samples. First, methods and standards that are correct in principle must be used. Second, a reason able degree of accuracy must be attained in all steps of the process not only of produc ing but also of using the standards. Basic Principles Essentially the measurement of a sam ple of a radioisotope is the measurement of the number of radioactive atoms present. This determines the strength of the source and involves three basic concepts: (1) the total number of radioactive atoms present, ΛΓ; (2) the disintegration, or decay, con stant, usually represented by λ, and ob tained from the half-value period, T, by the relation loge2 0.693 A
s
m
e
of course equal t o the number of nuclear particles, usually beta particles or posi trons, -which are emitted per unit time from all atoms which disintegrate within this interval of time. This is true since, in general, one of these particles is emitted for each disintegration which takes place. Important exceptions to this general rule will be discussed later. The requirement that all disintegration particles must be counted per unit time is by no means simple. The principal diffi culty is that these particles are emitted equally in all directions so that it is prac tically impossible to devise detecting equipment which, will record all of them. The Geiger-Mûiler beta-ray end-window counter, for example, has a window through which the beta particles enter from a source placed in front of the window. The counter can record only that fraction of the particles which pass through it. This represents, from geometrical considerations alone, not more than 25% of the particles actually emitted. At least three other factors may distort the results. They are: absorption of particles in the window; absorption of particles in the source; and backward scattering of particles by the support of the source which directs through the window some particles originally emitted in other directions. The first two factors reduce the number from the correct value ; the third increases it. Conceivably the contribution of all of these factors can be evaluated experi-
m
and (3) the disintegration rate, or "activ ity," X2V, the number of atoms disintegrat ing in unit time. Measurement of the rate of disintegra tion, that is, the number of atoms dis integrating in unit time, is necessary in order t o determine the total number of atoms present. The disintegration rate is THIS article -was adapted from NBS Circular 473, "Measurement of Radioactive Isotopes," obtainable from the Superintendent of Docu ments, U. 8. Government Printing; Offioe, Wash ington 25, D. C, at 5 cents per copy.
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mentally for a particular source and a definite arrangement of the apparatus. This not only requires considerable work but it must be repeated whenever changes are made in either the source or the apparatus. Radioactive Standards Most of the difficulties in making quantitative determinations of radioisotopes can be eliminated if standard sources of the radioisotope under measurement are available. A standard source consists of a preparation of the isotope in a form convenient for use with the detector of radiation, and for which the disintegration rate is known from previous calibration. Obviously, to be useful as a standard, a radioisotope must have a relatively long half-life. Furthermore, of those which have a sufficiently long half-life only those which can be calibrated in absolute disintegration rates are acceptable for preparation of standards. The number of isotopes which satisfy both these requirements is at present very limited. When the standard is prepared from the same isotope as that to be measured only three simple precautions are required to secure reliable results: (1) readings must be made with the standard in the same position as that at which readings are made on the sample; (2) the sample must be uniformly distributed over approximately the same geometrical area as the standard; and (3) the sample must be supported on a layer of material identical with that supporting the standard, or at least one that produces the same backscattering effect. Isotopes which have short half-periods and a known disintegration scheme may be measured with a beta-ray standard of some other isotope with fair accuracy if it is known that a beta ray or a positron is emitted for every disintegration, and if the maximum energy of the beta-ray spectrum of the standard is not too different from that of the beta-ray spectrum of the isotope to be measured. In comparing samples of isotopes differing from the standards, all precautions outlined for the comparisons of the same isotope must be employed and the thickness of the counter window in mg./cm.2 must be known. In addition, an absorption curve for the beta rays from the standard and a similar absorption curve for the beta rays from the sample must be taken. This procedure consists in observing the counting rates for each when different known thicknesses of aluminum in mg./cm.2 are interposed between the source and the counter. When the data are plotted on semilogarithmic paper, with counts per second on the AND
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Co*0 gamma-ray standards from the Bureau of Standards for calibration of unknown Co*0 solutions logarithmic y-axis and thickness on the linear g-axis, the graphs will be very nearly straight lines. Extrapolating to zero thickness gives zero absorber counting rates for both standard and unknown sam ple, from which the disintegration rate of the unknown sample may be computed. Working Standards Since calibrated standards supplied by the National Bureau of Standards can be expected to maintain their calibration only when handled carefully by trained person nel, it is desirable that laboratories prepare their own working standards. This ap plies particularly to beta-ray standards deposited on metal planchets, such as the RaD + Ε standards. There is another advantage in the preparation of working standards in the laboratory. This permits the primary standards in many instances to be prepared as gamma-ray standards consisting of a solution sealed in a glass ampoule from which it need never be re moved for use. An example is furnished by the Co60 standards now available from the National Bureau of Standards. They consist of 5 ml. of solution containing a total of 1.5 rather fords (1.5 X 10e disintegrations per second) in one series, and 0.15 rutherfords in another. They may be used to cali brate unknown solutions of Co60 by com paring the gamma-ray activity of the un known solution in a similar ampoule, using a gamma-ray electroscope for the purpose. The ratio of these readings, corrected for background, provides the information for computing the strength of the unknown solution in terms of a disintegration rate. After the unknown sample has been calibrated, it can be made up to a standard volume, diluted in known ratios, and aliquots taken to produce deposits of appro priate activity for beta-ray standards.
disintegration is such that disintegration rates can be obtained from observations which can be made conveniently. For ex ample, if the isotope is a positron emitter but also disintegrates in part by electron capture, there is no convenient way of measuring the disintegration rate since it is difficult to determine the number of disintegrations occurring by electron cap ture. This capture can be detected only by virtue of the resulting characteristic x-radiation. In the case of those isotopes for which disintegration schemes are lacking or for which it is known that the mode of dis integration does not lend itself to measure ment of disintegration rates, an alternative method of comparison of activities of sources is available if the isotope emits gamma rays. This alternative does not give disintegration rates but it can, when properly applied, yield reliable compari sons of sources in various laboratories. For a particular isotope which emits gamma rays it is obvious that the intensity of the gamma radiation emitted is proportional to the amount of the isotope present. However, it is a well-known fact that elec troscopes and ionization chambers used for measuring gamma rays have sensitivi ties which vary greatly from each other due to variations in size, materials, geo metrical disposition of the source, and similar factors. Therefore, the comparison of source strengths by the gamma-ray method is a valid method only when con fined to the same isotope and when using the same electroscope in the same geo metrical relation to the source. This is the basic principle of all measurements of radium by the gamma-ray method.
A method has been proposed for exten sion of the gamma-ray method of compari son of sources to all laboratories. To obtain uniform results with such a method, no matter where the comparisons are made, a standard instrument and standard geometry are necessary. The requirement for the standard instrument is that it shall yield the same response for two equal sam ples of any isc ope, regardless of the ener gies of the gamma rays emitted by these isotopes. There is a unit of gamma radia tion which is denned without reference to the energy of the gamma ray. This is the roentgen, defined as "that quantity of roentgen or gamma radiation such that the associated corpuscular emission (sec ondary beta radiation) per 0.001293 g. of air, produces, in air, ions carrying 1 elec trostatic unit of electricity of either sign." Therefore, an ionization instrument prop erly designed to measure roentgens will satisfy the requirement of a standard instrument. To determine the strength of a radioactive source the roentgens per unit time must be measured at a standard distance. These considerations have led to the suggestion (1) that the unit of time be one hour and the distance one meter. Con venient magnitudes are thus provided for sources in common use. Furthermore, the distance of the source from the measuring instrument is relatively large and, therefore, small accidental variations in this distance will not produce serious errors, because the response of the instrument varies inversely as the square of the distance from the source. With this experimental arrange ment a unit is actually being used for comparison of gamma-ray sources which
Samples of radioisotopes are compared with the National Bureau of Standards RaD+E beta-ray standard (disk on base of stand) using a bell type beta-ray counter (on stand) and recording instrument. In order to correlate measure ments of radioisotopes in different laboratories^ identical 25-ml. samples Pn were distributed to approximately 40 hospitals, universities, and similar institutions using this isotope or interested in its measurement. The range of values reported by these laboratories, which varied as much as 80% from the average, has been plotted on the graph at the right
Unknown Disintegration Schemes It has been shown that, without a knowledge of the disintegration scheme, it is impossible to make reliable measure ments of radioisotopes in terms of disinte gration rates. The disintegration scheme serves to determine whether the mode of VOLUME
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is defined as 1 roentgen per hour a t a dis tance of 1 meter (roentgen per hour at a meter, abbreviated rhm.). This leads to a consideration of units for expression of disintegration rates. In 1910 at the Radiology Congress, in Brus sels the name curie was applied to the amount of radon in equilibrium witl· one gram of pure radium. In 1930 the Inter national Radium Standards Commission extended this definition t o include the equilibrium quantity of any decay prod uct of radium (#). However, the use of the curie in refer ring indiscriminately to disintegration rates has occurred in numerous places in the literature on radioactivity. As originally defined and as amended in 1930, the curie refers t o the rate of disintegration of 1 gram of radium since 1 curie of radon, by virtue of its definition as t h e quantity in equilibrium with one gram of radium, is also that amount of radon (or any other member of the radium family), which has a rate of disintegration equal to one gram of radium. Consequently, the curie can be applied only when it is intended to refer to rate of disintegration. Although t h e rate of dis integration of 1 gram of radium has never been measured with great accuracy, and the exact value is in dispute to the order of 3 or 4 % , the National Bureau of Standards has adopted the arbitrary value of 3.700 Χ 10 10 disintegrations per second for the curie when applied to isotopes other than members of the radium family. For many purposes a unit for the dis integration rate can be used which is smaller than the curie and which can be specified exactly and independently of any natural constants, such as t h e rate of decay of radium. A convenient quantity for a unit is that quantity of a radioisotope which disintegrates at the rate of a million dis integrations per second. T h e name ratherford (rd.) (1) has been suggested for this unit. Features which make this unit sim ple to use include a numerical magnitude that is easy to remember, and a size that is frequently used in the laboratory. For example, a therapeutic dose of many iso topes will be of the order of 100 rd., betaray sources for use with mica-window, belltype counters will b e of the order of 100 to 500 Mrd. and the weakest source that can be measured with any accuracy with these counters is of the order of 1 μτά. Tracer samples will usually be of t h e order of 1 rd. The use of the rutherford in data pre supposes that a disintegration rate has been measured and that this rate is ex pressed in terms of disintegrations per second. This procedure, if rigidly fol lowed, removes all confusion regarding units and renders data reported from dif ferent laboratories directly comparable on an absolute basis.
Literature ( 1) (2)
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Condon, E . U., and Curtiss, L. F. t Phys. Be»., 69, 672 (1946). Reo. Mod. Phys. 3 , 432 (1931).
Food Storage and Fortification Studied A STAFF REPORT IF IT can be made more palatable, alfalfa could be used as a base for the preparation of a low-cost, protein-rich food a recent meeting of the Institute of Food Tech nologists in Los Angeles was told. Henry Borsook, biochemist at California Insti tute of Technology, explained t o the joint membership of the Northern and Southern California sections that alfalfa produces more protein per acre than does any other crop. If our food larder should ever be come bankrupt to the point where popula tions are willing to "eat h a y / ' this un palatable legume might be utilized in a formula similar to that used in the allpurpose dry food now being sent to relieve starvation in Europe. Dr. Borsook briefly reviewed the de velopment of M P F (multipurpose food) of which soybean grit is one of t h e most im portant ingredients. High i n nutritive value, the new food is produced at a cost estimated at three cents per meal. Flavor ing is added, and amino acids and vita mins may be added t o fortify the mixture. W. M. Martin, director of research and development, Schwarz Engineering Co., described the method being developed b y his company for sterilizing nonacid liquid food. T h e empty cans and lids are sterilized in superheated steam at 450° t o 500° F. The liquid food is sterilized a t about 280 r to 300° F . in a tubular heat in terchanger for i..ree to seven seconds. It is then cooled aseptically, filled into the sterile cans a t below boiling temperature, and aseptically sealed with sterile lids. Such products as pureed baby foods, soups, and condensed milk can be handled in this manner with marked improvements in retention of vitamin B, color, aroma, and flavor. T h i s method compares favor ably with the usual sterilization in the cans at 240° to 2 5 0 ° F. in which sterilization periods of 30 t o 90 minutes must be used.
Fumigating
Fruits
Reporting o n the fumigation of dried fruits, Paul Richert of Coast Laboratories stated that epoxides such as ethylene oxide and propylene oxide are very effective fumigants. T h e y not only destroy in sects and their eggs but also microorgan isms such as molds and yeasts. Eventu ally they hydrolize into harmless glycols by reaction with t h e water of the dried fruits. H e said, however, that for bulk fumigation methyl bromide is more power ful against insects Ttnd i s widely used for that reason. Since propylene oxide is a liquid, it is used for sterilization of dried fruits in the final package. Ethylene oxide, D r . Richert continued, has an ad verse effect on the germination of grains treated with it. H . J. Deuel, University of Southern
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California, discussed fat problems in food technology. He stated that growth of ex perimental animals is equally good on natural and dehydrogenated fats. In 22 generations of laboratory animals on an oleo plus basal diet there was n o decrease in fertility, size, and general condition.. Vaccinic acid had no growth promoting effect ; therefore, it probably is not a vitamin. Shark liver oil production is decreasing. Carotene m a y be* added to oleo or butter; however, the color produced is extremely intense. In time synthetic vitamin A may be available for fortification of oleo.
The Glass
Electrode
History of the glass electrode and im provements that have been made in it were presented in a paper b y A. O. Beckman, National Technical Laboratory. Early glass electrodes failed in N a O H solutions: the new Ε t y p e of glass electrode is reliable up to 3.5 pi I unit3 above the limit of the old soda glass electrodes. Recently a very rugged glass that "can be used to hammer tacks" has been perfected. I t is expected to b e a great boon to teachers of laboratory classes and to "heavy handed" bench chemists. T h e new product is considered practically indestructible; i t will with stand high temperatures without giving erratic results. T h e industrial and research uses of pH measurements include the liming of cane juice. This is now controlled auto matically b y a glass electrode plus elec trically actuated valves and other equip ment. The glass electrode is also useful in micro-Kjeldahl titrations, water purifica tion, saponification of fats, pickling of olives, standardizing fruit juices, checking the reaction of canned foods, and similar operations. Dr. Beckman also described a glass electrode "on a string" for testing the pH of stomach contents. W. T . Pentzer of the U. S. Department of Agriculture reported on the handling, storage, and transportation of fresh fruits and vegetables. A very dilute mixture of sulfur dioxide and air ( 0 . 1 % sulfur di oxide) effectively checks mold growth on packaged fresh fruits after loading into "reefer'' cars. Nitrogen trichloride has been used similarly for cantaloupes. Wrappers containing, a copper compound are an effective protection against mold for pears. Antiseptic washes containing borax, quaternary ammonia compounds, hypochlorite, chlorine, or sulfur dioxide have been used for various fruits and vege tables before packing; benzoate is used for fish to be iced. Such treatments require Food and D r u g Administration approval. President H. C. Diehl addressed the membership and discussed t h e program of the institute for the coming year.
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