B. T. Kenna university of Arkansas Fayetteville
I I
The Search
A
historical account of element 43 is presented in this paper in four main phases: (a) the many searches and claims prior to its actual discovery in 1937; (b) its discovery and the preparation and identification of its isotopes; (c) its discovery in stars, and the theories to explain its presence there; and (d) the recent searches for naturally occurring terrestrial technetium. The chemistry of technetium is not treated here since i t is more than adequately discussed by Anders (1) and Boyd (8). In his periodic table, Mendeleev (5,4) left certain gaps and predicted that new elements would he discovered in time to fill these gaps. The atomic weights and physical, and chemical properties also were forseen. Two of these "missing" elements were congeners of manganese, which we know today as technetium and rhenium, elements 43 and 75. Because their properties would seem to be similar to manganese, he named them tentatively eka-manganese and dwi-manganese (sometimes written dvi-manganese), with the symbols Em and Dm. Their atomic weights were predicted to be 100 and 190, respectively. Further predictions were that their compounds would be colored and there would he a series of oxides corresponding to the oxides of manganese. The periodic table has provided a blueprint which indicated how many elements were possible up to the heaviest natural element, uranium. By about the middle of the third decade of this century, all 92 elements had been discovered, with the exceptions of elements 43, 61, 85, and 87. Even these found their way into the periodic table under various names. However, these "discoveries" were erroneous and, in the 19301s, it could be shown that these elements are all radioactive, having such short half-lives that their existence in appreciable amounts on the earth is not possible (5). Early ARempb To Isolate
Polinium: The first recorded claim of finding a new element which would correspond to element 43 was in 1828. Osann, (6) while studying platinum ores, claimed three new elements, among which was one he named polinium. However, he later reported that polinium was actually impure iridium (7). Ilmenium: While examining various minerals, Hermann (a),in 1846, found what was thought to be a new This paper is based on a portion of the dissertation submitted hy the author to the Graduate School of the University of Arkansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Present address: Sandia Corporation, Sandia Base, Alhuquerque, New Mexico. This wor!: was performed under the auspices of the U. S. Atomic Energy Commission, AEC Contract AT-(40-1)-1313.
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h Technetium in Nature
metal which he named ilmenium. The metal accompanied niobium and tantalum in the minerals investigated, and was closely allied with them in general characteristics. Hermann (8) has given details of its properties, such as the fact that the dioxide was redbrown and that a red color was produced with potassium ferrocyanide. Almost 30 years after his initial claim, Hermann (9) gave an account of the separation of ilmenium from the minerals by fractional crystallization of the double fluoride, and advanced a mass of evidence to show the elementary character of ilmenium. He added that a new element, neptunium,' occurred in the minerals, The new element would fill the vacant space left for dwi-manganese. However, owing to the minute quailtities of the supposed elements obtainable and due to the lack of modern X-ray methods to identify them, confirmation was not possible. Marignac (10) criticized Hermann's work and asserted that ilmenium was a mixture of titanium, niobium, and tantalum; Rose (11) claimed ilmenium was impure niobium. Pelopium: Rose (11,18) in 1847 investigated the mineral tantalite and deemed himself the discoverer of a new element, pelopium, which was reputed to resemble tantalum and niobium. Hermann (8,9) refuted Rose's claim by showing that pelopium was impure niobium. Dauyum: I n 1877, Kern (15,14) perceived the presence of a supposedly new element belonging to the platinum group while separating this group from platinum ores. The name, davyum, with the symbol Da, was given to this metal in honor of Sir Humphrey Davy. It was stated that the new metal would occupy the place between molybdenum and ruthenium in the periodic system, and would have an atomic weight of 1.54 (15). Three reactions were found to be most characteristic for davynm (16): (a) Potassium hydroxide in a davyum chloride solution produced a light yellow precipitate which was insoluble in an excess of the reagent, but easily soluble in acids; (b) potassium nitrite, warmed, gave no precipitate; but in the cold, a light, silky, redbrownish precipitate formed; and (c) there was a reaction of davyum with potassium thiocyanate, similar to that of iron. Using a spectroscope powerful enough to see only the principal lines, Kern (16) studied the spectrum of davyum, but no interpretation of the lines was given except to say that they were characteristic of davyum. I t is interesting to note that in a periodic chart of the elements developed by Rang (17) davyum is listed for element 43. I t has been pointed out that serious doubt surrounded the validity of davyum being a new element (18). The atomic weight of davyum was stated to he about 154, 'The name neptunium is not to be confused with that of element 93, discovered by McMillan and Abelson in 1940.
whereas the atomic weight should have been about 100 if it were to fit into the periodic scheme of Mendeleev. Mallet (19) severely criticized the work of Kern. Going through the same separation procedure as did Kern, Mallet obtained a sample of the questionable element davyum. After dissolution of the metal, it was found tbat potassium thiocyanate gave a red coloration, as st,ated by Kern; however, Mallet also tested the solution with potassium ferrocyanide and obtained a positive test for iron. On saturating the solution with ammonium chloride, a dark brown-red precipitate formed, from which the ordinary reactions of iridium were obtained. After evaporating the filtrate to dryness and igniting the residue, rhodium was found to be present. From these tests, Mallet concluded that, due to the lack of better proof of the elementary character of davyum than that given by Kern, the metal had to be considered as a mixture of iridium, rhodium, and iron. Lucium: I n the course of researches on monazite sand, Barriere ($0) in 1896, claimed to have found a new elementary body, lucium, wbich he patented for use in incandescent gas lighting ($1). The spectrum was reported to be peculiar to lucium only and the atomic weight was reported to be 104. The claim was reportedly substantiated by other investigators. However, one of them ($2) stated that he had published no communications which could justify the assumption that a new element like lucium was present in the substance which had been submitted to him. Crookes (25) tested a quantity of Barriere's lucium and concluded that the claim that lucium was a chemical element was not justified. By looking a t the phosphorescent spectrum, t,he lucium was found to contain didymium, erbium, ytterbium, and mainly yttrium. The ultraviolet spectrum of yttrium and lucium were found to be almost ident,ical. The error in Barriere's chemistry was established by Crookes: Whereas Barriere bad presumed that by adding sodium thiosulfate to a solution, lucium precipitated while yttrium did not, Crookes found that by treating a solution of yttrium with this reagent, a small amount of precipitate formed. When ammonia was added to the filtrate, a large amount of precipitate occurred. The spectra of the two precipitates confirmed they were of the same material, ytt,rium. Boucher's New Element: In 1897, Boucher (24) and Ruddork (25) found what they believed to be a new element in boiler dust, cast iron, and pig iron. After isolating "the pure metal," they determined a number of chemical reactions for the substance, among which were the following: (a) on evaporating a hydrochloric or sulfuric acid solution of the metal to a small volume, a blue rolor resulted; (b) the oxide was almost insoluble in hydrochloric, sulfuric, and nitric acids; (c) the metal plus a stannous chloride solutioi~gave a blue coloration in the cold and a brown color on boiling with hydrochloric acid; (d) boiling a concentrated solution of the metal with sodium sulfite yielded a blue coloration. Jones (26)immediately pointed out that these were all reactions of molybdenum and suggested that molybdenum had been mistaken for a new substance. Although Boucber ($7) insisted that he had tested for molybdenum with negative results, the claim of having isolated a new element has never been accepted (28). Nipponium: The isolat.ion of a new element from
the minerals thorianite, reinite, and molybdenite was asserted in 1909 by Ogawa (29). The provisional name nipponium, with the symbol Np, was used to indicate the new element. The chemical properties of nipponium seemed to correspond fairly closely to those predicted by Mendeleev. The hydroxide had a white color with a pale yellow tinge and was soluble in alkalies. The ignited oxide had a dark brown color, was insoluble in acids, but soluble in water after a potassium hydrogen sulfate fusion. Solution of the metal in hydrochloric acid yielded a yellow-green color, which gave a chocolate brown precipitate on boiling with sodium thiosulfate. The chloride dissolved in water to give a pale green solution. The atomic weight was determined to be about 100, wbich placed it 'between molybdenum and ruthenium. Ogawa reported that there appeared to be two oxidation states, with the lower oxide behaving as a basic oxide and following aluminum hydroxide in its chemistry. At the same time the discovery of nipponium was announced, another claim to the discovery of a new element was made by Evans (SO) and it is generally accepted that both claims referred to the same substance. Evans tried to obtain the X-ray spectrum of the new element, hut found no new lmes. An equivalent weight determination was attempted electrolytically, using silver as a reference, but a good deposit was not obtained and no results could be derived. Neither the claim by Ogawa nor the claim by Evans has been substantiated (28). Element /a: Moseley (SI), in 1913, using the X-ray method of determining atomic numbers, established that the atomic number of molybdenum was 42 and that of mthenium w a 44. Thus, it was shown that the space left in Mendeleev's periodic table for eka-manganese was real and that a definite element should occupy that space-element 43. Neo-molybdenum: Gerber (52) attempted to separate elements 43 and 75 from various molybdenum and tungsten ores, hut without success. He criticized the names, eka- and dwi-manganese on the basis tbat elements 43 and 75 should follow the chemistry of molybdenum and tungsten more closely than the chemistry of manganese. On this basis, in 1917, he proposed the names neo-molybdenum and neo-tungsten for these elements ($3). Moseleyum: Bosanquet and Keeley (54) however, thought that element 43should resemblemanganeseand, in 1924, searched for this element in manganese ores, using X-ray spectroscopy. Molybdenum was used as a reference. Although all their results a t the time were negative, this line of investigation seemed so promising that they believed positive results would he forthcoming a t an early date, and the name moseleyum was proposed for the yet undiscovered element 43, in honor of Moseley who had died shortly before (18). Masurivm:2 However, before Bosanquet and Keeley could pursue their line of investigation further, The name masurium, for element 43, is not to be confused with an alleged dkaline earth element which wns named similarly (38). The name masurium has been severely criticized by Friend (39), who states: "The choice of masurium for element 43 was a stupid psychological blunder which no civilized country would make. It commemorates the crushing defeat inflicted on the Russians by the Germans in the Mesurirtn district . . . "
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Tacke and Noddack (55), with Berg (56) working in the laboratories of Nernst (57), announced the discovery of elements 43 and 75. Element 43 was named masurium, with the symbol Ma, and element 75 mas named rhenium, with the symbol Re. The discovery of masurium and rhenium by the Noddacks (this name will be used to designate thc scientists, W. Noddack and I. Noddack, n6e I. Tacke) was not one of chance, but rather resulted from a logical and methodical search. The Noddacks postulated that platinum ores and certain minerals, notably columbite, should contain eka- and dwi-manganese. From these sources they obtained a residue alleged to contain 0.5% eka-manganese and 5% dwi-manganese. The presence'of these two elements was determined by t,he presence of the Kal, Ka2, and Kpl line of masurium and the La,, La2,Lp,, and Lp2lines of rhenium (86,401. Optical and spark spectra were obtained, and thc Noddacks (41) predict,edt,hat t,helower oxides, to which they assigned the formulas MaO, Maz03,and MaOz, would be dark and insoluble in acid. The higher oxides, MaOa and MazOr, were expected (a) to he light in color and (b) to combine with water to form acids analogous to manganese acids. The highest oxide wa,s predicted to have a melting point of 350-400°C. With referenceto thephysical properties, the anticipated molecular weight was between 98 and 99; the density of the metal, 11.5; and the melting point, about 2300°C. Other investigators attempted to repeat the Noddacks' investigations, but without success (@, 43). Herszfinkiel (44) stated that tungsten and zinc also yielded the same X-ray lines observed by the Noddacks and Berg and that, although the absence of zinc has been demonstrated by the Noddacks, a trace of tungsten had been found by ot.her investigators (45). Herszfinkiel concluded that eka- and dwi-manganese were either very rare or were radioactive and had decayed away, since he had studied many minerals without finding any new elements. The Noddacks sent a preparation reputed to contain masurium and rhenium to Prandtl (42) to test chemically and with his X-ray spectrometer. The results were negative, and the claim of masurium and rhenium was criticized by pointing out that lines of zinc and tungsten were present, which interfered with the lines of rhenium. Prandtl further criticized their work by st,ating that, on Berg's spectrogram, a line of zinc was always blurred (which Berg took as evidence of the presence of rhenium), and by pointing out that Berg had relatively few positive results. Both Berg (46) and the Noddacks (47) replied to t~he criticisms of other scientists. The controversy might have raged for years mere it not for the fact that t,he Noddacks (48) isolated milligram quantities of rhenium. However, as discussed by Hackney (49) and Hardy (60),because of the work by the Noddacks relatively large quantities of rhenium were made available, but the claim of element 43 was never confirmed. Other Attempts: Dolejsek and Heyrovsky (51), while making polarographic studies of manganese salt solutions, noticed a half-wave a t -1.15 volts in the current voltage curves. This was att,ributed to masurium after gathering supporting evidence by X-ray spectroscopy. This claim, however, has never 438
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been verified and is not considered valid. Other investigators have reported feeble alpha activities in hydrochloric-acid insoluble impurities of commercial zinc and have assumed that this possibly could be due to masurium, (52) but no confirmation of this has been reported (49). Additional attempts have been made to concentrate element 43 chemically and detect it by X-ray spectroscopy, but without success (65). Early Theoretical W o r k
In 1926, the atomic weight of element 43 was predicted to be about 98 by Washburn (54). He found that a plot of the difference of the ratio of the atomic weight of each element to that of the next preceding or succeeding zero-group element and the ratio of the corresponding atomic numbers versus the atomic numbers of the elements exhibited a considerable degree of regularity. A Harkins diagram (A-2Z versus Z) of the stable isotopes yielded an almost unbroken sequence, with a few gaps which Brown (55) predicted would be filled by stable isotopes not then discovered. Two of these were the 97 and 99 isotopes of element 43, a result which also had been predicted by other investigators (56,579. In 1934, Mattauch's rule (58) xvas formulated. This is an empirical generalization which states that stable isobar pairs do not exist when they differ by only one charge unit, i.e., when AZ is unity. Although a few exceptions to this rule exist a t present, the general rarity of exceptions has been one of its most substantiating factors. Because of this rule and the total lack of evidence that element 43 had been isolated, it, became generally accepted as an experimentally and theoretically established fact that element 43 was absent in terrestrial sources. Discovery of Technetium by Artiflcial Synthesis
The dat,e of the beginning of knowledge concerning the chemistry of element 43, based on experimental evidence, coincides with that of the first reports by Perrier and Segr6 (59, 60). The discovery of element 43 was a direct consequence of the invention of the cyclotron. Molybdenum, vhen irradiated with deuterons in the Berkeley cyclotron, exhibited a strong, unknown radioactivity. By studying this new activity, it was established that element 43 had been produced by the reactions: M 0 ~ ~ ( d , n ) (60-day 4 3 ~ ~ ~half-life), and M0Q6(d,n)43~'" (90-day half-life)
The absorption and decay curves of radioelement 43 were determined, and the results agree surprisingly well wit,hpresentday values (61) (see Figure 1). With unweighable amounts, observations were made on the chemical behavior of element 43. They found that all but two chemical reactions bore a close resemblance to those of rhenium: (a) when moist hydrogen chloride gas is passed through an 80% sulfuric acid solution for ll/zhours a t a temperature of 180-20O0, all rhenium appeared in the distillate and the activity remained in the residue; (b) in 10 M hydrochloric acid solutions, rhenium would precipitate as the sulfide, whereas element 43 would not. However, element 43 was carried by a precipitate of rhenium (VII) sulfide on
addition of hydrogen sulfide to hydrochloric acid solutions up to 6 M. They found that the stability of the VII oxidation state was greater than that for manganese and less than that for rhenium. Various other chemical properties of element 43 also were described hy Perrier and Segr6. In 1946, Perrier and Segr6 (65') proposed the name technetium, with the symbol Tc, for element 43. The name, technetium, was derived after a Greek work which signifies "artificial." This was appropriate for the first artificially produced element. The claim of the discovery and the proposed name have been accepted officially since 1946 (6.9). Jensen (64) in 1938 presented the following disrussion, from which he predicted that technetium would possess no stable isotopes: (a) according to Mattauch's rule, element 43 should have no stable isotopes; (6) even if one considers that there are no stable isotopes of technetium, it might he postulated that a t least one hct:~:wtive isorope would haw n hdf-life so grwt th:,r i t vonld he rc.mnld 3s t~~sentiallv tuhle for :]I1 ornvtivnl purposes (however, Jensen stated that for this to be true, the nuclear masses of the technetium and daughter isotopes would have to be almost identical, and he considered this to be very implausible) ; (c) considering nuclei of odd mass numbers only, it is apparent that one goes from one stable nucleus of odd mass to the next stable odd isotope by adding either a neutron and a proton, or two neutrons. An exception to this process apparently occurs a t molybdenum (as predicted by Mattauch's rule) where the transition to the next stable odd nucleus occurs by the addition of two protons and, accordingly, technetium is passed by. By plotting
-
I
=
- 0.7]/[A411
+ 1281)
versus A , Kowarski (6.5) amplified part (c) of Jenscn's discussion. I t was indicated that technetium was naturally unstable because of the tendency to eliminate stable isotopes of odd Z in the region where the nuclei have neutrons in an open shell. Suess (66) has explained this more fully on the basis of present-day knowledge. In proceeding through
Figure 1. (01 Absorption curve of rodiotion from rodioelement43. Each unit of the abwirsa corresponds to 1 0 mg/cm2 Al. Ibl Decoy curve of the activity of rodioirotoper of element 43. Each unit of the abscissa corresponds to 20 days.
this discussion, it must be kept in mind that: (a) in the region of Z = 43 and N = 50, the pairing energy for two protons is greater t.han the pairing energy for t,wo neutrons, i.e., protons are paired in preference to neutrons; (b) if a stable isotope of t,echnetium exists, it is probably one with even N, since 2 is odd. Suess stated that the small pairing energy for neutrons with N values just beyond N = 50 explains the instability of all isotopes of technetium. An example of this is U T C Mwhich ~ ~ , undergoes negatron emission to aRurs9s. This transition produces a change of neutrnn excess from 13 to 11, but even more important is the fact that a neutron pair is broken to form a proton pair. The preference of proton pairing over neutron pairing is responsible for the instability of technetium-99. Similar arguments can he made for other technetium isotopes. After 1940, because of the t,heoretical predictions regarding the nonexistence of stable technetium isotopes which apparently had been verified by experimental results, coupled with the great interest in uranium fission and the fission products, most of the studies concerning technetium were turned to the artificial production and identification of the various technetium isotopes. Twenty-one isotopes of technetium are known today, ten of which are isomeric states. The means of producing these various isotopes, their modes of decay, their half-lives, and their complete references, have heen summarized by Anders (1) and Kenna (67). Technetium in Stars
In 1932, Merrill (68), while making spectroscopic observations of vari2us stars, detected fairly strong Tc(1) lines a t 4031 A, 4238 A, 4262 A, and 4297 A8, I t appeared that the technetium lines were stronger in the stars with the more dominant s-type characteristics which suggested that t,he Class S stars represented a comparatively transient phase of stellar existence. More recently, technetium lines have been observed in N-type stars (69). The observations by Merrill allowed numerous previously unidentified lines in R Andromedae to be assigned to Tc(1). Moreover, this discovery stimulated renewed speculation by astrophysicists concerning stellar processes. Moore (70) had reported previously the presence of technetium lines in sun spectra. However, this was proven to be erroneous (71,75'). Jordan (73) has described technetium as "the touchstone of cosmological theories" insofar as any theory must be able to account for the production or existence of technetium in the stars. He also has pointed out that there are three alternatives possible to explain the observed presence of technetium in stellar matter: (a) discovery of a hitherto unknown stable isotope. This would be a surprising and great discovery since it would be a prime example of a violation of Mattanch's rule. Moreover, it would be difficult to understand, since a stahle isotope of technetium has not been found to occur in terrestrial matter. (b) Technetium-99 is being formed continuously in stars (it was pointed out that, although there have been explanations set forth for the conversion of hydrogen into helium Readers are referred t o Figure 2 of the article hy Bovn, G. E., in mrs JonnNar., 36, 4 (1959). Volume 39, Number
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and the consumption of lithium, beryllium, and boron, there is no knowledge of how the synthesis of heavier elements, such as technetium might occur). (c) The process of element formation could be understood very well by considering the ylem t,heory, with the one modification that this process occurred not only in the beginning of the cosmos, but is a continuous phenomena. I.e., not only technetium, hut all the elements are being formed continuously in the universe by some process which produces the heavier elements from the lighter elements. Operating on the assumption that Jordan's second alternative was correct, Nahimas (74) postulated that certain elements were formed in stars by a closed cycle in which neutrinos played a major role. Nuclides such as technetium-99 were said to arise from processes such as: Ru"
+
++
v = Tcaa @+, and TcM= RuPQ 8- f s
Others, however, adopted the third alternative given by Jordan and derived a theory of continuous synthesis of the elements in the universe, which is accepted presently to be the best explanation of the known facts (75-79). According to this theory, technetium-99 occurs as a product of neut,ron-capture processes which other nuclides undergo.& One of the major neutron-capture processes postulated by the continuous synthesis theory is the s-process or slow-time-scale neutron-capture process. During the evolution of a star, there is a point a t which the helium core expands and mixing of the hydrogen in the envelope occurs. When the core recontracts, t,he central temperature rises sufficiently to support (e, n) reactions, which releases large numbers of neutrons within the stellar core. These neutrons are released iuto essentially a helium moderator which slows them down, t,hereby permittiug (n,a) reactions to proceed. The neutrons are captured by nuclei in proportion to their capture cross sections. This process is known as the s-process. The products of the s-process will lie on a path close to the center of the stable valley of the nuclear mass surface. Molybdenum-98 lies on this main capture path; nentrou capture leads to molybdenum-99 (half-life,67 hours). This, then, decays quickly into technetium-99 which has a half-life of 2.12 X lo5 years: thus, technetium-99 may be considered to be a "stable" member of this capture path. Hence, during the time that the s-process is operative, technetium-99 is maintained in abundance and would be observable in the star's spectrum. This process would also explain why no technetium is present in our own sun. Because our sun is in a comparatively early stage of development, the s-process is not occurring and, therefore, no technetium is being produced. Recent Attempts To Isolate
The theories and work in the fields of nucleosynthesis and cosmochemistry reawakened the interest in seeking naturally occurring technetium. It is known now that the half-lives of all known technetium isotopes are less According to Cameron in The Ast~ophy8ieal Jowmal, 130, 452 (1959), the technetium isotope responsible for the presence of Tc lines in S spectra. is probably Tc9' rather than Tcse.
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than lo8years, which prevents them from being present in terrestrial materials as primordial technetium. However, before 1958, the half-life of technetium-9s (2.6 X lo6years) (80-8g) was not known. Therefore, before this time, it was thought that technetium-98 might be present on the earth as primordial technetium. There were several notable attempts to isolate primordial technetium. The first of these was the work of Herr (83) in 1933. A number of rhenium-rich minerals from various locales of Norway and South Africa were decomposed with fuming nitric acid. Repeated distillation of perrhenate and pertechnetate from perchloric acid and sulfide precipitations from 9 M hydrochloric acid solutions to remove rhenium were performed to obtain purified technetium. Copper(11) sulfide was used to carry any technetium present in a final sulfide precipitation, and the precipitate was exposed to an intensive neutron bombardment with the expected reaction being IT
T C ~ ( ~ , ~ ) TL C " " Tea" 6.0 hr.
However, it was found that the decay curve did not follow the 6.0-hour technetium-99m decay, but. a rhenium-186 activity appeared which showed that further chemical purification was necessary. Alperovitch and Miller, (84-86) in 1955, reported the natural occurrence of primordial technetium-98 in various minerals such as columhite. A comhinatiou of precipitations, distillation, and ion-exchange proredures were employed to obtain any technetium present. After irradiation, the gamma radiation of the 6.0-hour technetium-99m was counted. Positive results were recorded for many of the samples. I t must be pointed out that there are several isotopes which possibly could interfere with the production and identification of technet,ium-99m from technetium98 by activation analysis, in that the interfering isotopes will also produce technetium-99m. These isotopes include MoQ8,TcS9,and R.uYS. Thus, if these nuclides are present, results indicating the presence of technetium-98 would be invalidated. Later work by Anders, Sen Sarma, and Kato (87) appeared to support the previous work of Alperovitch and Miller. In this later work, the technetium fraction was separated and purified from various minerals in a manner similar to that used by Alperovitch and Miller. To circumvent the danger of molybdenum contamiuation, a knowu amonnt of inavt,ive molybdenum carrier was added to the sample after irradiation. After separating technetium from molybdenum, the 140-kev gamma ray of technetium-99m was counted in both samples. If the growth from molybdenum-99 were the only source of technetium-99m, the ratio (the activity of technetium fraction versus the actirity of molybdenum fraction, both extrapolated to time of separation and corrected for chemical yield) was expected to have been 0.96. In about half the samples studied, the ratio gave positive indications of the presence of technetium-98. Shortly after this work, however, Boyd and Larson (88) found that the half-life of technetium-9s was less than lo8 years, which excluded the presence of primordial technetium-98 in terrestrial materials. Moreover, a search was made for thisisotopein anumber
of concentrates and minerals by these investigators, with negative results. Technetium was separated by using a combination of precipitation and ion-exchange methods. Spectrochemical, spectrophotometric, polarographic, mass spectrometric, and activation analyses were performed. Out of 20 samples, only 2 yielded positive results. It was shown later that one of these was contaminated by technetium-99, while the other apparently contained small quantities of rhenium. It was concluded that technetium was absent, or else, if present, it occurred in amounts below those which could be detected by the means employed (less than lo-" grams). It was stated that, because there are three technetium isotopes which are long lived and which may occur as contaminants in terrestrial substances, it is essential that proof he given that any primordial technetium claimed to have been discovered is not technetium-97g, technetium-98, or technetium-99g. The latest attempts to isolate naturally occurring technetium have not been concerned primarily with primordial material. If it is accepted that previous investigations have proven that no detectable amounts of primordial technetium exist within the earth system, one must look for the natural processes which could be producing technetium in nature. As discussed by Kenna (67),it may be concluded that the primary process which could produce technetium is the fission process, with subsequent decay of the short-lived mass 99 nuclides into technetium-99g. Strong evidence for this conclusion is provided by the work of Parker and Kuroda (89, 90) who found lo-'& curie of spontaneous fission-produced molybdenum-99 per gram of uranium238. Thus, technetium-99 also would be expected. Kuroda and co-workers (90-94) have recently reported the equilibrium ratios of a number of shorG lived fission products to uranium-238 in natural and depleted uranium salts. The results are sufficient to show the general shape of the mass-yield for spontaneous fission of uranium-238 (see Fig. 2).
MAS
NUMBER
Figure 2. Equilibrium ratiosof the flsion products/U-238 in non-irradiated natural and depleted uranium mltr. IAvalue of 2 for the number of rpontoneous Rsioo neutrons m i n e d w m used M plot the mirror poinb.1
If, for the moment, it is assumed that technetium-99 is produced predominantly in the ore by the spontaneous fission of uranium-238, then
where Ng9 and NZS8are the number of atoms of technetium-99 and uranium-238, respectively; X238i and Xss are the spontaneous fission decay constant of uranium-238 and the decay constant of technetium-99; and y99 is the fission yield for the mass 99 chain. From Figure 2, y~ is approximately 6'%. Substituting this and other numerical values into equation (I), it is easily seen that there should be about 2.5 X 10-lo g of technetium-99 (10.5 disintegrations per minute) per kilogram of pitchblende (50% uranium). Contribution from the slow neutron-induced fission of uranium-235 appears to be negligibly small in the case of nonirradiated natural uranium salts, but in the case of uranium minerals, the contribution from the induced fission cannot be totally neglected, as has been demonstrated recently by Kuroda (95) and others (94). However, Kenna and Kuroda (96) have estimated that the induced fission contribution in Katanga pitchblende is of the order of 25% of the total fission contribution or about 2.6 disintegrations per minute of technetium-99 per kilogram of pitchblende. Parker (97) sought to isolat,e the naturally occurring fission product, technetium-99g, from pitchblende ores. After dissolution of the ore in nitric acid, a combination of precipitation and distillation techniques were used to obtain a technetium fraction. Arather large amount of rhenium was used as carrier for the technetium. Tetraphenylarsonium perrhenate was precipitated to concentrate the technetium. The counting results were negative, which indicated that either no technetium was present or the sample was so thick that all weak beta rays of the techneti~~m were absorbed. The first isolation and identification of naturally occurring technetium was reported recently by Kenna and Kuroda (98). They dissolved kilogram quantities of pitchblende in nit,ric acid and employed specific precipitations, solvent extraction, and ion-exchange procedures to obtain the naturally occurring "fissiogenic" technetium-99g. Three to four milligrams of copper(I1) were used to coprecipitate technetium (VII) as the sulfide prior to counting. Three samples of pitchblende yielded positive and compatible results. A total of about 1 mp of technebium-99g was isolated from 5.3 kg of African pitchblende. There was no indication of a stable or extremely long-lived isotope. Almost one and a half centuries have transpired since the beginning of the search for naturally occurring terrestrial technetium to the actual isolation of this material. I n order to achieve this, the efforts of many scientists in the fields of nuclear physics and nuclear chemistry, physics, chemistry, astronomy, astrophysics, and cosmo- and geochemistry were required. And yet the problem may not be completely solved. Anders (8f) has suggested that the known technetium-98 isotope (2.6 X 106 years) might be only an isomeric state. If this is true, then it is possible there does exist primordial technetium on the earth. However, in view of all previous work, the isolation and identification of this isotope will have to wait until more sensitive Volume 39, Number 9, Sepfember 1962
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methods of detection are available. This also serves to point out a well-known fact-the complete or partial solution of one problemonly gives rise to other problems.
(46) BERG,O., Z. angew. chern., 40,254 (1927); C. A., 21, 17509 (19271.
Acknowledgment
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