Figure 1. Modern Periodic Chart of the Elements
The Transuranium Elements WILLIAM Q. HULL, Associate Editor
The discovery of elements b e y o n d uranium has o p e n e d w h o l e n e w fields of scientific concepts a n d investigation with a resultant b r o a d e n i n g of all scientific k n o w l e d g e U NTIL the past decade and a half, authors of textbooks of chemistry were not in the least reluctant to state that uranium, with an atomic number of 92, filled in the last space in the periodic table. They predicted that, in all probability, there remained no undiscovered elements. Since 1940, however, periodic charts have been under constant revision, and, at the present time the list of known chemical elements totals 98. Discovery of elements 93 through 98, the transuranium elements, is accredited to two University of California scientists, Edwin Mattison McMillan and Glenn Theodore Seaborg, and their associates. Their combined discoveries opened the doors to a great unknown and led to science's highest accolade, the Nobel prize in chemistry, 232
which was jointly awarded to them last month in Stockholm, Sweden. Early Claims of Transuranium Elements Investigation of nuclear fission was the starting point which eventually led to the discovery of elements heavier than uranium. In 1934 in Italy, Enrico Fermi placed a small piece of radium and beryllium against some oxide of uranium in an experiment, crude when compared with techniques of our modern cyclotron era, but showing the imagination of a genius. Some of the neutrons emitted from the radium-beryllium source were lodged in the nuclei of uranium, and it was later recognized that these atoms became an isotope of atomic weight 239. The new form of uranium showed beta ray emisCHEMICAL
sion and it looked as though the uranium isotope was converting itself into a new element, one beyond uranium in the periodic table. Chemical analyses showed that the new material had properties similar to manganese and quite different from those of uranium, and Fermi thought he was dealing with transuranium elements as did others who' followed him. Announcements of the discovery of elements number 93, 94, 95, and 96 ensued. But it was only a short period before the total number of building blocks of matter reverted to the long established 92. The work of O. Hahn, F . Strassmann, and Lise Meitner in 1938 and 1939 showed that chemists had not been creating elements heavier than uranium after all—instead, they h a d been splitting uranium 235 into AND
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two parts; and Fermi, rather than finding the first of the transuranium elements, had actually split uranium into elements like lanthanum, element number 57, and bromine, number 35. At the very time that Hahn and Strassmann made their announcement, several scientists in this country, including Philip H. Abelson who later worked with McMillan, as well as foreign scientists were on the verge of discovering fission. All of their work indicated that the bulk of material resulting from the neutron bombardment of uranium was fission products, and though element 93 may have been present, it was not identified. Once more nothing existed beyond uranium. It was at about this time that Edwin M. McMillan prepared for his first fission experiment in a crowded laboratory at the University of California. Elusive Atoms The young scientist was not looking for a new element when he sprinkled uranium oxide on a leaf of thin, French-made cigarette paper, added several layers of the same tissue on top and bottom, and exposed the sandwich to a stream of neutrons from the 60-inch cyclotron at Berkeley. Instead, he was interested in determining the range of the fission products from the uranium 235 atom. After bombardment, the fragments were lodged in the several layers of paper and McMillan accomplished the purpose of his experiment by determining that the range of the atomic fragments was equivalent to 2.3 centimeters of air. He also identified V O L U M E
3 0,
NO.
3
*
uranium 239, which Hahn h a d found a few >^ears previously, and the half-life of which tiad been established as 23 minutes. But McMillan also found an unexplained radioactivity with a half-life of 2.3 days in the target material, and which did not travel into the parper stack. Chemical studies seemed to identify it as one of the r a r e earths. McMillan concluded that he h a d uranium fission only and published his conclusions to that effect in the same year. However, the young physicist was dissatisfied in not identifying the unexplained half-life of 2.3 d -s. He theorized that uranium atoms ^. atomic weight 238 should, by absorption of neutrons, convert themselves into uranium 239, and that this isotope, upon decay, would produce a new element to occupy space 93 in the periodic table. In the spring of 1940, he went back t o the experiment. This time, McMillan painted the surface of a small Sat piece of Bakelite with ammonium uranate and bombarded it with neutrons from the cyclotron. He expected to find the n e w element in the layer of ammonium uranate; however, his chemical separations showed a material that sometimes acted like a rare earth and sometimes not-and he could not make a definite identification. Over and over again, the experiment was repeated, using first one cliemic-al reagent and then another, for McMillan was convinced that element number 93 'was there. However, time and time again, he was unable to identify it. Identification of First Transuranium Element Shortly afterwards, McMillan and Abelson, who had been attempting to find element 93 at: the Carnegie Institution of Washington, compared their independent work while the latter was vacationing in California. They decided to team up and work on the problem together, McMillan preparing trie samples and conducting the bombardments and Abelson performing the chemical separations. A week later, McMillan and Abelson were able to show, on the basis of their chemical and physical experiments, that the beta-particle emitting radioactivity of 2.3 days half-life formeel during the irradiation of uranium with neutrons was definitely due to the presence of a new element with an atomic number of 9 3 . Long before, McMillan had decided that the new element should be called neptunium after Neptune, the planet immediately beyond Uranus, for which uranium was named. In announcing their discovery, McMillan a n d Abelson observed that neptunium gave off beta particles—which meant that it was decaying into a still higher element. McMillan xvas confident that the second element existed and set out to find it. At this point, however, he was called by the Govcranient to assist the radar development program at Massachusetts Institute o f Technology, and he was not able to go farther in identifying the second new element wHich he thought to exist. Glenn T. Seaborg, A. C. Wahl, and J. W. Ken-
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2 1,
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nedy continued the search, and, late in 1940, element 94 was discovered as a result of their work. These workers reasoned that plutonium 2.39 might be too long-lived to observe or was otherwise undetectable and tried to prepare another isotope. In so doing they discovered a new isotope of neptunium which decayed into a new element, which they dubbed "plutonium." Since that time, additional new elements, including those with atomic numbers of 95, 96, 97, and 98 have been identified by Seaborg and his coworkers and much has been learned of the chemistry of these transuranium elements. Heavy Elements as an Aetinide Series Before looking further into the discovery and formation of the individual transuranium elements, it is interesting to consider the position of these elements in the periodic system. Prior to the discovery of neptunium, there had for some time been some uncertainty concerning the position of the elements following actinium in the periodic table. More generally, it was thought that these elements were analogous to preceding elements in which the rf-electron shell is being filled. As early as 1920, the work of Niels Bohr cast some light on some of these uncertainties. His work, among other things helped explain the chemical similarity between the rare earth elements. In this group, it is not the outermost electronic shell that is developed, nor the shell beneath it, but the one that underlies, the so-called 4/ subshell. This contrasts to addition of electrons to the outermost shell of the atoms of lower elements, which accounts for the unit by unit increase in positive nucleus charge for each step u p ward in the element series. Thus, in the rare earth elements, the exterior part of the atomic structure remains virtually unchanged, whereas in lighter elements, the chemical characteristics, which depend on the structure of the atom in the outermost shell, permit the successive elements, for the most part, to b e clearly distinguished from one another in respect to their chemical properties. The rare earth elements are thus a group of what have been called "quasi-isotopes" and because they are like lanthanum, the first element in the series, they were called lanthanides. Bohr reasoned that, if there existed an extension of elements beyond uranium, these would form a new series of closely associated elements which would all resemble uranium and, by analogy with the lanthanides, form a series of uranides. This prophesy that in the transuranium elements there is a group of substances of the same sort as the rare earth metals has been confirmed. However, investigation of the transuranium elements discovered by Seaborg, McMillan, and associates have indicated that the new series begins with actinium, element number 89, rather than uranium, and, corresponding to the lanthanides, the aetinide series has been proposed. A certain agreement is found between every member in the two series. Studies of the chemistry of neptunium, 233
and later of the other transuranium elements have shown that in the actinide series, the main trend is that electrons are added to the 5/ rather than to the 6d subshell. All available evidence has substantiated the existence of the actinide series, including investigations of chemical properties, absorption spectra in aqueous solution and crystals, cry stallo graphic structure data, and spectroscopic data. The actinides differ from t h e rare earth series in having more oxidation states above the III state, as well as in other ways, all of which are connected with the lower energy of 5 / compared to 4 / electrons. Of utmost importance is the fact that the characteristic oxidation state is the III state, and the group is placed in the periodic system on this basis. It follows that the discovery and identification of the transuranium elements and the examination and establishment of their chemical properties have led to a number of modifications in the periodic* table. A modern periodic chart, which shows the heavy elements as an actinide series, is shown in Figure 1. Data resulting from the extensive studies have also made it possible to show the probable electronic configuration for gaseous atoms of the actinides. Investigations of the systematica of alpha radioactivity have gone far toward completing the picture of stability of heavy nuclei. It has been found that alphadecay energies fall into a pattern that allows one to predict accurately values for unknown species, and, in general, gives a coherent picture of nuclear stability in the heavy element region. For example, plotting of alpha energy, which can be accurately measured, against mass numbers for the known isotopes of each element provides curves from which, by interpolation and extrapolation, the energies of unknown isotopes and elements can be
T a b l e 1 . E l e c t r o n i c C o n f i g u r a t i o n s of t h e T r a n s u r a n i u m Elements 1 Element
s
2 s p
spd
3
Neptunium Plutonium Americium Curium Berkellum Californium
2 2 2 2 2 2
2,6 2,6 2,6 2,6 2,6 2,6
2,6,10 2,6,10 2,6,10 2,6,10 2,6,10 2,6,10
estimated, and their half-lives calculated. This, a s w e l l as other correlations, h a v e been valuable tools in transuranium research. As a result, many of the properties of the elements were known before their identification which meant that the duration o£ bombardment and time which could b e spent o n their chemical separation were understood i n advance. The impressive range in stability of the known isotopes of t h e transuranium elements i s shown in Table II. The Transuranium Elements Now Total Six Four additional transuranium elements —americium, curium, berkelium, and californium-followed the discoveries of neptunium and plutonium. Excepting plutoniurn, none exists in appreciable concentrations in nature, and from the properties of these artificially prepared elements, it is known that those which may have been present earlier must essentially have disappeared fairly early in the history of the earth. The first ol the transuranium elements, neptunium, symbol N p , was discovered b y McMillan and Abelson in 1940. It has an atomic number of 9 3 and resulted from the bombardment of uranium with neutrons from the 60-inch cyclotron. First, a
5
4
sp d
spdf
6
f|
PI
2,6,10,14 2,6,10,14 2,6,10,14 2,6,10,14 2,6,10,14 2,6,10,14
2,6,1 0, 2,6,10, $31 2,6,10, §6§ 2,6,10, 2,6,10, 2,6,10, &£
i
7
spd
5
2,6,1 2,6,1 2,6,1 2,6,1 2,6,1 2,6,1
2 2 2 2 2 2
new isotope of uranium is formed by radiative neutron capture in the uranium2 3 8 atom, according to the reaction: U233 +
n - » U239 +
7-radiation
T h e n e w isotope of uranium is much more unstable than natural uranium, having a half-life of only 2 3 minutes. Its spontaneous disintegration through loss of beta particles leads to formation of neptunium : MU
S
£KNP*
This isotope of neptunium has a halflife of 2.3 days. A second isotope was discovered late in 1940 by Seaborg, McMillan, Kennedy, and Wahl. Uranium oxide was bombarded with fast deuterons from the 60-inch cyclotron: 92U238 + xH2 - »
MNP:I3»
+
2n
Neptunium 238 is beta active and has a half-life of 2.0 days. Both of these isotopes are far too unstable for properties of neptunium to be observed by methods other than tracer techniques. In 1942 Wahl and Seaborg identified a third and much more stable isotope, which permitted much to be learned about the new element. Uranium was exposed to fast neutrons and the beta active isotope was formed: ^U 238 + n - » ssU2*7 +
Five of the six transuranium elements were first produced in t h e 60-inch cyclotron at the University of California. Shielding used when in operation is not shown
2n
Uranium 237 yields neptunium 237 as its beta decay product: MU
2
0.Np 21 6.8 davs
Neptunium 237 is an alpha emitter with a half-life of 2.2 X 106 years. It is the longest lived of any of the isotopes of the six transuranium elements and, in a uranium-graphite pile, is produced at a rate corresponding to the order of 0 . 1 % of that of the primary product, plutonium 239. Neptunium is a silvery metal, not particularly affected by air, and has a density of 17.7 grams per cubic centimeter. Early work in the investigation of its chemistry was done with the unstable isotopes using tracer techniques. With the synthesis of neptunium 237, however, and its availability in weighable amounts as a product of the operation of the chain reacting units at Hanford and Oak Ridge, later investigations have been on the ultramicrochemical scale and then on the milligram scale as larger amounts became available. Neptunium was first isolated as a pure 234
C H E M I C A L
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compound by L. B. Magnusson and T. J. LaChapella at the Metallurgical Laboratory of the University of Chicago in 1944. These workers, through a series of reactions, isolated approximately 10 micrograms of pure neptunium dioxide, Np(\>, from the material secured from the bombardment of uranium with fast neutrons. Elemental neptunium has been prepared by heating the trifluoride to 1200° C. in the i>resence of barium vapors. From a viewpoint of health, neptunium 237 is relatively safe to handle because of its low specific activity of 1.5 million alpha particles per minute per milligram. This is only about one thousand times that of ordinary uranium, and material of this level of radioactivity can be handled without special equipment provided reasonable care and precautions are observed. All of the other transuranium isotopes are so highly alpha radioactive that special techniques and precautions are mandatory when they are handled in as much as microgram amounts. A total of 10 isotopes of neptunium have now been identified and are listed with their properties in Table II. Plutonium, the second transuranium element to be identified, was discovered by Seaborg, McMillan, Wahl, and Kennedy at the University of California late in 1940. It was named after the planet, Pluto, in keeping with the convention used in naming neptunium, and assigned the symbol, Pu. The first isotope of mass 238 was formed by the deuteron bombardment of uranium. First, neptunium 2 3 8 is formed as previously described, and this isotope decays to an alpha-emitting isotope of element 94: 0-
*Np 2
KPU*
2.0 days Plutonium 238 is an alpha emitter with a half-life of approximately 9 0 years. The chemistry of plutonium was first investigated using this isotope and tracer techniques. Plutonium 239—discovery by Kennedy, Seaborg, E. Segre, and Wahl—is the isotope of principal importance because of its property of being fissionable with slow neutrons, which led to its use as the explosive ingredient for the atomic bomb. It is an alpha emitter and has a half-life of approximately 24,400 years. In the uranium-graphite chain reacting units, this isotope is formed as a result of many complex reactions, the following two being of principal importance: U235 + n —> fission products -+• neutrons -f energy The energy of the fast neutrons liberated is reduced in the uranium lattice structure of the pile, and the neutrons return from the graphite to the uranium. After a proper proportion undergoes the above reaction, perpetuating the chain reaction, the majority of the neutrons remaining form plutonium 2 3 9 : Ua V O L U M E
u* 3 0,
Np 239 - » P u * NO.
3
.
.
239 was bombarded with neutrons leadUranium 237, which decays into neping to the formation of plutonium 2 4 1 : tunium 237, is also formed in the chain reacting units. Separation of plutonium Pua Pu239 -f- n •-» Pu'-"0 -f- n 239 from unchanged uranium and from numerous fission products has been acPlutonium 241 yields americium 241 as complished by processes making use of a result of beta particle emission: precipitation reactions. T h e first pure chemical compound of 0" wArn2" wPu1"3 plutonium free from carrier material and 14 y e a r s other foreign matter was prepared by B. A m e r i c i u m 2 4 1 h a s a half-life o f 4 7 5 B. Cunningham and L. B. Werner in years a n d is a n alpha p a r t i c l e e m i t t e r . Chicago in 1942, and, by the end of that E a r l y i n v e s t i g a t i o n of t h e c h e m i s t r y of year, a few hundred micrograms of plutoamericium w a s conducted with t h e 241 nium 239 were available and the chemisi s o t o p e i n tracer c o n c e n t r a t i o n , b u t as a try o f the element was examined on the result of t h e isolation of p u r e a m e r i c i u m ultramicrochemical scale. c o m p o u n d s b y C u n n i n g h a m i n 1945—46, T h e specific alpha radioactivity of plus t u d i e s w e r e m a d e first o n t h e u l t r a m i c r o tonium 239 is 140,000,000 alpha disintec h e m i c a l s c a l e a n d n o w t h e y are b e i n g grations per milligram per minute, requirm a d e o n t h e m i l l i g r a m scale. ing the use of special equipment and preA m e r i o i u m h a s a specific a l p h a a c t i v i t y cautions in the investigation of its properof s o m e 7 billion a l p h a disintegrations ties. Though more available than the odier heavy elements, plutonium may not find per m i n u t e p e r m i l l i g r a m , w h i c h is h i g h e r widespread distribution among research t h a n e v e n r a d i u m . T h o s e i n v e s t i g a t i n g its properties s h o u l d b e w e l l t r a i n e d i n h a n laboratories because of its high alpha d l i n g h i g h l y alpha a c t i v e m a t e r i a l s a n d radioactivity. special techniques a n d equipment must b e Investigations made by Seaborg and M. used at all times. L. Perlman indicate that plutonium is The element does not occur in nature. found in pitchblende in the order of 1 part However, eight isotopes, with half-lives per 10M parts of uranium and later work has shown this estimate to b e somewhat ranging from approximately 2 5 minutes to 10* years, have been identified. conservative, some uranium ores containSeaborg, James, and A. Ghiorso identiing as much as 1 part in 1012. This plutonium is not primeval but is fied the fourth transuranium element in 1944 before the discovery of americium. made and maintained in a steady state concentration in uranium by a small but They named it after Marie and Pierre steady supply of neutrons originating from Curie, pioneers in the study of radioalpha-particle induced nuclear reactions chemistry, corresponding to the naming of gadolinium, its rare-earth homolog, and spontaneous fission of uranium. Concentrations of this order preclude the Figure 2. Many of the properties of the transuranium elements are strikingly similar to those of their homologs in t h e rare use of uranium minearth series. On this basis the elutions of elements 99 and 100 erals as a practical can b e predicted with accuracy source for the production of plutonium. RELATIVE E L U T I O N OF HOMOLOGOUS The 11 known A C T I N I 0 E S AND RARE EARTHS isotopes of plutonium are listed in Table II. Element number 95 was named after America b y analogy with its rare-earth homolog, Europium, the namesake of Europe. The former has the symbol, Am. While third in the transuranium family, it was fourth to be identified, Seaborg, R. A. James, and L. O. Morgan making the discovery late in 1944 at the Metallurgical Laboratory, University of Chicago. T h e first identified isotope of americium was one of a mass of 241. Two reactions were involved. Plutonium
JANUARY
2 1,
1952
-.-,* •'OR.OPS'OF'eLUTklANf ^{fREfe COLUMN VOLUME SUBTRACTED* .
235
after the Finnish chemist, J. Gadolin, w h o was an early investigator of the rare earths. Curium has an atomic n u m b e r of 96 and symbol, Cm. Curium was first pro d u c e d by the bombardment of plutonium 239 with alpha particles: ,oCm3li +
wPu330 -h J l e 4
η
This isotope is an alpha particle emitter with a half-life of about 160 days. It has also been formed by the strong neutron irradiation of americium 2 4 1 : Am342 +
Anr"
T h e short-lived isotope decays to form curium: 95 A m
7-radiation of
americium
B-
5
,Am 3 1 1 - f
16 hours From the latter source, I. Pcrlman and Werner obtained pure curium compounds in 1947, marking the first isolation of a carrier free compound of t h e element. Table II. Summary of I s o t o p e s of Tran suranium Ele ments Isotope
Half-life
Radiations
Neptunium Xp«' N|»9«
\'|.2" \'p234 X r S3R X , 2V
Xp»Np»8 Xp™ 9 Xp«>
-.->0
iiiin
~ 1 3 min 3 o min 4.40 dav 4 1 0 dav" 22 hr 2 L>0 X 1 0* \ r 2 1 0 dav 2.M d a v 0 0 min
or
i-:r
KC. « K C . ex KC. ct K C . 0~ or
0~ 00-
Plutonium Pu»" P U 234 p,,235 p„23« p„237 p„238 p,,239 p„240 p.,24! Pu242 Pw243
3 6 min 0 . 0 hr 2 0 min 2 . 7 vr ~ 4 0 day 9 0 vr 24.400 vr 0 0 0 0 vr 14 vr "> X 10* vr .-, 0 hr
K(\ KC.
« «
r-:r,
a
or
EC' or or a
0 •«
or
0-
All.219 Al!|2>0
A m "2 '4 2
A 111 A m 24 2'"
Am " ' Am2"
1 . 2 hr 12 hr o0 hr 47."> vr - 1 0 0 vr 1.V7 hr - 1 0 4 yr — 2") min
KC KC. KC
a
or
0 -. a 0 ', E C . I T o-
0~
Curium Cm*** Cm2" • ("11)241 C 11)242
Cm""* Cm***
2..') hr 20.8 dav 3 o rlav 102 ri day - 1 00 vr - 1 0 yr
EC a a a
a
4 fi hr 4 .05 d a y
EC. EC.
a a
4 5 min 3 5 . 7 hr
a. E C ? ex
or or
Berkelium Bk-2*3 Bk246
Californium Cf244
Cf246
a—alpha p a r t:icle. EC—electron capture. IT—isomeric transition. 0 ~—beta particle
Many difficulties are encountered in studying curium as a result of the high alpha activity, corresponding to 1013 alpha disintegrations per minute per milligram. In the w a t e r of solution occur decomposi tion, evolution of gas, and heating effects. Too, all work must he performed in closed 236
2 He
4
- * ^Bk^ 3 -f 2n
Berkelium 243 has a half-life of 4.6 hours a n d decays chiefly by orbital-elec tron capture a n d to a less degree b y alpha particle emission. The element has not of course b e e n isolated in weighable amounts, and all investigations of its p r o p erties h a v e b e e n conducted through tracer techniques. However, it has been sepa rated from americium a n d curium by t h e ion-exchange method a n d exhibits proper ties similar to its rare-earth homolog, ytterbium. A second isotope, mass of 2 4 5 and halflife of 4.95 days, has b e e n identified. T h e most recent of the transuranium elements k n o w n to date was n a m e d cali fornium, honoring the state a n d university where it w a s discovered b y Thompson, K. Street, J r . , Ghiorso, and Seaborg in F e b r u a r y 1950. There is no direct analogy with the n a m i n g of its homolog, dyspro sium. Californium has a n atomic n u m b e r 98 and is represented b y the symbol, Cf. T h e first of its isotopes, with a mass of 244, was a result of the b o m b a r d m e n t of curium with rielium ions: ™CmS12 -h 2 He J
Americium Ani»»
systems because of the serious h a z a r d to health. Six isotopes of curium have b e e n identified at the present time a n d are listed in T a b l e II. Berkelium, symbol Bk, atomic n u m b e r 97, was discovered by S. G. Thompson, Ghiorso, and Seaborg at the University of California in 1949. It was n a m e d after the city of Berkeley, site of t h e university, analogous to the n a m i n g of its homolog, ytterbium after Ytterby, Sweden, w h e r e several rare earth minerals h a v e b e e n found. Berkelium -was o b t a i n e d a s a result of the b o m b a r d m e n t of americium with a l p h a particles from the 60-inch cyclotron:
*CP" -f 2n
Californium 244 has a half-life of 4 5 minutes and is an alpha emitter. Only a few milliontHs of a grain of curium 2 4 2 was used as a target material for t h e formation of californium, and the amount of californium obtained in the initial work was only a b o u t five thousand atoms. I n a s m u c h as it h a s not been isolated in weigh able a m o u n t s , its chemical properties h a v e been studied with invisible amounts utiliz ing its radioactive properties for identifica tion purposes. Ultramicrochemistry D e v e l o p e d as Aid To T r a n s u r a n i u m Research T h e techniques developed b y Seaborg's group, especially C u n n i n g h a m and P . L. Kirk at t h e Metallurgical Laboratory oi the University of Chicago in 1942 for the separation of plutonium u n d e r condi tions in w h i c h t h e plutonium concentra tion would b e the same as that anticipated for Oak R i d g e and Hanford, were in t h e m selves an outstanding contribution t o diemical history. At t h a t time, no one h a d actually seen plutonium and all d e ductions as t o its chemistry h a d been m a d e on the basis of tracer studies. Plutonium from t h e chain-reacting units was a year C H E M I C A L
away, but the design of the separation p l a n t had to begin immediately in order that the separation plants would be avail able upon completion of t h e chain react ing units. T h e gigantic problem was solved tiirough work in two fields. First, a weigh able a m o u n t of plutonium was p r o d u c e d through b o m b a r d m e n t of large amounts of uranium with neutrons from the cyclo tron. This in itself was an epoch-making occasion, a n d m a r k e d the first time that weighable a m o u n t s of transmutation p r o d ucts had been produced b y a particleaccelerating machine. E v e n so, it was anticipated that not greater than micro g r a m amounts of plutonium would b e produced, and t h e second aspect of t h e solution of the problem w a s through d e velopment of methods b y which only microgram amounts of material would b e used but concentrations would be those expected in later plant operations. Through the use of extremely small volumes a n d b y weighing microgram amounts of plutonium, solutions of rela tively high concentration resulted. It was through t h e study of material on this ultramicrochemical scale of operation t h a t compounds of plutonium, and later t h e other transuranium elements, were p r e p a r e d and their properties measured, and it was possible to test the separation proc esses under consideration for Hanford at actual concentrations that w o u l d prevail. T h e extremely small volumes involved in ultramicrochemical studies, usually of t h e order of 10"3 ml., are h a n d l e d with t h e aid of specially constructed small capil lary containers, burets, micromanipulators, a n d other pieces of equipment. Several types of ultramicro-balances w e r e d e veloped w h i c h could weigh amounts as small as a microgram with an accuracy of 0.03 microgram, and, if necessary, could w e i g h something of mass as small as 0.1 microgram. T h e material being w e i g h e d c o u l d be in containers weighing as m u c h as 20 mg. or 20,000 micrograms. On the basis of the ultramicrochemical work done, the separation process used at Hanford w a s selected and from the b e g i n ning its performance exceeded all expecta tion. High yields and decontamination factors w e r e achieved a n d continued t o improve with time—a remarkable fact con sidering t h a t t h e scale-up between t h e ultramicrochemical experiments and plant operation amounted to a factor of a b o u t 10 10 , the greatest scale-up ever a t t e m p t e d . By-products of T r a n s u r a n i u m Research Through irradiation of different kinds of h e a v y atoms with neutrons, protons, d e u terons, helium nuclei, and carbon nuclei, a total of 39 isotopes of t h e six transura n i u m elements have been produced to t h e present t i m e ( T a b l e I I ) . A wealth of sci entific data has resulted from t h e study of t h e formation of these isotopes and their properties which led to other important scientific findings. A significant revelation was t h e discov ery of t h e missing (4n-\- 1) radioactive series by Seaborg during research extendAND
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NEWS
Many of the isotopes of the transuranium elements listed in Table II were produced in this 184-inch synchrocyclotron in the radiation laboratory at Berkeley
ing from 1940 to 1945 at the University of California and the Metallurgical Laboratory. During the work on the plutonium project, many originally isolated observations on the radioactive transmutation series were made and brought together. Prior to this time, the mass numbers of the known radioactive families were of the form (An) (thorium series), (4ra -|- 2), (uranium series), and (4n -|- 3) (actinium series). The discovery of the (4n -f- 1) series, named the neptunium family from its most long-lived member, filled the missing gap. Unlike the other three series, it ends on an isotope of bismuth. It also differs from the other series in not containing a rare gas, or emanation, member. Upper members of the series include: w Pu
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mPa*33 - > nU*™ "Collateral" members of this series include Th233 and U2"7, which also have been described previously. The decay products of U233 trace a major part of the series. Beta-emitting Pb209 and stable Bi200 are the remaining members of the series, and these were studied independently by a Canadian team. The discovery of a new fissionable isotope of uranium, U233, by Seaborg and his colleagues in 1942 while studying the reaction of slow neutrons with thorium is of possible far-reaching importance. The new isotope, which gives off alpha rays and has a half-life of 120,000 years, can be used as an atomic fuel, which means that it may be possible to use indirectly thorium, much more plentiful in nature than uranium, for atomic energy purposes. Crystal Ball Gazing Will other transuranium elements be identified and the periodic table extended beyond 98? Drs. Seaborg and McMillan think so. In fact, Seaborg and his associates feel that they can predict rather well the chemical properties of a considerable V O L U M E
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number of further transuranium elements, though californium is the last to have been identified at the present time. (See elution curves in Figure 2.) Many of the transcalifornium elements will be extremely though californium is the last to have been synthesized, may or may not be assigned space in the periodic chart depending upon whether or not they have sufficient lifetime to permit chemical tests. Is practical use of the transuranium elements other than plutonium foreseen in the near future? Probably not, the discoverers say; however, it is entirely conceivable that they may some day lead to discoveries of great utility. Actual and potential uses of plutonium fuel have become the conversation of laymen; and possible applications of isotopes and fission products resulting from its
production have grown at a phenomenal rate. But the scientist will look upon the subject of transuranium chemistry in still another light. Inasmuch as the transuranium elements are members of a transition group, the chemical and physical properties of all members of the group become of comparable interest. Of importance not at all overshadowed by the dramatic introduction of the new elements themselves was the further development of ultramicrochemistry necessary before the chemistry of the transuranium elements, usually available in millionths-of-a-gram amounts, could be confirmed. The many other by-products of transuranium research, which have contributed to a better understanding of nuclear chemistry, will also receive continuing attention and lead to other advancements of scientific value.
At the Berkeley laboratory Dr. McMillan operates the synchrotron, which h e invented. This powerful atom smasher accelerates electrons to 300 M.e.v. p"*"
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» J A N U A R Y
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1952
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