Summer 1997 Los Alamos National Laboratory • A U.S. Department of Energy Laboratory The Actinide Research 55 Quarterly TA o f t h e N u c l e a r M a t e r i a l s T e c h n o l o g y D i v i s i o n In This Issue Source of the Actinide Concept by Dr. Glenn T. Seaborg 1 Source of the This issue of the Actinide Research Quarterly Actinide Concept is blessed with the guest article “Source of the Ac- tinide Concept” by Dr. Glenn Seaborg. One could 4 not ask for a more significant and timely introduction to the “Plutonium Futures—The Science” conference NMT Division than the personal perspective of the discoverer of the Recycles, Purifies element. The Santa Fe conference will also feature Plutonium-238 Dr. Seaborg’s plenary lecture videotaped in April this Oxide Fuel for Future year. In reminiscing about the past and anticipating Space Missions the future, we hope that this conference helps to her- ald a new beginning in the future science of actinides. 6 K. C. Kim New Mexico Welcomes My romance with the transuranium ele- "Plutonium ments started 63 years ago, in 1934, soon after Futures—The I became a chemistry graduate student at the Science" Conference University of California, Berkeley. These were the undiscovered elements with atomic num- 7 bers greater than 92 (the atomic number of "Plutonium uranium), the heaviest naturally occurring XBC 942-905 Lawrence Berkeley National Laboratory Futures—The element. Science" Conference We (the transuranium elements and I) the chemical properties using the periodic Program were first introduced at the weekly chemistry table as it was then known. The heaviest natu- seminar on nuclear science held in venerable ral elements, thorium, protactinium, and ura- 16 Gilman Hall. Actually, I was introduced to nium (atomic numbers 90, 91, and 92), were NewsMakers what were thought to be the transuranium placed in that table just below the sixth-period elements. I read articles by Enrico Fermi and “transition elements”—hafnium, tantalum, coworkers about the induced radioactivities and tungsten (in these elements, the “5d” elec- observed when elements such as uranium tron shell is being filled). Thus it was assumed were bombarded with neutrons. Since some that the 6d electron shell was being filled in were published in their native Italian, they these heaviest elements, and the chemical were a challenge to decipher. properties of the transuranium elements, the These induced radioactivities were, of undiscovered elements 93, 94, 95, and 96, course, produced in trace (unweighable) would be homologous with the 5d elements quantities, so radiochemistry methods were immediately above them in the periodic table, needed. For guidance, researchers predicted rhenium, osmium, iridium, and platinum. 90 232.0381 91 231.0359 92 238.029 93 237.0482 94 (244) 95 (243) 96 (247) 97 (247) 98 (251) 99 (252) 100 (257) 101 (258) 102 (259) 103 (260) Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Thorium Protactium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Medelevium Nobelium Lawrencium Nuclear Materials Technology Division/Los Alamos National Laboratory 1 The Actinide Research Quarterly The limited chemical identification experi- McMillan then started looking for a ments of Fermi and coworkers seemed consis- shorter-lived isotope of element 94 through tent with this view. The work of Otto Hahn, the deuteron bombardment of uranium. When Lise Meitner, and Fritz Strassman in Berlin McMillan was called to MIT for war work, seemed to further confirm it. I continued this quest with the help of my Little did we know then how we were graduate student Arthur C. Wahl and another being misled by accepting what was easiest to instructor in chemistry at Berkeley, Joseph W. accept. I bought this interpretation “hook, line, Kennedy. We succeeded on the night of Febru- and sinker.” In the fall of 1936, I described the ary 23–24, 1941, in chemically identifying (i.e., work and interpretation of Otto Hahn and discovering) element 94 (the isotope 23994) in coworkers during a required graduate stu- room 307, Gilman Hall (designated as a Na- dent talk to the chemistry faculty, staff, tional Historic Landmark on the 25th anniver- Dr. Glenn Seaborg graduate students, and visiting scientists. sary of the discovery). Most importantly, we Then in January 1939, the bubble burst! found that the chemical properties of element At the physics journal club meeting, we heard 94 weren’t like those predicted from the peri- something extraordinary. Niels Bohr, who had odic table of that time (i.e., not like osmium), arrived in New York the previous week, but were chemically similar to uranium. Joined brought news from Otto Hahn’s laboratory by physicist Emilio Segrè, we soon identified that the neutron-bombardment of uranium 23994 and, most importantly, demonstrated that produced isotopes of light elements, like it was fissionable by slow neutrons. barium and lanthanum. The meaning was Following McMillan’s suggestion for nam- simple: the uranium had been split approxi- ing element 93 “neptunium” (after Neptune, mately in half, and all the radioactive “transu- the first planet beyond Uranus), with the ranium” isotopes studied by Hahn, Strassman, chemical symbol Np, Wahl and I suggested and Meitner during the previous four years “plutonium” (after Pluto, the next planet) for were actually isotopes from the middle of the element 94. We first debated whether the name periodic table. should be “plutium” or “plutonium,” the This was exciting! After the seminar, I sound of which we liked better. Although the walked the Berkeley streets for hours, cha- chemical symbol might have been “Pl,” we grined that I hadn’t recognized that the “tran- liked the sound of “Pu,” for the reason you suranium elements” in which I had been so might suspect, and therefore decided on “Pu.” interested were nothing of the kind. I felt stu- I had the pleasure of meeting for the first pid for failing to admit the possibility. Subse- time Clyde Tombaugh, the discoverer of the quent work showed that the radioactivities planet Pluto, in Albuquerque, New Mexico, on that had been ascribed to transuranium ele- June 9, 1991. At that time, he told me he had ments were actually due to fission products! also considered naming his planet after the With poetic justice, the actual discovery of Greek god Cronus or Roman goddess Minerva the first transuranium element resulted from (rather than after Pluto). In that case, I suppose experiments aimed at understanding the fis- we would have given element 94 the name sion process. In 1940, Edwin M. McMillan and “cronium” or “minervium,” and therefore, Philip H. Abelson showed that a radioactive people throughout the world would never product of the bombardment of uranium with have heard the word “plutonium” which is neutrons was an isotope of element 93, with a so much in the news today. mass number 239 (23993). The isotope 23993, a negative beta-particle emitter, should decay to the product 23994, but they were unable to ob- serve this daughter product because of its long half-life. 2 Nuclear Materials Technology Division/Los Alamos National Laboratory Summer 1997 The chemical properties of neptunium and their rare earth homologues, europium (ele- plutonium were found to be similar to those of ment 63) and gadolinium (element 64). The uranium and quite unlike those of rhenium new elements were subsequently named am- and osmium, which, according to the existing ericium (95) and curium (96) by analogy with periodic table, they should have resembled. the naming of their homologues. Thus we concluded that a new series of 14 This bold revision of the periodic table rare-earth-like elements, starting at uranium, was a hard sell. When I showed it to some would be the “uranide” (uranium-like) series, world-renowned inorganic chemists, I was just as the 14 rare-earth elements were known advised not to publish it—such an act would This bold as the “lanthanide” (lanthanum-like) series. “ruin my scientific reputation.” revision of the Wrong again! However, I did publish it after the war, periodic table Soon after Pearl Harbor and the U.S. entry and it became a guide for the chemical identi- was a hard into World War II, I moved to the wartime fication of most of the subsequent members of Metallurgical Laboratory of the University of the actinide series. The series was predicted to sell. When I Chicago. Here, we solved many of the prob- end at element 103, and the subsequent inves- showed it to lems attendant with plutonium-239 produc- tigations confirmed this. some world- tion, and I turned my attention to the quest for At element 104 (now known as rutherfor- renowned the next two transuranium elements, 95 and dium), we jumped back up to the body of the inorganic 96. I was joined in the endeavor by my col- periodic table, and rutherfordium took its chemists, I leagues Albert Ghiorso, Ralph A. James, and place under hafnium (element 72). (This spot Leon O. (Tom) Morgan. But when we pre- had been occupied by thorium before I moved was advised dicted the chemical properties on the basis of it to a separate row at the bottom of the peri- not to publish the “uranide” concept, we failed to make any odic table). Then we proceed across the peri- it—such an identification of our transmutation products. odic table along now-known elements act would We weren’t successful until I suggested 105–112, to undiscovered elements 113–118; “ruin my that we needed a bold revision of the periodic element 118 will be a noble gas.