prïostloy medal
address
THE NEW MILLENNIUM Darleane C Hoffman is scheduled to present the Priestley Medal Address on March 28 during the award ceremony at the American Chemical Society's 219th national meeting in San Francisco. Hoffman is a chemistry professor in the graduate school of the University of California, Berkeley, and faculty senior scientist and co-group leader ofthe Heavy Element Nuclear & Radiochemistry Group at Lawrence Berkeley National Laboratory. She is being honored for her work investigating chemical properties of the heaviest elements on a one-atom-at-a-time basis that has been important in elucidating the placement of those elements in the periodic table. Hoffman also is being honored for her contribution to education in the field of nuclear and radiochemistry. Following is the text of her address. have chosen to call this article 'The New Millennium" to signify change and new beginnings. In the interests of brevity, I must limit my discussion to two topics that are near and dear to my heart and with which I can claim some familiarity—the recent spectacular advances in heavy-element nuclear and radiochemistry and changes in the status of women in chemistry over the past 50 years.
1999: A Banner Year for Heavy-Element Nuclear and Radiochemistry Historically, the production of new elements and new heavy-element isotopes and studies of the chemical and nuclear properties of the radioactive elements have been primary goals of the University of California, Berkeley/Lawrence Berkeley National Laboratory (LBNL) Heavy Element Nuclear & Radiochemistry Group. Under the direction of the late Glenn Seaborg and then Albert Ghiorso, the transuranium elements were extended through element 106, later to be named seaborgium. The sometimes unexpected and unique chemical properties of these elements were also investigated at Berkeley and elsewhere. However, after discovery of element 106 in 1974, the focus for the discovery of new elements shifted to 36
MARCH 27, 2000 C&EN
This island was calculated to be in the region of the spherical nuclear shells (or "magic numbers") at atomic numbers 114 or 126 and neutron number 184, as shown in the topographical representation in Figure 2. Predictions of half-lives as long as a billion years prompted searches in natural ores for element 114 as ekalead and even for element 126, now expected to be part of a "superactinide" series which begins with element 122 and ends with element 153 after filling of the 5g and 6f electronic subshells (Figure 3). First attempts to produce SHEs at accelerators were initiated at Berkeley in 1968, but with no success. Although there were many reports of evidence for SHEs both in nature and at accelerators, no confirmed results had yet been obtained by 1976. During the August 1976 ACS national meeting in San Francisco, Glenn Seaborg gave his ACS presiGesellschaft fur Schwerionenforschung dential address entitled "Chemistry— (GSI) in Germany, where the new in- Key to our Progress" to mark the 100th flight separator for heavy-ion reaction Anniversary of ACS. He not only pointed products had been built. It was used to out the many advances in fundamental identify the next six elements: bohrium knowledge and the contributions and ap(107) through meitnerium (109) between plications of chemistry to society over the 1981 and 1984, and elements 110 through previous 100 years, but made numerous predictions about future developments. 112 between 1994 and 1996 (Figure 1). As the pace of discovery of new eleOne of these that particularly caught ments slowed, interest in studies of my attention was, "Nuclear chemists their chemical properties declined pre- will be involved in the synthesis of addicipitously from the 1970s until the mid- tional chemical elements, hopefully in 1980s. Then a renaissance of interest in the Island of stability.' " Although many the chemistry of lawrencium (103) and extremely sensitive physical and chemithe transactinides was sparked by pre- cal methods were developed and used dictions that relativistic effects might in the search, no confirmed evidence for cause changes in their electronic struc- the existence of SHEs remained by the tures, which would result in deviations end of the 1980s, and the searches were in their chemical properties from those abandoned. predicted by simple extrapolation of But the production of elements 110 known trends in the periodic table. to 112 in 1994-96 at GSI created renewed optimism because they decayed The long-sought superheavy elements predominantly by α emission rather The existence of superheavy elements than spontaneousfission,contrary to ear (SHEs) was predicted as early as 1955 by lier predictions. The subsequent at John Wheeler and discussed by Gertrude tempts to produce element 113 were un Scharff-Goldhaber in 1957. But the big successful, and the GSI group decided to wave of interest and serious searches delay further attempts while they im for SHEs did not begin until the 1960s, proved their separator. spurred by theoretical predictions of an At about the same time, our group em "island of superheavy elements" well be- barked on a project to build the Berkeley yond uranium. Gas-filled Separator (BGS) based on a
of Robert Smolanczuk, a Ful- and a half-life of about 5 seconds based on bright Scholar visiting with our observation of a spontaneous fission halfgroup from the Soltan Institute life of about 4.5 minutes for the daughter. 178a.— — U$2) for Nuclear Studies, Warsaw, we They believed this to be the same as the decided to try the "cold" fusion re 1.4-minute spontaneous fission activity action of 208Pb targets with 86Kr that they produced previously in the reac BAm(95), Cm(96) tion of 48Ca with 238U targets and attribut projectiles. These experiments were un ed to element 112 with mass 283. But B Es(99), Fm(100) dertaken at the LBNL 88-inch cy identification based only on the similarity clotron in April and May 1999. in half-life of this unconfirmed spontane BGS was used to separate three ous fission activity is rather uncertain. events attributed to element 118 So suddenly and rather unexpected with a mass of 293 based on de ly, the new superheavy elements 118, cay by a unique series of high-en 116, and 114 have burst upon the scene. ergy α particles to the new ele In addition, there is evidence for three ments 116 and 114, and to pre isotopes of element 114, three new iso viously unknown neutron-rich topes of element 112, two new isotopes 1993 isotopes of elements 112, 110, of elements 110 and hassium (108), and 108, and 106. The three observed one new isotope of seaborgium (106). 114, 116, 118 α-decay chains from element 118 If we assume that all these reports are are shown in Figure 4. The decay confirmed, then the number of known energies and estimated half-lives nuclides beyond element 105 has in New Elements (number per year) of 0.12 millisecond, 0.60 millisec creased from 23 to 39 within a single Figure 1 ond, 0.58 millisecond, 0.89 mil year, and most of them are longer lived lisecond, 3.0 milliseconds, and than previously known (Figure 5). This new design by Ghiorso to achieve greater 1.2 seconds for the chain members were certainly bodes well for the extension of sensitivity than existing separators and in remarkable agreement with Smolan- chemical studies to still heavier ele allow us to reenter the quest for still czuk's predictions. In two of the chains ments. The "gaps" shown in the time line 269 heavier elements. Ken Gregorich, one there is evidence that the half-life of Sg for the discovery of the transuranium might be as long as 20 seconds. elements (Figure 1) emphasize that the of Seaborg's last Ph.D. students and my first postdoctoral fellow after coming to In July 1999, a multinational group, discoveries come in spurts after the de Berkeley in 1984, was appointed to head also working at Dubna, published evi velopment of new concepts, new produc the project, and Victor Ninov from GSI dence for two different decay chains of el tion reactions, new techniques, or new was brought to help with the project. ement 114 consisting of α decay followed instrumentation needed for the produc By the fall of 1998, BGS was ready for by spontaneous fission produced in the tion, separation, and identification of each testing, and the first experiments were reaction of 48Ca projectiles with 242Pu tar successive "group" of new elements. performed in December 1998. Then the gets. They attributed these events to the What are the chances for filling in excitement began in January 1999 when α decay of element 114 with mass 287 the missing elements 119,117,115, and we learned that a collaborative group from the Flerov Laboratory of Nuclear 120 1 1 ι ι ι \ ι ι Reactions, Dubna, Russia, and Lawrence HEAVY ELEMENT TOPOLOGY Livermore National Laboratory had found evidence for a single event attribut able to element 114 in data obtained from 110l· Half life some 40 days of running time with their •+ + » · « Ν gas-filled separator at the Dubna cyclo J Stable .'-^ · : . ("°H8) tron during November and December • >
