SPECIAL REPORT
The
Heaviest Elements Darleane C. Hoffman, University of California, Berkeley, and Lawrence Berkeley Laboratory
Difficult to make and highly radioactive, the elements heavier than fermium are revealing some unexpected chemistry
ow long does an atom need to "exist" before it's possible to do any meaningful chemistry on it? Is it possible to learn anything at all about the reactions of an element for which no more than a few dozen atoms have ever existed simultaneously? These are some of the questions my students and I and our colleagues in a few laboratories worldwide attempt to answer as we investigate the chemistry of the heaviest elements—elements produced one atom at a time in accelerators by bombarding radioactive targets with high-intensity beams of heavy ions. All of these elements spontaneously decay; the most stable of them have half-lives of only a few minutes, some that are less stable exist for only milliseconds. So far, no chemical studies have been performed on elements whose longest lived isotopes last only milliseconds because the difficulties of doing chemistry on this time scale under highly radioactive conditions are enormous. Over the past 10 years, however, nuclear chemists have developed new techniques or adapted existing ones to begin to probe the chemical properties of those very heavy elements that have half-lives in the range of seconds to minutes. The designation of elements as "heaviest" changes, of course, as new and heavier elements are discovered. Prior to 1940, the heaviest element for more than 150 years was uranium, element 92, discovered in 1789. Today, periodic tables include all the elements through meitnerium, element 109. The first nine transuranium elements—from neptunium (93) through mendelevium (101)—were produced between 1940 and 1955, in an era that might be called the golden age for discovery of new elements. Since then, eight more elements have been discovered, the most recent of which, hassium (108), was reported in 1984. For all the elements through element 106, discovery has now been confirmed by groups of
H
24
MAY 2, 1994 C&EN
scientists other than the original discoverers. A natural dividing line for the "heaviest" elements is perhaps at fermium, element 100. Elements of higher atomic number than fermium are so extremely unstable that they must be made an atom at a time in an accelerator by bombarding a heavy-element target with a high-intensity beam of ions at least as heavy as helium. Their chemistry also has to be explored one atom at a time. For the transfermium elements, merely identifying what element has been produced is difficult and can be controversial. Mendelevium is the heaviest element whose initial atomic number assignment was based on chemical separation and the first to be produced and chemically identified one atom at a time. Its initial discovery by Albert Ghiorso and colleagues at Lawrence Berkeley Laboratory in 1955 was based on the detection of only 17 atoms. The discovery was confirmed in subsequent experiments in which the use of larger targets permitted production of thousands of atoms. Although these classic experiments are now nearly 40 years old, they are worth describing, as they were the first of their kind and illustrate many of the techniques that are still used and essential in studying these very short-lived, radioactive elements. The researchers prepared a thin target of the highly radioactive and rare isotope ^3Es. Not only was less than a picogram of 2:>3Es available from production by neutron irradiation of plutonium, but the isotope decays with a 20-day half-life. The researchers irradiated the target with helium ions at the cyclotron at the Radiation Laboratory at the University of California, Berkeley, and used a then-new technique^—the recoil method—to separate the products of the nuclear reaction from the einsteinium target. In this method, the product atoms are produced with enough energy to recoil out of a thin target, thereby allowing the target to be used over and over again. The technique, which has become a standard one, also provides considerable purification of the few atoms produced from the billion or so target atoms.
In Ghiorso's experiments, only about one atom of mendelevium was produced during each three-hour bombard ment of the target. The recoiling products were rapidly sep arated by elution from a cation-exchange resin column us ing the complexing agent α-hydroxyisobutyrate (HIB). Trivalent actinides and lanthanides elute from such a col umn according to their ionic radii, with smaller radii eluting first. The researchers repeated the bombardment about a dozen times, detecting altogether 17 atoms at the elution position predicted for element 101. Radioactivity detected at both the element 100 and 101 positions indicated that, at each position, nuclei were breaking into two large frag ments in a process called spontaneous fission. The sponta neous fission activity decayed with a half-life of about three hours. The researchers postulated that a compound nucleus reaction had occurred in which a helium ion fused with an einsteinium target atom to form an excited system that rap idly emitted a neutron to become 256Md. The newly formed element decayed with a half-life of about 1.6 hours to 256Fm, which then fissioned with a half-life of about three hours. Starting with nobelium (102), the reported discoveries of new elements have been contested and controversial, pri marily because of very low production rates, half-lives of minutes or less, and the necessity for initial identification by methods other than known chemical separations. The first claim to the discovery of element 102 came in 1957 when an international team of scientists from Argonne National Lab oratory in Illinois; the Atomic Energy Research Establish ment in Harwell, England; and the Nobel Institute of Phys ics in Stockholm reported that they had found an alphaemitting activity with a half-life of about 10 minutes in chemically separated products of the irradiation of 244Cm with 13C ions. They attributed this activity to element 102 because of its early elution position (before mendelevium) with HIB from a cation-exchange resin column.
Unfortunately, other ions can elute in this position, and these initial experiments did not rule out the other possibil ities. In addition, these researchers had assumed that the most stable oxidation state of nobelium in aqueous solution would be 3+, as it is for most actinides and for ytterbium, the lanthanide homolog of nobelium, although Yb2+ can be prepared in aqueous solution with very strong reducing agents. In 1969, long after a second claim to discovery of nobelium was made in 1958 based on alpha decay methods, Robert J. Silva and coworkers at Berkeley found, surprising ly, that the most stable oxidation state of nobelium in aque ous solution is 2+. Thus, the original report of the discovery of this element was erroneous for two reasons: The chemi cal separation was based on the inaccurate assumption that the element would form a trivalent actinide ion in aqueous solution, and the separation was not definitive enough to exclude nonactinide elements. To my mind, actual discovery of nobelium came in 1958 when a group at the Radiation Laboratory at Berkeley identi fied an isotope of the element based on chemical identification of a known alpha-emitting fermium daughter isotope. How ever, the Transfermium Working Group, a joint committee of the International Union of Pure & Applied Chemistry and the International Union of Pure & Applied Physics, in a report is sued in 1992, credited conclusive discovery of nobelium to two groups at the Joint Institutes of Nuclear Research at Dubna, near Moscow, based on experimental results simulta neously published in 1966. One group, led by Eugeny D. Donets, based its identification of the element on chemical separation of known alpha-emitting fermium daughter iso topes; the other, headed by B. A. Zager, used identification of alpha emission from known fermium daughter isotopes. Lawrencium (103) was first produced and identified at the Berkeley Heavy Ion Linear Accelerator in 1961 by Ghiorso and coworkers in bombardments of californium iso-
37
38
39
40
44
45
46
47
48
49
50
51
52
53
54
Rb
Sr
Y
Zr
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
55
56
72
76
77
78
79
80
81
82
83
Cs
Ba
Hf
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
H
88
Ra
104 i 105 Rf i Ha
106
(Sg) Ns
86
108 109 Hs
58
59
Ce
Pr
60 Β • 62 Nd [ 9 Sm
89
90
91
92
Ac
Th
Pa
U
Lanthanides
Actinides
107
11 Po E H Rn 84
63
64
65
66
67
68
69
70
71
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Heavy and recently discovered elements fill in the end of the penodic table. Elements shown in bine have been discovered since 1937, those in red have not yet had their discovery confirmed by a group of researchers other than the onginal discoverers. An important change in the chemistry of these elements is predicted to occur between lawrencium, element 103, which ends the actinide senes, and rutherfordium, element 104, the group 4 element that begins the 6ά transition senes. MAY 2, 1994 C&EN 25
SPECIAL
REPORT
topes with boron beams. The recoiling products were collected on a Mylar tape that was moved past a series of alpha detectors, which recorded activity from a few new 257Lr and 2^8Lr nuclei with half-lives of seconds or less. In 1965, Donets and colleagues at Dubna used a double recoil technique to identify a longer lived isotope, 256Lr, now known to have a half-life of 26 seconds. They linked its decay—via either electron capture followed by alpha decay or alpha decay followed by electron capture—to its known granddaughter, 252Fm, with a half-life of 25 hours, which was identified chemically and by its characteristic alpha decay. Another nonchemical technique that can positively identify both the atomic number and mass number of a heavy element is alpha-alpha correlation. Here, the alpha decay of an unknown new species is measured and correlated in time with the alpha decay of a known daughter nuclide, thus establishing their genetic relationship. Additional alpha correlations with subsequent decay of granddaughter or even great-granddaughter species can make the identification even more compelling. The method requires rather sophisticated detection and timing techniques, but it provides unequivocal identification because the parent atom has to be just one helium atom heavier than its known daughter. Alpha-alpha correlation was pioneered by Ghiorso and coworkers, who used it to identify rutherfordium (104), hahnium (105), and element 106. It was also used in the discoveries of nielsbohrium (107), hassium, and meitnerium, each with half-lives of only milliseconds, made between 1981 and 1984 by Peter Armbruster, Gottfried Munzenberg, and colleagues at the heavy-ion research facility known as
GSI (Gesellschaft fur Schwerionenforschung) near Darmstadt, Germany.
Why do atom-at-a-time chemistry?
Although atom-at-a-time studies of the heaviest elements began with the study of mendelevium, that element can now be produced in copious quantities by alpha bombardment of radioactive einsteinium targets. A million or so atoms of 256Md can be produced in one- or two-hour bombardments, and this isotope is the one typically used for chemical experiments. Nobelium, too, was originally studied an atom at a time. However, in later studies, bombardments of 249Cf targets with 12C ion beams produced more than a thousand atoms of 255No per 10-minute collection. Beginning with lawrencium, though, very short half-lives and low production rates mean that all the chemical experiments performed on these elements necessarily have been done one atom at a time. Many of the recent studies of them have been made by our heavy-element nuclear and radiochemistry group at Lawrence Berkeley Laboratory. There are many challenges to performing atom-at-a-time chemical studies on these elements. Normal analytical techniques are not necessarily applicable. The atoms must be detected by measuring their radioactive decay, principally alpha or spontaneous fission decay. With half-lives usually measured in seconds, these elements can only be studied chemically using comparably rapid techniques. The techniques used must give the same results for a few atoms as for macro amounts. Useful techniques include ion exchange and gas chromatography, solvent extraction, and other methods in which atoms are rapidly subjected to many identical chemical reactions and equilibrium is rapidly attained. For transfermium elements, much is in the name The elements are produced in exBy custom, the right to choose the the Nomenclature Committee of the tremely small amounts and with a name of a new element belongs to the American Chemical Society last Nohost of unwanted contaminants. Their element's discoverers. Although the vember endorsed the use in the U.S. production requires special handling International Union of Pure & Ap- of the names rutherfordium (Rf) and to accommodate the use of radioacplied Chemistry (IUPAC) officially hahnium (Ha) for elements 104 and tive actinide targets, accelerators that approves the names of elements, it's 105; and nielsbohrium (Ns), hassium can furnish microampere beams of the elemenf s discoverers who get the (Hs), and meitnerium (Mt) for 107,108, light to heavy ions, and ways to prohonor of proposing a name to IUPAC. and 109, respectively. tect both the accelerator and personFor several of the transfermium eleLast March at the ACS national nel from contamination should the raments, however, competing claims to meeting in San Diego, the name seadioactive targets rupture. Other rediscovery from different laboratories borgium (Sg) was proposed for elemake the question of what to call ment 106 by the team of researchers quirements include a rapid method these new elements a sensitive one. from Lawrence Berkeley Laboratory for removing reaction products from Though the Transfermium Work- and Lawrence Livermore National the target region, facilities for target ing Group, a joint committee of IU- Laboratory that first discovered the preparation and for handling radioacPAC and the International Union of element in 1974. This initial discovery tive materials, personnel trained in nuPure & Applied Physics, recently is- was recently confirmed by a different clear and radiochemistry techniques, sued a report assigning priority of group of researchers, including the auand instrumentation that can detect the discovery for the transfermium ele- thor. This confirmation makes it apemitted radiations. Such facilities and ments, the report has generated con- propriate to suggest a name, according capabilities exist at only a few laboratosiderable controversy, especially con- to the "Criteria for Discovery of New ries in the world. cerning element 104. Names for ele- Elements" proposed by an internaments 104 through 109 have not yet tional group of scientists in 1976. In Although experiments with the been officially approved by IUPAC. addition, the Transfermium Working heaviest elements are difficult and de[The names mendelevium (Md) for Group awarded priority for discovery manding, they offer special opportunielement 101, nobelium (No) for 102, of element 106 to the Berkeley-Liverties to obtain information about trends and lawrencium (Lr) for 103 were ap- more group, which also paved the way in the periodic table and to assess the proved by IUPAC in 1970.] However, for proposing a name. magnitude of the influence of relativistic effects on chemical properties at
26
MAY 2, 1994 C&EN
these very high atomic numbers. In addition, these experiments provide a way to examine the dramatic change in chemical properties that occurs between the actinides, which end at lawrencium, and the transactinide elements. For example, the most stable oxidation state of all the transplutonium actinides except nobelium is 3+, but beginning with rutherfordium, the transactinide elements are predicted to belong to a 6d transition series. By analogy to their lighter homologs hafnium, tantalum, and tungsten in the 5d transition series, elements 104, 105, and 106 would be expected to exhibit rather complex chemistry and have most stable oxidation states of 4+, 5+, and 6+, respectively. One of the major questions experimentalists hope to help answer is whether the properties of these heaviest elements can be predicted by simple extrapolation from the properties of their lighter homologs in the periodic table. The heaviest elements also provide a unique opportunity to evaluate the influence of relativistic effects on chemical properties because these effects on the atomic orbitals should increase approximately as the square of the atomic number. Thus, the heaviest elements for which we can obtain information are the best place to look for deviations caused by these effects from the chemical properties predicted by extrapolation from lighter homologs. As Pekka Pyykkô of the University of Helsinki pointed out in a 1988
review article, when the charge of the nucleus becomes very large, it causes a decrease in the effective Bohr radius for inner s and ρ shells, making them more stable. Because the out er shells must be orthogonal with the inner ones, the effect is transmitted all the way to the s and ρ electrons in the outer valence shells. This contraction more effectively screens the d and / orbitals, causing their destabilization and an increase in their radial extension. Spin-orbit splitting of the p, d, and / electron energy levels may also occur with the lower angular momentum orbitals such as pl/2 being strongly stabilized. Pyykkô proposed that the similarity in the radii and chemical properties of the group 4 elements zirconium (40) and hafnium (72) is caused by a coincidental cancellation of nearly equal relativistic and shell-structure effects. That is, the relativistic effects, which would be expected to decrease the radii of the s and pl/2 orbitals in hafnium relative to zirconium, are matched by the increased radii of the filled 4/ shell and two 5d electrons in hafnium, compared with zirconium, which has no/electrons. Measurement of the chemical properties of rutherfordium, presumed to be the heaviest known group 4 element, should help to assess the relative magnitude of these counterbalancing effects. Other theorists have predicted that relativistic effects might be large enough to stabilize 7s and 7pl/2 orbitals so that the valence electron structure of rutherfordium might
^ ^ H 266 ^ ^ ^ • u I j M H 3 . 4 ms ^^^H
All known transfermium isotopes are ananged according to their number of neutrons and their atomic numbers. Nearly two thirds of the isotopes have half-lives of less than a minute. Vie mass number of each isotope is give at the top of its box; underneath is its half-life and major modes of decay [a = alpha decay, SF = spontaneous fission, EC = electron capture, IT = internal transition]. Half-life range • 1 millisecond (ms) to 1 second (s)
OC Λ
108 Hs
261 262 12 ms 8ms|0.1 s a a, SF? a
07 Ns
D 1 second to 1 minute (m) •
1 minute to 1 hour (h)
•
1 hour to 1 day (d)
•
more than 1 day
106 Sg
105 Ha
254? 253 255 1 . 5 s 0.5 ms 1.4 s SF SF,a? SF,a?
104 Rf
103 Lr
102 No
101 Md
247 3s α 146
248 7s EC,oc
255 1.5s SF
253 1.3s a
254 13 s a
252 2.3 s a, SF
Urn
253
250 0.3 ms SF
251 0.6 s α
249 24 s a, EC
2 5 0 : 251 I 2 5 2 50 s 4m 2m EC,a ; « , a
148
150
a
m$a
260 261 4 ms 0.3 s a, SF a, SF?
259 0.5 s a, SF
257 1.3s a, SF
258 4.4 s EC,a
256
257
Ο^Γ
264 265 0.08 ms 2 ms a, SF? a
265
263 0.9 s a, SF
2-30sl20-30s a a
260 1.5s a, SF
261 1.8s a, SF
259 3.0 s a , SF
260 262 261 20 ms 65 s 47 ms SF a, SF? S F
262 34 s EC,a
263 27 s SF, a
a, SF
a, SF
258 13 ms SF,a?
255 22 s a, EC
256 26 s a, EC
257 0.65 s a, EC
258 3.9 s a
259 6.1s a, SF
262 3m 3 9 m 216 m a » E 0 ; ;'"SF ,; E C
256 2.9 s a, SF I _ n
257 25 s a
258 1.2 ms
$ê m
• w
a, EC K n
JHBN «lM*?j E&a "]
256 1.3 h EC,a
FUTM 4.8 s
255 254 0.3 si55 s 3.1 m IT J a a , EC
254 253 - 6 m 30 mho m EC I EC; a 152
154
280
tm
I 266
m
260 106 ms ^ ^ ^ H
262 5 ms SF
260 257 258 259 5.5 h 57 m 52 d 1.6 h 27.8 d SF EC,a EC ' a SF 156
158
160
Num ber of neu trons MAY 2, 1994 C&EN
27
To collection site
SPECIAL REPORT
Protective window 18
Recoils Obeam
Beam collimators Helium with aerosol
Cooling gas
be 7s27pl/22 rather than the 6d27s2 structure expected by analogy to hafnium. Such a change could lead to different volatilities for the neutral atom or even to a stable 2+ oxida tion state in aqueous solution. However, recent relativistic calculations indicate that these effects may be quite subtle and complex. The calcula tions give a ground-state configuration for rutherfordium of [Rn]5/46d7sVp1/2, while that of hahnium (105) might be [Rn\5f%d37s2, as expected by analogy with tantalum. If the Is electrons are sufficiently stabilized, hahnium might even exhibit an oxidation state lower than 5+ in aqueous solu tion. Thus, the ability to compare fundamental chemical properties of rutherfordium and hahnium—such as their most stable oxidation states and the complexes they form in aqueous solution—with the same properties in lighter homologs can provide important information about the mag nitude of relativistic effects. Better knowledge of relativistic effects and shell effects can also help theorists refine their models and so be better able to predict chemical properties of new elements. Once some of the fundamental chemical properties of the heaviest elements are established, these properties can also be used to develop methods to separate the element from the multitude of products produced in heavy-ion reactions. Such clean samples are needed to study the nuclear proper ties of new isotopes of elements and also to positively iden tify the element. Chemical separation is particularly impor tant for elements that decay by spontaneous fission. Spon taneous fission, which is only known to occur in uranium and heavier elements, becomes a major mode of decay for the heaviest elements. Understanding this process better is of utmost importance because this tendency of the heaviest nuclei to break apart spontaneously is what will ultimately limit the number of new chemical elements that can exist. Heavy-element research, at the frontiers of both chemical and nuclear properties, appeals to students. Although very demanding work, this kind of science is both exciting and fun. It also provides knowledge and skills that are valuable in a host of applications; many career options are open to students with this training. 28
MAY 2, 1994 C&EN
Lawrencium (103), rutherfordium (104), and hahnium (105) are produced in this target chamber of the 88-inch cyclotron at Lawrence Berkeley Laboratory. TJte actinide target—a 1-mg-per-sq-cm or thinner deposit of248Cm or 249Bk on a thin backing—is held in the center of the back wall of the chamber. Tliis target is bombarded with a beam of18Ο ions, which first passes through a thin window that isolates and protects the cyclotron beam line from the expenmental system. Bombardments of249Bk targets can produce both 260Lr and i62Ha; 261Rfis produced by bombarding 248Cm targets. In a typical expenment, with a cross section of about 5 nanobarns, a target of 1-mg-per-sq-cm thickness, and a beam of 0.5-particle microamp, about two atoms per minute of the isotope are produced. Product atoms recoil out of the target (in the photograph, they would recoil toward the reader) into the chamber where they attach to potassium chlonde aerosols. Die product-laden aerosols are transported by a stream of helium through a capillary either to a collection site or directly to a detection system. Lite system has a 50% transport efficiency, an 80% chemical yield, 35% detection efficiency, and 50% decay. About one alpha is detected every 10 minutes. Manual or automated chemistry expenments can be performed at the collection site.
Chemical studies of the heaviest elements Atom-at-a-time chemical studies in our laboratory of law rencium, rutherfordium, and hahnium use isotopes pro duced by bombardment of heavy actinide targets with high-intensity ion beams, usually at the Berkeley 88-inch cyclotron. We electroplate the target material to a thickness of about 1 mg per square centimeter on a thin backing, such as 2.8-mg-per-sq-cm beryllium. We typically use the longest lived known isotope for each of these elements: 26uLr (with a half-life of three minutes), 261Rf (with a half-life of 65 sec onds), and 262Ha (with a half-life of 34 seconds). 249Bk+18O^260Lr 248
18
Cm + 0->
249
B k +
18
Q
261
+
4
H e +
3 n
Rf + 5n
^262
H a
+
5 n
Only a very small fraction of the target atoms undergo nuclear reactions. About two atoms per minute of the de sired isotope are produced, resulting in detection of about one alpha every 10 minutes. Essential to these experiments is the availability of 248Cm and 249Bk target materials. Targets made with 248Cm, which has a half-life of 3.5 χ 1CP years, can be used over and over again. However, because 49Bk has a half-life of only about 300 days, these targets must be replenished. The material we use comes from the transplutonium production program at
To detect separated atoms and confirm that they really are the element we want to study, we use the alpha-alpha correlation technique, measuring the kinetic energies of emitted alpha particles using passivated, ion-implanted planar silicon detectors and recording the time of each decay event. In some experiments, we also measure the kinetic energies of coincident spontaneous fission fragments in order to obtain information about fission properties. A variety of radiation detection systems are used, including our own multiple detector system, the Merry-Go-Around rotating wheel system, and a moving tape system that collects and transports the activities to detectors. The first chemical experiments on lawrencium were conducted in 1970 by Silva and coworkers at Berkeley using 2% Lr, which has a half-life of 26 seconds. They performed extractions with thenoyltrifluoroacetone to show that lawrencium extracts similarly to the trivalent actinides and unlike divalent and tetravalent tracers. These pioneering experiments demonstrated that, unlike nobelium, the most stable oxidation state of lawrencium in aqueous solution is 3+. Although a longer lived isotope of the element with a half-life of three minutes, 260Lr, was discovered in 1971, making chemical studies much easier, no additional investigations of lawrencium chemistry had been reported when we began our manual studies of its chemistry in 1986. One of the first of these was the deduction of the atomic radius of Lr3+ by comparing its elution position from a cation-exchange resin column using the eluting agent HIB with those of other trivalent actinide and lanthanide tracers whose ionic radii are known. The elapsed time from collection of the newly produced 260Lr atoms to the beginning of counting of samples eluted from the column was five to six minutes, about two half-lives of this isotope. Although only seven alphas from the decay of lawrencium were detected, we were able to show that the ion eluted in nearly the same position as erbium and, therefore, had a similar ionic radius. These experiments were performed again later that year in collaboration with Kratz and Schâdel and their groups, who brought ARCA to our laboratory. These automated experiments—in which the separation time was reduced to about three minutes and 25 alphas from 260Tr were detected—confirmed the earlier result. An average ionic radius of 0.0881 ± 0.0001 nm was obtained for Lr3+. Mendelevium isotopes form in the same bombardment via transfer reactions, and the elution gave an average ionic radius of 0.0896 ± 0.0001 nm for Md3+ based on detection of about 250 decays of mendelevium. A semiempirical model was used to calculate the heats of hydration for the two elements from these ionic radii. The difference of only 0.0015 nm between the radii of lawrencium and mendelevium—which differ by two atomic numbers—is much smaller than the separation of 0.0021 nm between the analogous trivalent lanthanide ions thulium and lutetium. This finding is unexpected since GSI's Matthias Schâdel (left) and the University of Mainz' Jens V. Kratz set up similar experiments comparing the ionic radii their automated rapid chemistry apparatus at Lawrence Berkeley Laboratory for of mendelevium and fermium (which differ by one in a series of collaborative expenments. 0.0015 nm) and fermium and einsteinium Oak Ridge National Laboratory in Tennessee. This program, supported by the Chemical Sciences Division of the Office of Basic Energy Sciences in the Department of Energy, is the only one in the world that produces such rare, relatively short-lived transplutonium isotopes on a regular basis. We study these isotopes using both manual and automated chemical procedures. The recoiling reaction products, attached to potassium chloride or other aerosols, are transported through a capillary by a stream of helium. They can be deposited on collecting disks for subsequent chemical processing or, alternatively, the aerosols can be transported to one of several computer-controlled automated systems. In collaborative studies of aqueous chemistry, we have used the automated rapid chemistry apparatus (ARCA) developed by Jens V. Kratz and students at the University of Mainz in Germany and Matthias Schâdel and his group at GSI. Heinz Gàggeler and his group, first at GSI and later at the Paul Scherrer Institute at Villigen, Switzerland, have pioneered the use of continuous isothermal gas chromatography. In collaborative studies with them, we have used the on-line gas-chemistry apparatus (OLGA) they developed. We have also used the heavy-element volatility instrument (HEVI), which was designed and built by graduate student Babak Kadkhodayan with help from postdoctoral fellow Andreas Turler and other members of our group. Most of the people in our heavy-element nuclear and radiochemistry group are graduate students from the chemistry department of the University of California, Berkeley. Each is working on an individual research project. Experiments often require many consecutive eight-hour shifts at the accelerator, however, so that more than one person is required to carry them out. Everyone in our group works on almost all of our experiments, although a specific graduate student will have primary responsibility for each one.
MAY 2, 1994 C&EN
29
SPECIAL REPORT eluded that this ion is unlikely to exist in aqueous solution and calculated an upper limit of -1.56 V for the 3+/1+ couple. The solution chemistry of rutherfordium was also first studied in 1970 by Silva and coworkers, who used 261Rf pro duced via irradiation of 248Cm with 18 0. Detection of its char acteristic alpha decay positively identified the element. The researchers showed that rutherfordium eluted from a cationexchange resin column using ΗΊΒ in a way that was similar to that of the group 4 elements zirconium and hafnium and not like that of the trivalent actinides. Based on these experi ments, the researchers proposed placing the element in the periodic table as the heaviest group 4 element, thereby con firming that the actinide series ends with lawrencium. In a series of experiments beginning in 1990, Kenneth R. Czerwinski of our group extracted "61Rf into triisooctylamine, tributyl phosphate, and thenoyltrifluoroacetone from hydrochloric acid solutions using a manual procedure that takes about a minute and uses only 10 μ ι of each phase. Extractions into tributyl phosphate gave the first ev idence that rutherfordium's chemistry deviates from that of its group 4 homologs zirconium and hafnium. Czerwinski found that rutherfordium forms anionic chloride species sim ilar to those of Pu4+ (a pseudo-group 4 element) at high chlo ride concentrations; zirconium, hafnium, and the pseudogroup 4 element thorium (90) do not. Rutherfordium can be extracted into triisooctylamine from 12M hydrochloric acid, as can zirconium and hafnium, but thorium cannot. In thenoyl trifluoroacetone, however, rutherfordium behaves most like thorium. The first experiments on the solution chemistry of hahnium were performed by our group in 1987 to determine the ele ment's most stable oxidation state in aqueous solution. Visit ing professor Gunter Herrmann of the University of Mainz suggested that we take advantage of the fact that the group 5 elements niobium and tantalum sorb on glass, but group 4 el 7 seconds ements zirconium and hafnium do not, to determine whether Remove glass plate on hahnium was more like a group 4 or group 5 element. which hahnium and Kenneth E. Gregorich, then a postdoctoral fellow, devel potassium chloride 10 seconds aerosol have been Transfer to oped a 51-second, manual procedure to study 262Ha, which deposited from detector and has a half-life of 34 seconds, and, with other members of collector and place begin counting on hot plate our group, carried out 801 of these manual separations on radioactivity hahnium as well as similar experiments on radioactive iso 7 seconds topes of group 4 and 5 homologs. These experiments Add 3 μί_ concentrated showed that hahnium does, indeed, sorb on glass like a nitric acid to group 5 element and, therefore, should be placed in the pe dissolve riodic table as the heaviest member of group 5. However, in potassium chloride and other experiments in which anionic fluoride species of tan fume to dryness talum were extracted into methylisobutylketone, hahnium remained in the aqueous phase with niobium. Thus, the 8 seconds Fume again using complexing properties of hahnium are different from those 7 μ ι concentrated 6 seconds of tantalum and cannot be predicted by simple extrapola nitric acid Dry thoroughly tion from the properties of tantalum. with hot air We have explored the extraction behavior of hahnium in 8 seconds 5 seconds Wash with 1.5M Wash with more detail in a series of fruitful collaborations with Kratz nitric acid acetone and Schadel and their groups. In 1988—and again in 1990, 1991, and 1993—they brought their mini-ARCA to Berkeley A 51-second experiment to determine whether hahnium ions sorb to to allow us to perform more complex procedures. In 1992, glass was the first aqueous chemistry to be performed on this our Berkeley group went to GSI for similar collaborative element. Speed was of the essence since the element's longest lived experiments. In these experiments, hahnium sorbed from isotope, Ha, has a half-life of only 34 seconds. In this experiment, developed by Kenneth E. Gregorich when he was a postdoctoral 12M hydrochloric acid on microchromatographic columns fellow in Hoffman's lab, hahnium adheres to glass, as do other (1 mm in diameter and 3 mm long) containing triisooctylamine on an inert support, as did group 5 elements niobigroup 5 elements but not group 4 elements and actinides. (which differ by 0.0016 nm) show differences larger than the 0.0012 nm measured for the analogous lanthanide ions. Thus, the contraction of ionic radii near the end of the actinide se ries is relatively stronger than that near the end of the lanthanides. For lawrencium, however, the last member of the actinides, the radius of the 3+ ion appears to be larger than expected based on simple comparisons with the lighter actin ides and homologous lanthanides, possibly because of the polarizability of the 5f orbitals. Unfortunately, fully relativistic calculations are not avail able for ions heavier than No3+, so we were not able to draw definitive conclusions concerning the relevance of rel ativistic effects. Measurement of the ionic radius of No3+ would provide important data to investigate this effect, but this experiment is very difficult because of the necessity of using strongly oxidizing conditions to maintain nobelium in its trivalent state. Our group has not yet succeeded in con ducting the experiment. We have also tried, unsuccessfully, to reduce Lr3+ with V2+ and Cr2+ in dilute hydrochloric acid using ARCA, suggesting that lawrencium's two 7s electrons may not be sufficiently stabilized by relativistic effects to make it possible to prepare Lr1+. In 1988, after 262Lr, a new isotope of lawrencium with a half-life of 3.6 hours was discovered, a group of researchers headed by Ronald W. Lougheed and E. Kenneth Hulet of Lawrence Livermore National Laboratory used the new iso tope in one of the few examples of atom-at-a-time chemistry coupled with a precipitation method. Because of the isotope's longer half-life, they were able to attempt to reduce Lr3+ to Lr1+ with the strong reducing agent Sm + and coprecipitate the Lr1+ on rubidium tetraphenylborate or chloroplatinate. The precipitate was then physically separated and counted. Although 20 spontaneous fission decays from 262Lr were de tected, the researchers found no evidence for Lr1+. They con-
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Work on heaviest atom chemistry provides excellent training for students Frontier studies of the chemistry of the heaviest elements attract many graduate and undergraduate students to the field of nuclear and radiochemistry. Although the isotopes used in the studies in this report are so short-lived that they do not exist in the environment, study of their chemistry offers excellent training for work with longer lived radioac tive actinide isotopes, such as those of uranium, neptunium, plutonium, americium, and curium. The moni toring and environmental fate of many of these actinides is of great concern, and professionals with the training in nuclear and radiochemistry needed to understand that these isotopes in the environment are in critically short supply. Recent graduates from the heavyelement nuclear and radiochemistry group in the chemistry department of the University of California, Berke ley, are now working in a number of areas. Some of these are studying the behavior of actinides in the environ ment and relating laboratory mea surements to actual field situations; providing fundamental information to permit development of models for predicting the long-term behavior of
actinides in the environment; moni toring the behavior of actinides and other radionuclides in the environ ment; studying nuclear waste isola tion, particularly for long-lived spe cies such as 239Pu; performing ultra sensitive analysis of radioactive and other toxic materials; using remote handling procedures to treat and pro cess radioactive, chemical, and other toxic materials; developing and using computer-controlled automated chem ical processing and data acquisition systems; producing radioisotopes for nuclear medicine; and studying actinide element chemistry in nucle ar reactors and in the nuclear fuel cycle. In 1992,1 contacted the 34 universi ty departments identified by the American Chemical Society's Division of Nuclear Chemistry & Technology as having programs in nuclear chemis try. In total, the 25 departments re sponding had graduated an average of only four doctorates per year over the previous five years with any training in heavy-element chemistry (elements heavier than actinium). A total of three masters' degrees in this area had been awarded during the entire five-year period.
um and tantalum and pseudo-group 5 element protactini um (91). However, hahnium's elution behavior from a mix ture of 4M hydrochloric acid and 0.02M hydrofluoric acid was most like that of protactinium. Like niobium and tanta lum, hahnium eluted promptly from cation-exchange resin columns using unbuffered 0.05M HIB, unlike tetravalent and trivalent metal ions, again verifying that its most stable state in aqueous solution is 5+.
Gas-phase experiments In further collaborations with oirr Swiss and German col leagues, we have investigated the gas-phase chemistry of lawrencium, rutherfordium, and hahnium. The first of these gas-phase experiments was conducted in 1986 when Gaggeler and his group brought OLGA to Berkeley to study the volatility of elemental lawrencium. Modern relativistic the oretical calculations predict a ground-state electronic config uration for lawrencium of [Rn]5/147s27p1/2, instead of [Rn]5f%d7s2 as expected by analogy to lutetium, its lanthanide analog. If so, Gaggeler reasoned, elemental lawren cium might be highly volatile like thallium, which also has a single ρ electron. Our on-line gas chromatographic exper iments, however, found no evidence of lawrencium volatil ity under reducing conditions at 1,000 °C using either quartz or platinum chromatographic columns. Adsorption of the lawrencium onto the column might promote move
Only three or four universities of fer opportunities for graduate re search in this field. During the past decade, the number of such programs has actually decreased as faculty in the field reached retirement age at several universities and were not replaced. In addition, these programs require ac cess to facilities not commonly found in university chemistry departments as well as support from health and safety professionals. To try to address this problem, the Glenn T. Seaborg Institute of Transactinium Science was established in 1991. The institute makes facilities, complex and specialized equipment, and diverse expertise available for student training, visiting faculty, and collaborative research. It is the brain child of Christopher Gatrousis, re cently retired associate director for chemistry and materials science at Lawrence Livermore National Labo ratory. Although centered in Livermore, Calif., the institute's initial participants include the University of California and both Lawrence Livermore and Lawrence Berkeley Labora tories. I hope other national laborato ries and U.S. universities will join as well.
ment of an outer electron from a ρ to a d orbital, however, so this experiment does not exclude a ground-state electron in a 7pi/2 orbital for lawrencium. Early investigations of the gas-phase chemistry of ruther fordium carried out in the late 1960s by Ivo Ζ vara and co workers at Dubna are complicated by the extremely contro versial history of the discovery of that element, probably the most controversial of any element. The controversy arises from the short half-lives of its isotopes—its longest-lived known isotope has a half-life of only 65 seconds—and the fact that the first claims to discovery were based on detec tion by measurement of spontaneous fission alone. Al though the two large fragments into which a nucleus de cays via spontaneous fission are relatively easy to detect, it's impossible to deduce unequivocally from them the atomic number and mass number of the original parent nucleus. The earliest discovery claim for this element, by Georgi N. Flerov and his coworkers at Dubna, was based on the de tection in the bombardment of 242Pu targets with 22Ne of spontaneous fissions with a half-life of about 0.3 second. The researchers assigned this activity to 260104 based on rather ad hoc arguments about nuclear-reaction and halflife systematics. The existence of such a 0.3-second activity has never been confirmed. Zvara and coworkers attempted to form volatile chlorides of this isotope, which they also detected by spontaneous fisMAY 2, 1994 C&EN
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SPECIAL
REPORT
sion. The Transfermium Working Group recalculated some of Zvara's data and concluded in its report in 1992 that these ex periments probably did produce gaseous chloride com pounds of 2:?9Rf, thereby establishing the Dubna group's claim to codiscovery of the element. This retrospective conclusion, however, is disputed and remains controversial. We have used HEVI to study the retention behavior of the volatile chlorides of 262Rf and short-lived isotopes of zirconium and hafnium using quartz chromatography col umns. The retention times (a measure of volatility) of the chlorides of rutherfordium and zirconium are quite similar and much greater than that for hafnium. These experiments show a reversal in trend for rutherfordium and indicate that its volatility cannot be simply extrapolated from those of its lighter group 4 homologs, zirconium and hafnium. A pre liminary result for rutherfordium bromide indicates that it is less volatile than the chloride, in agreement with halide volatilities for zirconium and hafnium based on their vapor pressures. More research is needed on the halides of rutherfordium and their lighter element homologs. It would be especially valuable to devise some means—such as a simple time-offlight mass spectrometer—to determine unequivocally that we are measuring the tetrachlorides in our chloride experi ments. Examination of retention on other surfaces and with a greater variety of halogenating agents would also be useful. The Zvara group also conducted thermochromatographic ex periments on the halides of 261Ha in 1974-76, again using spon taneous fission as their only detection method. They concluded that hahnium is a homolog of the group 5 elements niobium and tantalum, but it is not certain that spontaneous fissions de tected in these experiments belonged only to hahnium. In 1988, the Gaggeler group came to Berkeley again, bring ing OLGA-Π, an improved version of their gas chromatogra phy apparatus, to use in studies of the volatilities of the ha lides of hahnium and its homologs. Further collaborations to study hahnium followed at Berkeley in 1990 and 1993. In
Helium and aerosol 180
beam
Λ
Halogenating agent
Chromatography column
248 Cm or Bk target
Oven Aerosol and carrier gas
Polypropylene film Aerosol & recoils 32
1992, we took HEVI to GSI for collaborative experiments on hahnium and to the Paul Scherrer Institute for experiments on short-lived zirconium and niobium fission products produced at the reactor there. Studies comparing the behavior of the bromides of 262Ha with short-lived isotopes of niobium and tantalum indicate that hahnium bromide is less volatile than either of the other two bromides. This result is in disagreement both with recent relativistic molecular calculations of Valeria Pershina, now at GSI, and colleagues, which predict a higher volatility for HaBr5, and with a nonrelativistic thermodynamic model of Bernd Eichler, now at the Paul Scherrer Institute, which pre dicts nearly equal volatilities for all group 5 pentabromides. In preliminary studies, the chlorides of these elements appear to be more vol atile than their respective bromides. Recent experiments by the Gâggeler group show that the oxyhalides of
'MAn^fca
Temperature, °C 900' 249
Recluster chamber
Graduate student Babak Kadkhodayan adjusts the heavy-element volatility instrument that he designed.
MAY 2, 1994 C&EN
Changeable wheel (80 positions)
To study the gas-phase chemistry of short-lived heavy elements, the recoiling products from the reaction of780 with 249 Bk or i48Cm are attached to potassium chloride aerosols and transported in helium gas to an oven where they are caught on a glass-wool plug and halogenated. Products then pass through a gas chromatography column. After separation, the nuclides of interest are again attached to aerosols and transported to a detection system. Shown here is the Merry-Go-Around system, in which the aerosol-laden gas is deposited on thin polypropylene disks held on a horizontal wheel that can be rotated so as to position the disks successively between pairs of detectors to measure alpha and spontaneous fission half-lives and energies.
lighter group 4 and 5 homologs are much less volatile than are the pure halides. Particularly with the group 5 elements, it's difficult in our experiments to ensure that the fully halogenated pentahalide species are produced, rather than the less volatile oxyhalides. Here, too, determination of the molecular weight of the species needs to be made. Our comparisons of the chemical behavior of both aqueous and gas-phase rutherfordium and hahnium with their lighter homologs in groups 4 and 5 and with the pseudogroup 4 and 5 elements show unexpected differences. They demonstrate that the chemical properties of the transactinide elements cannot be predicted by simple extrapolation from the properties of their lighter homologs. Additional studies need to be performed and compared with theory in order to help develop predictive models. Much more work also remains for theorists. Only a few investigators are currently performing the relativistic and molecular orbital calculations needed to predict the electronic configurations and chemical properties of the heaviest actinides and transactinides. Among them are Galina V. Ivonova at the Institute of Physical Chemistry of the Russian Academy of Sciences in Moscow; Burkhard Fricke of the University of Kassel, Germany; Elijah Johnson at Oak Ridge National Laboratory, and Pershina. Several other groups of Russian scientists are performing calculations concerning relativistic effects in rutherfordium chemistry; they include Viktor A. Glebov and colleagues at the Bochvar All-Union Institute for Inorganic Materials in Moscow and Boris L. Zhuikov of the Institute for Nuclear Research at Moscow-Troitsk, working in collaboration with researchers in the Flerov Laboratory of Nuclear Reactions in Dubna.
Future directions Like heavy-element researchers in Germany, Switzerland, and Russia, we hope to extend our studies of chemical properties to still heavier elements. Elements 106 through
Hoffman (left) and staff scientist Diana M. Lee display their detection system, the Merry-Go-Around.
112 should complete the 6d transition series that begins with rutherfordium and would thus be chemical homologs of tungsten through mercury. Our recent results on rutherfordium and hahnium show that the chemical behavior of these heavier elements will be difficult to predict, particularly the relative influence of the 7p and 6d orbitals. Glenn T. Seaborg, professor of chemistry at the University of California, Berkeley, and codiscoverer of many of the transuranium elements, has even suggested that these orbitals might be close enough in energy and radial extension to give a mixed effect, similar to what happens to the 5f and 6d orbitals in uranium, neptunium and plutonium. Kenneth S. Pitzer, also a chemistry professor at Berkeley, suggested as early as 1979 that, because of relativistic effects, element 112, which would fall in the periodic table directly below mercury, might be a volatile liquid or even a gas. By analogy to the lighter group 6 elements molybdenum and tungsten, element 106 is expected to form very volatile halides and oxyhalides, making gas-phase studies attractive. The name seaborgium (Sg) was recently proposed for this element. Based on their recent Dirac-Slater discretevariational calculations, Pershina and Fricke warn that SgCl6 should be less stable than WC16 and probably so unstable in the gas phase that thermal chromatography experiments will be very difficult. They suggest that the oxychlorides will be better candidates for such studies. Other chemical studies of the very short-lived 263Sg, which has a half-life of 0.9 second, have been considered using an automated solvent extraction system developed for rapid measurement of partition coefficients. The system has been used successfully by a German-Scandinavian collaboration led by Norbert Trautmann at the University of Mainz and Gunnar Skarnemark of Chalmers University of Technology, Gothenburg, Sweden, for radiochemical separations and measurements of short-lived fission products on a time scale of seconds. The method has proven difficult to adapt for measurement of alpha and spontaneous fission activities in the continuous liquid stream produced by the apparatus, but recent advances in scintillation counting techniques make such experiments appear to be feasible. Last fall, a Russian-American collaboration led by Yuri A. Lazerev of Dubna and Lougheed and Hulet of Lawrence Livermore National Laboratory reported production and identification of two new isotopes of seaborgium, 265Sg and 266Sg, which may have half-lives as long as two to 30 seconds. The researchers suggest that these unexpectedly long half-lives are due to the recently postulated stabilizing effect of deformed nuclear shells in the region of 162 neutrons and 108 protons. Isotopes of this stability make both the proposed aqueous and gas-phase chemical studies much more practicable. The known isotopes of nielsbohrium, hassium, and meitnerium have half-lives of only milliseconds, which makes any currently known chemical method of study impossible. However, more neutron-rich isotopes of these elements in the new region of stability around 162 neutrons may well have half-lives long enough to permit studies of their chemical properties. The big problem is not only how to produce these new nuclides, but how to do so with sufficiently large cross sections to produce enough atoms at a time to make studies of their chemical properties feasible. Nuclear reactions using 24 Bk and 254Es targets and neon and oxygen projectiles have been proposed for some time MAY 2, 1994 C&EN
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SPECIAL REPORT for reaching isotopes of nielsbohrium and meitnerium near the region of extra stability. U.S. researchers have proposed experiments using the highly radioactive (276-day half-life), rare isotope 254Es as a target material, but producing this isotope in multimicrogram quantities in existing high-flux reactors is both difficult and expensive, and such a program has never been funded. The very neutron-rich, long-lived (7,400-year half-life) isotope Cm is another promising target that, together with sodium beams, might make nielsbohrium isotopes near the region of 162 neutrons. With argon ions, this target might even produce products in the long-sought island of superheavy elements predicted in the region of 114 protons and 184 neutrons. Yuri Ts. Oganessian and other scientists at Dubna have recently proposed what they call fusionevaporation reactions that would use readily available 238U as a target and 34S projectiles to produce 268Hs, another isotope close to the predicted new stability region. Experiments to produce hassium are currently in progress at Dubna by a collaboration of researchers from Dubna and Lawrence Livermore National Laboratory, but no results are yet available. A similar reaction with 242Pu might produce 272 110. Ghiorso and coworkers at Berkeley have used stable 209 Bi targets and 59Co ions to try to make element 110 via a compound nucleus reaction, but the cross section is apparently very low, and the data are still being analyzed. How far studies of chemical properties can be extended depends on the results of these very exciting attempts to synthesize more stable isotopes of the known elements, and perhaps, ultimately, even the superheavy elements. Studies of nuclear reactions to optimize cross sections are also of utmost importance. Clearly, studies of the chemical and nuclear properties are inextricably intertwined and must proceed hand in hand to be most fruitful. Based on the unexpected results already found in studies of the chemistry of the heaviest elements, this is a frontier that not only challenges scientific ingenuity, but offers unparalleled insights into completely new chemistry not readily extrapolated from current knowledge. •
Suggested Readings Gaggeler, Heinz W. et al. "Gas Phase Chromatography Experiments with Bromides of Tantalum and Element 105/' Radiochim. Acta 57 (1992): 93. Gregorich, Kenneth E. et al. "Aqueous Chemistry of Element 105/' Radiochim. Acta 43 (1988): 223. Harvey, Bernard G., et al. "Criteria for the Discovery of Chemical Elements," Science 193 (1976): 1271-72. Hoffman, Darleane C. "Chemistry of the Transactinide Elements," in "Proceedings of the Robert A. Welch Foundation Conference on Chemical Research XXXIV: Fifty Years with Transuranium Elements." Houston: Welch Foundation, 1990, 255-76.
Reprints of this C&EN special report are available in color at $10 per copy. For orders of 50 or more, subtract 30% from the total order cost. On orders of $50 or less, please send check or money order with request. Send orders to: Distribution, Room 210, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036.
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Hoffman, Darleane C "Nuclear and Chemical Properties of Elements 103,104, and 105," in "Transuranium Elements: A Half Century." L. R. Morss and J. Fuger, editors. Washington: American Chemical Society, 1992,104-15. Hoffman, Darleane C , "Atom-at-a-Time Chemistry," Radiochim. Acta 61 (1993): 123. Hulet, E. Kenneth, "Chemistry of the Elements Es through Element 105," Radiochim. Acta 32 (1983): 7. Hyde, Earl K., Darleane C. Hoffman, and O. L. Keller, Jr. "A History and Analysis of the Discovery of Elements 104 and 105," Radiochim. Acta 42 (1987): 57. Kratz, Jens V. et al., "Chemical Properties of Element 105 in Aqueous Solution: Halide Complex Formation and Anion Exchange into Triisooctylamine," Radiochim. Acta 48 (1989): 121. Schadel, Matthias et al., "ARCA-II: A New Apparatus for Fast, Repetitive HPLC Separations," Radiochim. Acta 48 (1989): 171. Seaborg, Glenn T., "The Transuranium Elements," /. Chem. Ed. 62 (1985): 463. Seaborg, Glenn T. and Walter D. Loveland. "The Elements Beyond Uranium." New York: John Wiley & Sons, Inc., 1990. Darleane C. Hoffman has been a professor of chemistry at the University of California, Berkeley, and leader of the heavyelement nuclear and radiochemistry group at Lawrence Berkeley Laboratory since 1984. During that time, nine of her students have received Ph.D. degrees and three have received masters' degrees. She currently has seven graduate students. In 1991, she became director of the Glenn T. Seaborg Institute for Transactinium Science, centered at Lawrence Livermore National Laboratory. Before joining Berkeley, she was at the Los Alamos National Laboratory, serving most recently as leader of the Isotope & Nuclear Chemistry Division. A native lowan, she received a bachelor's degree in chemistry and a Ph.D. in physical chemistry (nuclear) from Iowa State University. Besides the chemistry of the heaviest elements, Hoffinan's research interests include spontaneous fission properties, nuclear reaction mechanisms and production of neutron-rich heavy-element isotopes, the search for heavy elements in nature, and studies of radionuclide migration in geologic media. Among her accomplishments are the discovery of Pu in nature and the first observation of enfanced symmetric mass division in spontaneous fission in heavy fermium isotopes. Her group performed the first studies of the aqueous chemistry of element 105, hahnium; provided the first direct proof of electron-capture delayedfission;and recently confirmed the discovery of element 106. Hoffman has been honored with a number of awards, including the ACS Award in Nuclear Chemistry in 1983 and the Garvan Medal in 1990. She was elected to membership in the Norwegian Academy of Sciences & Letters in 1990. She is an active member of the ACS Division of Nuclear Chemistry & Technology and is a past chairwoman of the division. She also has chaired the Committee on Nuclear & Radiochemistry of the National Academy of Sciences' National Research Council and the IUPAC Commission on Radiochemistry & Nuclear Techniques.
