|PRIESTLEY MEDAL PROFILE
Volume 78, Number 13
CENEAR 78 13 pp.
Darleane C. Hoffman, 2000 Priestley Medalist, is honored for her discoveries on the frontiers of the periodic table
Nuclear chemist Darleane Christian Hoffman formally retired in 1991, but you'd never know it. Rather than slowing down, she now finds herself busier than ever doing science. She is one of the leaders of a research team that has been making stunning discoveries at the frontiers of the periodic table. Just last year, for example, the team created elements 118 and 116--the heaviest yet seen. She continues to advise her graduate students, organize international conferences, and serve on scientific advisory and review committees. And since she doesn't teach classes anymore, she spends more time jetting around the world, giving invited talks and conferring with colleagues.
Hoffman agreed to go on "retirement" because the University of California, Berkeley, where she was a professor in the chemistry department, offered its eligible employees a monetary incentive they couldn't refuse: "I could get as much not working as working," Hoffman tells C&EN. So now she draws retirement pay, but no salary. "I just work for free," she remarks.
Which is to say, she works for the love of it.
In recognition of the pioneering work she has done for almost 50 years, the American Chemical Society this week is awarding Hoffman its highest honor, the Priestley Medal, at the society's national meeting in San Francisco.
Not bad for someone who started out, in her own words, as "a naive, small-town Iowa girl" with less than full confidence in her abilities. Today, she is considerably more self-assured. But Hoffman freely admits that she was "totally shocked" when told that she had won the medal. "I knew that the chemistry department had nominated me, but I thought there was no chance at all," she says quietly. "So I was really surprised. Nuclear chemists haven't been all that popular."
Maybe so. But in the nuclear science community, Hoffman is a highly regarded researcher. Several years ago, the late Glenn T. Seaborg , who shared the 1951 Nobel Prize in Chemistry, called Hoffman "the premier woman nuclear scientist in the world today." He also considered her "a highly valued colleague" during the last decade or two of his life.
Relatively few women of Hoffman's generation have achieved her stature in the scientific world. She has been a trailblazer, and as a result, many more women are likely to follow in her footsteps and achieve leadership positions in science.
Hoffman was born (as Darleane Christian) on Nov. 8, 1926, in Terril, Iowa, a town whose population has hovered around 400 for most of the past century. Her father was a school superintendent who also taught mathematics. Her mother, who worked in the home but occasionally did substitute teaching, was most interested in music and dramatics. The young Hoffman developed interests in math, art, and music. When she graduated from high school, she hadn't yet taken any chemistry courses and was trying to decide whether to pursue studies in math or art.
Hoffman enrolled in what was then Iowa State College (later to become Iowa State University of Science & Technology) in Ames, Iowa, as an applied art major. "To take applied art," she explains, "you had to enter into the home economics curriculum." One of the required courses was freshman chemistry, which was taught by Nellie Naylor. "She was an incredibly good teacher" and made the subject come alive for Hoffman. "I decided it was the most interesting thing I'd ever heard about. So after my second quarter, I switched my major to chemistry," Hoffman says.
Before she did that, though, Hoffman informed her adviser, a woman who was also the applied art teacher, that she was thinking of switching to chemistry. The adviser's comment was: "Well, do you really think chemistry is a suitable profession for a woman?" Hoffman replied, "Of course--my chemistry professor is a woman."
"It just so happens that both these women were what we used to call spinsters," Hoffman says. And that is definitely not what she wanted to become. "I wanted to be more like Marie Curie, whose story I had read in junior high. I really admired her because she was able to work in the lab with her husband, and one of her daughters became a nuclear physicist while the other one was a well-known war correspondent, author, and accomplished pianist."
Hoffman's father was supportive of his daughter when she announced her intention to study chemistry in graduate school. But he suggested she get a teaching certificate so she would be able to find a job. Hoffman rebelled against this idea: "I said the one thing I never want to do is teach."
Even today, Hoffman isn't sure why she felt that way. It may have been youthful rebellion, a reaction to the reality that she came "from a long line of teachers" and that teaching was one of the main professions that women traditionally pursued. "I guess I also kind of sensed that women didn't get that fair a shake being teachers because at that time [before World War II] I observed that if a woman married, she had to quit her teaching job. And heaven forbid if she became pregnant! I don't think I thought out all these things that clearly, but I sensed that this was not the profession I wanted to follow."
Later in life, however, her views on teaching would do a flip-flop.
Hoffman usually was the only woman in her chemistry classes. But she didn't feel alone. She had female friends who were science students, and she would get together with them for lunch. She says she never was discouraged from studying chemistry, and she never felt discriminated against because she was a woman.
To support herself in college, Hoffman worked at various jobs. One in particular changed her life because it introduced her to nuclear chemistry and, ultimately, to her future husband: One summer she began working as an undergraduate research assistant at the Institute for Atomic Energy, which was on the Iowa State campus. The institute, which was partially financed by the state of Iowa and the federal government, later became the Department of Energy's Ames Laboratory.
Darleane Christian at five years old (above) with baby brother Sherril in Dec. 1931. Darleane's photo (middle) in the Iowa State College yearbook, 1945. Darleane at about three years of age (right) on her gradfather Christian's farm near Decorah, Iowa.
The 68-MeV synchrotron, "probably the highest energy synchrotron at that time," according to Hoffman, was being built at the institute when she started working there under Donald S. Martin Jr., an inorganic and nuclear chemist. When the synchrotron became available, she used it to irradiate inorganic metal complexes that she synthesized in the lab. When a stable metal atom, such as cobalt or platinum, is irradiated with photons of synchrotron radiation (similar to rays), Hoffman explains, the nucleus can spit out a neutron, a proton, or a neutron-proton pair (a deuteron). The result is a new isotope. She and Martin discovered a number of new isotopes in the course of studying such photonuclear reactions.
Early on, Hoffman became fascinated with the study of radioactivity. "The work was exciting because nobody had done much on photonuclear reactions," she points out. What fascinated her was "the potential for finding new isotopes and new elements--seeing something that nobody else had seen before."
Hoffman worked summers and during the school year at the institute. When she received her B.S. degree in chemistry in 1948, Martin was ready to recommend her to a top-notch California university for graduate studies. But being "a small-town Iowa girl, I couldn't imagine going to California to go to school," Hoffman says. "So I didn't."
She opted to stay at Iowa State and get her doctorate by further investigating photonuclear reactions at the synchrotron. That decision turned out to be fortunate, scientifically as well as personally, she says. When her father died suddenly of a heart attack two years later, she was close enough to home to take care of family matters. That responsibility fell to her because she was five years older than her brother Sherril, her only sibling.
During Hoffman's first semester in graduate school, she met her future husband, Marvin M. Hoffman, who was also a first-year graduate student. He was studying nuclear physics and was able to run the synchrotron in the evenings, allowing Darleane to carry out her irradiations almost any time she wanted.
In December 1951, after spending just over three years on her thesis research, Darleane received her Ph.D. degree. She and Marvin were married that same month on the day after Christmas.
In January 1952, Darleane took a position as a chemist at Oak Ridge National Laboratory, while Marvin stayed at Ames to finish his Ph.D. work. It was "fairly unconventional in those days" for a woman to go off to a job without her husband, she says, but she was eager to begin her career "and I needed the money!"
During the summer of 1952, Marvin joined her in Oak Ridge and finished writing his thesis. After he received his Ph.D. in nuclear physics, he considered taking a position at Oak Ridge. Darleane was busy working on what she calls "the ill-fated aircraft nuclear propulsion project." It was interesting, she says, but "I wasn't really doing the nuclear chemistry I wanted to do."
So when Marvin suggested that they both go to work at Los Alamos National Laboratory, where he had had a summer job, Darleane was thrilled. Los Alamos "had a very large radiochemistry effort, primarily concerned with diagnosing nuclear weapon tests." Marvin assured her that there would be no trouble finding her a position there.
Marvin started work at Los Alamos in October 1952, and Darleane followed him there right after Christmas. But when she called the personnel department to inquire about the job she thought was waiting for her in the radiochemical diagnostics division, she was told, "No, that couldn't possibly be--we don't hire women in that division."
"That was my first real head-to-head encounter with discrimination against women," Hoffman says. "And it came as a great shock to me."
Shortly thereafter, she and Marvin went to a cocktail party for new hires and their spouses at the Los Alamos lab. There she met Roderick W. Spence, the head of the radiochemistry group. As Hoffman recalls, she told him she was trying to find the job she had been promised, and he said, "Oh, I've been looking for you. Where have you been?" He arranged an interview for the following Monday, and hired Hoffman on the spot.
Spence needed to hire additional radiochemists because his group was analyzing the debris from the first thermonuclear test, code-named "Mike," which had been conducted in the South Pacific just two months earlier. Unfortunately, Hoffman couldn't begin working immediately because the security office couldn't locate the security clearance that had allowed her to work at Oak Ridge. It was eventually found, but the snafu kept Hoffman sidelined for three frustrating months.
Finally, in March 1953, she began working at Los Alamos. But by that time, Spence and his colleagues, including Seaborg and Albert Ghiorso at what later came to be called Lawrence Berkeley National Laboratory (LBNL), had discovered two new transuranium elements--einsteinium and fermium--in the debris from the Mike explosion. Missing that opportunity "was one of the biggest disappointments of my life," Hoffman says. "One of the few, I might say."
Darleane and Marvin Hoffman, seated at the piano, sing along with their children Daryl and Maureane in their home in Pajarito Acres, Los Alamos, N.M., in 1974. [LANL photo]
Scientists sifting through the Mike debris also discovered a new isotope of plutonium (244Pu) with a half-life of about 83 million years. In the years following this discovery, researchers found indirect evidence that 244Pu was present in the early solar system. And this led them to hypothesize that traces of primordial244 Pu, thanks to its very slow decay into a thorium isotope, might still be found on Earth. But no one was able to say for sure--until Hoffman and her coworkers embarked on the quest.
Hoffman's team looked in a rare-earth ore believed to contain thorium and other elements with a chemistry similar to plutonium's. They obtained a partially processed ore fraction equivalent to about 85 kg of ore from a mining company and performed chemical separations to concentrate that down to a small sample. In what Seaborg later called "an experimental tour de force," Hoffman and her colleagues were able to detect 20 million atoms of 244Pu--equivalent to a concentration of one part in about 1018 of ore. "Before this discovery," Seaborg later wrote, "238U was thought to be the heaviest naturally occurring isotope of primordial origin."
At Los Alamos, Hoffman also was involved in searches for new heavy elements and isotopes in the debris from nuclear tests. The heaviest nuclide they were able to isolate, though, was a fermium isotope, 257Fm, formed when 238U gobbles up 19 neutrons. Her team was able to isolate enough 257Fm from the debris of an underground nuclear test to measure its spontaneous fission properties. This enabled Hoffman and coworkers to make what Seaborg, writing a few years ago, called "probably the most important discovery in the understanding of the fission process in the last quarter century."
When a nucleus undergoes fission, it splits into two chunks that generally eject a few neutrons. In all the previous studies of spontaneous fission or low-energy fission, Hoffman explains, the nucleus invariably split into fragments of unequal mass, a process called asymmetric fission.
Ideally, Hoffman would have liked to study 264Fm because it could split symmetrically into two 132Sn nuclei. The132 Sn nucleus is thought to be especially stable because it has the "magic" number of protons (50) and neutrons (82) required to completely fill their respective shells in the nucleus. This stability would therefore influence the fission to follow this path.
Unfortunately, 264Fm was unknown. The best Hoffman could do in 1971 was examine the spontaneous fission of257 Fm, which has too few neutrons to split into two of the "doubly magic" 132Sn nuclei.
"I was home, sick with the flu, analyzing the data," Hoffman recalls. And it became apparent to her that, although most of the 257Fm nuclei were splitting into two unequal fragments, a significant percentage appeared to be splitting more symmetrically. As her group later showed, this tendency toward symmetric fission, beginning with 257Fm, increases with mass number until it is the most probable process for 259Fm.
That one or two mass numbers could make such a difference was "a monumental step" in the understanding of fission, according to Seaborg. But the first time Hoffman reported her findings at a physics conference, the physicists were skeptical. "They said, 'Well, you chemists don't know how to measure these things. You better go home and look at your detectors,' " she says with a chuckle. "But we convinced them."
Hoffman's results on fission were at odds with previous theoretical models and predictions, and have served to spark the development of new theories to explain them.
As a member of the radiochemistry group, which was in the weapons testing division at Los Alamos, Hoffman took trips with colleagues to the site in Nevada where the government conducted underground nuclear tests. There, they would drill into the ground and collect samples of the debris from the tests for radiochemical analysis. Hoffman was the first woman to do this.
On her first sampling trip to the Nevada test site, she relates, the local test director laid eyes on her and uttered, "Oh, my God, not a woman!" The test director, who later became good friends with Hoffman, reacted that way, she explains, because "the drillers were very superstitious about having women around."
In 1975, Hoffman became one of the leaders of a field study designed to look at the potential for radionuclide migration away from the site of underground nuclear tests. Her work on this project, Seaborg later noted, was a seminal contribution to our understanding of radionuclide behavior in the environment. The project later led to an effort aimed at finding a suitable site for an underground repository for nuclear waste at the Nevada test site, a program now known as the Yucca Mountain Project.
In 1978, while still at Los Alamos, Hoffman was awarded a Guggenheim Fellowship. This allowed her to spend nine months at Lawrence Berkeley National Lab, continuing her study of the mechanisms of nuclear fission. There, in association with Seaborg's group, she was able to use the lab's 88-inch cyclotron to make isotopes that were not otherwise available and to study their spontaneous fission properties. She also became involved in the Berkeley group's then-unsuccessful search for superheavy elements that were predicted to have much longer half-lives than the heaviest known elements.
On her return to Los Alamos in 1979, Hoffman became head of the 160-member Chemistry-Nuclear Chemistry Division, the first woman to fill such a position at the lab. Unfortunately, her administrative and managerial duties made it more difficult for her to conduct research. She occasionally went back to Berkeley to do experiments with Seaborg's group, but it became increasingly difficult to do that.
Then, in 1984, Hoffman was offered a full professorship in the chemistry department at UC Berkeley. Because of the university's close association with LBNL, the offer also included the position of leader of the Heavy Element Nuclear & Radiochemistry Group in the Berkeley lab's Nuclear Science Division. The offer was presented to Hoffman by a group of nuclear chemistry professors, including Seaborg who, at age 72, was stepping down as leader of the heavy-element chemistry group.
Hoffman and coworker Diana M. Lee show off the Merry-Go-Around rotating wheel system at Lawrence Berkeley National Laboratory's 88-inch cyclotron in 1979. They used the system initially to study the spontaneous fission properties of fermium isotopes. [LBNL photo]
"My husband said, 'You'd be a fool not to accept the offer,' " Hoffman recalls, and after much debate with herself, she did accept it. So after spending 31 years at Los Alamos, she made the leap to Berkeley.
The move allowed her to return to a more active involvement in the investigation of the heaviest elements--and to do it at LBNL, one of the premier heavy-element research centers in the world.
The move also allowed her to do something she had vowed she would never do--teach. By the time she moved to Berkeley, Hoffman points out, "I had recanted and felt that it was very important to train the next generation of students in nuclear and radiochemistry." At Los Alamos, in fact, she had been involved in a number of programs to attract young women into science.
Now, at Berkeley, Hoffman finally had the opportunity to teach, and "I was really devoted to it," she says. But the first year there "was the hardest year of my entire life" because the undergraduate teaching lab in nuclear chemistry was full of antiquated equipment, and she had to oversee its conversion into a more modern lab.
Another reason Hoffman decided to plunge into the academic world was because "there were so few women professors at major universities. I was only the second woman on the chemistry faculty at UC Berkeley," she notes. Since her arrival, four additional women professors "have come up through the ranks" to join the chemistry faculty, she says with some satisfaction.
But snaring a tenured professorship at a major university is still difficult for women, she says. Because of the long hours involved, many women seeking this goal delay having children until they get tenure, but some women aren't willing to do that. There's definitely a disconnect between the number of women who receive a Ph.D. in chemistry and the number that become professors at major universities, Hoffman says.
At Berkeley, Hoffman and her coworkers spearheaded a renaissance in the one-atom-at-a-time study of heavy-element chemistry. The elements her group has been studying are produced one atom at a time in an accelerator by bombarding a heavy-element target with a highly intense beam of heavy ions. And because the newly created atoms exist for only minutes, seconds, or less, their chemical behavior must be examined immediately--before they decay into other elements.
In addition to studies of element 104 (rutherfordium), Hoffman's group was the first to investigate the aqueous solution chemistry of element 105. Those studies were carried out using an isotope of 105 having a 34-second half-life. At the time, 105 was known informally as hahnium (one of its proposed names), but was later officially named dubnium.
In 1993, Hoffman and her colleagues confirmed the 1974 discovery of element 106 (seaborgium) by a team of researchers from LBNL and Lawrence Livermore National Laboratory. Hoffman's team then undertook the first investigations of seaborgium's chemical properties, also one atom at a time.
Her group has shown that the chemical properties of 104, 105, and 106 cannot simply be extrapolated from what is known about the chemical behavior of their lighter homologs in groups 4, 5, and 6 of the periodic table. That's because the high positive charge of the nucleus perturbs the atom's electronic structure, producing chemical properties different in some cases than would be expected.
During the past year, Hoffman and her colleagues have gone a step further, embarking on the first studies of the chemistry of element 107 (bohrium).
In the past two decades especially, Hoffman's growing list of accomplishments has been recognized and honored by ACS and other organizations. In 1983, for instance, she became the first and only woman to receive the ACS Award for Nuclear Chemistry. In 1990, she garnered the ACS Garvan-Olin Medal, which honors the accomplishments of women chemists. And in 1997, President Clin-ton awarded her the National Medal of Science.
Last year was a banner year for Hoffman. First she was selected to receive the Priestley Medal--only the second woman to be so honored (Mary L. Good received it in 1997). Then in June, the heavy-element group that she now co-directs with nuclear chemists Kenneth E. Gregorich and Heino Nitsche announced the creation of elements 118, 116, and 114--the first superheavy elements and arguably the most exciting discovery of her career ( C&EN, June 14, 1999, page 6 ). "I would have been the maddest woman in the whole universe if I had really retired before this last discovery was made," she says with a laugh.
Hoffman and Seaborg are all smiles at a summer 1994 party to celebrate confirmation of the discovery of element 106 and the proposal to name it seaborgium. [LLNL photo]
The observation of elements 118, 116, and 114 at Berkeley followed an earlier--and somewhat weaker--claim for the observation of 114 from a Russian-American group working in Russia ( C&EN, Feb. 1, 1999, page 8 ). All of these observations will have to be confirmed before any conclusions about priority of discovery can be made, Hoffman cautions.
Nuclear researchers have been looking for superheavies for decades, Hoffman notes, and the discoveries open up "a whole new universe" for scientists to explore. That's because the new elements confirm theoretical predictions that certain superheavy nuclei will be much more stable (and longer lived) than has been typical for the transfermium elements.
Element 118 was produced by bombarding a 208Pb target with86 Kr ions. Atoms of 118 then went through a sequence of -particle emissions, decaying first to element 116, then 114, and then to new isotopes of 112, 110, 108, and 106. Hoffman says the group will be doing further experiments on this decay chain. They also hope to make element 119 for the first time and watch it decay through a chain of three more new elements--117, 115, and 113. "So we have lots of big plans," she says.
At the award ceremony in Washington, DC., President Clinton congratulates Hoffman on her winning the 1997 National Medal of Science.
When asked how long she expects to continue working, Hoffman laughs and says, "I don't know. I at least want to see this [superheavy element work] through."
Even as she and her colleagues continue their efforts to further extend the periodic table into unexplored territory, Hoffman is trying to wind down on at least one aspect of her scientific activities: She didn't take on any new graduate students in 1998 and accepted only one in 1999--"because he really talked me into it," she says. "But I will stop taking new students after this academic year."
Hoffman also has wound down her involvement with the Glenn T. Seaborg Institute for Transactinium Science , which she helped establish at Lawrence Livermore in 1991. The institute aims to further the education and training of students and other researchers in heavy-element science by providing them access to the national labs' facilities and expertise, which are not available at most U.S. universities. Hoffman served for five years as the first director of the Seaborg Institute. Today, she is a senior adviser there.
In addition, Hoffman has a new book out, which she coauthored with Seaborg and Ghiorso, another veteran heavy-element discoverer at LBNL. "The Transuranium People: The Inside Story" relates the story of how elements 93 (neptunium) to 112 (still unnamed) were discovered and studied. An epilogue on the recent observations of the superheavy elements was added during production. The book also includes personal information on the authors and their families.
As important as her work has been to her, Hoffman also is devoted to her family. Darleane and Marvin's daughter, Maureane, an M.D.-Ph.D., is a tenured professor at Duke University Medical School. Their son, Daryl, is a plastic surgeon in Palo Alto, Calif. Daryl and his wife, an internist, have provided them with three young grandchildren to dote on.
But Hoffman, being who she is, can only dote so much. And real retirement seems to hold little appeal. After all, new elements and isotopes, as well as new chemistry, wait to be discovered. And as her colleague Gregorich observes, "She likes being very busy."