Darleane Hoffman: Adventures in the nature of matter

Darleane Hoffman in 2006 and in 1944Darleane Hoffman was born in the small town of Terril, located in Dickinson County in northwest Iowa. Her parents, Carl and Elverna Christian, had moved there as newlyweds in 1925 when Carl was selected as superintendent of the consolidated public school there. It was his first position as a superintendent.

Terril has a population of only 404 persons, about the same now as in Nov. 1926, when Elverna Christian decided to give birth at home. She enlisted the help of her sister, Nellie Clute. On Nov. 8, Darleane Hoffman was born into the hands of these independent Midwestern women.

Perhaps her first experiences shaped her personality and sealed her fate. Hoffman would become a pioneering nuclear chemist and would receive the nation’s highest public honor for a scientist, the National Medal of Science, and the highest private honor for a chemist, the Priestley Medal of the American Chemical Society.

Hoffman herself, in her down-to-earth manner, sees her birthday in a slightly different light. “Actually, I think I was born a day late,” she says. “Had I been born on November seventh, I would share a birthday with both chemist Marie Curie and physicist Lise Meitner, two of my role models.”

Hoffman’s mother was a full-time homemaker who kept busy hosting school functions and parties. She also found time to write for local newspapers. She used a pseudonym because it was not considered appropriate for a school superintendent’s wife to be a local journalist as well.

Although her father often taught mathematics, schools then were the domain of unmarried women. “If a young teacher got married,” says Hoffman, “it was mandatory for her to resign.” Hoffman’s family placed a high value on education, and her father encouraged her to get a teaching credential, but she demurred. For her, teaching was a drab and constrained world of widows and spinsters. “I vowed never to become a teacher,” Hoffman emphasizes.

Hoffman enrolled in Iowa State College in 1944. She took a required freshman chemistry course from a female professor, Nellie Naylor, whose teaching captivated her. Says Hoffman, “Her teaching convinced me that chemistry was the most logical, interesting and practical science in the world.”

Hoffman changed her major to chemistry from applied art, which was then part of the home economics curriculum. Her advisor, also a woman, asked her if she thought chemistry was a suitable profession for a woman.

Like other female teachers, Naylor had remained single. While Hoffman was strongly influenced by Naylor, she did not wish to emulate her life. Instead, Hoffman was drawn to the story of Marie Curie, whom she had read about as a middle-school student.

Curie seemed to have it all—a family, talented children, a husband who respected her abilities and a career doing important scientific research. As it turns out, Hoffman achieved all those things as well.

After she graduated in 1948 with a B.S. in chemistry and math, Hoffman decided to stay on at Iowa State (by then a university) for her Ph.D., where she continued to work with nuclear chemist Donald S. Martin. Together they had discovered several new isotopes using the new synchrotron, a device that used accelerated electrons to generate high energy X-ray beams.

In her first semester of graduate school, Darleane met Marvin Hoffman. He was a first-year physics student who helped build the synchrotron and was allowed to run it at night. Darleane and Marvin would often work late into the evening on experiments. Darleane completed her thesis research in three years, earned her Ph.D. in December 1951, and married Marvin the day after Christmas. Last December they celebrated their 60th wedding anniversary.

OAK RIDGE AND LOS ALAMOS

In early 1952 Darleane left Marvin behind to begin working at Oak Ridge National Laboratory, TN. Marvin joined her later that year while he completed writing his dissertation. It was Marvin who suggested that they head west to Los Alamos National Laboratory, NM, where he had been a summer graduate student assistant. Marvin liked the climate, which reminded him of Colorado, where he had lived in his youth. Marvin left first, in the fall of 1952, and Darleane followed a few weeks later.

After living in Iowa and then among the green hills and lakes of Tennessee, Darleane found Los Alamos to be a dramatic change of scenery. Located 25 miles northwest of Santa Fe at an altitude of about 7,500 feet, the lab sprawls across several acres of volcanic tuff atop the austere yet spectacular Pajarito Plateau. The location was chosen during WWII for top-secret nuclear weapons research precisely because of its remoteness, a characteristic that it retains even today.

Within a decade of their arrival at Los Alamos, the Hoffmans had settled down and begun raising a family, although Darleane continued to work full time. Daughter Maureane was born in 1957, followed by a son, Daryl, in 1959. Maureane, an M.D./Ph.D., works with the Veterans Administration and at Duke University in Durham, NC. Daryl is a plastic surgeon in Palo Alto, CA.

RESEARCH IN NEVADA

Darleane’s job during the 1950s didn’t always allow her to be home at night. There were several days when her office was the Nevada Test Site, a desolate 1,360-square-mile tract of desert north of Las Vegas. There, during the heyday of the Cold War, the site was the location of almost 1,000 nuclear explosions, more than 90 percent of them underground.

Darleane Hoffman

Darleane Hoffman pipettes radioactive solutions at the Institute for Atomic Research in Ames, IA, in this 1950 photo.

At the test site, Hoffman monitored the specimens that the crew recovered as they drilled down into the cavity created by the explosion and extracted the highly radioactive core samples. These were carefully shipped back to Los Alamos for detailed radiochemical analysis to determine device performance.

“For the early drillbacks,” she says, “the drill rig was placed right in the explosion cavity and the drilling was straight down. But after it was realized that a few of the cavities had experienced further collapse, the drill rig was set to one side on the ground above, and ‘slant’ drilling was conducted.” During that fieldwork, in the middle of the desert, surrounded by researchers and drilling-rig roughnecks, Hoffman was often the only woman for miles.

A non-commercial flight back from the test site to New Mexico became Hoffman’s most memorable airplane trip. She was driven to a nearby airstrip, where the flight crew assisted her with her parachute and guided her to the Plexiglas nose cone of a WWII B-25 bomber. She flew home admiring the panoramic view.

Recalls Hoffman, “They actually tried to pull a joke on me and said we might have to make an emergency landing. I told them there was no way they would ever get me to jump out—I was going down with the plane.” Hoffman was no stranger to flying. She had taken flying lessons and completed her solo flight requirement for her pilot’s license but could not find the time in her busy schedule for the required cross-country trip.

At Los Alamos, Hoffman conducted research on the samples that sometimes revealed new isotopes. One nuclear test in particular, the “Mike” test in the South Pacific, yielded an unexpected find, a new isotope of plutonium, plutonium 244. Subsequent measurements showed that it had a half-life of more than 80 million years.

Says Hoffman, “For years it had been assumed that uranium 238, which has a half-life of 4.5 billion years, was the heaviest naturallyoccurring radioactive isotope. With the discovery of plutonium 244, we began to speculate about whether it might have been created during the last nucleosynthesis in our solar system some 4.5 to 5 billion years ago. If that event created uranium 238 it might have also created plutonium 244, and some of it might still be left on planet Earth.”

The hunt was on for plutonium 244 in nature. Hoffman knew that the redox chemistry of plutonium was similar to cerium, one of the lanthanide (rare earth) elements located just above the actinides in the periodic table. That led her to a location that is currently in the news, California’s Mountain Pass mine. Recently reopened after China began restricting exports of rare earth minerals, the Mountain Pass mine is the best-known source of lanthanides in the United States.

Hoffman acquired nine liters of a recycled solvent used to purify the rare earth element cerium from the Mountain Pass mine. Given her knowledge of plutonium chemistry, Hoffman hypothesized that any plutonium 244 would concentrate in the solvent.

Using standard, large-scale and complicated wet lab chemistry, Hoffman and co-workers were able to purify the sample and confirm the presence of the plutonium 244 isotope. They relied on extremely sensitive mass spectroscopy conducted at the Knolls Atomic Power Laboratory. Located in upstate New York, this lab provides research support for the U.S. Navy’s nuclear propulsion program.

Hoffman and co-workers estimated they had detected 20 million atoms, a concentration of one part in 1018, or one atom in a billion billion atoms of ore. The research was published in Nature in November, 1971. Seaborg later called this work “an experimental tour de force.”

KEY RESEARCH BREAKTHROUGH

That year, 1971, was a banner year for another one of Hoffman’s research interests. She had continued to look for new isotopes of heavy elements in the core samples from underground nuclear tests at the Nevada Test Site. The heaviest element she was able to detect was fermium 257, an actinide with an atomic number of 100 and a half-life of about 100 days.

Her team isolated enough of the isotope to study its spontaneous fission, and one day while she was at home nursing a bad case of flu, she reviewed the data and made a surprising discovery. The fermium nucleus was not splitting in ways that were consistent with existing theory. Although most nuclei split into two unequal parts, a substantial number split symmetrically into two equal-mass fragments, something existing theory did not account for.

When Hoffman presented these results at a meeting of the American Physical Society, the physicists filling the room were dismissive. “They thought I, as a chemist, must have made a mistake,” says Hoffman. It didn’t matter for long, because Hoffman’s research group at Los Alamos, along with colleagues from Lawrence Livermore National Laboratory (LLNL), soon validated the results.

Hoffman had discovered the importance of nuclear shells in spontaneous fission. It had been postulated many years earlier that the atomic nucleus had shells that were similar to electron shells. Hoffman continued her work on clarifying the role of these nuclear shells in spontaneous fission of fermium isotopes. Her research verified the importance of nuclear shells in spontaneous fission decay.

Hoffman and colleagues produced more neutron-rich isotopes of fermium. She says, “The very short-lived isotope fermium 258 can divide symmetrically into two equal-mass fragments, both consisting of nearly filled neutron and proton nuclear shells.”

Adds Hoffman, “It was one of my most important discoveries. Until then all spontaneous fission was assumed to be asymmetric. The importance of nuclear shells in fission fragments greatly advanced the fundamental understanding of spontaneous fission.” When Seaborg in Berkeley heard of the findings, he called it “a monumental step” in understanding the nature of fission.

LAWRENCE BERKELEY NATIONAL LABORATORY

Throughout the 1970s, Seaborg had developed a growing respect for Hoffman’s scientific work. In 1978 she was awarded a Guggenheim Fellowship, and Seaborg invited her to work at Lawrence Berkeley National Laboratory, where she spent almost a full year working on spontaneous fission at the 88-inch cyclotron.

She returned to Los Alamos in 1979 to lead the Chemistry and Nuclear Chemistry Division, the first woman to lead a scientific division at Los Alamos. Although for a time she did try to travel back and forth to Berkeley, her administrative duties left her little time for her own research, and it became increasingly difficult to find time for it.

Seaborg turned 70 in 1982. His retirement pending, he sensed Hoffman’s desire to continue her research and saw an opportunity to lure her to LBNL to insure that nuclear chemistry would continue at Berkeley. With help from other nuclear chemists at LBNL and at UC Berkeley, he was able to put together an offer to bring Hoffman back in 1984—a full professorship at UC Berkeley and the leadership of the Heavy Element Nuclear and Radiochemistry Group at LBNL. Advised husband Marvin, “You’d be a fool not to accept the offer.”

“It did cause some disruption for Marvin,” she says. “But by then he had earned an advanced degree from the Anderson School of Management at the University of New Mexico in Albuquerque, and he was working as a business consultant.”

After 31 years at Los Alamos, Darleane and Marvin took the big step of moving to Berkeley in August 1984. She says, “I was hesitant. I certainly didn’t come back to try to replace Seaborg—nobody could replace Glenn Seaborg. And I was very devoted to my division at Los Alamos, but I felt it was in good shape and that it was time for me to help educate the next generation of students in nuclear and radiochemistry.”

She adds, “Of course, this meant that I had to ‘eat my words’ and renege on the vow I had made in my youth never to teach! But in the ’80s, there were very few women professors in chemistry departments at major universities and I felt an obligation to help train nuclear and radiochemists—women as well as men. I was only the second woman on the chemistry faculty at Berkeley, coming shortly after Judith Klinman, who later became the first woman chair of the chemistry department.”

Hoffman is grateful for the years she was able to work with Seaborg, who died in 1999. “He was kind and generous, and a wonderful mentor,” she says. “At times, he could be hard to keep up with—especially on those steep stairs between campus and the lab.”

LBNL, sometimes referred to as “the Hill,” perches 300 vertical feet above the eastern boundary of the UC Berkeley campus. There is a little-known staircase that winds up the hillside, used only by the most fit researchers.

Says Hoffman, “Seaborg was a lanky six feet three inches tall. I am barely over five feet. He preferred the stairs for attending meetings on campus. Climbing back up the hill, it was literally hard for me to keep up. Even into his 70s he could climb those stairs at a fast clip.”

Once settled in at LBNL, Hoffman continued her work discovering and characterizing the isotopes of heavy elements. Following her earlier work on fermium, element 100, she continued to work her way up to bohrium, element 107, in addition to verifying the existence of even heavier elements. Her group also confirmed the 1974 discovery of element 106, thus allowing it to be named seaborgium in honor of Glenn Seaborg.

ACCOLADES FOR RESEARCH

Darlene Hoffman and President Clinton

Darleane Hoffman accepts the 1997 National Medal of Science from President Bill Clinton. Hoffman led teams that confirmed the existence of element 106, seaborgium, and discovered plutonium 244, the only isotope heavier than uranium found in nature.

In 1991 she retired from active teaching to help create the Seaborg Institute for Transactinium Science at LLNL and served as its first director from 1991–96. In 1997, President Bill Clinton awarded her the National Medal of Science, the highest public honor a scientist can attain in the United States. In 2000 she received the Priestley Medal of the American Chemical Society, the nation’s highest private honor for a chemist.

In November 2011, she flew to Oslo, Norway, to speak at a symposium honoring the 100th anniversary of Marie Curie’s Nobel Prize in chemistry. The symposium was held on November 7, the 144th birthday of Curie, born in 1867.

This was not the first trip to Norway for Hoffman. In 1964, she had been awarded an NSF postdoctoral fellowship, while husband Marvin was awarded a Fulbright fellowship. The whole Hoffman family, including Darleane’s mother, spent a year in Olso. They visited the ancestral homes of Darleane’s great-grandparents, and her daughter attended first grade in the neighborhood school, where she became fluent in what she jokingly refers to as “first-grade Norwegian.” Hoffman was elected to membership in the Norwegian Academy of Science and Letters in 1991.

In 1964, Darleane met nuclear chemist and human rights activist Ellen Gleditsch. Gleditsch had been Curie’s assistant in France but returned to Norway and became a prominent nuclear chemist in her own right. She sheltered and employed women scientists who escaped from Nazi Germany and was active in the resistance movement when Norway was occupied during WWII.

At the Curie symposium in November Hoffman spoke near the end of the conference. Says Hoffman, “I spoke just after Hélène Langevin-Joliot, the granddaughter of Marie Curie.” Langevin-Joliot is a nuclear physicist in France. Hoffman continues, “I had a hard act to follow, but I concentrated on quoting Marie Curie’s 1911 Nobel Prize lecture in which she predicted the use of ‘atom-at-a-time’ chemistry to discover new radioisotopes and elements.”

Hoffman has not only been a role model for women scientists, she has worked with organizations to encourage young women’s interest in the sciences. When asked why so many prominent nuclear chemists were women, Hoffman replies, “It was a new field, and women were able to step into new roles. There was a less-established hierarchy, and women encountered fewer obstacles than in the more traditional scientific fields.”

Still active at 85, Hoffman retains a restless quality. She enjoys keeping in touch with many colleagues in the nuclear chemistry field and acting as co-organizer for various scientific conferences.

For Hoffman, nuclear chemistry is a uniquely fundamental form of research, one that probes the deepest nature of what we call matter. But she adds, “There is also an array of practical issues that require the expertise of nuclear chemists—new and safer nuclear reactor designs, better medical diagnostics and radio-pharmaceuticals, more sensitive techniques for detecting proliferation, safer nuclear waste storage and environmental remediation, to name but a few. The field is wide open—there are many great discoveries yet to be made.”