Franco Dino Rasetti, professor emeritus of physics at Johns Hopkins and the last living holder of the patent on the atomic reactor, died on Dec. 5 in a retirement home in Waremme, Belgium, where he lived with his wife, Madeleine. He was 100.

Rasetti was a member of a key group of Italian physicists led by Nobelist Enrico Fermi who created the world's first nuclear reaction in the 1930s. Although he avoided media coverage, Rasetti won widespread notice when he refused Fermi's invitation to join the Manhattan Project, the group of scientists that created the world's first atomic bomb. Rasetti said he based his decision on his moral objections to harnessing nuclear research for wartime purposes.

According to the Italian newspaper Il Messagero, Rasetti wrote of his decision, "Discovering the secrets of nature is among the most fascinating things that one can do, but I must say that the most fascinating is also the most perilous. Men have to ask themselves about their motivations in their hearts. And scientists don't do that very often."

In addition to his accomplishments with the Fermi group and on his own as a physicist, Rasetti was an internationally recognized expert in trilobites, a type of fossilized arthropod, and on the wildflowers of the Alps.

Rasetti was born Aug. 10, 1901, in Pozzuolo Umbro, near Perugia, Italy. He began studying engineering at the Scuola Normale di Pisa, but after meeting Fermi changed his focus to physics. Fermi later recruited Rasetti to come to the University of Rome's Physics Department, known as the Institute via Panisperna after the name of the street on which the laboratory was located. While there in the 1920s, Rasetti obtained key evidence that scientists' understanding of the nucleus of an atom might be based on a faulty assumption.

At the time, scientists thought an atom's nucleus was composed entirely of subatomic particles known as protons, each of which had a mass of one and a positive charge of one in atomic units. However, scientists had shown that the nucleus of the nitrogen atom had a charge of seven and a mass of 14, which didn't seem to add up. To explain the discrepancy, researchers speculated that a nitrogen nucleus might contain 14 protons and seven electrons. Because electrons have less than one-thousandth the mass of a proton and are negatively charged, they could produce the right charge without throwing off the mass.

Rasetti used spectroscopy and a quantum mechanical phenomenon known as spin to show that this couldn't be the case, and his experiments were cited as key supporting evidence in 1932 when another physicist discovered the neutron, a subatomic particle with a mass of one and no net electrical charge.

Fermi, Rasetti and many other members of the Institute via Panisperna left Italy in 1939 after Italian dictator Benito Mussolini declared his intentions to institute in Italy the racist policies of Germany's Adolf Hitler.

Later that year, Rasetti became director of the Physics Department at Laval University in Quebec, where he remained until coming to Hopkins as a professor of physics in 1947.

Rasetti, previously the recipient of a number of Italian physics prizes, would receive his next prominent scientific award in the field of geology in 1955, when he won the National Academy of the Sciences' Charles D. Walcott Medal. Although he was professionally continuing to do research in particle physics, Rasetti's amateur endeavors as a collector and identifier of fossilized trilobites had won him great respect among professional paleontologists. His remarkable ability to identify different species of trilobites led not only to the prestigious Walcott Medal, awarded only once every five years by the NAS, but also to many requests for help.

"People who found they couldn't identify trilobites all over the world would mail them to him," recalls Aihud Pevsner, Hopkins professor of physics. "Every morning he'd get a shoebox full of them, and every afternoon, he'd have them all identified and ready to mail back.

"He said he could identify 10,000 different species of trilobites. He said they weren't so hard to learn because he had been an amateur entomologist and had learned to identify 10,000 species of insects. But he admitted that once he'd learned 10,000 trilobites, he lost about 5,000 insects," Pevsner says with a laugh.

According to Pevsner, when Rasetti retired in 1967 he donated his extensive collection of trilobites, amassed over many summer vacations in the Alps, to the Smithsonian Institution.

In addition to fossil-hunting, Rasetti spent his vacations classifying the wildflowers of the Alps. As with his other avocations, Rasetti amassed a considerable body of expertise in Alpine wildflowers and published a definitive book on them, I Fiori Delle Alpi, or Flowers of the Alps, in 1980.

Rasetti was a member of the Pontifical Academy of the Sciences, a panel of 80 scientists that advises the Vatican on scientific issues, promotes research and examines the ethical and environmental responsibilities of scientists. He was also a fellow of the American Physical Society and the Geological Society of America, and a member of the Paleontological Society.