Frances Pleasonton seals a vacuum chamber in 1951.
The old photos show her casually writing data in a logbook with stacks of lead bricks nearby, or sealing a vacuum chamber with a wrench. ORNL researcher Frances Pleasonton was instrumental in some of the earliest explorations of the properties of the neutron as the X-10 Site was finding its postwar footing as a research lab.
In 1950 then-Physics Division Director Arthur Snell reported he and Pleasonton had measured beta decay in neutrons, a process in which neutrons, which are naturally short-lived outside the stability of a nucleus, emit a proton and an electron.
In the late 1940s Snell and colleague Leonard Miller observed the decay by focusing a neutron beam from the Graphite Reactor. After publishing that observation they intended to measure the decay. However, Miller died in a vehicle accident. Snell then enlisted Pleasonton and fellow physicist Reuben McCord in building a detector to count neutron decay products.
Pleasonton, born in 1912 in Harrisburg, Pa., was a product of Bryn Mawr College who had published a model of the crystal structure of Rochelle salt as a graduate student. She came to ORNL from the Naval Ordnance Plant at Indianapolis in 1947 as an associate physicist. McCord was one of the original crew of the “Clinton Pile” — the Graphite Reactor — that went critical in November 1943. He moved to the plutonium production operations in Hanford, Wash., and returned to ORNL as a physicist, also in 1947.
In September 1951 Snell described in an article, headlined “Proof of Neutron’s Radioactivity Obtained by Physicists at ORNL,” how he and Pleasonton “opened a hole in the 7-foot-thick concrete shield” of the Graphite Reactor. “A stream of neutrons then came through the hole in a well defined beam.”
The beam was passed through a vacuum tank outfitted with detectors in a complex arrangement to detect protons. Less than 20 years earlier the neutron discoverer James Chadwick had realized that neutrons were slightly heavier than protons and theorized they could turn to protons through radioactive beta decay. Snell, Pleasonton and McCord’s experiment essentially confirmed that.
“The observations we have been able to make appear to us to constitute experimental proof beyond reasonable doubt that free neutrons are not ‘fundamental’ particles, but that they spontaneously transform themselves into protons,” Snell wrote in Lab News.
A British researcher at Canada’s Chalk River Laboratories, John Robson, had been studying the same theory and reached similar conclusions. The Snell and Robson teams published their results simultaneously in 1950 in Physical Review, with Snell estimating neutron half-life at 10 to 30 minutes and Robson reporting 9 to 25 minutes. Researchers are still trying to pin down the exact half-life.
Snell valued working with Pleasonton. He wrote in 1985 in Nuclear Science and Engineering:
While I was director of the Physics Division (early 1950s), the bureaucracy had not matured, and I had time to leave the office and join my able colleague, Frances (“Tony”) Pleasonton in our experimental laboratory.
Snell then describes two pursuits: the observation of neutrino recoil and charge spectrometry of radioactive rare gases, of which he wrote:
The charge spectra were qualitatively new; nobody had seen them before, and Tony and I were quite excited in seeing them for the first time.
Pleasonton authored and co-authored a number of scientific publications. In 1958 she and Snell studied the decay of helium-6 and confirmed the electron-neutrino theory of beta decay. Her lab was visited in 1958 by Queen Frederika of Greece and in 1959 by a young King Hussein of Jordan. After retiring from the Lab she remained in Oak Ridge and was active in local environmental causes. She died in 1990.
