Phyllis S. Freier

Career

During World War II, Freier was employed as a physicist at the Naval Ordnance Laboratory from 1944 to 1945. Following the war, she continued her graduate studies in physics at the University of Minnesota. Freier worked on her doctoral research with Edward Ney and Frank Oppenheimer, using high altitude balloons to study cosmic radiation. In 1948, this research led to Freier becoming the first person to see tracks in nuclear emulsions, proving that nuclei of heavy elements were included in cosmic radiation. After completing her Ph.D., Freier was a research associate at the University of Minnesota, Minneapolis from 1950 to 1970. She stayed at that university and from 1970 to 1975 she was an associate professor, and from 1975 to 1992 she was a professor of physics.

In 1988, Freier was recognized by the University of Minnesota with a distinguished teaching award for her outstanding contributions to the education of physics undergraduates. She taught for eighteen years where she originated the application of student textbook learning to the laboratory settings.

Research contributions

Freier's work focused on understanding the composition of the high-energy particles that continually strike Earth's atmosphere from outer space. Although cosmic rays had been studied since the early twentieth century, their exact composition remained uncertain in the 1940s. Physicists knew that energetic particles were arriving from space, but the types of particles involved and their origins were not yet fully understood.

Freier's research used nuclear emulsions, a highly sensitive photographic detection technique that allowed scientists to record the tracks left by individual subatomic particles. When charged particles passed through these emulsions they produced microscopic trails that could be examined under a microscope, allowing physicists to determine properties such as the particle's charge and mass.

Working with the cosmic-ray research group at the University of Minnesota led by physicist Edward P. Ney, Freier analysed nuclear emulsions that had been exposed to cosmic radiation during high-altitude balloon flights. These balloons carried detectors more than 20 kilometres into the atmosphere, where incoming cosmic particles could be recorded before they were significantly altered by interactions with the denser air below.

In 1948 Freier identified particle tracks produced by heavy atomic nuclei within cosmic radiation. Her work provided the first direct evidence that cosmic rays contain nuclei of elements heavier than hydrogen. This finding helped establish the modern understanding of the composition of cosmic radiation and demonstrated that cosmic rays include fragments of atomic matter travelling through space at extremely high energies.

The discovery was an important step in the development of cosmic ray physics and helped link the field more closely with astrophysics, since the presence of heavy nuclei suggested that cosmic rays originate in powerful astrophysical environments capable of accelerating atomic particles to enormous energies.

Freier also contributed to the development and application of nuclear-emulsion techniques for studying high-energy particles. Before the widespread use of large particle accelerators and electronic detectors, nuclear emulsions were among the most effective tools available for identifying rare cosmic-ray events, and they became widely used in mid-twentieth-century particle physics and cosmic-ray research.

There is a notable historical transition reflected in Freier's work. In the late 1940s cosmic rays functioned as a natural source of extremely energetic particles at a time when laboratory accelerators were still relatively limited. Within the following decade, however, the rapid development of particle accelerators transformed high-energy physics and moved much of this research into controlled laboratory environments.

Freier's career therefore sits at an important moment in the history of physics, when cosmic-ray studies helped bridge the emerging fields of particle physics and astrophysics.

Death

Freier died at home in St. Paul, Minnesota, on December 18, 1992, from Parkinson's disease.

Recognition and honours

• Fellow, American Physical Society — elected in recognition of her contributions to cosmic ray physics and the study of high-energy particles.
• Fellow, American Association for the Advancement of Science — elected for her research on cosmic radiation and astrophysics.
• Horace T. Morse–Minnesota Alumni Association Award (1988) — the University of Minnesota's highest honour for undergraduate teaching.
• Service on NASA advisory committees concerned with cosmic ray research and high-energy astrophysics.