Frederick Reines

Frederick Reines
Reines in the early 1950s
Frederick Reines
Born	16 3, 1918
Birthplace	Paterson, New Jersey, U.S.
Died	Template:Death date and age Orange, California, U.S.
Nationality	American
Occupation	Physicist
Known for	Co-detection of the neutrino
Awards	Nobel Prize in Physics (1995), National Medal of Science (1985)

Frederick Reines (Template:IPAc-en Template:Respell; March 16, 1918 – August 26, 1998) was an American physicist whose name became inseparable from one of the most elusive particles in nature: the neutrino. Together with his colleague Clyde Cowan, Reines first detected the neutrino in June 1956, confirming the existence of a particle that had been theorized by Wolfgang Pauli in 1930 but long considered undetectable. For this achievement, Reines was awarded the 1995 Nobel Prize in Physics, nearly four decades after the original experiment. A graduate of the Stevens Institute of Technology and New York University, Reines spent his early career at the Los Alamos Laboratory, where he worked on the Manhattan Project and later directed nuclear weapons test operations in the Pacific. He dedicated the greater part of his scientific life to the study of the neutrino's properties and interactions, contributing to the detection of atmospheric neutrinos produced by cosmic rays and, in 1987, the detection of neutrinos emitted from Supernova 1987A, which inaugurated the field of neutrino astronomy. He has been described as "the father of neutrino physics" and may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the subsequent thorough investigation of its fundamental properties."

Early Life

Frederick Reines was born on March 16, 1918, in Paterson, New Jersey, to a family of Russian Jewish immigrants. He was raised in the New York metropolitan area and grew up in an environment that, while not particularly scientific, encouraged curiosity and intellectual development. In his Nobel Prize autobiographical statement, Reines recalled being asked as a boy what he wanted to be when he grew up; among his answers was "a scientist," though by his own account this was alongside other childhood ambitions.

Reines developed an early interest in science and engineering. He was an Eagle Scout as a youth, a distinction he later mentioned with pride. He attended schools in New Jersey and showed aptitude in mathematics and the physical sciences. His formative years coincided with a period of rapid advancement in nuclear and quantum physics, as scientists across Europe and the United States were probing the fundamental structure of matter. The intellectual excitement of this era would shape Reines's career trajectory.

The Reines family's immigrant background instilled in the young Frederick a drive for achievement through education. His parents supported his academic pursuits, and he gravitated toward physics and engineering as fields where his talents could be applied. The combination of practical engineering skill and theoretical imagination that Reines cultivated during these early years would prove essential to his later experimental work, which required both ingenious apparatus design and deep understanding of particle physics theory.

Education

Reines enrolled at the Stevens Institute of Technology in Hoboken, New Jersey, where he studied engineering. He earned both a Bachelor of Science and a Master of Engineering degree from Stevens.

He subsequently pursued graduate studies in physics at New York University (NYU), where he earned his Ph.D. in 1944. His doctoral research was conducted under the direction of Richard D. Present and focused on nuclear physics topics. His doctoral thesis dealt with the liquid drop model of nuclear fission, a subject that placed him at the frontier of contemporary nuclear physics and provided a foundation for his subsequent work at Los Alamos.

The combination of an engineering background from Stevens and a physics doctorate from NYU gave Reines a distinctive skill set—he could design and build sophisticated experimental equipment while also engaging deeply with the theoretical questions his experiments sought to answer.

Career

Manhattan Project and Los Alamos

Upon completing his Ph.D. in 1944, Reines joined the Manhattan Project at the Los Alamos Laboratory in New Mexico. He was assigned to the Theoretical Division, where he worked in a group led by Richard Feynman. The work at Los Alamos during this period was directed toward the development of nuclear weapons, and the laboratory assembled an extraordinary concentration of physicists, including many who would go on to distinguished postwar careers.

In 1946, Reines became a group leader at Los Alamos, taking on increased responsibility for theoretical and experimental work related to nuclear weapons effects. He remained at the laboratory for several years after the end of World War II, participating in a series of nuclear weapons tests. His involvement in these test programs grew steadily, and in 1951 he served as the director of Operation Greenhouse, a series of nuclear weapons tests conducted at Enewetak Atoll in the Pacific Ocean. Operation Greenhouse was significant for testing new weapon designs and provided data important to the development of thermonuclear weapons.

While the weapons work was technically demanding, Reines increasingly felt drawn to fundamental physics questions. By his own account, he took a sabbatical-like period in 1951 to think about problems in basic physics that he could address, eventually settling on the challenge of detecting the neutrino—a particle whose existence had been postulated by Wolfgang Pauli in 1930 to explain the apparent violation of energy conservation in beta decay, but which had never been directly observed.

The Neutrino Experiment

The neutrino had been incorporated into Enrico Fermi's 1934 theory of beta decay, which provided a quantitative framework for understanding the weak nuclear force. However, the neutrino's interaction cross section with matter was extraordinarily small, leading many physicists to consider its direct detection impossible. Pauli himself had famously worried that he had postulated a particle that could never be experimentally verified.

Reines recognized that the intense flux of neutrinos (more precisely, antineutrinos) produced by nuclear reactors might provide a sufficient source to make detection feasible, provided a sufficiently sensitive and large detector could be built. He recruited Clyde Cowan, a fellow Los Alamos physicist, as a collaborator, and the two began developing what would become one of the landmark experiments in twentieth-century physics.

The experimental method was based on inverse beta decay: an antineutrino interacting with a proton would produce a positron and a neutron. The positron would quickly annihilate with an electron, producing two gamma rays, while the neutron would be captured a few microseconds later, producing another gamma ray signal. This distinctive "delayed coincidence" signature—two gamma ray pulses separated by a characteristic time interval—would serve as the fingerprint of a neutrino interaction, distinguishing it from background radiation.

Reines and Cowan first conducted preliminary experiments at the Hanford Site in Washington state, using a nuclear reactor as the antineutrino source. The initial results were suggestive but not conclusive, hampered by high background levels from cosmic rays. They then moved their experiment to the Savannah River Site in South Carolina, where the reactor was more powerful and the experimental setup could be placed underground to reduce cosmic ray backgrounds.

At the Savannah River Site, Reines and Cowan refined their detector, which used large tanks of water laced with cadmium chloride as a target and liquid scintillator detectors to observe the gamma ray signals. The cadmium served as a neutron absorber, providing the delayed coincidence signal that was central to the detection method. After painstaking work to reduce and characterize backgrounds, the experiment yielded a clear, statistically significant signal in June 1956.

On June 14, 1956, Reines and Cowan sent a telegram to Wolfgang Pauli informing him that the neutrino had been detected. Pauli reportedly shared the news with colleagues at a conference and replied with a letter of congratulations. The result was published in the journal Science in 1956. The measured cross section for the inverse beta decay reaction was consistent with theoretical predictions, providing strong confirmation of Fermi's theory of the weak interaction and establishing the neutrino as a real, detectable particle.

Continued Neutrino Research

Following the initial detection, Reines dedicated the remainder of his career to investigating the properties and interactions of neutrinos. He pursued a broad program of neutrino physics that extended over several decades and encompassed multiple experimental approaches.

One major area of research involved the detection of neutrinos produced in Earth's atmosphere by cosmic rays. When high-energy cosmic ray particles strike atoms in the upper atmosphere, they produce showers of secondary particles, including neutrinos. Reines was among the first to detect these atmospheric neutrinos, using deep underground detectors to shield against other forms of radiation. This work laid the groundwork for later discoveries about neutrino oscillation, the phenomenon in which neutrinos change from one type ("flavor") to another as they travel, implying that neutrinos have mass.

Reines also studied neutrino interactions with various nuclei and investigated the properties of different types of neutrinos. His experimental program contributed to the understanding of the weak nuclear force and the Standard Model of particle physics. Through these studies, he and his collaborators measured neutrino cross sections with increasing precision and explored the limits of neutrino physics with the technology available at the time.

Supernova 1987A and Neutrino Astronomy

One of the most dramatic episodes of Reines's later career came in February 1987, when Supernova 1987A was observed in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. This was the closest observed supernova since Kepler's Supernova in 1604, and it provided an unprecedented opportunity for neutrino detection.

Reines was part of the Irvine-Michigan-Brookhaven experiment (IMB), a large underground detector located in a salt mine near Cleveland, Ohio, originally designed to search for proton decay. When the supernova occurred, the IMB detector registered a burst of neutrino events, as did the Kamiokande-II detector in Japan. These detections marked the first time neutrinos had been observed from an astronomical source beyond the Sun, inaugurating the field of neutrino astronomy.

The observation of supernova neutrinos confirmed theoretical models of stellar core collapse and provided direct evidence that the energy released in a supernova is carried predominantly by neutrinos. It also opened a new observational window on the universe, complementing electromagnetic observations with neutrino-based information about astrophysical events.

Academic Career at UC Irvine

In 1959, Reines left Los Alamos to join the faculty of Case Institute of Technology (later Case Western Reserve University) in Cleveland, Ohio, where he served as head of the physics department. During this period, he continued his neutrino research and helped establish a strong experimental physics program.

In 1966, Reines moved to the University of California, Irvine (UCI), where he became the founding dean of the School of Physical Sciences. He played a central role in building the physics department at the then-new university and recruited faculty who would establish UCI as a center for particle physics and astrophysics research.

Reines remained at UCI for the rest of his career, continuing his experimental work and mentoring generations of students and postdoctoral researchers. His presence at UCI helped attract significant research funding and established the university's reputation in experimental physics. The annual Reines Lecture at UCI, inaugurated in his honor, continues to bring distinguished physicists to the campus.

Personal Life

Frederick Reines married Sylvia Samuels, and together they had two children. Outside of physics, Reines had a lifelong interest in music and was an accomplished singer, performing in amateur productions and choral groups. He was also known for his warmth and sense of humor among colleagues and students.

In his later years, Reines suffered from declining health. He was diagnosed with a neurological condition that progressively impaired his cognitive abilities during the 1990s. When he was awarded the Nobel Prize in 1995, his health had declined to the point where he was unable to fully participate in the traditional Nobel festivities in Stockholm. He died on August 26, 1998, in Orange, California, at the age of 80, after a long illness.

Recognition

Reines received numerous honors and awards over the course of his career, reflecting the significance of his contributions to experimental physics.

His most prominent recognition was the Nobel Prize in Physics, awarded in 1995. Reines shared the prize with Martin Lewis Perl, who received his half for the discovery of the tau lepton. Reines's half was awarded "for the detection of the neutrino," recognizing the 1956 experiment with Clyde Cowan. The award came nearly 40 years after the original discovery, and Cowan, who had died in 1974, was not eligible for the posthumous award under Nobel Committee rules.

In 1985, Reines received the National Medal of Science, presented by the President of the United States, for his contributions to physics. He was also awarded the Bruno Rossi Prize by the American Astronomical Society for his work on the detection of neutrinos from Supernova 1987A.

Reines was elected to the National Academy of Sciences and was a fellow of the American Physical Society. He received honorary degrees from several institutions, including his alma mater, Stevens Institute of Technology.

The University of California, Irvine, honored Reines by naming the Frederick Reines Hall after him, a building that houses the physics department. The annual Reines Lecture series at UCI brings prominent scientists to campus and serves as a continuing tribute to his legacy.

Legacy

Frederick Reines's detection of the neutrino stands as one of the foundational achievements of twentieth-century physics. The neutrino, once dismissed as a mere theoretical convenience that might never be observed, has become central to the Standard Model of particle physics and to modern astrophysics. Reines's experimental work demonstrated that particles interacting only through the weak nuclear force could be detected with sufficient ingenuity and persistence, opening an entirely new domain of experimental physics.

The experimental techniques pioneered by Reines and Cowan—particularly the use of large liquid scintillator detectors and the delayed coincidence method for identifying inverse beta decay events—became standard tools in neutrino physics. Subsequent generations of neutrino experiments, from reactor neutrino oscillation studies to solar neutrino detectors, have built upon the principles established by the Reines-Cowan experiment.

Reines's work on atmospheric neutrinos laid the experimental groundwork for the later discovery of neutrino oscillations, which demonstrated that neutrinos have nonzero mass. This discovery, confirmed by the Super-Kamiokande experiment in 1998 and recognized with the 2015 Nobel Prize in Physics awarded to Takaaki Kajita and Arthur B. McDonald, represented a departure from the original Standard Model and pointed toward new physics beyond it. Reines's early measurements of atmospheric neutrino fluxes were among the first experimental hints that the Standard Model's treatment of neutrinos was incomplete.

The detection of neutrinos from Supernova 1987A, in which Reines participated through the IMB experiment, inaugurated neutrino astronomy as a field. This event demonstrated that neutrinos could serve as messengers from violent astrophysical events, providing information complementary to that obtained through electromagnetic radiation. The development of large-scale neutrino observatories such as IceCube at the South Pole can be traced, in part, to the scientific vision that Reines helped establish.

A 1998 Nature obituary described Reines as "rightly known as the father of neutrino physics," noting that he was "perhaps more than any other scientist" identified with the discovery of a fundamental particle and the thorough investigation of its properties. His career exemplified the combination of theoretical insight, experimental skill, and dogged persistence that has characterized the most productive experimental physicists.

At UCI, his influence persisted through the department he helped build and the researchers he trained. The ongoing Reines Lecture series serves as a reminder of his contributions and their continuing relevance to contemporary physics.