Jerzy Pniewski (1913 - 1989)

The Warsaw scientists, analyzing the traces of cosmic radiation particles recorded in a block of photographic emulsion, discovered the hypernucleus—a type of atomic nucleus that, in addition to protons and neutrons, contains at least one hyperon. In this case, it was a Λ hyperon. It was a bold hypothesis – at that time, no one suspected that the recently discovered hyperon could be a component of an atomic nucleus. However, subsequent observations confirmed the validity of the Polish researchers' idea. This discovery inaugurated a new field of physics: the study of hypernuclei. Nobel Prize archives reveal that Danysz and Pniewski were repeatedly nominated for the Nobel Prize for this groundbreaking discovery.

For nearly two years, Marian Danysz worked in Bristol in the laboratory of Cecil Frank Powell (1903–1969). During Danysz’s stay, the English scientist was awarded the Nobel Prize in Physics "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made using this method." Powell’s research utilized blocks of photosensitive photographic emulsion, which could be divided into layers. Particles of radiation passing through the emulsion left visible traces. To increase the likelihood of capturing cosmic radiation, the experiments were conducted high above Earth's surface.

When Danysz returned to Warsaw in May 1952, he brought with him a block of emulsion exposed during a stratospheric balloon flight. He persuaded Jerzy Pniewski, with whom he had developed closer ties during a six-month stay in Liverpool, to collaborate on analyzing the emulsion. After dividing the block into layers, each was examined under a microscope. On the evening of September 19, during one such examination, Danysz exclaimed, "Look at this strange beast!" His attention was caught by an image resembling a "double star." The first "star" marked the point where a high-energy cosmic radiation particle struck an atomic nucleus in the emulsion, creating a burst of linear tracks emanating from the collision. Each track was left by a fragment of the shattered nucleus. Unexpectedly, one track split into four parts, forming the second "star," indicating that a fragment of the nucleus had undergone a secondary decay.

The researchers were captivated by this anomaly. Twice a day, they visited the recently opened Niespodzianka café near the Institute of Physics on Hoża Street in Warsaw, rejecting one hypothesis after another until only one explanation remained: the observed trace must have come from an atomic nucleus containing not only protons and neutrons but also a Λ hyperon. This was the first- ever observation of a hypernucleus.

The scientists’ findings, published in the prestigious journal The Philosophical Magazine, sparked a wave of reports on hypernucleus observations worldwide. This achievement was one of the greatest successes of Polish post-war physics. Two years later, M. Gell-Mann (1929–2019) and A. Pais (1918–2000), independently of K. Nishijima (1926–2009), demonstrated that hyperons possessed a peculiar property they named "strangeness." This property explained their unusually long lifetimes before decay and confirmed the validity of the conclusion drawn by Danysz and Pniewski.

A hypernucleus is an atomic nucleus in which at least one nucleon (proton or neutron) has been replaced by a hyperon. Nucleons also have their own internal structure. Both protons and neutrons are composed of three quarks: a proton consists of two u quarks and one d quark, while a neutron consists of two d quarks and one u quark. Quarks are currently recognized as one of the fundamental building blocks of matter.

In addition to the u (up) and d (down) quarks, there are s (strange), c (charming), b (bottom), and t (top) quarks. This classification was not yet known when Danysz and Pniewski observed the first hypernucleus. A hyperon is defined as a particle composed of three quarks, one of which is a strange quark. The Λ hyperon, for example, is composed of u, d, and s quarks.

Shortly after this discovery, the paths of the two scientists diverged for several years. In November 1953, Pniewski became the director of the Institute of Physics, where administrative duties absorbed much of his time. Danysz, meanwhile, was appointed deputy director of the Joint Institute for Nuclear Research in Dubna and spent most of his time in the Soviet Union.

By the late 1950s, the researchers, along with a group of young physicists—many of whom would later become professors at Warsaw universities—returned to studying hypernuclei and made further significant discoveries. In 1959, they achieved the best estimation of the mass of the Λ hyperon available at the time. In 1962, they initiated hypernuclear spectroscopy, examining hypernuclei in excited states, and in the same year, they discovered the first double hypernucleus containing two Λ hyperons. This breakthrough relied on hypernuclei produced in a particle accelerator capable of achieving high energies. Their collaborator, Jerzy Zakrzewski (1932–2008), was the first to recognize the trace that turned out to be evidence of a double hypernucleus.

Observing a double hypernucleus proved to be extremely challenging. Over the next 30 years, apart from the Warsaw group, only one similar case was recorded, in the Brookhaven laboratory in the United States. Further research into hypernuclei was primarily conducted by large international teams utilizing the world's most powerful particle accelerators.

In the 1960s, bubble chambers began to replace emulsion blocks for recording radiation. The particle traces created in the chamber were photographed, and isolating an event of interest required painstaking analysis of the film. On Pniewski's initiative, a device was constructed in Warsaw to aid this process. It consisted of an interconnected scanner and a KAR-65 computer for image analysis.

This innovative system, groundbreaking for its time, was built using germanium transistors by a team led by Jacek Karpiński (1927–2010). Employing Karpiński was a bold decision by Pniewski, as anonymous letters questioned Karpiński's qualifications. However, the decision proved justified. The computer was successfully constructed and served Warsaw physicists for 20 years.

Biography

Jerzy Pniewski was born on June 1, 1913, in Płock, into a family with a multi-generational teaching tradition. His interest in science emerged early, as his family lived in a school building where his father taught mathematics. The school was the prestigious St. Małachowski High School in Płock, which Jerzy graduated from in 1930. He then began studying mathematics at the University of Warsaw. During his third year, after attending physics lectures delivered by Stefan Pieńkowski (1883–1953), he developed an interest in the field and started pursuing physics studies as well.

In 1935, while still a student, Pniewski became Pieńkowski’s assistant at the Experimental Physics Department of the University of Warsaw. He earned his master’s degree in mathematics in 1936 and completed his physics studies a year and a half later. His master’s thesis focused on optics.

During the occupation, Pniewski participated in underground education and attended weekly physics seminars organized by Pieńkowski. He supported himself through work at the Laboratory of Physical Measurements, established by Pieńkowski.

After World War II, Pniewski was promoted to assistant professor and took on the responsibility of overseeing the reconstruction of the war-damaged university building under Pieńkowski’s leadership as rector. He combined his work in physics with lecturing in mathematics for chemistry students and managing the Physics Department at the Warsaw University of Life Sciences.

In November 1948, Pniewski went on a two-year internship at the University of Liverpool’s nuclear physics laboratory, led by Warsaw-born Józef Rotblat (1908–2005). There, he focused on a new area: beta radiation spectroscopy. Although he was offered the opportunity to stay in England, he returned to Warsaw in 1950. In 1951, he defended his doctoral dissertation based on his research conducted in England.

After the death of Stefan Pieńkowski in November 1953, Jerzy Pniewski succeeded him as Director of the Experimental Physics Institute at the University of Warsaw. In 1954, he was appointed associate professor, and in 1963, he became a full professor. By 1958, he managed to delegate his responsibilities as Director, allowing him to return to scientific research. He also began collaborating with the Institute for Nuclear Research in Świerk, where he became the head of the High Energy Department. In the early 1960s, he took on the leadership of the newly established Department of Elementary Particles at the University of Warsaw.

Pniewski served as Director of the Institute again from 1962 to 1975 and was Dean of the Faculty of Physics at the University of Warsaw from 1975 to 1981. Hoża Street, where the Institute was located, remained his second home for the rest of his life. As Director, he ensured the Institute's high standards and sought to hire the best graduates, providing them with opportunities for development, including international internships. However, he was also decisive in parting ways with those who did not show potential to become significant scientists.

Pniewski was known for his great kindness toward people and his ability to see their positive qualities. His office at the Physics Institute on Hoża Street was always open to everyone, not only to researchers but also to other staff members. However, his numerous responsibilities meant his time was tightly scheduled. As a result, he would begin oral exams for students as early as 6 a.m. In March 1968, during the university's efforts to suppress a student strike that had caused significant unrest, Pniewski addressed the students at a rally. His intervention helped both sides—the students and the university authorities—approach the situation more rationally. Years later, in December 1981, when student strikes resumed, Pniewski visited the protesters every morning, bringing them freshly baked rolls. Even in retirement, Pniewski remained a frequent visitor to Hoża Street. His final co-authored publication was submitted for print just two months before his death.

In recognition of his scientific achievements, Pniewski became a member of the Polish Academy of Sciences in 1961, serving as Chairman of its Physics Committee from 1975 to 1981. He was also a member of the Academy of Sciences in Heidelberg and greatly valued his honorary membership in the Płock Scientific Society. He received honorary doctorates from the universities of Lyon (1975) and Heidelberg (1980). In 1969, he was awarded the Smoluchowski Medal, the highest honor bestowed by the Polish Physical Society.

Jerzy Pniewski passed away in Warsaw on June 16, 1989.