September 14, 1913-July 8, 2013
Rubby Sherr was born September 14, 1913 in Long Branch, New Jersey, of immigrant parents form Lithuania. He attended New York University, where he received his B.A. degree in physics in 1934, then entered graduate school at Princeton University and completed his Ph.D. in experimental physics in 1938. Research for his Ph.D. thesis, “The Separation of Isotopes by Diffusion” was a daunting experimental challenge involving twenty-nine glass mercury diffusion pumps with hundreds of vacuum seals. It was a testament to his experimental talent.
Rubby (pronounced “Ruby”) completed his Ph.D. at a time when new developments in physics and international affairs had far reaching consequences. The Hahn-Strassmann data on the breakup uranium by slow neutrons in late 1938, followed by the theoretical interpretation by Meitner and Frisch in early 1939, implied a huge release of energy by slow-neutron fission of uranium. The discovery meant the possibility of a new beneficial source of power, and also, the prospect of powerful bombs. Later that year, Germany invaded Poland and set off World War II. These sudden changes propelled Rubby into a career in nuclear physics that eventually took him to Los Alamos and the development of the first atomic bomb.
Rubby’s first nuclear physics experiment was done at Princeton shortly after he received his doctorate. In 1935 Professor Milton White had constructed a cyclotron in the basement of Palmer Hall (now the Frist Student Center). Rubby and White did a study of short-lived radioactive isotopes produced by bombarding various targets with the cyclotron beam. This was a time when little was know about many of these nuclei.
In 1939 Rubby left Princeton to join the faculty at Harvard where he continued nuclear research with the recently constructed Harvard cyclotron. He did two studies of nuclear reactions with high-energy neutrons that were produced when 11 MeV deuterons from the cyclotron struck a lithium-7 target. The neutrons came out of the target with a wide range of energies up to a maximum of 25 MeV. At that time, this was the highest energy beam that had been produced. (By contrast, three quarters of a century later, the energy of the beam in the Large Hadron Collider in Geneva is 500,000 times greater.)
For the first study, Rubby directed the neutrons onto a mercury target causing nuclear reactions that converted some of the mercury into gold. Their results were published in 1941 and attracted considerable attention. They were modern alchemists. While the nuclear reaction did produce a small number of gold atoms, the gold isotopes were radioactive and decayed back to mercury within a few days.
In the second study, the 25-MeV neutron beam was used to measure the sizes of nuclei of several elements ranging from carbon to mercury. Rubby was the sole author of the paper “Collision Cross Sections for 25-MeV Neutrons.” He did the measurements while he was at Harvard, sometime between 1940 and 1942, but submitted the paper for publication a few years later, on September 27, 1945, six weeks after the end of WWII when Japan surrendered on August 15, 1945.
In the course of his second measurement, Rubby made an accidental discovery that was significant for the war effort to control nuclear fission to make a bomb. He was told to keep his discovery secret, and published his nuclear studies after the war, with only a hint of his discovery.
One of the elements Rubby was investigating was carbon in the form of graphite. This was one of two possible materials needed as a “moderator” to slow down the neutrons emitted in fission so they could be absorbed by other uranium atoms to produce a chain reaction. A chemical impurity that absorbed neutrons would prevent the chain reaction from developing.
Rubby unintentionally had the perfect apparatus to see if neutrons were absorbed in materials. His measurement of nuclear size was based on measuring the loss of neutrons as the beam went though the target. If a chemical impurity could absorb neutrons more than the effect he was looking for, the scattering off carbon nucleus, he would have seen it. Indeed, it turned out that boron was present in the graphite. Because of its nuclear structure, boron is about a million times more effective at absorbing slow neutrons than carbon. Even at a concentration of 1 ppm, boron in graphite is a problem.
At about the same time, scientists in the US and Germany found that graphite absorbed neutrons too much to be useful as a moderator for uranium fission. The German program abandoned graphite and attempted to use the second possible moderator, “heavy water” (deuterium oxide) produced at the Norsk hydroelectric facility in Norway. The destruction of this plant in 1943 by Norwegian and British saboteurs shut down this sole source of heavy water, and blocked the German nuclear bomb program.
In the US, there was suspicion that a chemical impurity could cause a loss of neutrons in graphite. Fermi and Szilard visited the National Carbon Company in December 1940 to discuss the chemical purity of graphite. National Carbon then developed a chemical purification procedure that produced low-boron graphite and by 1942 it was ready for use in the first successful nuclear reactor.
All of this research was classified, so it is difficult to know when and if Rubby’s experiment added to the suspicion of a chemical impurity in the graphite. He was certainly involved with a sensitive and unique apparatus at that time, and he passed on his findings to his superiors. In the last year or two before he died, he said to colleagues that his contributions might one day be known. In the 1945 paper, Rubby mentions the graphite in the following footnote: “The author is indebted to the National Carbon Company of Cleveland Ohio for high grade artificial graphite necessary for this experiment.”
With the U.S. entry into the war in 1941, many members of the scientific community were drawn into research that would aid the war effort. The Harvard cyclotron would soon be dismantled and sent to Los Alamos, where a secret laboratory was being set up to develop the atomic bomb. Rubby would eventually join Bainbridge in Los Alamos, but from 1942 to 1944 he was diverted to the development of radar at the MIT Radiation Laboratory. At MIT he invented a Doppler radar system that determines the speed of an aircraft relative to the ground. He patented the system under the title, “Aircraft Velocity Indicating Pulse Echo System” (number 2,669,710); the system was used during and after the war. His flying experience while testing the device, crammed in the rear of a small Army aircraft high above a New England highway, terrified him so much he avoided flying for several years. Some of you may have fallen victim to an offshoot of his system, the automobile radar gun.
From 1944 to 1946 Rubby was at Los Alamos as a member of the Manhattan Project. When he arrived he learned that he was assigned to the “Initiator Group” under Charles Critchfield. The initiator is a key component of the bomb that produces a sudden burst of neutrons that initiates the explosive chain reaction of fissions of uranium or plutonium. Rubby was co-inventor of the Fuchs-Sherr polonium-beryllium initiator, nicknamed the “urchin.”
Rubby was one of the last surviving witnesses of the atomic bomb test on July 16, 1945. The test was made at the Trinity site near Alamogordo, New Mexico. Rubby and others were in a bunker 10 km from ground zero, watching their instruments to record signals that would reveal important details of the blast. To see their oscilloscopes better, the room was darkened with the only light in the room coming through narrow slits in the wall. The explosion lit up the room before the blast arrived. Rubby was totally focused on his instruments, waiting out the count down. When bright light from the blast obscured his instrument, he yelled, “Who the hell turned on the lights? “Then he looked outside and thought, “This is the greatest experiment of all time - at least it was certainly the biggest. Then the horror sank in that /the thing/ had actually worked, followed by relief that the atmosphere hadn’t ignited, as some had feared.”
Klaus Fuchs, Rubby’s co-inventor of the initiator, was a brilliant young German-born theoretical physicist who came to the project with the British group. Fuchs was close to Rubby, and even helped the Sherr’s by babysitting their two young daughters. Fuchs returned to Britain after the war and in 1950 during on-going investigations, he confessed to being a spy for the Soviet Union while at Los Alamos. Fuchs went released in 1959 and emigrated to East Germany where he continue his scientific career until retirement in 1979.He died in 1988 in Berlin.
Rubby returned to Princeton in 1946 as a member of the physics faculty. He taught and carried out many research projects with his students and colleagues for 36 years. From 1955 to 1971 he was the principal investigator of the nuclear physics group that carried out theoretical and experimental low energy nuclear research.
Using the Palmer cyclotron, in 1953 he published a paper with J. B. Gerhard on “Experimental evidence for the Fermi interaction in the β -decay of oxygen-14 and carbon-10 (PR 91, 4, 909, 1953). In a 2014 review, Hardy and Towner wrote “It was less than five years since Sherr had first discovered these two nuclei, yet already the two authors were using the decays to probe for the first time the fundamental nature of β -decay.” As Rubby appreciated in 1953, the study of these fundamental beta decays sheds light on the fundamental properties of weak interactions.
Through the 1960s the group studied the structure of light nuclei. Former graduate student Charles Glasshauser recalled, “It was the summer of 1961. Four of us, Gary Crawley, Gerry Nolen, Chuck Whitten and I arrived in Palmer Lab, ready to enter the new world of nuclear physics. Some of us were more ready than others, and some of us were pretty sure wehad jumped into deeper waters than we had realized. But Rubby was there to help; it was the largest class of nuclear experimentalists that he ever had… The atmosphere in the lab was electric, highly charged with physics, and we thought of little else.”
In 1961 Rubby initiated a nine-year effort to finance and construct the Princeton Azimuthally Varying Field (AVF) cyclotron. The cyclotron became operational in 1970 in Jadwin Hall and paved the way for studies of many nuclei.
Gerry Garvey, a senior colleague from that time, recalled “Rubby showed me how to embrace physics, particularly those parts of it that could not be readily encompassed by a single concept. He showed me how important it is to assemble a vast body of experimental results, phenomenological descriptions and theory into a personal view of how nuclei were assembled. … His humility was remarkable. Even though we got together several times in Los Alamos he never mentioned his contributions to the Manhattan project.”
Richard Kouzes recalled the warmth and hospitality of Rubby and his wife Pat. “They had many dinner parties and other social events for students and faculty in the Nuclear Physics group. He was a wonderful mentor and an active researcher his entire life.”
Rubby’s research spanned the golden age of experimental nuclear research. In his late twenties at Los Alamos, where he was a close friend of the Oppenheimers, he held some of our nation’s most prized nuclear secrets. By his 90th birthday, the cyclotron in Jadwin Hall had been replaced by a condensed matter physics lab. Much of the deep knowledge of nuclear physics that grew out of his group was used to design experiments that were as free as possible from radioactive materials. Further investigations of weak-interaction physics, which Rubby had such an important role in establishing, led to a measurement of the nuclear reaction that powers the Sun, by Princeton’s Frank Calaprice, Cristiano Galbiati and colleagues, and the discovery that the neutrino has mass, by Art McDonald and colleagues. Art, who was with the cyclotron group from 1982 to 1989, won the 2015 Nobel Prize for his research.
The experimental nuclear physics group that Rubby nurtured at Princeton continues to apply nuclear methods to explore exciting new research areas. The latest topic that grew out of his group is the search for hypothetical elementary particles that could explain the mysterious dark matter that fills the universe.
In his career, Rubby published over a hundred papers on a wide variety of topics. As a leader of the cyclotron group, he enabled many more. Unlike others, his publication rate increased in his later years when his vast experience came to the fore. With Terry Fortune at the University of Pennsylvania, he published 7, 8, and 11 papers respectively in 2011, 2012, and 2013 when he was between the ages of 97 and 99. From his daughter Fran, "Dad was many people: A dedicated physicist, a serious fly fisherman, very active with artists and musicians, and an avid birder. But his identity was as a physicist. His sense of self was as a physicist. The rest was just his personality. His life was very much his physics and the excitement of it, of solving problems. To the end."
Rubby is survived by his daughters Fran (Robert Hess) of Wynnewood, Pa., and Elizabeth Sherr Sklar (Lawrence Sklar) of Ann Arbor, Mich., as well as by his granddaughter, Jessica Sklar of Seattle.
Mister President: For the Committee I move that this Resolution be spread on the records of the Faculty; that a copy be sent to his daughters Fran and Elizabeth, his granddaughter Jessica, and to the Archivist of the University.
Frank P. Calaprice, Professor of Physics
Lyman A. Page, Jr., James S. McDonnell Distinguished University Professor in Physics