Benjamin S. Fraenkel

Benjamin S. Fraenkel, a pioneer in developing instruments for high resolution vacuum UV and x-ray spectroscopy and using them for a wide range of applications, died in Jerusalem, Israel, on May 4, 2013, a few days before what would have been his ninetieth birthday. He was professor emeritus at the Racah Institute of Physics at Hebrew University, and was the founder of the Laboratory of X-Ray and VUV Spectroscopy.

Ben was born in Marburg, Germany, on May 13, 1923, the son of Abraham (Adolf) H. Fraenkel, originally from Munich, and Wilhemina (Wilma) A. Prins, originally from Amsterdam. Abraham Fraenkel was a mathematician, best known for his role in formulating the standard axioms of set theory, known as the Zermelo-Fraenkel axioms. The family moved from Marburg to Kiel when Abraham got a faculty position there in 1928. Abraham and Wilma were both ardent Zionists, and in 1929 Abraham obtained a position at Hebrew University, took a two-year leave of absence from Kiel, and moved to Jerusalem, but for economic reasons the family returned to Kiel in 1931. In April 1933, the Nazis dismissed most Jews from academic positions, and Abraham, who could have stayed because he was a World War I veteran, resigned his position. Worried about the deteriorating situation, the family fled to Amsterdam, where they stayed with Wilma’s family. In October 1933, after Abraham had obtained an appointment as professor of math and director of the Mathematical Institute at Hebrew University, they returned to Jerusalem. Abraham later became rector of Hebrew University.

As a boy, Ben was very interested in math and physics, but wasn’t sure exactly what he wanted to do as a career. One day, he overhead his father and a mathematical colleague arguing about why 1 + 1 equals 2. Ben thought that this was a ridiculous thing to argue about, and determined to go into applied physics, because it was as far from pure mathematics as possible. When telling this story, Ben had the same deadpan face, understated smile, and twinkle in his eye that he always had when he was telling one of his many funny stories.

During World War II, Ben volunteered to join the Royal Engineers of the British Army and was stationed in Cairo, where he used his glassblowing skills to make periscopes for tanks. He also served in the Haganah, which became part of the Israel Defence Forces, in the Israel War of Independence.

Ben did his PhD research under Ernst Alexander in the Physics Dept. at Hebrew University, analyzing VUV spectra of the solar corona that had been obtained after the war using captured German V2 rockets. In order to interpret the measured spectra of the corona, it was necessary to develop laboratory sources of VUV, and higher resolution VUV spectroscopes, which were later extended to extreme UV and x-rays, and applied to many other problems. Giulio Racah was an important mentor to Ben at Hebrew University.

After a post-doctoral fellowship at Liverpool, Ben spent his entire professional career associated with Hebrew University, but had visiting positions at many research institutions abroad, including Stanford University, NASA Moffett Field, the Princeton Plasma Physics Laboratory, the Naval Research Laboratory in Washington, and the Rutherford Laboratory at Oxford. The instruments he developed are widely used internationally. Many of his former students continue to pursue research and development of instruments for VUV and x-ray spectroscopy, building on Ben’s work, in the United States and Europe, as well as in Israel.

The first high resolution VUV spectroscope was developed by Ben and his colleagues in 1963, and used with a laboratory plasma, making it possible to identify some of the prominent VUV lines seen in the sun’s corona, from argon-like iron and nickel. The high resolution made it possible to use the Doppler effect to measure ion temperatures in laboratory plasmas, and to classify spectral lines observed in laboratory plasmas according to the degree of ionization. In the 1970s, the work was extended to studying very high density plasmas such as pulsed power discharges, and grazing incidence high resolution spectrometers were developed in collaboration with J. L. Schwob. Also at that time, and in the 1980s, Ben worked on diagnostics for plasmas used in controlled fusion research, including tokamak and laser plasmas. For looking at extended sources such as tokamaks, he developed slitless x-ray spectrometers using cylindrically curved single crystals, installed first on the Princeton Large Torus in 1975. In the 1990s, this work was extended to crystals curved in two dimensions, spherically or toroidally, which were expected to provide the increased sensitivity needed for more advanced fusion experiments that have plasmas with lower levels of impurities.

For compact x-ray sources, Ben designed x-ray cameras and spectrometers using double reflection of x-rays in a single crystal, making it possible to obtain high spatial resolution monochromatic images of laser plasmas. These monochromatic imaging techniques are widely used in crystallographic research, and more recently in phase contrast imaging experiments. For strong x-ray lines, the double reflection spectrometer was also used to measure independently the density and ion temperature distribution in tokamaks and other extended sources. Other applications of the double reflection technique that were developed by Ben and his group include measuring x-ray wavelengths absolutely, monochromatically imaging synchrotron sources that emit a broad spectrum of x-rays, and measuring the spot size of monochromatic emission from x-ray tubes.

Ben and his wife Judith were deeply involved in efforts to improve the quality of elementary and secondary school education in Israel, especially in development towns far from the center of the country, where the quality of schools was not as good. They founded an organization called Ma’of, which trained students from development towns, most of whose families had come to Israel from the Middle East or North Africa, to teach elementary school math and science, providing them with full tuition and board if they agreed to go back and teach in their towns after they graduated. Ben and Judith would visit the classes these graduates were teaching, and would encourage their pupils to believe they could succeed. Ben considered his work in education the most important thing in his professional life. He would say, “Education is my profession. Physics is just my hobby.”

As a religiously observant Jew, Ben did not believe there was any conflict between science and religion, but on the contrary felt that they complemented each other. He was one of the founders of the Institute for Science and Halacha, which develops technological solutions to problems involving conflicts between Jewish religious law, particularly restrictions on activities on the Sabbath, and the needs of hospital workers, emergency workers, people with disabilities, etc.

In addition to physics, Ben had an encyclopedic knowledge of history, geography, Judaica, family history, and philosophy—he was an admirer of the philosopher Yeshayahu Leibowitz. He was a talented artist, who once made a wood carving that was put on exhibit at the Betzalel Academy of Arts and Design. He loved walking, and hiked extensively on the many hiking trails in Israel. He also enjoyed hiking in the Alps, especially around St. Moritz. His talent, his energy, his warmth, and his droll sense of humor are greatly missed by his many friends and colleagues, and by his family.

I thank Michael Finkenthal of Johns Hopkins University and emeritus at the Hebrew University in Jerusalem, for providing me with details about Ben’s research in VUV and x-ray spectroscopy.