Nils Göran was born in Eksjö on 28 March 1925. He showed a great interest in physics already at young age. His father was a high-school teacher who lectured in mathematics, physics and chemistry, and his mother was member of parliament between 1949-56.
Nils Göran had enrolled to the newly started Engineering Physics program of the Royal Institute of Technology (KTH) in Stockholm and he got his diploma in 1949. He continued with graduate studies and his choice fell on a subject representing one of the most promising and exciting new branches of physics of the time – nuclear engineering, more concretely reactor physics. This was a very exciting time worldwide and in Sweden, and Nils Göran became one of the true pioneers of this field. This was the time of the start of the “Atomic age”, with Eisenhower’s talk “Atoms for Peace” in the UN in 1953, which opened up the scene for peaceful applications of nuclear energy with, among others, the historic Geneva conferences with start 1955. Due to her neutrality, Sweden had a good position to have access to technical and scientific know-how, as well as to raw materials such as Norwegian heavy water. Together with rich access to low-grade uranium ores in Sweden, there was a promise of developing a domestic nuclear energy program and to become self-supporting in electricity production.
It was in such a setting that Nils Göran started his PhD at KTH, while having an assistant researcher status at the newly founded AB Atomenergi, the governmental research institute for the development of nuclear power in Sweden. His advisor was Sigvard Eklund, head of the research group, who later became General Director of the IAEA. During this time Nils Göran took part in the construction and operation of the first research reactor in Sweden, the R1, which was built at the campus of KTH in an underground location. He took part in the start-up of R1 which started operation in 1954. During his PhD studies N. G. Sjöstrand was also visiting scientist at Brookhaven National Laboratories in 1952-53 and in AERE Harwell in 1957-58.
The R1 reactor was used for research in reactor and neutron physics, mostly pulsed neutron experiments. The goal of these measurements was to determine the properties and dynamic response of the reactor in the critical state. However, for obvious reasons, such pulsed experiments can only be performed in the subcritical state of the reactor. To extrapolate the results to the critical state was extremely complicated, which started to become a hinder for large-scale development of nuclear technology. During his PhD time, Nils Göran Sjöstrand found an elegant, exact solution to the problem, by developing a methodology of measuring reactivity in subcritical systems, the so-called “area ratio method”, also called the “Sjöstrand method”. It was published in the journal of the Swedish Physical Society, Arkiv för Fysik, in 1956, and it is one of the most cited Swedish articles of all categories. Nils Göran’s PhD thesis had the title ”A theoretical and experimental investigation of the pulsed source method as a means for neutron diffusion studies”. He got his PHD degree at KTH in 1959.
The cover of article in Arkiv för Fysik (left) and the schematics of the area ratio method (right) from the article
The area ratio method earned Nils Göran international fame. The theory, performance and capabilities of the method were studied intensively around the world, among others at the Department of Reactor Physics in the Central Research Institute for Physics in Budapest, where undersigned was PhD student in the early 1970’s. The method plays a continued important role today in the recent development of the so-called Accelerator Driven Systems (ADS) for transmutation of nuclear waste, and the area ratio method is cited just as frequently today.
The area ratio method in the book Bell and Glasstone: Nuclear Reactor Theory
Research and education in reactor physics was started in Sweden in the early 1960’s, where departments for reactor physics were established in Chalmers and KTH. Chalmers founded the Department of Reactor Physics in 1960. Nils Göran became the first professor in reactor physics in Sweden when he was appointed first as acting, then in 1961 as a full professor in Chalmers and moved to Göteborg. Since the research at the “syster department” in Stockholm was focused on neutron scattering and diffraction, for a long time Chalmers had the only department with research and education in reactor physics in Sweden.
Since no education in nuclear engineering subjects was in existence in Sweden at the time of Nils Göran’s appointment, he had to build up education in the Swedish language at the undergraduate and graduate level from scratch. The lecture notes he wrote were the only teaching material in Swedish, and were used at other Swedish universities as well.
The experimental circumstances were very rudimentary and restrictive at the beginning. The Department rented two floors in a block of flats in central Göteborg, where also a low-yield neutron generator was also installed. This naturally led to some complications. During the turbulent times around the national vote on nuclear energy in 1980, some journalists believed that the neutron generator was a nuclear reactor, which of course gave headlines in the newspapers.
After some time the Department could move to the Chalmers campus and have access to a good laboratory with international standards. The Department got access to an own building, which was originally built as a transformer station for the trams in Göteborg. A new powerful neutron generator was installed, which was suitable for research and PhD work. During his time as professor, about 20 PhD students graduated at the Department, and a large number of master theses and licentiate exams were taken. His students have taken many leading positions in academia and industry both in Sweden and world-wide.
During his time as professor at Chalmers, Nils Göran shifted his research to semi-analytical and numerical solution methods of the transport equation, in particular high accuracy calculation of its higher order eigenfunctions and eigenvalues. He has been pursuing this activity with large productivity under a number of years, partly in collaboration with D. C. Sahni from BARC, India. He initiated in 1983 the biannual series “Reactor Physics Research in the Nordic countries”. His reputation in the field was an important factor that in 1997 Chalmers hosted the 15th International Conference in Transport Theory.
Nils Göran continued with this research long after his retirement in 1991. He also used to have some hours in the courses in reactor and neutron physics when he told many interesting stories and anecdotes from the pioneering years, which were a highlight of the courses. One of these was the legendary story when he escorted a highly secret transport of heavy water from Rjukan in Norway to Stockholm, for the construction of the R1 reactor. About this, and about the history of the Department from the beginnings to his retirement in 1991, he wrote two very entertaining internal reports.
Nils Göran Sjöstrand around 1995
Nils Göran was a member of the Editorial Board of “Nuclear Instruments and Methods A” under 28 years, and reviewed about 600 manuscripts submitted for publication. He was the Vice Dean of the School of Physics in Chalmers for 15 years. He was also a member of the Committee for the Safety of Nuclear Installations at the Swedish Nuclear Power Inspectorate.
Nils Göran’s publications are known not only for their technical content, but also for their literary qualities. He was an archetype of a writer of high quality articles, both scientific and literary ones. His help as a language reader for younger colleagues was very much appreciated. He was not only interested in language and terminology, but he played an active role in elaborating and maintaining the terminology of reactor physics and nuclear power both in Swedish and English.
The lifetime achievement of Nils Göran Sjöstrand was acknowledged internationally in many ways. The international journal Progress of Nuclear Energy published a “Festschrift”, a special issue, to honour him on his 70th birthday in 1995. In late 2006, a mini-symposium was held in Chalmes, to commemorate the 50th anniversary of the publication of his seminal paper in Arkiv för Fysik. He is also recipient of the prestigious Eugene P. Wigner Reactor Physicist Award of the American Nuclear Society, which he received as late as 2011. Since by that time Nils Göran could not travel, Dimitrios Cokinos, the Head of the Honours and Awards committee of the ANS Reactor Physics Division, came to Sweden to present the Award to Nils Göran at a ceremony in the Chalmerska Huset in Göteborg.
The Wigner Award ceremony in the Chalmerska Huset in 2011
Nils Göran was not only a prominent scientist, but also very interested and knowledgeable in arts and humanities. He had a large collection of the books on the history of nuclear energy, which he donated to the Department in 2006. During 44 years he was a Fellow of the Royal Society of Arts and Sciences in Göteborg, and in 1996 he was its president. His humble and helpful attitude together with his huge knowledge and insight earned him much respect and many friends. As a learned friend and colleague put it, “he was a fine man and a scholar of distinction”. The reactor physics community lost one of its most outstanding founding members, but he will always be remembered for his achievements, wide knowledge, and kind personality.