Ursula Wagner (1933-2021) and Friedrich Wagner (1937-2023)

Ursula and Friedrich Wagner in front of the Munich Research Reactor (2000)
Photo taken from Mössbauer Effect Reference and Data Journal (MERDJ), March 2004, Vol. 27, No. 3

Ursula and Friedrich (Fritz) Wagner – A couple in Mössbauer Science

Fritz Wagner was born in 1937 and studied physics at the Technical University of Munich. In 1960, he was looking for a topic for his diploma thesis. Professor Heinz Maier-Leibnitz suggested the topic of resonance absorption of gamma rays, which was to be supervised by his assistant Rudolf Mössbauer. Fritz Wagner's diploma thesis was on ¹⁷⁰Yb, and he used the then fairly new Munich research reactor to produce the ¹⁷⁰Tm sources. During this time he saw little of Rudolf Mössbauer who soon went to CalTech in Pasadena but Mössbauer spectroscopy did not let go of Fritz Wagner in the following decades. He completed his doctoral thesis on the total reflection of ¹⁶⁹Tm gamma radiation in 1965 under Maier Leibnitz and Paul Kienle.

Ursula Wagner studied chemistry and did her diploma and doctoral theses at the Technical University of Munich in organometallic chemistry with E. O. Fischer who received the Nobel Prize for his work in 1973. Her doctoral thesis was on the hot-atom chemistry of organometallic compounds using the then new Munich research reactor. After her doctorate, she went to Brookhaven to continue this line of research with Garman Harbottle.

Ursula had her first encounter with Mössbauer spectroscopy as early as 1962, when her training in organometallic chemistry led her to work with Paul Kienle and Mike Kalvius on Mössbauer experiments on ferrocene and iron carbonyls. She was back in Munich when Rudolf Mössbauer returned from CalTech and started to set up his group at the Technical University of Munich. Ursula became the head of the radiochemistry lab, which was mainly needed to produce sources for a variety of Mössbauer isotopes and materials for investigation with the Mössbauer method. With the reactor on site, the Munich group was indeed able to excel in working with many of the less-common Mössbauer isotopes over the years.

These included ¹³⁹La ¹⁴¹Pr, several Er and Yb isotopes as well as the numerous Mössbauer isotopes of osmium, iridium and platinum. Great efforts were made for ⁹⁹Ru and even ¹⁰¹Ru. A significant and lengthy attempt was made for ¹⁹³Ir and ¹⁹⁷Au. For these isotopes with their short-lived parent isotopes, which had to be produced at least once a week, the Munich research reactor next door proved to be a real stroke of luck. In 1971, Rudolf Mössbauer left Munich to become director of the research reactor in Grenoble (ILL), and G. M. Kalvius came to Munich to replace him. When Fritz Wagner returned from a stay at Argonne National Laboratory, he joined Mike Kalvius' group.

Ursula and Fritz married in 1973, Fritz obtained his Habilitation in 1974 and soon after became a professor of physics at the Faculty of Physics of the Technical University of Munich. Towards the end of the 1970s, the study of metal-hydrogen systems became an important area of research in Fritz's group. Then, in 1979, a devastating fire destroyed almost all the Mössbauer equipment in the group. Fortunately, the remaining equipment was patched up within a few days and measurements resumed. Ursula continued her hot-atom chemistry work, now also with the help of Mössbauer spectroscopy.

In 1977, a turning point in Ursula's scientific career occurred when Josef Riederer, by then head of the research laboratory of the Archaeological Museums in Berlin, suggested using the Mössbauer spectroscopy of the Physics Department to study some ancient Egyptian ceramics in which he was interested. Ursula was intrigued by the idea and soon some promising results were obtained. The subject developed into Ursula's main area of research. Studies eventually turned away from Egyptian ceramics when she began to work on pre-Columbian Latin American ceramics with Izumi Shimada, the director of the Projecto Arqueologico de Sican. Later, her interest also turned to Celtic ceramics from Central Europe, especially Bavaria. This became her second major area of research in ceramics. Ursula combined Mössbauer spectroscopy with other techniques, especially neutron activation analysis for provenance studies of ceramics. Again, the neighborhood of the Munich Research Reactor was of utmost importance.

The good times lasted until the summer of 2000, when the reactor was shut down because a new reactor, the FRM-II, had been built next to it, which was to be commissioned within six months. In fact, it went critical for the first time in March 2004, after a delay of about three years, mainly due to political squabbles. Ursula's archaeology project suffered somewhat as a result, but with Mössbauer spectroscopy combined with X-ray diffraction and thin section analysis, the archaeometric work still went well. As a result, Ursula edited two special volumes of Hyperfine Interactions on Mössbauer spectroscopy in archaeology. This research field  then fascinated the two Wagners for all the years that followed.

The closure of the reactor had a strong impact on Fritz's research, which in the 1990s had largely shifted to work with short-lived sources from the Munich reactor. Gold played a major role in this, for example in the study of gold ores that contain very small amounts of gold in chemically bound form, which was of great interest for gold production and mineralogy. He and his group also studied gold in gold ruby glass and a range of catalysts. Catalysis became one of the other topics that excited Fritz in the 1980s and since, with ⁹⁹Ru, ¹⁹³Ir and ¹⁹⁷Au playing the main role and the more mundane Mössbauer isotopes merely a supplement.

Since the end of the Munich Research Reactor, catalyst and mineralogy research has been limited to work with commercial sources, mainly ⁵⁷Fe and ¹¹⁹Sn and ¹²¹Sb. Although this seemed to be a step backwards, the Munich Mössbauer group has always been proud of the fact that they had the expertise to work with less common Mössbauer isotopes. Indeed, it is to their great credit that they have built up this knowledge base, which is so valuable for generations to come.

With the availability of synchrotron radiation sources, it is again becoming possible to turn to the less common Mössbauer isotopes, as they can be excited from the ground state, making the production of parent isotopes unnecessary. An example of this is the 73 keV transition of ¹⁹³Ir, recently established at PETRA III at DESY in Hamburg where a seminal review article by Fritz Wagner in Hyperfine Interactions in 1983 was of great help. In this sense, the lives and science of the Wagners continue to be influential for future science.

To honor the scientific merits of Ursula and Fritz Wagner, they were appointed Fellows of the International Board on the Applications of the Mössbauer Effect (IBAME) in 2021.

Their lifelong interest in Mössbauer Science and their contributions to conferences and lifely personal exchange will be deeply missed.

(Compiled by Ralf Röhlsberger, in great parts based on Mössbauer Effect Reference and Data Journal (MERDJ), March 2004, Vol. 27, No. 3)