This chapter is available as free (personal) off-print from the new book version.

20-02 Nuclear Magnetic Resonance

In 1946, two scientists in the United States, independently of each other, de­scri­bed a physico-chemical phenomenon which was based upon the magnetic pro­per­ties of certain nuclei in the periodic system. This was Nuclear Magnetic Re­so­nan­ce, for short NMR.

The two scientists, Edward M. Purcell and Felix Bloch, were awarded the Nobel Prize in Physics in 1952 [⇒ Bloch, Hanson, Packard, ⇒ Purcell, Torrey, Pound].

Purcell (Figure 20-04) was born in Illinois in the United Sta­tes of America and studied at Purdue University in In­di­ana. After a research year at the Karlsruhe Technical Uni­ver­si­ty in Germany, he worked at the Massachusetts In­sti­tu­te of Technology. Figure 20-04: Edward M. Purcell (1912-1997).

The discovery of nuclear magnetic resonance absorption was made just after the end of the World War II when he became Professor of Physics at Harvard. His paper on the subject was published in Physical Review in early 1946, as Bloch's paper was (Figure 20-06).

Bloch (Figure 20-05) was born in Zurich and taught at the University of Leipzig until 1933; he then emigrated to the United States and was naturalized in 1939. He joined the fa­cul­ty of Stanford University at Palo Alto in 1934 and be­came the first director of CERN in Geneva in 1962. In 1983 he died in Zurich. Figure 20-05: Felix Bloch (1905-1983).

Figure 20-06: Bloch's short communication in Physical Re­view, received in January 1946. Pur­cell's paper had reached the journal four weeks ear­lier, at Christmas Eve 1945.

 Both Purcell and Bloch were protagonists for the interaction between Eu­ro­pe and the United States. NMR and MRI would not exist without this interaction.

At some stage of their career, many European scientists contemplated and per­haps still today contemplate emigration to the USA. Some stay for good, others re­turn. There is hardly any movement in the other direction. The historical reasons were different prior to and after the Second World War. Before the war, plain phy­si­cal survival for many depended on emi­gra­tion, or it was at least guided by po­li­ti­cal necessity. The attraction of the Statue of Liberty made scientists move west­ward.

After the war, research facilities in the United States were more attractive than those in Europe because the academic system in the USA was more flexible than the university structures in Europe – and dollars were plentiful for research and for personal income.

Bloch and Purcell were not the only scientists working in the field. The 1920s had been roaring and inflationary, but also extremely fruitful in science.

In 1924, Wolfgang Pauli (1900-1958) suggested the possibility of an intrinsic nuc­lear spin. The year after, George Eugene Uhlenbeck (1900-1988) and Sa­mu­el A. Goudsmit (1902-1978) introduced the concept of the spinning electron. Two years later Pauli and Charles Galton Darwin (1887-1962) developed a theo­re­ti­cal frame­work for grafting the concept of electron spin into the new quantum mechanics de­ve­lop­ed the year before by Erwin Schrödinger (1887-1961) and Werner Hei­sen­berg (1901-1976).

Pauli, Uhlenbeck, and Goudsmit went to the United States to work. The British stayed in Britain – at that time. This development continued in the 1930s.

After initial pacemaking work, in 1933, Otto Stern (Figure 20-07) – together with Walther Gerlach (1889-1979) – was able to measure the effect of the nuclear spin by de­flec­tion of a beam of hydrogen molecules (the so-called Stern-Ger­lach effect). Figure 20-07: Otto Stern (1888-1969).

Stern was awarded the Nobel Prize in Physics for 1943 "for his contribution to the development of the molecular-ray method and his discovery of the magnetic moment of the proton."

During the early 1930s, Isidor Isaac Rabi's (Figure 20- 08) laboratory at Columbia University in New York be­came a major center for related studies. Figure 20-08: Isidor Isaac Rabi (1899-1989).

Rabi's research was successful, but only with the visit by Cor­ne­lis Jacobus Gorter (Figure 20-09) from the Ne­ther­lands in September 1937 they were finally able to measure the nuclear magnetic moment. Gorter had tried similar ex­pe­ri­ments earlier and failed (Figure 20-10). Figure 20-09: Cornelis Jacobus Gorter (1907-1980).

Several years later, Gorter was the first to use the term nuclear mag­ne­tic re­so­nan­ce in the publication about his failed experiments which appeared in the war-torn Ne­ther­lands in 1942, attributing the coining of the phrase to Rabi [⇒ Gorter].

Figure 20-10: Beginning of Gorter's article (1942).

After Gorter's visit Rabi took up his suggestions concerning the experiments, chan­ged them, and was able to observe resonance experimentally. This led to the pub­li­ca­tion of "A New Method of Measuring Nuclear Magnetic Moment" in 1938 [⇒ Ra­bi].

Figure 20-11: Beginning of Rabi's article about his first ob­ser­va­tion of nuclear magnetic re­so­nan­ce (1938). In a footnote he stated how in­deb­ted he was to Gorter and his idea of how to run the experiment.

 The Second World War had a major influence upon research – and its in­ter­rup­tion. Germany, for instance, the leading country in science and medicine at the time, quit the race in the 1930s. The final breakthrough came with Bloch and Pur­cell in 1946 and during the next few decades NMR developed in a wide range of ap­pli­ca­tions.

But there was another country in which major contributions to nuclear mag­ne­tic resonance were made. They originated in Kazan in Tatarstan, which was part of the Soviet Union at that time and is now an independent republic within Rus­sia.

Electron spin resonance was discovered at Kazan's uni­ver­si­ty by Yevgeni K. Zavoisky (Figure 20-12 – Евгений Кон­стан­ти­но­вич Завойский) towards the end of the war [⇒ Zavoisky]. Zavoisky had first attempted to detect NMR in 1941, but like Gorter he failed. Figure 20-12: Yevgeni K. Zavoisky (1907-1976).

Then, in January 1944, he was able to register ESR (electron spin re­so­nan­ce) signals. They were recorded on celluloid films and also copied by hand into the laboratory logbook (Figure 20-13). Figure 20-13: Pictures of the first ESR signals as copied from the screen of Zavoisky's os­cil­lo­graph in 1944 (Picture source: Memorial La­bo­ra­to­ry of Yevgeni K. Zavoisky at Kazan State University).

He published a number of scientific papers on his discoveries in the late 1940s (Figure 20-14). Figure 20-14: Zavoisky's publication from March 1945: "Spin-Magnetic Resonance in Pa­ra­mag­ne­tics” (English translation) in which he refers to Gorter.