- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
The neutron was discovered in 1932. Because it carries no electric charge, many physicists assumed it would have no magnetism, but indirect evidence suggested that it did behave like a tiny magnet. Measuring that magnetism directly on free neutrons was a difficult challenge.
Felix Bloch, a Jewish theorist from Zurich, left Germany in the spring of 1933 after Hitler came to power and in 1934 took up a post at Stanford University. On a visit to Copenhagen in 1935, Niels Bohr encouraged him to think about neutron physics. Bloch's knowledge of ferromagnetism gave him an idea: neutrons passing through magnetised iron should scatter differently depending on how their own magnets were oriented. In 1936 he published the theory, showing that the effect could produce beams of polarised neutrons, all with their magnets pointing the same way. Enrico Fermi had advised him to try experiments because it was fun, and Bloch took the advice.
His key insight came in 1937. Bloch planned to send a polarised neutron beam through a steady magnetic field while a weak radio-frequency field acted on it, and to look for a change in the beam as the frequency passed through resonance. A first attempt at Stanford failed because the neutron source was too weak. Ernest Lawrence then suggested that a young experimenter, Luis Alvarez, join him at the 37-inch cyclotron in Berkeley. Starting in the autumn of 1938, they worked through the spring and summer of 1939 on a machine that ran only sporadically.
Their reward was a resonance dip of about two percent in the beam. Bloch and Alvarez found the neutron's magnetic moment to be 1.935 nuclear magnetons, with an accuracy of about one percent, pointing opposite to the proton's. The result, published in 1940, was the first accurate, direct measurement and settled lingering doubts about the neutron's magnetism.
After the war Bloch combined this resonance approach with his new nuclear induction technique and, with D. Nicodemus and H. H. Staub, remeasured the neutron's moment with even greater precision in 1948. The experiment is still remembered as a celebrated case of a theorist becoming an experimenter, and the reasoning behind it led Bloch directly to nuclear magnetic resonance.
Why it matters
Polarised neutron beams and the neutron's magnetism are now used to probe the atomic and magnetic structure of materials, and the experiment pointed the way to NMR.
About Felix Bloch
Swiss-born physicist who laid the quantum foundations of solid-state physics, discovered nuclear magnetic resonance – the basis of MRI – and became CERN’s first director-general.
More about Felix Bloch
- Felix Bloch helped explain how particles slow in matterIn Copenhagen in the early 1930s, Felix Bloch refined Hans Bethe’s theory of how charged particles lose energy in matter, work remembered in the Bethe–Bloch formula.
- Felix Bloch's equations underpin MRIIn 1946 Felix Bloch wrote the Bloch equations for nuclear magnetisation, introducing the T1 and T2 relaxation times that later helped make MRI a powerful medical tool.
- Felix Bloch was CERN's first director-generalIn 1954 Felix Bloch became the first director-general of CERN, the new European laboratory for particle physics, as construction began on its site at Meyrin near Geneva.
- Felix Bloch's method became chemistry's key toolChemical shift experiments in Felix Bloch's own laboratory helped turn nuclear magnetic resonance into the most important spectroscopic tool in chemistry.
- Felix Bloch discovered how to listen to atomic nucleiIn 1946 Felix Bloch, William Hansen and Martin Packard at Stanford developed nuclear induction, one of the two founding methods of nuclear magnetic resonance.
- Felix Bloch described magnetic domain walls and spin wavesFelix Bloch described the boundaries between magnetic domains, now called Bloch walls, and proposed spin waves, partly in a paper written in hospital after a climbing accident.
- Felix Bloch explained how electrons move through metalsIn his 1928 thesis as Werner Heisenberg's first graduate student, Felix Bloch showed how electrons travel as waves through a crystal, founding the quantum theory of solids.
Sources
- Felix Bloch: Biographical NobelPrize.org
- Felix Bloch 1905–1983: A Biographical Memoir by Robert Hofstadter National Academy of Sciences
- Neutron magnetic moment Wikipedia
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