- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
When a fast charged particle, such as a proton or an alpha particle, passes through a material, it gradually loses energy by exciting and ionising the atoms along its path. How quickly it slows down, a quantity called stopping power, was one of the central questions of early nuclear physics. Niels Bohr had written a famous classical treatment of the problem in 1913, and he spent years hoping to bring theory into closer agreement with experiment.
In the autumn of 1931 Felix Bloch, then a young theorist who had already made his name with the quantum theory of metals, arrived at Bohr's institute in Copenhagen on an Oersted fellowship. In long conversations Bohr gradually drew him into the stopping problem. There was a puzzle to solve. Bohr's classical calculation predicted larger energy losses than were seen for alpha and beta particles, while Hans Bethe had produced a more accurate, fully quantum mechanical result in 1930. It was not clear why the two approaches disagreed.
Bloch resolved the discrepancy. He showed that the Bohr and Bethe formulas were opposite limiting cases of a single, more general result, depending on how the phase of the interaction changed as the particle passed close to an atom. Bohr was pleased. Bloch finished the paper after returning to Leipzig as a lecturer in 1932, and it appeared in the Annalen der Physik in 1933. Late in 1932 he added a second, very practical refinement: using the Thomas–Fermi model of the atom, he showed that the average excitation energy of an atom, the one number that describes the material in Bethe's theory, is roughly proportional to its atomic number. That paper appeared in the Zeitschrift für Physik in 1933.
Bethe's equation combined with Bloch's estimate became widely known as the Bethe–Bloch formula, and a further term that improves its accuracy is still called the Bloch correction. Within months of this work Hitler came to power, and Bloch, who was Jewish, left Leipzig for good.
The formula served for decades as the basis for more refined calculations that added screening and straggling effects, and its modern descendants predict stopping power to within a few percent at higher energies. Physicists rely on them to design particle detectors and to work out how beams of protons and heavier ions deposit their energy in the body, knowledge of great practical importance for radiation therapy against cancer.
Why it matters
Knowing how charged particles lose energy is essential for designing particle detectors and for planning radiation treatments that use proton and ion beams.
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'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 measured the neutron's magnetismIn 1939, working with Luis Alvarez at the Berkeley cyclotron, Felix Bloch measured the neutron's magnetic moment to about one percent accuracy, a theorist turned experimenter.
- 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 1905–1983: A Biographical Memoir by Robert Hofstadter National Academy of Sciences
- Felix Bloch: Biographical NobelPrize.org
- Bethe formula Wikipedia
- Stopping power (particle radiation) Wikipedia
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.