The Nobel Prize in Physics 1994
Bertram N. Brockhouse, Clifford G. Shull
Presentation Speech by Professor Carl
Nordling of the
Royal Swedish Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses,
Ladies and Gentlemen,
This year's Nobel Prize in Physics has been awarded to Bertram Brockhouse and Clifford Shull for their pioneering contributions to the development of neutron scattering techniques for the study of liquid and solid matter. In simple terms, one could say that Shull answered the question of where atoms "are," while Brockhouse answered the question of what they "do."
At the end of World War II, research conditions underwent a radical change, especially in the United States. For some years, every single neutron emitted by a radioactive source, produced in an accelerator or released in a nuclear reactor had been employed for one single purpose: to produce the first atomic bomb. Suddenly a major new resource was being placed in the service of peaceful research. Neutrons could henceforth perform other tasks besides splitting atomic nuclei. Words like TOP SECRET were no longer automatically stamped across the cover of each research report.
Researchers had been familiar with neutrons as building blocks in the atomic nucleus for more than a decade. They had also done some thinking and conducted experiments concerning the properties of neutrons as free particles outside the nucleus. For example, they knew that neutrons possessed a dual nature that was characteristic of their tiny world: the ability to behave both as particles and as waves. In their latter guise, neutrons had been reflected against the atomic planes of a crystal in the same way X-rays previously had.
This provided a hint that some day, neutrons might become a tool for studying the microstructure of matter at the atomic level. The door was already ajar, but had not yet been opened wide.
Brockhouse and Shull followed their own individual strategies, both with the aim of gaining new knowledge about liquid and solid materials, otherwise called "condensed matter."
Shull took advantage of the fact that the wavelength of neutrons from a reactor may be roughly equal to the distance between the atoms in a solid body or a liquid. When the neutrons bounce against atomic nuclei, they do not lose energy, but their scattering is concentrated in directions that are determined by the structure in which the atoms are arranged. Shull revealed that neutrons could answer questions that the X-ray diffraction method had failed to answer, such as where the atoms of the light element hydrogen are located in an ice crystal.
Another breakthrough concerned magnetic structures. Neutrons themselves are small magnets and can interact very efficiently with the atoms in a magnetic material. Shull demonstrated how neutrons can reveal the magnetic properties of metals and alloys. The X-ray method had been powerless to accomplish this task as well.
While Shull was studying elastic neutron scattering, that is, scattering that occurs without energy changes, Brockhouse was concentrating on inelastic scattering. In the latter, neutrons lose part of their energy to the material or pick up energy from it.
Brockhouse designed ingenious instruments with which he managed to record the energy spectrum of the scattered neutrons. This enabled him to gather new information about such phenomena as atomic vibrations in crystals, diffusion movements in liquids and fluctuations in magnetic material. As a consequence, the study of these types of phenomena underwent a renaissance.
Over the years since Brockhouse and Shull made the contributions for which they are now being awarded the Nobel Prize, their methods have found widespread applications. Thousands of researchers are using neutron scattering to study the structure and dynamics of the new ceramic superconductors, molecule movements on surfaces for catalytic exhaust emission control, the interaction between proteins and the genetic material of viruses, the connection between the structure and elastic properties of polymers, the rapidly fading memory of the atomic structure of a metallic melt and much more. The pioneers of this broad field of research are the recipients of this year's Nobel Prize in Physics.
Professor Brockhouse, Professor
You have been awarded the 1994 Nobel Prize in Physics for your pioneering contributions to the development of neutron scattering in condensed matter research. It is my privilege to convey to you the heartiest congratulations of the Royal Swedish Academy of Sciences, and I now ask you to receive the Prize from the hands of His Majesty the King.
From Nobel Lectures, Physics 1991-1995, Editor Gösta Ekspong, World Scientific Publishing Co., Singapore, 1997
Copyright © The Nobel Foundation 1994