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Shaping the Future

Scientific Milestones During Israel’s First Half-Century
Shaping the Future

First published in 1997

Nuclear Physics




The energy within

Institute scientists contributed to understanding the structure of atomic nuclei by developing methods for the quantitative calculation of energies of nuclei in various states.
Atomic nuclei are composed of protons carrying positive electrical charges, which repel each other, and neutrons, which are electrically neutral. The strong nuclear force between protons and neutrons holds them together, thus permitting the existence of stable atomic nuclei.
Since the 1930s, physicists all over the world have tried to understand the motion of protons and neutrons within the nucleus. By 1949 it was realized that the nuclear constituents move in orbits which are arranged like the shells of onion skins, similar to the motion of atomic electrons revolving around the nucleus ("shell model"). The protons and neutrons move constantly in their orbits in the average field of force due to all constituents. Since these forces are very strong and have a very short range, it was very difficult to predict the orbits of protons and neutrons and to calculate the energies of nuclei in various states.
Institute scientists developed a method to circumvent this difficulty. In the shell model, nuclear energies are determined only by "effective forces" acting between nuclear constituents. It is very difficult to calculate such effective forces due to the strength and short range of the real forces between protons and neutrons. Assuming that the effective forces act only between pairs of nuclear constituents, it was found that their values could be extracted from experimentally measured energies of some nuclear states. Using these values, it was possible to carry out for the first time quantitative calculations of energies of states in various nuclei. General features of the effective forces which were determined in this way led to an understanding of the structure of nuclei and its dependence on the number of protons and neutrons.
The method developed here for the determination of the effective forces was adopted throughout the world and even now, many years later, it still serves as a basic tool in the calculation of nuclear properties. Coupled with modern computing facilities, it allows for the prediction of energies of many nuclei in their various states.


The riddle of collective movement

Weizmann Institute scientists made a decisive contribution to understanding the collective movement of nucleons (protons and neutrons) observed in a number of regions of atomic nuclei. It was originally surmised that all constituent nucleons participate in the collective motion, but Weizmann scientists demonstrated that this is not the case and that, in fact, the number and nature of the nucleons involved is governed by rather subtle features of nuclear structure.
These breakthroughs employed innovative experimental techniques exploiting the magnetic interaction between the nucleons and the electrons that surround them. Through the intermediation of these electrons, the scientists generated very strong magnetic fields which acted on the nucleons. This allowed them to elicit the magnetic properties of the nucleus, which in turn is related to an important nuclear feature: the ratio of protons and neutrons in the collective motion. Those techniques were later expanded to cover a wide range of nuclear features- collective and otherwise: nuclear shapes in general and in one important case, the deviation of a particular nucleus from the mirror symmetry that nuclei usually exhibit. Some of these techniques have been taken up by research centers around the world.
The very high magnetic fields employed in these experiments are essential because the phenomena that are observed and studied occur only in excited nuclei of very short lifetimes (a billionth of a second or less); only with the application of extremely strong fields is the response of the nucleus to the applied field observable. To excite the nuclei, the scientists used the Institute's ion accelerator complex, the only one in Israel. The facility includes three accelerators (the Van de Graaff accelerator, the Heineman Tandem accelerator and the Koffler Pelletron accelerator), all of which produce beams of high energy ions impinging on fixed targets.