Great Scientists and their Inventions
Aage Bohr(1922 – 2009.)
image source: wikimedia commons
Aage Bohr was awarded the Nobel Prize in Physics in 1975 for his work detailing the structure of the atomic nucleus.
Like his father, Aage Bohr was intrigued by the structure of the atom. The atomic nucleus in particular; that tiny, densely packed, positively charged mass at the heart of every atom interested him intensely.
What was the nucleus really like – were there any structural details, and if so, what were they?
The Nucleus as a Drop of Liquid
One idea, which had been developed most fully by Niels Bohr and John Archibald Wheeler in the late 1930s, was the liquid-drop model. The liquid-drop model pictured the nucleus as a rotating drop of incompressible liquid held together by surface tension.
The drop of liquid could be deformed from its basic spherical shape and a large drop of liquid could fall apart to form two new drops. Similarly a large atomic nucleus, like uranium, could fall apart to form two new atomic nuclei – this is nuclear fission, the energy source behind both the uranium atom bomb and the uranium power plant.
The liquid drop model had its greatest successes in explaining the properties of heavy nuclei, such as uranium.
By 1950, however, the liquid drop model was in danger of being pushed aside by the newer shell model of the nucleus.
The Nucleus with Energy Shells
Much like electrons are said to occupy shells of different energy outside the nucleus, the shell model of the nucleus says protons and neutrons occupy distinct energy shells inside the nucleus.
By 1950, most physicists had decided the shell model looked more promising than the liquid-drop model.
In particular, the shell model explained why atomic nuclei with so-called magic numbers of protons+neutrons are particularly stable. This is similar to the concept taught in high school chemistry, where atoms with complete electron shells, for example, 2 or 8 electrons in their outermost shells are particularly stable, leading to the unreactive behavior of the noble gases.
In the case of atomic nuclei, the magic numbers of 2, 8, 20, 28, 50, 82 and 126 protons+neutrons result in particularly stable nuclei.
The shell model was particularly good at explaining the properties of lighter nuclei and nuclei with the magic numbers of protons+neutrons, but was less successful with heavy nuclei such as uranium.
In fact, the liquid-drop model and the shell model both had advantages and disadvantages – indicating that neither could be the full story.
In 1949, James Rainwater, a Columbia University physicist, decided to combine the best aspects of the liquid-drop and shell models into a single unified model of the nucleus.
At that time Rainwater shared an office at Columbia with Bohr and explained his ideas to him. Bohr was captivated, seeing the potential of Rainwater’s ideas to explain the behavior and structure of the atomic nucleus.
Bohr returned to Copenhagen, determined to pursue the unified model further. There he worked with Ben Mottelson, who had completed his Ph.D. at Harvard University and who was now in Copenhagen on a Traveling Fellowship from Harvard.
Together, Bohr and Mottelson worked out in intricate detail how a unified model could explain a huge number of experimental observations from different atomic nuclei. In 1953 they published a 173-page report describing their unified model and in 1954 Bohr published The Rotational States of Atomic Nuclei. Crucially, predictions they made about how nuclei would behave were verified in experiments.
One of their key findings was that some of the behaviour of nuclei could be explained by nuclei having different amounts of energy resulting from rotation. Furthermore, nuclei do not rotate as rigid objects but, instead, a surface wave travels around the nucleus. They also found that nuclei vibrate, changing their shape around an average value.
At first Bohr had trouble convincing his father that the liquid-drop model should be dropped – after all, Niels Bohr was one of the liquid-drop model’s main architects – but eventually he won his father round.
The unified model – often called the collective model – is sometimes likened to a swarm of bees, where each bee is a neutron or proton and the swarm is the nucleus. The swarm acts as a single entity, even though each bee within it is moving around independently with its own, individual energy. In the Bohr-Mottelson model, the outside of the swarm rotates and wobbles inward and outward.
Each neutron or proton has its own orbital energy within the nucleus. These orbits can sometimes deform the nucleus so that it is no longer truly spherical. For example, the nucleus of heavier atoms can become an oblate spheroid (discus shaped) or prolate spheroid (football shaped).
Of course, we need to remember that atomic nuclei have a diameter of between 1.7 x 10−15 m for hydrogen and about 15 x 10−15 m for uranium.
The fact that Bohr and others were able to mathematically model such incredibly small objects, producing fine structural detail, and predicting their behavior in agreement with experimental data is remarkable.
In 1975, Aage Bohr, Ben Mottelson and James Rainwater shared the Nobel Prize in Physics for their model of the nucleus. In the words of the award committee, the prize was:
Despite the huge strides taken by the trio of physicists, even today, the structural details of atomic nuclei have still not been fully resolved.
Aage Bohr died on September 8, 2009, aged 87. He was buried in the Mariebjerg Cemetery, Copenhagen. His first wife, Marietta, died in 1978.
Bohr was survived by his second wife, Bente Meyer Scharff, whom he had married in 1981, and by two sons and a daughter from his marriage to Marietta. One of his sons, Tomas, became a Professor of Physics at the Technical University of Denmark.
We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.
An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.