Prof. Maria Goeppert Mayer was only the second woman to win the Nobel Prize in Physics! The award was given for her work on the nuclear shell model where she had formulated the mathematical model for the nuclear shell on her own. This is the very same mathematical model of the nuclear shell that we are so well versed with right now. The prize was won along with Prof. Eugene Wigner and Prof. Hans D. Jensen in the year 1963.

Nuclear Shell Model

The nuclear shell model states that the nucleus consists of protons, electrons, and neutrons. But what brought about this discovery and how did they explain it? This is where Prof. Mayer’s contributions come into play because she was the one who realized that neutrons have a bigger role to play in the stability of the nucleus. Also, the nuclear shells are basically multiple orbits in which these particles revolve around the nucleus depending on the kind of energy level they are in!

In Physics, there are usually two approaches that one can take to solve a problem. While one is the assessment of the interaction between different elementary particles that gives us an idea about the forces involved. The other is using the experimental approach that eventually results in the development of models through the collection of data. In this case, even though this model was proposed by, multiple theoretical physicists, in the past, nobody ever came as close to the actual model as this team has.

Magic Numbers

It all began when she took a closer look at what we know as ‘magic numbers’ that denote the total number of nucleons that get equally distributed in the nuclear shells which gives the element great stability. This was first illustrated by the scientist Elsasser in a paper written in 1933. When they worked on the nuclear shell model, she came across this concept and along with the new data, she realised they held up with these numbers. Elements that had full shells (a fact they discovered), were more stable and this is where the model was created!

Magic numbers: 2, 8, 2 0, 2 8, 5 0, 8 2, 1 2 6

What does this mean?

For example, Helium is known to have 2 protons and 2 neutrons and it almost impossible to remove an electron from its nucleus. This is why it isn’t. They also figured out that for any nuclei that were heavier than Calcium (Z=40), the number of protons was lesser than the number of neutrons and this is how they realized that the number of neutrons had a bigger role to play than previously thought. Only when a suggestion was given by Enrico Fermi, the famous physicist, of whether or not spin-orbit coupling (a phenomenon where the spin motion of the nuclei is assessed with respect with its orbital motion) had a role to play in this scenario, did it click.

Eventually, this was all the information she needed to establish the mathematical model and that was that. The discovery has been best described by Prof. Mayer in her Nobel Prize Speech and here’s an excerpt from the speech:

Think of a room full of waltzers. Suppose they go round the room in circles, each circle enclosed within another. Then imagine that in each circle, you can fit twice as many dancers by having one pair go clockwise and another pair go counterclockwise. Then add one more variation; all the dancers are spinning twirling round and round like tops as they circle the room, each pair both twirling and circling. But only some of those that go counterclockwise are twirling counterclockwise. The others are twirling clockwise while circling counterclockwise. The same is true of those that are dancing around clockwise: some twirl clockwise, others twirl counterclockwise.

Nobel Prize Lecture, 1963

From the works of Prof. Mayer, it is clear that when women are given the opportunity to contribute to the field of science, they can go much far ahead than where we expect them to go. This discovery has changed not only the field of physics but has resulted in the advancement of nuclear physics over the years!

References

The Shell Model (1963). Retrieved from https://www.nobelprize.org/uploads/2018/06/mayer-lecture.pdf