The Story of Nuclear Energy, Volume 2 (of 3): The Neutron - Nuclear Spin

Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume II, THE NEUTRON: Nuclear Spin

THE NEUTRON

Nuclear Spin

What Rutherford did not (and could not) take into account were the consequences of a completely new type of nuclear bombardment involving a type of particle unknown in the 1920s (though Rutherford speculated about the possibility of its existence).

The beginnings of the new path came about through the reluctant realization that there was a flaw in the apparently firmly grounded proton-electron picture of nuclear structure.

The flaw involved the “nuclear spin”. In 1924 the Austrian physicist Wolfgang Pauli (1900-1958) worked out a theory that treated protons and electrons as though they were spinning on their axes. This spin could be in either direction (or, as we would say in earthly terms, from west-to-east, or from east-to-west). Quantum theory has shown that a natural unit exists for what is called the angular momentum of this spin. Measured in terms of this natural unit of spin, the proton and the electron have spin ½. If the particle spun in one direction it was +½, if in the other it was -½.

When subatomic particles came together to form an atomic nucleus, each kept its original spin, and the nuclear spin was then equal to the total angular momentum of the individual particles that made it up.

For instance, suppose the helium nucleus is made up of 4 protons and 2 electrons, as was thought in the 1920s. Of the 4 protons, suppose that two had a spin of +½ and two of -½. Suppose also that of the 2 electrons, one had a spin of +½ and one of -½. All the spins would cancel each other. The total angular momentum would be zero.

Of course, it is also possible that all 6 particles were spinning in the same direction; all +½ or all -½. In that case the nuclear spin would be 3, either in one direction or the 93other. If 5 particles were spinning in one direction and 1 in the other, then the total spin would be 2, in one direction or the other.

Wolfgang Pauli lecturing in Copenhagen in April 1929.

In short if you have an even number of particles in a nucleus, each with a spin of +½ or -½, then the total spin is either zero or a whole number, no matter what combination of positive and negative spins you choose. (The total spin is always written as a positive number.)

On the other hand, suppose you have lithium-7, which was thought to be made up of 7 protons and 4 electrons. If the 7 protons were all +½ and the 4 electrons were all -½ in their spins, the nuclear spin would be ⁷/₂ - ⁴/₂ = ³/₂.

If you have an odd number of particles in the nucleus, you will find that any combination of positive and negative spins will never give you either zero or a whole number as a sum. The sum will always include a fraction.

Consequently, if one measures the spin of a particular atomic nucleus one can tell at once whether that nucleus contains an even number of particles or an odd number.

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This quickly raised a problem. The nuclear spin of the common isotope, nitrogen-14, was measured accurately over and over again and turned out to be 1. There seemed no doubt about that and it could therefore be concluded that there were an even number of particles in the nitrogen-14 nucleus.

And yet, by the proton-electron theory of nuclear structure, the nitrogen-14 nucleus, with a mass number of 14 and an atomic number of 7, had to be made up of 14 protons and 7 electrons for a total of 21 particles altogether—an odd number.

The nuclear spin of nitrogen-14 indicated “even number” and the proton-electron theory indicated “odd number”. One or the other had to be wrong, but which? The nuclear spin was a matter of actual measurement, which could be repeated over and over and on which all agreed. The proton-electron theory was only a theory. It was therefore the latter that was questioned.

What was to be done?

Suppose it is wrong to count protons and electrons inside the nucleus as separate particles. Was it possible that an electron and a proton, forced into the close confinement of the atomic nucleus might, by the force of mutual attraction, become so intimately connected as to count as a single particle. One of the first to suggest this, as far back as 1920, was Rutherford.

Such a proton-electron combination would be electrically neutral and in 1921 the American chemist William Draper Harkins (1873-1951) used the term “neutron” as a name for it.

If we look at the nitrogen-14 nucleus in this way then it is made up, not of 14 protons and 7 electrons, but of 7 protons and 7 proton-electron combinations. Instead of a total of 21 particles, there would be a total of 14; instead of an odd number, there would be an even number. The structure would now account for the nuclear spin.

But could such a revised theory of nuclear structure be made to seem plausible? The proton-electron theory seemed to make sense because both protons and electrons were known to exist separately and could be detected. If an intimate proton-electron combination could also exist, ought it not exist (or be made to exist) outside the nucleus and ought it not be detected as an isolated particle?

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49820/49820-h/49820-h.htm#c24

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