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The Origin of the Near-Spherical Appearance of Nuclei

The Ellipsoidal Shapes of Nuclei

The neutrons and protons of nuclei are separately organized in shells. Remarkably however the maximum occupancies of the shells are the same for neutrons and protons. Other than the shell structures the spatial organization of neutrons and the protons has been a mystery.

Nuclei are believed to have near-spherical shapes; i.e., ellipsoidal. A quantity called the electrical quadrupole moment (EQM) has been defined. The crucial matter is how can it be measured. There are numerous methods that have been proposed for measuring the EQM of nuclei. While often these various methods give approximately the same that is not always the case. N.J. Stone tabulated these measurements from individual studies published in the journals of physics. They are available in his Atomic Data and Nuclear Data Tables which is updated every few years.

The expectation was that nuclei with filled shells would be spherical and thus have an electrical quadrupole moment of zero. This expectation is examined in the table below. The table for the smaller nuclides includes only those measurements obtained by the method labled NMR (nuclear magnetic resonance), including β-NMR (NMR with beta detection). A few cases in which Stone re-evaluated the data were included.

The EQM has the dimensions of electric charge times area. The area unit used for the tabulation is 10−24 cm², known as a barn, as in "big as a barn door."

The Electric Quadrupole Moments of the Smaller Nuclides
Number of
protons
Number of
Neutrons
Absolute Value of
Electric Quadrupole
Moment
(barns) |Q|
1 1 0.00286
3 3 0.0008
3 4 0.0406
3 5 0.0317
3 6 0.036
3 6 0.0253
3 8 0.031
4 5 0.0529
5 3 0.063
5 5 0.0847
5 6 0.0407
5 7 0.0134
5 8 0.037
5 9 0.0298
5 10 0.038
5 12 0.0386
6 5 0.032
7 5 0.0098
7 7 0.0208
7 9 0.018
7 11 0.0123
8 9 0.02578
8 10 0.036
8 11 0.0037
9 8 0.058
9 10 0.072
9 11 0.042
10 10 0.23
10 12 0.19
11 15 0.0053
11 16 0.0072
11 17 0.0395
11 18 0.086

The absolute values of the EQMs are given because in Stone's table in some cases the sign was not determined.

There is no clear pattern to the values and no apparent tendency for zero values at the magic numbers of nucleons.

The Difficulty of Maintaining a Spherical Structure for Nucleons

It is not possible to create a polyhedral structure of overall spherical shape that can maintain its structure by rotation against attraction toward the center of the structure. If the rotation of the particles at or near the equator balances the force on them the ones near the poles experience unbalanced force. What follows below is an explanation of how the dynamic appearance of ellipsoidal shapes can be created in nuclei.

The Alpha Module Structure of Nuclear Shells

The mass of a nucleus is less than the sum of the masses of the protons and neutrons that it is made of. The difference is called its mass deficit. When the mass deficit is expressed in energy units via the Einstein equation E=mc² it is called the binding energy of the nucleus. The binding energy has been computed for 2931 nuclides (types of nuclei).

The incremental binding energy of a neutron in a nuclide with n neutrons and p protons is its binding energy less that of a nuclide having one less neutron. The incremental binding energy of a neutron can be computed for about 2820 (2931−111) nuclides. For all of the incremental binding energies of neutrons (IBEN) there is a sawtooth, odd-even pattern in which the IBEN is higher for an even number of neutrons than for an odd number because of the formation of a spin pair of neutrons. This means that whenever possible two neutrons form a spin pair. Here is an example of the sawtooth pattern for IBEN.

Likewise the incremental binding energies of protons (IBEP) can be computed for 2769 (2931−162) nuclides. For all of these as well there is the sawtooth odd-even pattern indicating that that proton-proton spin pairs are formed whenever possible. Here is an example of the sawtooth pattern for IBEP.

Whenever the number of protons is less than the number of neutrons the addition of another proton will result in the formation of a neutron-proton spin pair. When the number of protons exceeds the number of neutrons no neutron-proton spin pair is formed and so the level of the IBEP drops. The same thing happens to the IBEN when the number of neutrons passes from a level below the number of protons to a level above it. The following graph shows the drop in incremental binding energies when the number of one type of nucleon exceeds the number of the other type.

Thus whenever possible a proton and neutron form a neutron-proton spin pair. The fact that the level of incremental binding energy drops when there is no excess of the other nucleon means that neutron-proton spin pair formation is exclusive; i.e., a proton can form a neutron-proton spin pair with only one neutron, and likewise for a neutron.

This means that neutrons and protons are linked into chains. A neutron is linked to one other neutron. That neutron is linked through a spin pairing to a proton, which in turn is linked to another proton. Thus there has to be a chain comprised of modules such as -n-p-p-n-, or equivalently -p-n-n-p-. These can appropriately be called alpha modules. An alpha particle is just an alpha module in which the end nucleons link up. More generally there are several alpha modules contained in a ring. Here is a depiction of a ring of four alpha modules. It is not intended to depict realistically the arrangement of the nucleons; instead it is a symbolic representation.

These chains of alpha modules must close. Otherwise there would be a nucleon at one end of the chain or the other without a linkage. An odd nucleon of one type is left out of the chain

In addition to the binding energies reflecting the energies involved in the formation of substructures like spin pairs there is the energies involved in the interaction of nucleons through the nuclear strong force. As noted previously neutrons and protons are organized in shells. When one shell is filled any additional nucleon goes into the next shell. The higher shells are at a greater distance from the nucleons in the other shells and therefore the interaction energies are less. Therefore the incremental binding energies decrease for higher level shells.

The Dynamics of a Ring of Alpha Modules

The neutrons and the protons in the same shell numbers are attracted to each other through the nuclear strong force. In order to maintain stability these nucleons must rotate about their centers of mass. This means that the neutrons and protons in a shell must rotate like a vortex ring; i.e., a so-called smoke ring.

A substructure linked together such as in a ring can be subject to motions that cannot occur for a single particle. For example, a ring can rotate about a diameter line. For a circular ring this produces the dynamic appearance of a sphere. There can be rotation about more than one diameter line. This reenforces the appearance of the rotating ring as a sphere. A ring can also rotate about an axis through its center perpendicular to its plane. This gives alone a trajectory for a single particle that is a toroidal helix. When this motion is combined with the flipping motion of rotation about a diameter the trajectory of all the particles becomes very complicated and it is easy to believe that each particle more or less covers a spherical shell.

Here is an animation that illustrates the combined motions. The blue torus depicts the vortex ring of the the neutrons and protons in a nuclear shell.

In the animation the flipping rotations about ring diameters are shown sequentially but in reality they would occur simultaneously. Under sufficiently rapid rotation the torus would appear to be a spherical surface.

When there is an odd nucleon the appearance would be a sphere with a string or ribbon wrapped around it. This would create a nonzero value for the quadrupole moment.

(To be continued.)

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