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Nuclear Rotations |
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Aage Bohr and Ben Mottleson found that nuclear rotations obey the I(I+1) rule; i.e., have energy levels E_{I} such that
where I is an integer, J is the moment of interia of the rotating nuclear shell and h is
Planck's constant divided by 2π. Note what this implies about the quantification of angular momentum.
If ω is the rate of rotation then
Thus angular momentum L is given by
This is in contrast to the Old Quantum Physics of Niels Bohr in
which L would be hI.
Note that
and thus the smaller the moment of inertia the faster a structure rotates.
However the angular momentum is always h[I(I+1)]]^{½}
for any mode of rotation no matter what the moment of inertia is.
Thus there is an equipartition of angular momenta among the various modes of rotation.
A deuteron consists of a proton and a neutron. The neutron has slightly more mass than the proton. For purposes of this order of magnitude estimate the differences between the neutron and the proton will be ignored. It is thus the computation for a pseudo-deuteron.
The diameter of the deuteron is approximately 4.2 fermi. The charge radius of a proton is 0.877 fermi. Deducting two proton radii from the diameter of the deuteron gives a separation distance of the particle centers of 2.446 fermi and thus an orbit radius of 1.223 fermi.
The mass of a proton is 1.673×10^{-27} kg. Thus the moment of inertia of a deuteron for rotation about an axis perpendicular to its longitudinal axis is
The minimum rate of rotation is thus
The number of complete rotations per second is 4.74×10^{21}.
There could also be rotation about the longitudinal axis. The moment of inertia of a solid sphere of mass M is (2/5)MR² where R is the radius of the sphere. The moment of inertia of the pseudo-deteron about the longitudinal axis is then
And therefore the rate of rotation is
Thus the order of magnitude of the rotations of a deuteron is about 10^{20} per second.
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