Can protons in the nucleus of an atom be aligned by electromagnetic fields? If so can it be done around $-135°C$ zero?
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1$\begingroup$ Refining your question a little bit: (1) Nuclei generally do not have permanent electric dipole moments (but see physics.stackexchange.com/questions/76339/… ). Therefore the field of interest would be a magnetic field. (2) The nuclear magnetic dipole moment is not just due to the protons. It's due to both the protons and the neutrons. $\endgroup$– user4552Commented Oct 27, 2013 at 1:24
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2 Answers
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Yes.
First, we could note that many nuclei possess nonzero magnetic moment. The presence of magnetic field may cause the change in its orientation and realign the nucleus as a whole. This is essentially the basis of Nuclear magnetic resonance. However, the orientation of individual nucleons relative to each other is not changed.
Second, we could consider that extremely strong magnetic field could potentially alter the spin coupling of strong nuclear force, reordering the nuclear shell structure and realigning individual protons and neutrons. This would be analogous to Paschen-Back effect for electron structure of atoms. To make a rough estimate for the magnitude of magnetic field needed for this we could assume that the change of 1 MeV with the change of spin orientation would be enough to alter the nuclear shell structure (since it is comparable to nuclear binding energy per nucleon which is usually several MeV) So the magnetic field to effect such change would be of the order $B = E /\mu_N$ which is around $10^{13}$ Tesla. This is an order of magnitude larger than the maximum magnetic field of neutron star (magnetar), and far beyond anything achievable in laboratory.
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Well, in the case of hydrogen H1, the nucleus (which is nothing more nor less than a proton) can certainly be aligned by a static magnetic field -- said alignment being the basis of proton magnetic resonance, and, not coincidentally, most clinical MRI imaging.
I won't venture beyond hydrogen, but that's one example at least.
