Please Describe Nuclear Magnetism

Please Describe Nuclear Magnetism


1 Answers

Sachin Tyagi
31 Points
14 years ago

The nucleus, which is composed of protons and neutrons in orbital motion under the influence of their mutual forces, has a magnetic moment with two parts: an orbital part, due to the motion of the protons (neutrons, being uncharged, do not contribute to the orbital magnetic moment even though they may have orbital angular momentum), and an intrinsic part, due to the intrinsic magnetic moments of the protons and neutrons. (It may seem surprising that the uncharged neutron has nonzero intrinsic magnetic moment. If the neutron were truly an elementary particle with no electric charge, it would indeed have no magnetic dipole moment. The nonzero magnetic dipole moment of the neutron is a clue to its internal structure and can be fairly well accounted for by considering the neutron to be composed of three charged quarks.)


Nuclei have orbital and spin magnetic dipole moments that can be expressed. However the mass that appears in these equations (the electron mass) must be replaced by the proton or neutron mass, which is about 1800 times the electron mass. Typical nuclear magnetic dipole moments are smaller than atomic dipole moments by a factor of the order of 10-3 and their contribution to the magnetic properties of materials is usually negligible.


The effects of nuclear magnetism become important in the case of nuclear magnetic resonance, in which the nucleus is subject to electromagnetic radiation of a precisely defined frequency corresponding to that necessary to causes the nuclear magnetic moment to change direction. We can align the nuclear magnetic moments in a sample of material by a static magnetic field; the direction of the dipoles reverses when they absorb the time-varying electromagnetic radiation. The absorption of this radiation can easily be detected. This effect is the basis of magnetic resonance imaging (MRI), a diagnostic technique in which images of organs of body can be obtained using radiation far less dangerous to the body than x ray.

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