FEATURE ARTICLE
Explosives Detection with Nuclear Quadrupole Resonance
An emerging technology will help to uncover land mines and terrorist bombs
Joel Miller, Geoffrey Barrall
Squashed Nuclei
Nuclear quadrupole resonance has much in common with nuclear
magnetic resonance (NMR), the fundamental physical process that
makes magnetic resonance imaging possible. Nuclear magnetic
resonance, first demonstrated in 1946, takes advantage of the fact
that certain atomic nuclei possess magnetic dipole
moments—that is, these nuclei act like tiny bar magnets,
each with a north magnetic pole at one end and a south magnetic pole
at the other. The laws of quantum mechanics dictate that when such
nuclei are subjected to an externally applied magnetic field, they
must align themselves along it. But the magnetic moments of these
nuclei, usually depicted as arrows, are allowed two possible
orientations: in the same direction as the applied magnetic field or
opposite to it.
Although alignment with the applied field is favored (this
being the lower-energy condition), the energy difference between the
two orientations is such that thermal agitation is usually
sufficient to ensure that only slightly more than half the nuclei
are in the lower-energy state. The key is that the nuclei can occupy
two distinct states separated by a well-defined increment in energy.
(It will be well defined as long as the applied magnetic field is
uniform.) In that sense, the situation is much like that of an
electron in an atom, which can be in the "ground" state or
in a higher-energy "excited" state.
A ground-state electron shifts to an excited state when the atom
receives a dollop of electromagnetic radiation of just the right
energy to put it there—that is, when it absorbs a photon of
just the right frequency. Conversely, if this excited-state electron
falls back to the ground state, the atom will emit a photon of the
exact same frequency to carry away the difference in energy. In NMR,
the energy difference between states is much less than for the
electronic states of an atom, so the relevant frequencies are much
lower. Rather than dealing with optical frequencies, NMR typically
involves oscillations of just a few tens to hundreds of megahertz,
which includes the band where broadcast FM radio stations operate.
Nuclear quadrupole resonance is similar in concept, but unlike NMR
it does not rely on the nuclei aligning themselves in an externally
applied magnetic field. Instead, NQR exploits the fact that some
nuclei possess an electric quadrupole moment, which can be
thought of as arising from two back-to-back electric dipoles
(positive and negative charges separated by a short distance). Why
do some atomic nuclei have an electric quadrupole moment? Physicists
would say because they have a spin quantum number greater than
½. A more intuitive explanation is because the positive
electric charge these nuclei carry is not distributed with perfect
spherical symmetry.
Consider for a moment a spherical nucleus with its positive charge
distributed uniformly throughout. Now squeeze that nucleus in your
mind's eye so that what was originally shaped like a basketball is
flattened into a pumpkin. A pumpkin of positive charge can be
thought of, to a rough approximation, as being the sum of a sphere
of positive charge and two oppositely directed electric dipoles, one
at the top and one at the bottom. That is, the only requirement for
an electric quadrupole moment is that the nucleus be squashed (or
stretched) along one axis.
When a nucleus possessing such an electric quadrupole moment is
subjected to an electric field that varies from place to place,
interesting things happen. The intrinsic electric quadrupole moment
of the nucleus and the electric-field gradient imposed from outside
together create distinct energy states. This result is analogous to
the multiple energy states in NMR, where the critical ingredients
were the intrinsic magnetic dipole moment of the nucleus and a
magnetic field imposed from the outside.
The key difference between NMR and NQR is the definition of
"outside." In NMR, the outside magnetic field arises
because the experimenter has invested considerable effort in setting
it up, perhaps using a superconducting electromagnet. In NQR, the
required electric field (or, more precisely, the required
electric-field gradient) comes for free: It reflects the local
arrangement of electrons around the nucleus under study. That
arrangement, in turn, depends not only on the nature of the atom but
also on its chemical environment. This feature accounts for one of
the chief benefits of NQR—the method is exquisitely sensitive
to chemistry.
Interestingly, an early motivation for investigating NQR was the
possibility that it might be useful for finding hidden explosives.
Shortly after World War II, Robert Pound, one of the pioneers of
NMR, became aware that people in the British army were speculating
about the possibility of using this technique to detect hidden land
mines. Pound was, however, skeptical that it would ever be possible
to project a magnetic field of the necessary uniformity into the
ground. So he decided to try NQR instead. As early as 1951, he
managed to produce some promising results, but for reasons that are
unclear, he did not pursue this avenue of research. A decade had to
pass before others began to appreciate the potential of this idea
and to study it in detail.
That later research has been carried out mostly in academic
laboratories in the United States and Europe, but NQR has attracted
military and commercial interest too. One of us (Miller) works at
the Naval Research Laboratory, where efforts to develop NQR for the
detection of explosives have been going on since 1987. The other
(Barrall) is employed at a private company, Quantum Magnetics, which
has been involved in similar efforts since 1993.
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