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Electric/Magnetic

The Ultimate Sensor

November 1, 2006 By: Ed Ramsden Sensors


In the original Star Trek series, DeForest Kelley played ship's surgeon Dr. Leonard McCoy. This character was especially memorable for his frequent medical pronouncement, "He's dead, Jim," and for his handheld medical scanner, a device that looked like a pepper shaker with a spinning cap. This magical sensor could instantly and noninvasively diagnose any medical condition.

Ed Ramsden
Ed Ramsden

Although present-day medical technology hasn't quite caught up with that world, some of what we do have in the opening years of the twenty-first century would likely astound many of the science fiction writers responsible for that television show.

Nuclear Magnetic Resonance

Shortly after the close of World War II, Felix Bloch and Edward Purcell discovered the phenomenon of nuclear magnetic resonance (NMR). The nuclei of atoms containing odd numbers of nucleons (protons and neutrons) have a measurable magnetic dipole moment, and will tend to line up with an externally applied magnetic field like tiny bar magnets. If this alignment is perturbed by the application of a secondary field, the nuclei will try to realign when the secondary field is removed. In the process of realigning, the nuclei will precess like toy tops (Figure 1), and radiate an electromagnetic signal whose frequency is proportional to the strength of the externally applied magnetic field. For the case of hydrogen atoms, where the nucleus consists of a single proton, the resulting nuclear precession, or resonance frequency, is about 42.5 MHz/tesla of applied field.

How to Detect NMR Effects

Nuclear magnetism is an extremely subtle phenomenon, at least compared with the more commonly observed magnetic effects such as ferromagnetism and diamagnetism, which have their origins in the behavior of an atom's electron shells. Consequently, sensitive instrumentation is required to discern this effect. Figure 2 shows a conceptual apparatus for detecting NMR effects. A strong magnet is used to establish the main field to which the nuclei will align. Other coils will be aligned orthogonally to perturb the nuclei, and also to detect the precession signal. An RF signal source is required to stimulate the nuclei, as is a sensitive RF receiver to detect the precession signal.

Figure 1. In the process of realigning with an external magnetic field after perturbation by a secondary field, nuclei will precess like toy tops
Figure 1. In the process of realigning with an external magnetic field after perturbation by a secondary field, nuclei will precess like toy tops

A Sensor Platform

Despite requiring sophisticated measurement techniques for detection, NMR effects can be used as the basis for several different types of sensors. For example, if you place a sample of an unknown material into a magnetic field of known strength, you can use the resonant frequencies you observe to identify the material's chemical composition. Because the electrons surrounding a nucleus have an effect on its resonant response, in many cases you can also use NMR techniques to determine a compound's molecular structure. This is the basis for what is known as NMR spectroscopy, a widely used analytic technique. 

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