Nuclear acoustic resonance is a phenomenon closely related to nuclear magnetic resonance. It involves utilizing ultrasound and ultrasonic acoustic waves of frequencies between 1 MHz and 100 MHz to determine the acoustic radiation resulted from interactions of particles that experience nuclear spins as a result of magnetic and/or electric fields. The principles of nuclear acoustic resonance are often compared with nuclear magnetic resonance, specifically its usage in conjunction with nuclear magnetic resonance systems for spectroscopy and related imaging methods. Due to this, it is denoted that nuclear acoustic resonance can be used for the imaging of objects as well. However, for most cases, nuclear acoustic resonance requires the presence of nuclear magnetic resonance to induce electron spins within specimens in order for the absorption of acoustic waves to occur. Research conducted through experimental and theoretical investigations relative to the absorption of acoustic radiation of different materials, ranging from metals to subatomic particles, have deducted that nuclear acoustic resonance has its specific usages in other fields other than imaging. Experimental observations of nuclear acoustic resonance was first obtained in 1963 by Alers and Fleury in solid aluminum.
History
Nuclear acoustic resonance was first discussed in 1952 when Semen Altshuler proposed that the acoustic coupling to nuclear spins should be visible. This was also proposed by Alfred Kastler around the same time. From his specialization in the field, Altshuler theorized the nuclear spin-acoustic phonon interactions which resulted with experimentation in 1955. The experiments led physicists to suggest that nuclear acoustic resonance coupling in metals could be formulated and observed, with modern physicists discussing the many properties of nuclear acoustic resonance, although it is not a widely known concept. Concepts of nuclear acoustic resonance in objects have been theorized and predicted by many physicists, but it was not until in 1963 when the first observation of the phenomenon occurred in solid aluminum along with observation of its dispersion in 1973, and subsequently, the first experimental nuclear acoustic resonance in liquid gallium in 1975. However, the aspect of acoustic spin resonance has been observed by Bolef and Menes in 1966 through samples of indium antimonide where nuclear spins were shown to absorb acoustic energy exhibited by the sample.
Theory of nuclear acoustic resonance
Nuclear Spin and Acoustic Radiation
The nuclei is deduced to spin due to its different properties ranging from magnetic to electric properties of different nuclei within atoms. Commonly this spin is utilized within the field of nuclear magnetic resonance, where an external RF (or ultra-high frequency range) magnetic field is used to excite and resonate with the nuclei spin within the internal system. This in turn allows the absorption or dispersion of electromagnetic radiation to occur, and allows magnetic resonance imaging equipment to detect and produce images. However, for nuclear acoustic resonance, the energy levels that determine the orientation of the spinning while under internal or external fields are transitioned by acoustic radiation. As acoustic waves are often between frequencies of 1 MHz and 100 MHz, they are usually characterized as ultrasound or ultrasonic (sound of frequencies above the audible range of 20 − 20 , 000 H z {\displaystyle 20-20,000Hz} ).
Comparison with Nuclear Magnetic Resonance
Similar to nuclear magnetic resonance, both phenomena introduces and utilizes external sources such as a DC magnetic field or different frequencies, and results from both methods produce similar data sets and trends in different variables. However, there are distinct differences in the methodologies of the two concepts. Nuclear acoustic resonance involves inducing internal spin-dependent interactions while nuclear magnetic resonance denotes interactions with external magnetic fields. Due to this, nuclear acoustic resonance is not solely dependent on nuclear magnetic resonance, and can be operated independently. Such cases where nuclear acoustic resonance is a better substitute for nuclear magnetic resonance include resonance in metals where electromagnetic waves can be difficult to penetrate and resonate, such as amorphous metals and alloys, while acoustic waves can easily pass through. However, the suitability for using nuclear acoustic resonance or nuclear magnetic resonance is reliant on the material to be used in order to achieve the most efficient and evident results.
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