We report experimental results from a first-of-a-kind ultrasonic transducer that generates a beam with a Bessel function profile. Using a technique of nonuniform poling, an axially symmetric Bessel function pattern is ‘‘polarized into’’ a piezoelectric ceramic element. The resulting circular-disk transducer has the usual full-plating electrode configuration, but produces an ultrasonic beam with a radial displacement profile approximating that of the Bessel function J0 (r), both in amplitude and in phase. The radiation field of a 1-in.-diam, 2.25 MHz Bessel transducer mapped out with a point probe shows good agreement with calculated results using a Gauss-Hermite model. Bessel transducers are of particular interest in attempts to achieve ‘‘diffractionless’’ beams.

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The elastic properties of tantalum have been investigated over the temperature range 4.2–300 K. The values of the adiabatic elastic constants C11, C44, and CL=(1/2) (C11+C12+2C44) at 4.2 K in units of 1012 dyn/cm2 are, respectively, 2.70, 0.873, and 3.02. The Debye temperature was calculated from the elastic constants and found to have the value of 265 K at low temperature. The elastic constants become temperature independent at low temperatures and approach linear behavior at high temperatures. In contrast to the normal behavior of the elastic constants, nuclear acoustic resonance experiments show anomalous line broadening.

Nuclear-acoustic-resonance dispersion has been observed for the first time in a metal. Absorption and dispersion signals in aluminum were investigated as a function of frequency, sound-wave velocity, and temperature. The line shapes of both absorption and dispersion signals show temperature-dependent asymmetries which are in striking agreement with theory.

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