Low temperature ultrasonic relaxation in a dilute Ta-H alloy

The ultrasonic attenuation of tantalum containing approximately 500 atomic ppm of hydrogen has been measured as a function of temperature below 5 K at frequencies between 10 and 55 MHz. In addition to hydrogen the tantalum contains approximately 120 atomic ppm of carbon and 100 atomic ppm of oxygen interstitials. A peak in the attenuation in the C11 mode was found below 2 K. The peak is caused by the relaxation of hydrogen trapped near the carbon and/or oxygen impurities. No peak was seen in the C44 mode in the same temperature range. Similar relaxation processes have previously been observed in dilute Nb-O-H alloys, but the peak temperatures observed in those systems are higher than those observed in Ta