Ultrasonic attenuation measurements are carried out for a quartz specimen irradiated with 4.7 × 1019n/cm2and annealed at 700[ddot]C during 1 hour. This dose is interesting because it is situated in the ‘threshold region'. The measurements are carried out as a function of temperature (0.3–20 K) for different frequencies (80–300 MHz). The data show a behaviour that is qualitatively similar to previous observations for a much lower dose, and indicate a tendency to return to the crystalline state.