2018 | ANNUAL REPORT 71 Performance of the magnetostrictive transducers was characterized using the normalized magnitude of the fast Fourier transform (FFT) of the first reflected acoustic signal (normalized to the time when the reactor first reached full power).The frequency- transformed signal is used because it is less sensitive to the interference effects of noise and signal transients The Galfenol transducer was stable over the course of the irradiation, though the total peak-to-peak signal amplitude was typically on the order of one third that observed for Remendur. Figure 3 shows the normalized peak-to-peak amplitude for a Galfenol transducer as a func- tion of accumulated fluence.The green trace shows the reactor power history.The Galfenol transducer shows steady operation during periods when the reactor power level was stable.There is little decrease in the signal strength over these periods.The decreases in signal strength observed when reactor power is increased appear to be due to increases in operating tempera- ture, as the signal strength stabilizes shortly after each power increase. The transducers were irradiated to a fast fluence of 8.8 × 1020 n/cm2 (E>1 MeV). Post-irradiation exami- nation of each irradiated material indicated negligible effects on the magnetostrictive behavior of either Figure 3. Galfenol element transducer signal amplitude as a function of accumulated fast fluence.