2003 年 38 巻 12 号 p. 686-692
A small, low-magnetic-noise nondestructive inspection (NDI) system that uses a high-Tc superconducting quantum interference device (SQUID) cooled by a coaxial pulse tube cryocooler (PTC) has been developed. The key components for realizing the compact, low-noise system developed in this study, include the following: a high-Tc SQUID gradiometer, the coaxial PTC, and a SQUID-mount stage separated from the cold head of the cryocooler. The high-Tc SQUID gradiometer contributes not only to stable system operation when subjected to the environmental noise without magnetic shielding, but also a decrease in magnetic noise caused by the mechanical vibration of the SQUID, which is transmitted from the cryocooler. By using the coaxial PTC, which was constructed of non-magnetic materials, both magnetic noise and the mechanical vibration are well suppressed compared to conventional two-axial PTCs. In addition, the coaxial PTC is designed to be compact (50 mm diameter, 400 mm height, and 4 kg weight) for practical use. The SQUID-mount stage separation contributes to stable operation by reducing the transmitted vibration to the SQUID, and thermal stability at the SQUID-mount stage. The magnetic flux noise spectrum of the cryo-cooled system was measured and compared with that of a liquid-nitrogen-cooled system using the same SQUID. From the comparison, no increase in magnetic noise due to the cryocooler was observed, although the magnetic flux noise level of the cryo-cooled system was dominated by SQUID operation noise, which was rather high, (i.e., around 100 μΦ0/Hz1/2 at 100 Hz). The relation between the mechanical vibration at the SQUID and magnetic noise is discussed.