254 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003 Gamma Ray Irradiation Tests of High- SQUID Tatsuoki Nagaishi, Katsuyuki Ara, Yosuke Morita, and Hideo Itozaki Abstract—Gamma ray irradiation tests on high- SQUIDs were carried out to examine their behavior under nuclear reactor environments. The SQUIDs were made of HoBa Cu O superconducting thin films on SrTiO substrates. Some were encapsulated in glass fiber rein forced epoxy resin. Gamma ray irradiation was performed with a Co-60 gamma ray source. Irradiation dose rates used were 8.1 to 12.2 10 Gy/h (1.0 to 1.5 10 R/h) and the maximum absorption dose was about 10.4 MGy. During and after the irradiation, the noise of the output of the SQUIDs was measured with a power spectrum analyzer. The modulation voltage did not change until a total irradiation dose of about 3 MGy, after which it decreased slightly. We conclude that high- SQUIDs are resistant to gamma ray irradiation and thus the application of high- SQUIDs as sensors for reactor component inspection is promising. Index Terms—Gamma ray, high- SQUID, nuclear reactor, ra- diation-resistant characteristics. I. INTRODUCTION F OR THE purpose of maintaining structural integrity and safety, nuclear power plants require periodic nondestruc- tive testing of structural materials and dismantle checkups of instruments while the operation is temporarily halted. Conven- tional methods of nondestructive testing currently used for the investigation of the structural materials of nuclear plants include visual observation and ultrasonic and eddy current methods. Re- search is also underway on methods to evaluate the deterioration of structural materials and estimate the lifetime of a power plant. Magnetic measurements of the material of nuclear power plants have recently been drawing attention and the basic research has been reported [1]–[3]. An important consideration of the mag- netic method is that the magnetic sensor being used must be resistant to damage by radioactive rays. The investigation of a nuclear power plant is to be done during a period while the nu- clear plant is halted. During this time the atomic fuel is moved from the nuclear reactor to the water pool of the plant and in- tense gamma radiation is emitted from the radioactive nuclear reactor itself as well as its outside structure. It is to this radiation that the sensor should be resistant. In general, semiconductor sensors are vulnerable to gamma rays. Hence, sensors made of metal, such as the flux gate sensor, GMR sensor, MI sensor, and SQUIDs, which are more resilient Manuscript received August 5, 2002. T. Nagaishi is with the Development Division, Sumitomo Electric Hightechs Co., Ltd., Hyogo 664-0016, Japan (e-mail: nagaishi@shs.co.jp). K. Ara is with Non Destructive Evaluation and Science Research Center, Iwate University, Iwate 020-8551, Japan (e-mail: kara@iwate-u.ac.jp). Y. Morita is with the Takasaki Research Establishment, Japan Atomic Energy Research Institute, Gunma 370-1292, Japan. H. Itozaki is with the Superconducting Materials Center, the Na- tional Institute of Materials Science, Tsukuba 305-0047, Japan (e-mail: itozaki.hideo@nims.go.jp). Digital Object Identifier 10.1109/TASC.2003.813698 Fig. 1. Picture of high- SQUID showing (a) the top view of a bear SQUID and (b) an expansion picture of the Josephson junctions. TABLE I SPECIFICATIONS OF SQUID USED IN THE EXPERIMENT under radiation exposure, are considered to be more appropriate for these purposes. Among these sensors the SQUID is con- sidered to be most suitable as it is the most sensitive magnetic sensor. The material of the nuclear structure is made of austenite steel, which is nonmagnetic, and the welding parts are weakly magnetic. It is of great interest to be able to investigate the mi- croscopic structural changes of the austenite base material as it ages, before cracks begin to develop in the material. As an auto- matic measurement system is required, we believe that the use of high- SQUIDs with liquid nitrogen coolant is preferable over low- SQUIDs with liquid helium cooling where there may be problems with the maintenance of the cooling system or due to the size of the system. Furthermore, liquid nitrogen can be re- placed with a mechanical cryocooler, and a trial of the use of a cryocooler is underway. In light of these considerations we have investigated the behavior of a high- SQUID under gamma ray irradiation in order to clarify the applicability and any possible points of improvement of high- SQUIDs for the environment of nuclear plants. The experiments were done in the Co-60 irra- diation facility at the Japan Atomic Energy Research Institute, Takasaki, Japan. II. EXPERIMENT A. SQUID Fig. 1 shows pictures of the SQUID and its Johsephson junc- tions. The superconducting thin film in the SQUID is made of HoBa Cu O and its specifications are shown in Table I [4]–[6]. The typical superconducting critical current is from 20 1051-8223/03$17.00 © 2003 IEEE