Development of an automated gamma-imaging system for nondestructive testing of welded pipes and its performance evaluation

As a continuation of our radiographic testing (RT) R&D, we have developed an automated gamma-imaging system to be suitable for nondestructive testing of welded pipes. The system consists of an imaging detector having a CdTe/CMOS-based pixel array (AJAT, SCAN1000), a collimated ⁷⁵Se gamma source (MDS, GammaMat SE) having about 78.7 Ci activities, a gear track mounted around test pipes for image scan, and a computer-controlled microstep motor for precise control of the scan speed. All the components of the imaging system were assembled tightly by a chain belt. From the imaging system, we obtained useful gamma images of a stainless steel pipe (30 cm in internal diameter and 1 cm in thickness) having a welding line. By using standard techniques for evaluation of the radiography quality, we assessed the imaging characteristics of the detector in terms of the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE). We also evaluated the gamma imaging characteristics of the system by using standard wire specimens (ASTM-1B and 1C), typically used in the field of industrial RT. In this paper, we present a detailed description of the automated gamma-imaging system and the experimental results we obtained.