Experiments on the detection limit of iron reinforcement rods and various types of defects in X-rays and gamma ray radiographic test of concrete. (In Japanese): 40682 Mizunuma, M.; Hanada, H.; Takagi, I.; Kadomatsu, Y.; Kitagawa, T. Sato, K. Journal of Japanese Society for Non-Destructive Inspection, Vol. 37, No. 9A, pp. 871–872 (Sep. 1988)