Terahertz (THz) spectroscopy is an important method in noninvasive detection and diagnosis for historic relics. A new nondestructive testing (NDT) method based on terahertz time-domain spectroscopy (THz-TDS) technology was developed to measure the hollowing deterioration of the Yungang Grottoes in this paper. Hollowing deterioration samples were strictly prepared, and a series of experiments were conducted to ensure the representativeness of the experimental results. A hollowing thickness model was established by the relationship between the thickness of the hollowing deterioration sample and the time difference of the front flaked stone surface and the stone wall surface of the hollowing deterioration samples. The results show that the R-squared value of the model equation reached 0.99795, which implies that this model is reliable. Therefore, the actual hollowing thickness of the Yungang Grottoes can be obtained by substituting the time difference in the proposed thickness hollowing model, where the time difference is obtained from measured THz spectra. The detection method of stone relic hollowing deterioration is easy to apply, which can not only realize qualitative NDT but also quantitative hollowing deterioration thickness determination. This method has crucial practical significance for the repair and strengthening of stone relics similar to the Yungang Grottoes.

Abstract-It is difficult to determine the feed radiation center and F/D of the reflector impulse-radiating antenna (IRA) in frequencydomain, due to its ultra-wideband (UWB) property. This paper presents an efficient approach to design the reflector IRAs, via evaluating the transient radiation patterns (TRPs) of the feed for some given different radiation centers. Only one time-domain simulation for the feed is needed in this method. Comparing with the global optimization algorithms, our method is fast and reliable in the design of the reflector IRAs.