针对复杂结构条件下钢壳混凝土交界面毫米级脱空无损定量检测难题，基于中子慢化理论提出一种脱空高度定量识别方法。首先，通过室内标定试验，分别建立14 mm和40 mm厚度的钢板与3种不同含水量混凝土块组合工况下的热中子计数率与脱空高度的相关关系，揭示钢壳混凝土组合结构中钢板厚度、混凝土含水量和脱空高度3个要素与热中子计数率的相关变化规律；其次，基于厚度40 mm钢板与特定含水量混凝土块组合工况进行测试，结果表明，使用中子散射法检测的等效脱空高度误差较小；最后，依托深中通道钢壳混凝土沉管隧道横断面足尺模型，开展现场先检后验的开盖验证试验，采用溢水法实测验证钢壳混凝土的平均脱空高度，并与检测结果进行对比。经统计，当允许误差为±2 mm时，脱空高度符合率约为94%。

Based on neutron moderation theory, a nondestructive method is proposed for quantitatively detecting millimeter-scale voids at steel shell-concrete interfaces under complex structural conditions. First, an indoor calibration experiment is conducted to establish the relationship between thermal neutron counting rates and void heights, using steel plates with thicknesses of 14 and 40 mm, as well as three concrete blocks of varying moisture contents. The experiment clarifies the relationship among steel plate thickness, concrete moisture content, void height, and thermal neutron counting rates in steel shell-concrete composite structures. Next, a verification test is performed that utilizes a theoretical curve for a 40-mm steel plate and a concrete block with specific moisture content. The results show that the equivalent void height measured through neutron scattering exhibits a relatively small error. Finally, a case study is conducted on the steel shell-concrete immersed tunnel of the Shenzhen-Zhongshan link, where a full-scale cross-sectional model is established for on-site void detection and open-cover validation. Following removal of the steel plate cover, void heights at the detection points are measured via the overflow method and compared with the detection results. Statistically, with an allowable error of ±2 mm, the compliance rate for void height measurements reaches 94%.