缓发中子有效份额βeff是反应堆动态特性的重要参数，也是相对反应性与绝对反应性之间的转换桥梁，对于以反应性作为宏观参数的检验工作具有重要意义。测量采用基于Rossi-α方法的Nelson数法开展了快临界装置βeff的实验研究。通过采用铅屏蔽、更薄的6Li玻璃闪烁体、脉冲幅度甄别三种措施，降低了γ射线对测量的影响。实验中测量了反应堆从-60 ￠到缓发临界之间的7个状态，最终测量得到βeff值为0.006 66，不确定度为7.88%；与理论计算数值偏差为2.15%。测量结果与理论值符合良好，表明了测量方法的有效性.

Effective delayed neutron fraction βeff is an important parameter in dynamic behaviour of nuclear reactor. Besides, βeff has the role of the conversion factor between a relative unit and an absolute unit in reactivity scale, which is important for examining work of macroparameter according to reactivity. An experiment was performed to measure the βeff in a fast critical assembly using Nelson number method which is based on Rossi-α method. Using lead for shielding, adopting the thinner 6Li glass scintillator and pulse amplitude discrimination, the influence of γ is decreased. The measurements were carried out for reactivity ranging from -60 ￠ to delayed critical state. The final measured value of βeff is 0.006 66, with 7.88% uncertainty, of which the deviation is 2.15% compared with the theoretical value. Because of the good matching between the measured value and the theoretical value, the experiment method is demonstrated to be effective.