To obtain information on the detector position dependence of the prompt neutron decay measurement, the neutron detectors were set at three positions as shown in Fig. 9.1: near the tungsten target [position of (17, D): 1/2-in.-diameter BF3 detec­tor]; and around the core [positions of (18, M) and (17, R): 1-in.-diameter 3He detectors]. The prompt and delayed neutron behaviors (Fig. 9.6) were

c

1000

о

£

c

з

100

c

о

з

CD

z

10

experimentally confirmed by observing the time evolution of neutron density in ADS, an exponential decay behavior and a slowly decreasing one, respectively. These behaviors clearly indicated that the neutron multiplication was caused by an external source: the sustainable nuclear chain reactions were induced in the sub­critical core by the spallation neutrons through the interaction of the tungsten target and the proton beams from the FFAG accelerator. In these kinetic experiments, the subcriticality was deduced from the prompt neutron decay constant by the extrap­olated area ratio method. The difference of measured results of 0.74 %Ak/k and 0.61 %Ak/k at the positions of (17, R) and (18, M) in Fig. 9.1, respectively, from the experimental evaluation of 0.77 %oAk/k, which was deduced from the combination of both the control rod worth by the rod drop method and its calibration curve by the positive period method, was within about 20 %. Note that the subcritical state was attained by a full insertion of C1, C2, and C3 control rods into the core.