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Ignition under high temperature and density is the ultimate destination of ICF. According to Lawson Criterion, only for the areal density <ρR> greater than 0.4g/cm~2 can the aim come true. <ρR> is directly related to self-heating, burn fraction and the gain, and is the critical physical parameter to ICF ignition. So it is of great significance to diagnose <ρR>. There are many methods to diagnose <ρR>. But now the diagnostics are mostly based on the neutron information because neutrons can escape from the largest and densest target core. Under the low density condition, the <ρR> can be inferred from the yield ratio of the secondary neutron yields to the primary neutron yields, which is a reliable method.The foundation of Shenguang III prototype facility supports us with the platform to diagnose higher areal density. The international diagnostic experience is referenced in this study, and we are also constructing a large areal neutron scintillation detector array (LaNSA). The LaNSA has characteristics of better sensitivity and energy resolution, which can be used to determine the secondary neutron spectrum from which the areal density can be deduced.Beginning with the application of the neutron detection, encircling with the problem to determine the higher areal density, using the hot-spot model, the quantitative relationship between the secondary neutron and the areal density has been deduced in this paper. In the deduction process, some of referred physical quantities, such as the DT reaction cross section and the stopping power of the fuel, were also analyzed. In the part of experiments, the international LaNSA facilities were introduced, and the structure, the components, the experimental processes and the influential factors of the LaNSA on Shenguang III prototype facility were discussed in detail. In the analysis part, the data of the low density experiments were showed firstly as a transition to the higher density experiments, and the results and its uncertainties were also estimated. To the data of the higher density experiments, firstly, the primary spectra, that is, the flight of time (TOF) spectra were transformed into the secondary neutron spectra on the basis of the relations between the neutron flight time and its corresponding energy; secondly, the obtained neutron energy spectra were used to deduce the slowing tritium spectra using the DT dynamics; finally, the areal density could be inferred from the information which could be reflected in the tritium spectra. In the last part, to validate the conclusions gotten above, from the methodology of simulations, assuming that some of the physical parameters (electron temperature, electron density and areal density) of the fuel had been known, the spectra were calculated. The results demonstrate that the experiments and the simulation are in good agreement.
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