We focused on the propagation property of an annular beam in strong scattering random media such as nimbostratus or dense fog. An annular beam as a lidar transmitted beam can propagate a longer distance even through atmospheric fluctuation. The reason is that an annular beam can self-transform to a non-diffracting beam, which is called non-diffractive effect. In this work, the center peak intensity as result of non-diffractive effect was generated after the propagation of an annular beam in random media with different concentrations and propagation distances. The linear relationship between the propagation distance and the transport mean free path calculated from the media concentration that caused the maximum center peak intensity was obtained. The generation condition of the non-diffractive beam was discussed under arbitrary parameters of beam diameter, propagation distance and media concentration.
Hydrogen explosion on Fukushima No.1 nuclear power plant caused huge damage in real life. Especially its radiation damage caused widespread. Field works on intermediate storages and reuses of decontaminated soils has been started in earnest under government promotion since 2017. This study put a compact polarization lidar at the intermediate storage site, where was only 3 kilometers away from Fukushima No.1 plant. There was a difficult-to-return zone. The lidar system was remotely controlled at Chiba univ. Its observation range was 300m. It covered almost the whole of working field. The lidar monitoring was conducted at one month during Nov. and Dec. in 2017. The lidar scanned horizontally by crossing the dropping port of the decontaminated soils and capturing the suspended dusts on the soil preparation. The lidar monitoring was synchronized with 3 dust-samplers, and we obtained spatial distributions of dust density and estimated the distribution of the radioactivity concentration. The lidar counting rate for dust and the radioactivity concentration was estimated 0.001 mg/m3 /count and 10-9 – 10-10 Bq/cm3 , respectively. This estimation was much lower than the working rule of safety assessment of 10-5 Bq/cm3 . The expressions of these distributions are helpful as communication tool to civilians’ safe and workers’ secure.
For the optical sensing in random media, an annular beam was used because of its nondiffractive feature. In our previous
work, we have confirmed the nondiffractive effect of the annular beam in diluted solution of milk as random media. In
the experiment of this study, increment of the nondiffractive effect has been confirmed by using the pulsed annular beam.
We have simulated propagation characteristics of the annular beam in random media by wave analysis method. The
attenuation rate and the forward scattering pattern of the propagation beam were calculated, and it showed agreement by
comparing them with results of the experiment.