Estimation of surface concentration of radon decay products from gamma dose rate change after rain

Abstract

A method to estimate the surface concentration of radon decay products 214 Pb and 214 Bi from gamma dose rate changes due to rainfall has been developed. The conversion from the dose rate to surface concentration was performed by a dose rate model considering geometric conditions such as buildings and ground undulations around each monitoring station (MS). This method of calculating dose rate increase (DRI) was successful in reproducing the difference in the shape of the DRI decrease due to differences in the initial 214 Bi/ 214 Pb. The estimated concentration exhibited differences among the MSs despite uniform depositions. This difference was considered to have been caused by surface run-off and infiltration of radionuclides.

Estimation of infiltration and surface run-off characteristics of radionuclides from gamma dose rate change after rain

“…This model takes account of the location of buildings around each MS and the height of the NaI(Tl) scintillation detector on the basis of field investigations. This model has been tested for nonaccidental and no-rain cases to show that it can reasonably reproduce the background dose rate from the radioactivity in the natural environment and in the building materials [1].…”

“…We estimated 214 Bi/ 214 Pb ratio of surface radioactivity concentration from the trend of the DRI measured after rain stopped by using the method described in our previous study [1]. This method is based on the fact that the shape of DRI decrease with time differs depending on the 214 Bi/ 214 Pb radioactivity ratio.…”

“…of radon decay products in nonaccidental cases [1]. It was reported by Fujiyoshi et al [3] that there was a similarity in the macroscopic behavior at the ground surface between anthropogenic radionuclides and radon decay products.…”

“…The authors estimated the surface radioactivity concentrations of radon decay products, 214 Pb and 214 Bi, from the increase in dose rate due to rain by using a dose rate model [1]. The dose rate model describes the physical processes of gamma-ray transport, and relates the ground-shine component of dose rate to the amount or concentration of radionuclides on the ground surface by taking into consideration their geometrical distribution.…”

“…Analyses are made for dose rate increases (DRIs) observed during rain in a sevenyear period at MSs around the Hamaoka NPP. Based on the results in our previous study [1], more in-depth considerations are made for the differences in the DRIs between MSs caused by different attenuation of gamma ray due to infiltration and by the different run-off characteristics.…”