Critical assessment of the MEDUSA gamma ray detection system for radon flux measurement on a tailings dam
Motlhabane, Tebogo Gladys Kgaugelo
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Worldwide measurement of radon flux on mine tailing dams has been performed using various instruments. Some of the methods used in South Africa are electrets, alpha tracks, accumulator cans etc. Although these techniques and methods have been used for many years, a number of shortcomings are still evident. The major shortcomings are that, the methods lack spatial representivity that is, they only measure the radon flux at a point where they' are placed and not the whole site in that way, the spatial variation is not shown in a site which is not homogeneous. Another shortcoming is that, they do not show seasonal variation and some have a back diffusion problem, and the time required for the result to be known is too long. For· example it takes several days for electrets to gather sufficient information required, yet it is a single point result. This makes it difficult to steer the measurement. Furthermore, the moisture and atmospheric pressure on the mine dump influence some of the measurements. The above shortcomings led to the investigation of a new· technique based on gamma ray spectrometry to quantitatively assess the radon flux from the mine tailings dam. The system is called Multi Element Detector for Underwater Sediment Activity (MEDUSA). Initially, this technique was uniquely designed to measure the radioactivity on the sea floor where it proved to be successful. The major focus of this research study was, therefore, to critically assess the MEDUSA gamma ray detector system for measurement of radon flux on a tailings dam. The process of determining the radon flux in this work involved field measurements using MEDUSA and laboratory measurements using Hyper Pure Germanium (HPGe} detector. The laboratory measurements were for correcting the field measurements. The HPGe has better advantage over MEDUSA in terms of resolution and this means that the gamma ray energy peaks have better visibility than on the MEDUSA. The field measurements on the tailings dam were done using the MEDUSA mounted on a 4x4 vehicle, and simultaneously sample points were identified and samples collected. The samples were measured for specific activity in the laboratory using gamma spectrometer with HPGe detector. These measurements enabled the determination of a factor that was used to calculate the activity of radium in the field. This activity was found to have an average of 309 Bq.kg"1 with data range of 60 -540 Bq.kg-1.A radon flux equation was then derived and used to calculate the radon flux on the field. Based on the radium content, the radon flux was calculated to average about 0.105 ± 0.023 Bq.m-2 .s-1 The results are within the same range as the previous flux measurement on the same tailings dam but with better statistics. This research work has demonstrated that the MEDUSA can be adapted for radon flux determination from tailings dam. The method promises to address some of the key shortcomings of existing techniques and the usefulness of this method can be extended to measuring radioactivity on contaminated sites for rehabilitation purposes.