In-situ gamma-ray mapping of environmental radioactivity at Ithemba Labs and associated risk assessment
In February and July 2005, following up on June 2004, the Environmental Radioactivity Laboratory (ERL) of /Themba LABS performed in-situ and ex-situ measurements of environmental radioactivity on the /Themba LABS (/TL) site. The ERL's Multi-Element-Detector-for-Underwater-Sediment-Activity (MEDUSA) and high-purity germanium (HPGe) detector systems were used to make in-situ and ex-situ measurements, respectively. MEDUSA consists of a CsI(Na) crystal (length 15 cm, diameter 7 cm) for y-ray detection. MEDUSA was mounted - 0.5 m above the ground on the front of a 4x4 vehicle to traverse [at ~ 2 m.s"'] the accessible portions of the /TL site. Spatial data (via a GPS receiver mounted above the crystal) were acquired every 1 s, and y-ray spectra (0-3 MeV) every 2 s. Maps of count rate were produced from the MEDUSA data to show the spatial distribution of radioactivity on the site. The lead-shielded HPGe detector is a Canberra p-type model with built-in pre-amplifier, crystal diameter 62.5 mm, and length 59.9 mm. The HPGe was used to measure the radioactivity in soil (and also in some grass) samples collected at particular spots on the /TL site. The sampled spots include six identified high activity spots ("hotspots") and two ''calibration spots". The activity concentration results that were obtained for the naturally occurring radioactive materials (NORM) [from both HPGe and MEDUSA data from off-road /TL site sections] have the ranges 7.8 - 39.0, 5.9 - 21.4? and 29.8 - 63.3 Bq.kg"1 for 238U, 2j2Th, and "^K, respectively. The absorbed and effective doses (from external gamma irradiation) that were calculated for the NORM radionuclides have the ranges 10.4 -27.2 nGy.h"1 and 12.8 - 33.4 /iSv.y"!r respectively. The average NORM activity concentration results (from February and July 2005) for the jTL road are 30.3, 29.6, and 345.0 Bq.kg1 for 238U, 232Th, and 40K, respectively- When averaging over the off-road sections of the iTL site, and excluding data from the hotspots, the average results obtained are 10.6, 8.5, and 42.4 Bq.kg"1. When averaging over the off-road sections plus hotspots, the average results are 23.1, 8.0, and 50.4 Bq.kg"1. The activity concentration results that were obtained for the anthropogenic radionuclides have the ranges 1.2 - 6533.5, 0 - 100.8, 0 - 20.6, 0 - 0.7, and 0,2 -192.5 Bq.kg'1 for 68Ga, 652n, ^Na, I37Cs, and 54Mn, respectively. The absorbed and effective doses (from external gamma irradiation) due to anthropogenic sources were calculated for two plane source depth locations, namely, 1 and 10 cm. The absorbed and effective dose results that were obtained at 1 cm plane source depth have the ranges 0.2 - 152.7 nGy.h"1 and 0 - 187.3 ^Sv.y"1. The absorbed and effective dose results that were obtained at 10 cm plane source depth have the ranges 0-38.1 nGy.h" 1 and 0 - 46.7//Sv.y1. The count rate maps showed that the six identified hotspots consistently appear at the same locations for June 2004, and February and July 2005 maps. The maps also showed that the radionuclide deposition at the hotspots is not a typical result of radionuclide spread via irrigation. The activity concentration results showed that the radioactivity' level due to NORM sources on the /Themba LABS grounds is below the world average of 35; 30, and 400 BqJcg"1 for 238U, ^Th, and ^K, respectively. The effective dose results showed that the maximum effective dose to humans on the iThemba LABS grounds as a result of external exposure to natural and anthropogenic radionuclides is well below the regulatory 1 mSv per year per member of public.
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