More than fifty surface (2-10 cm) soil samples were collected in the Gran Sabana and Sierra de Pacarima, State of Bolivar
(Venezuela) and analyzed for their 137Cs activity. The measurements were performed by high resolution gamma-ray spectrometry employing a hyperpure germanium detector
with an energy resolution of about 1.8 keV for the 1.33 MeV line of 60Co and a counting efficiency greater than 25%. About 500 ml portions of dried soil samples with less than 85 mm particle size
were counted for 20,000 seconds and the data were collected, stored and analyzed with an IBM-compatible computer. The concentrations
of 137Cs in most samples were below the detection limit of 1.2 Bq/kg, but many of these soil samples could have been disturbed not
only by man but the nature itself, for example, by the large amount of rainfall in the wet season. Only 15% of the soil samples
were determined to have values equal or greater than 2 Bq/kg and only two of these were greater than 10 Bq/kg. The study area
was divided into five zones, which had different environmental characteristics: climate, annual average rainfall, temperature
ranges, elevations (m.a.s.l.), soil types and vegetation. Finally, it was concluded that the high 137Cs activities could be associated with the areas of higher annual rainfall and with very humid pre-mountainous climate rather
than with humid tropical climate.
Fault traces have been previously located from measurements of 222Rn in soils taken a constant soil-depth across the fault trace. In this paper, we have studied the uranium-series disequilibria of the 226Ra, 222Rn (gas) and 214Bi radionuclides, not only for their horizontal spatial patterns across the fault trace, but also for their vertical spatial patterns near and at the fault trace itself. Radon-222 activities in the soil-gas were measured on-site with a radiation monitor and a Lucas cell. Radium-226 and 214Bi were determined in soil samples in the laboratory by gamma-ray spectroscopy. A new technique employing the measurement of 222Rn versus soil-depth shows a decrease in 222Rn activity at the fault trace due to the much higher soil-gas permeability as a result of the fractured soil, as well as relative larger uranium-series disequilibria, in respect to an increase in 222Rn activity at normal sites, where the soil is not fractured. Finally, it is suggested that fault trace detection could possibly also be performed by measuring 214Bi in surface soils (0-100 cm) along a transect.
The activity of cesium-137 (Bq/kg) in surface soils between 2-5 cm was determined for more than ninety sample sites on the Araya Peninsula, the Paria Peninsula and the isthmus in between them in the state of Sucre (Venezuela). The measurements were performed by high-resolution gamma-ray spectroscopy, employing a compatible IBM computer with a Nucleus analog to digital interface card. In general, the values for the 137Cs activities were much greater on the Paria Peninsula than the Araya Peninsula and slightly lower on the isthmus. Even though the peninsulas are only separated by a small isthmus, their environmental characteristics are very different. The large difference in the annual rainfall of the peninsulas could have resulted in a greater amount of deposition of 137Cs on the Paria Peninsula, as well as other environmental characteristics, such as altitude and vegetation. The difference of the environmental characteristics could also affect the degree of disturbances of the soils by natural processes on the peninsulas and the isthmus, which could also have affected the 137Cs activity distribution. Finally, an anomalous high 137Cs activity distribution was found at the farthest southeastern sample sites and one of lesser extent at the most northern sample sites. These anomalies are possibly due the direct affect of the northeasterly winds which transported the 137Cs fallout from the nuclear weapon test sites.
The original purpose of this investigation started in 1996 was to study the radiological impact on the local population of
the village of Chichiviriche de La Costa. But, soon after the major earthquake (Ms=6.8) in the state of Sucre on July 9, 1997,
the objective was changed to study the fluctuation of radon (222Rn) to see if it could be correlated to seismic activity and/or if the amonlous change just before the earthquake can be considered
a precusor for it. Measurements of222Rn by simply de-gassing about 250 ml of natural thermal water employing a Pylon AB-5 radiation monitor and counting the radiation
after it reached equilibrium were performed. The values for four sampling periods in the first half of 1996 were about 17
Bq/l of222Rn, a month before the earthquake they were less than 15 Bq/l and increased about 70% to 25 Bq/l two days before the seismic
event. In about two weeks, they returned to about 18 Bq/l. But, surprisingly, they have gradually increased to about 35 Bq/l,
before leveling off at about 27 Bq/l.
The 137Cs activities (Bq . kg-1) were determined in more than ninety soil samples between 2 and 5 cm depths surrounding and near the Guri Reservoir (state of Bolivar, Venezuela). The measurements were performed by high-resolution gamma-ray spectroscopy, employing Soil-6 as a comparator. In general, the values of the 137Cs activities were about double on the west side of the reservoir than on the east side, the environmental parameters were similar on both sides, but the soils were very different, they were untisols on the western side and entisols on the eastern one. The soils were highly mineralized and on the western side they were above rich iron deposits. Many of the sampling sites on the eastern side were annually covered with water, when the reservoir was at high levels. The anomalously high 137Cs values, southeast of the reservoir were found in a small area that had very different environmental characteristics and can be explained by the direct deposition of the fallout by the clouds on the vegetation and surface, since this area is in a dense cloud forest.
As a result of routine soil sampling to determine the 137Cs background activities country-wide in Venezuela, it was decided to further investigate El Mirador (Lookout) area at the
base of the Sierra de Lema mountain range. In April 2003 (A), soil samples were collected at eight sites on and around the
edge of the diabase outcrop to confirm that this area had anomalously high 137Cs activities. In July 2003 (B), not only soil samples were collected again, but also black mat, palm tree leaves and trunks,
fruit bushes leaves and its fruit and fern leaves. The 137Cs content was measured by high resolution gamma-ray spectroscopy by a comparative method with reference materials. The 137Cs activity values range from 16.3 to 30.8 Bq·kg-1 in the soil samples collected in July 2003, 20.7–32.1 Bq·kg−1 for the black mat, 26.3–38.4 Bq·kg−1 for the palm leaves, 16.8–31.2 Bq·kg−1 for the palm trunks and 17.6–27.3 Bq·kg−1 for the fruit bush leaves, while, the 137Cs activity values for the whole fruit were between 23.4 and 30.7 Bq·kg−1; but, the value of the 137Cs activity in the center of the fruit (the edible part) was 51.6 Bq·kg−1, and the value of the 137Cs activity for the fern leaves was 51.8 Bq·kg−1. Thus, most of the 137Cs activity values determined in the soil, black mat and vegetation samples from El Mirador (Lookout) were considered anomalously
high with respect to those found near the equator and in other areas of Venezuela. Only the center of the fruit from the Clusia grandiflora bushes and the fern leaves had high activity ratios, about a factor of three and could be considered as biomonitors that
concentrate and retain the 137Cs. Finally, these anomalously high 137Cs activities have been attributed not only to the rich organic soils, as sinks, but also due to the affect of the cloud forests.
The activity of 137 Cs in surface soils (2-5 cm) was determined at twenty-one sampling sites along the northwestern and eastern coast of the Paraguana peninsula (Venezuela), as well as, at nine locations, between 95 and 535 m.a.s.l. on Cerro Santa Ana. The measurements were performed by high-resolution gamma-ray spectroscopy employing a compatible IBM computer. Most of the values were much higher than those found along the coastline of the mainland; four sites were found to be anomalously high, with 137 Cs values greater than 10 Bq/kg. It is difficult to explain these anomalous 137 Cs values by geographical or climatological factors since there is little rainfall here and the clouds and fog are rarely if never present along the coast of the peninsula. Possibly, some mechanism of the mist that is blown ashore could explain these anomalies. The values of the 137 Cs versus altitude on the Cerro Santa Ana show an increase of two or three times at 500 m.a.s.l. level, thus we have concluded that the base of the clouds was at this height when the fallout was directly deposited by condensation in this cloud forest. These results in the Cerro Santa Ana cloud forest are similar to those of other cloud forests along the Venezuelan coast, but the altitude (m.a.s.l.) of the base of the clouds here are much lower.
Soils were collected at different elevations (m.a.s.l.) near the two roadways, that pass through the Henri Pittier National Park (Edo. Aragua, Venezuela) in order to determine the distribution of the concentrations of the 137Cs fallout and its relation to the tropical cloud forest. Duplicate samples were taken at most elevations between 2–5 cm below the soil surface to confirm that the samples were representative of the area. In many cases, it was difficult or impossible to locate areas that were undisturbed by man or nature. The 137Cs (Bq/kg) content was determined by conventional high resolution gamma ray spectroscopy employing a standard comparison method. The background of the 137Cs fallout in soils, below the cloud (fog) baseline was calculated to be about 5 Bq/kg on both the south (land) side and north (ocean) side for both roadways. The concentrations of 137Cs (Bq/kg) were between 2–3 times higher at the baseline of the cloud (fog) on both sides of the mountain range. The 137Cs values at the highest elevations (1105 and 1625 m.a.s.l.) near the roadways were about 5–6 times higher than the determined background levels. Our estimates of the baseline of the cloud (fog) are in good agreement with other visual observations. Finally, we have concluded that the distribution of 137Cs in soils in cloud forests can be employed to estimate the baseline and the concentrations of 137Cs fallout can be related to the relative density of the cloud (fog) when it was deposited.
The optimum sample size and arrangement for flat-disc samples employing a commercial annular241Am radioactive source and a planar pure germanium detector were studied for quantitative X-ray fluorescence analysis. The effective overall sample areas at different heights (distances between the source and sample) were estimated simply by drawing to scale the source-sample-detector arrangement and its accessories. The effective sample areas were also determined experimentally with standard samples and aluminium shields. The rough estimate was in good agreement with the experimental values for the effective overall areas. Finally, data concerning the shift of the centroid of the Am-scatter peak due to the 59.6 keV gamma-rays from 48.7 to 52.0 keV, with its respective resolution for the different heights, are discussed with respect to the characteristic rare-earth element X-rays in this energy region.