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  • Author or Editor: J. LaBrecque x
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Abstract  

A rapid procedure for determining palladium in catalysts and geoiogical materials is presented. This method involves the use of an241Am (100 mC) source for excitation and employs a PDP-11/05 processor for the data acquisition and analysis by energy-dispersive X-ray fluorescence. The relative standard deviation for 5 independent determinations is less than 3% for 100 seconds of fluorescent time. The calculated and measured values agreed well for both, the prepared standards and the doped Venezuelan laterites. Finally, data on the different sources of error is given including counting time, sample preparation, and total error.

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Abstract  

The determination of soil-gas anomalies especially 222Rn anomalies, are important to precisely locate fault traces, as well as to investigate earthquake precursors. In this paper, we have studied and compared new rapid methods for on site determinations of radon (222Rn), thoron (220Rn) and total radon (222Rn+220Rn) in soil-gas. These new techniques pump the soil-gas continuously from the soil through a simple sampling tube to the counting cell for one-minute with discarding the excess. Then, either four one-minute counting periods (5-minute technique) or nine one-minute counting intervals (10-minute technique) are followed immediately. In all the methods, conversely to Morse"s method, the first counting period (C1) was not employed for calculations. Three calculation methods for the five-minute technique, two for the ten-minute technique and a modified Morse"s method are compared with theoretical values and different real soil-gases with different radon/thoron ratios. The affect of different flow rates of soil-gases into the counting cell was also investigated. Finally, the ten-minute technique seems to be a little more accurate, but the 5-minute technique is much more suitable for seismic field studies when a much larger number of determinations are required in a short time.

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Abstract  

In this study, we have investigated the fluctuations of radon levels in two experimental rooms constructed for secondary standard dosimetric calibration. The values over the short-term (the first two weeks) were in the range of 10–14 pCi/L of radon in air, while those of the long-term were between 6–14 pCi/L of radon. Even though the temperature and humidity was relatively constant during the second half of the rainy season, there was a trend towards lower radon values. It is concluded that ventilation is an important factor in indoor radon levels.

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Abstract  

More than one hundred undisturbed soil samples from Northern Venezuela and the islands of Margarita and Los Roques have been analyzed for137Cs,40K,238U and232Th by -ray spectroscopy. The specimens were taken from between 5–10 cm below the earth's surface. Thus, they are valid not only for the137Cs deposition studies but also for the estimation of the natural -ray dose from primordial radionuclides that form the terrestrial component. The concentration of40K was directly determined from its 1461 keV -ray, while those of137Cs,238U and232Th were performed using a -ray from one of their daughter radionuclides: the 661 keV -ray of137mBa for137Cs, the 1760 keV -ray of214Bi for238U and the 2620 keV -ray of208Tl for232Th. Finally, the concentration values were compared with those of global estimates.

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Abstract  

A rapid, non-destructive method is presented for the determination molybdenum and cobalt in hydrodesulfurization catalysts employing a PDP-11/05 computer-base radioisotope x-ray fluorescence technique. Precision for molybdenum and cobalt better than 2.5% and 4.0%, respectively, are achieved in 100 sec of fluorescent time. The values obtained by this method are in agreement with values from atomic absorption and neutron activation methods. Besides being nondestructive, only about 50 mg of sample is needed, and no time-consuming acid dissolution is required as in atomic absorption and colorimetric methods. The results for six different commercial catalysts are compared with determinations by atomic absorption, neutron activation analysis methods as well as an x-ray fluorescence system with manual calculations of the percentage of molybdenum and cobalt.

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Abstract  

The activity of137Cs was determined in mosses and their surrounding soils in three Venezuelan cloud forests. The concentration of137Cs in both the mosses and their respective soils were correlated with elevation (m.a.s.l.). This relationship was the result of the cloud forest effect, no direct deposition by condensation below the base of the clouds and increasing deposition of137Cs with the density of the clouds. The ratio of the activity of137Cs in the mosses to their surrounding soils was in general the same for sites near the top of the mountains, but the ratio at the Altos de pipe location was 3.5 times higher than that of the La Sierra mountain. This was explained by the difference in the water content of the surface and the top soil layer. It was concluded that the137Cs measured in the mosses was from the soil effect rather than new deposition of137Cs in the last three years.

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Abstract  

The determination of thirteen elements, eleven rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Ho, Tm, Tb and Lu), thorium and uranium were performed by instrumental neutron activation analysis for four geological reference materials (G-2, AGV-1, GSP-1 and SY-2) as well as our laboratory rare-earth standard. The analyses were performed using a low energy photon detector coupled to an Apple IIe microprocessor employed as a multichannel analyzer not only to collect the data but also for data analysis. Finally, based on this work we have proposed a simple, rapid routine method for the determination of eleven rare-earth elements, thorium and uranium in geological materials employing IVIC's RV-1 nuclear reactor.

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Abstract  

The concentration of137Cs, potassium, thorium and uranium for 6 monitoring sites and 32 other sites at the Venezuelan Institute for Scientific Research (IVIC) were presented, as well as, the estimated air dose and exposure rates for each site. The concentration of137Cs was found to be much higher at many sites at IVIC in respect to the average estimated value of137Cs in Venezuela. But, this was determined to be due to a natural cause, the cloud forest which surrounds the higher elevations. The values of potassium, thorium and uranium were compared to values from other parts of northern Venezuela and were found to agree for sites at similar elevations. They were also shown to be about two to three times higher than global estimates. Thus, the air dose and exposure rates were correspondingly two to three times higher too, but the annual dose from the primordial radionuclides in the soil was estimated to be less than 0.1 mSv, which is considered negligible for health risks.

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Abstract  

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.

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