222Rn Loss Research Articles

An innovative technique of harvesting soil gas as a highly efficient source of 222Rn for calibration applications in a walk-in type chamber: part-1

The paper describes a novel technique to harvest 222Rn laden air from soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a walk-in type 222Rn calibration chamber. The technique makes use of a soil probe of about 1 m to draw soil gas, through a dehumidifier and a delay volume, using an air pump to fill the calibration chamber. 222Rn concentration in the range of a few hundred Bq m−3 to a few tens of kBq m−3 was easily attained in the chamber of volume 22.7 m3 within a short pumping duration of 1 h. A new technique referred to as “semi-dynamic mode of operation” in which soil gas is injected into the calibration chamber at regular intervals to compensate for the loss of 222Rn due to decay and leak is discussed. Harvesting soil gas has many important advantages over the traditional methods of 222Rn generation for calibration experiments using finite sources such as solid flow-through, powdered emanation, and liquid sources. They are: (1) soil gas serves as an instantaneous natural source of 222Rn, very convenient to use unlike the high strength 226Ra sources used in the calibration laboratories, and has no radiation safety issues, (2) does not require licensing from the regulatory authority, and (3) it can be used continuously as a non-depleting reservoir of 222Rn, unlike other finite sources. The newly developed technique would eliminate the need for expensive radioactive sources and thereby offers immense application in a variety of day to day experiments—both in students and research laboratories.

The age of Homo naledi and associated sediments in the Rising Star Cave, SouthAfrica.

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/-70 ka, whilst a minimum age scenario yields an average age of 200 +70/-61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology.

210Pb-226Ra disequilibria in young gas-laden magmas

We present new 238U-230Th-226Ra-210Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant 238U and 226Ra excesses, whereas those from the ocean islands have moderate 230Th and 226Ra excesses, reflecting mantle melting in the presence of a water-rich fluid in the former and mantle melting by decompression in the latter. Differentiation to erupted compositions in both settings appears to have taken no longer than a few millennia. Variations in the (210Pb/226Ra)0 values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a 210Pb deficit, which we attribute to ~8.5 years of steady 222Rn loss to a CO2-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (210Pb/226Ra)0 values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of 222Rn in a gas phase. We infer that degassing during decompression of water-saturated magmas either causes these magmas to crystallize and stall in reservoirs where they reside under conditions of near stasis, or to quickly rise towards the surface and erupt.

A gamma-ray spectroscopy survey of Omani meteorites

The gamma-ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani-Swiss meteorite search campaigns 2001-2008 were nondestructively measured using an ultra-low background gamma-ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean 26Al activities in groups of ordinary chondrites with a) weathering degrees W0-1 and low 14C terrestrial age and b) weathering degree W3-4 and high 14C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in 26Al. Among the least weathered samples one meteorite (SaU 424) was found to contain detectable 22Na identifying it as recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is ~120 events/106 km2*a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass ~4.5 t) show significant variations of 26Al activities, including the highest values measured, consistent with a meteoroid radius of ~115 cm. Activities of 238U daughter elements demonstrate terrestrial contamination with 226Ra and possible loss of 222Rn. Recent contamination with small amounts of 137Cs is ubiquitous. We conclude that gamma-ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.

Correcting for potential 222Rn loss in 210Pb dating of sediments from the South Adriatic Pit

Measurement of 226Ra activity concentrations is often used to estimate supported levels of 210Pb for purposes of geochronology. However, the implicit assumption that supported 210Pb and 226Ra are in secular radioactive equilibrium may not always be true because of the migration of an intermediate product, gaseous 222Rn. As a consequence, supported 210Pb activity concentration might be lower than the measured 226Ra value, which was the case in a core collected from the South Adriatic Pit. Here we present a new approach to improve the determination of supported 210Pb, which is based on correction of 226Ra activity concentrations using the average (210Pb/226Ra) activity ratio in deeper sediment layers. Using this approach, sedimentation rates for the South Adriatic Pit with a constant rate of supply model and a constant initial concentration model were 0.054 g/cm2/a and 0.070 g/cm2/a, respectively.

226Ra measurements through gamma spectrometric counting of radon progenies: How significant is the loss of radon?

Most determinations of 226Ra are based on gamma spectrometric measurements of 226Ra progenies 214Pb and 214Bi. This approach assumes radioactive equilibrium between these radionuclides which is normally achieved by sealing the sample containers so that the direct decay product of 226Ra, 222Rn, is not lost. We investigated to what extent 222Rn is lost from various sample containers usually used for gamma spectrometric measurements and which have been sealed applying different techniques. The sample containers containing reference material with known amounts of 226Ra were placed in an air-tight measurement chamber and we measured the 222Rn accumulation over time in the chamber. Taking the leakage of the experimental set-up into account, we related the measured 222Rn to the 226Ra content of the reference material to calculate the 222Rn loss from the sample containers. For most of the reference materials investigated the sealing techniques applied reduced the 222Rn loss to <6%. We found that BaSO4 sample matrix and plastic containers sealed with epoxy resin most effectively minimize the 222Rn loss (<1%). For those sample matrixes like phosphogypsum and Mn-fibers, which are characterized by high emanation and diffusion of radon, sealing is very important to reduce the 222Rn loss.

Gas transport model for the magmatic system at Mount Pinatubo, Philippines: Insights from ( 210Pb)/( 226Ra)

Measurements of 226Ra– 210Pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide, 222Rn, follows the gas phase. We present measurements of 210Pb– 226Ra disequilibria for whole-rock samples representing a time sequence of tephra through the June 15, 1991 cataclysmic eruption of Mount Pinatubo volcano, Philippines. Mount Pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption, and we can, therefore, isolate the 210Pb– 226Ra disequilibria due to gas accumulation without complications due to 222Rn loss during degassing. Pyroclastic samples have ( 210Pb)/( 226Ra) 0 ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has ( 210Pb)/( 226Ra) 0 = 1.12. Previous uranium-series degassing studies have suggested that 210Pb excesses are created by rapid volatile transport (carrying the intermediate daughter 222Rn) and subsequent volatile accumulation and decay of 222Rn to 210Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222Rn large enough to cause measurable disequilibria in the 210Pb– 226Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of 222Rn. Therefore, we present a model in which 210Pb– 226Ra disequilibria is established during basaltic recharge of the Pinatubo reservoir. The relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210Pb excess in localized basaltic melt. Transport of volatiles and 210Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of 222Rn (3.8 days), but rather by the half-life of 210Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of 210Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in 210Pb values with time may suggest changing conditions of magma supply at volcanoes.

Testing for loss of 222Rn from Pacific halibut ( Hippoglossus stenolepis) otoliths

Radiometric ageing of fish by measuring the disequilibrium of 210Pb/ 226Ra found in otoliths has been used by numerous researchers in over 15 different studies. The ultimate goal of the previous research has been to validate conventional ageing methods. If 222Rn, an intermediary between 226Ra and 210Pb, escapes from the otoliths, the radiometric age would be biased young. The goal of this study was to test for this bias using Pacific halibut ( Hippoglossus stenolepis) otoliths collected in 1960 and analyzed in 1997. If 222Rn loss occurred, the estimated radiometric age would be less than the 37 year span between collection and analysis. In comparison to other fish species, Pacific halibut otoliths were found to have an unusually low activity of 210Pb and 226Ra. Low activity measurements lead to radiometric age estimates with very large estimated errors. All estimated radiometric ages for the Pacific halibut otoliths were much greater than 37 years. This suggests that 222Rn is not escaping from the otoliths and that radiometric ages are not biased low. However, considering the estimated error, the results are not definitive. This study should be repeated using a different species that is known to have a higher activity of 210Pb and 226Ra.

Isotopically heterogeneous initial Pb and continuous 222Rn loss in fossils: The U-Pb systematics of Brachiosaurus brancai

Diagenetic incorporation of uranium into bone material establishes a U-Pb system that provides a minimum age for the fossil. Loss of Rn and Ra from the 238U-decay series, recent mobility of U and Pb, and incorporation of radiogenic Pb from an old basement, which was available during diagenesis, disturbed the U-Pb system: All data fall in a 206Pb∗/238U-207Pb∗/235U (concordia) diagram to the right of the concordia. While samples that do not experience redistribution of U, Pb, 222Rn, and 226Ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic Pb, loss of 222Rn and 226Ra has led to displacement of the present data set to lower 206Pb/238U and, therefore, to younger intercept ages and to older 207Pb/206Pb ages. The 207Pb/204Pb-208Pb/204Pb systematics demonstrates that (1) three out of 19 samples experienced significant postdiagenetic fractionation of U and Pb and that (2) the diagenetic Pb addition was isotopically heterogeneous. The 235U/207Pb∗-206Pb∗/207Pb∗ and 206Pb∗/238U-207Pb∗/235U systematics combined demonstrate that (3) the samples experienced 5 to 18% 222Rn loss for a 1.9-Ga-old source of the diagenetically introduced Pb (and, correspondingly, more for a younger Pb source) and that (4) there is no unique solution for the age of the source of diagenetic Pb, since Pb from a younger source appears in the 235U/207Pb∗-206Pb∗/207Pb∗ diagram in the same way as higher 222Rn loss. No precise age can be determined from the data, although the most consistent solution falls in the range of 140 to 150 Ma. Addition of radiogenic Pb to the fossil bone is obvious only through its anomalous, highly radiogenic character and the isotopic heterogeneity in 208Pb/204Pb (no Th, i.e., no in situ 208Pb growth). The effect of unrecognized diagenetic additions of radiogenic Pb is an overestimation of the age, whereas unrecognized 222Rn loss results in a too-young discordant age. Since fossil material formed at times for which the discordia only shows a small curvature, anomalies originating from diagenetic Pb additions and 222Rn loss may not always be recognized.

Testing assumptions of otolith radiometric aging with two long-lived fishes from the northern Gulf of Mexico

In this study, we evaluate the assumptions for estimating ages with radiometric and elemental analyses of otoliths of red snapper (Lutjanus campechanus) and red drum (Sciaenops ocellatus) from the northern Gulf of Mexico. In whole otoliths of both red snapper and red drum, 226Ra activity was inversely correlated with the otolith mass increase with age, validating the assumption that 226Ra accumulation in otoliths occurs at a rate proportional to that of Ca. 222Rn emanation from otoliths decreased as otolith mass increased. 222Rn loss occurred in all red snapper otolith samples ([Formula: see text]4.1%) and, to a lesser extent, in red drum otolith samples ([Formula: see text]0.6%) and decreased with increasing size (age) of the otolith. For red drum, the assumption that the initial activity ratio of 210Po to 226Ra in otoliths is essentially zero was indirectly validated with elemental analyses of Pb and Ba. Radiometric age estimation of shallow-water fishes from the northern Gulf of Mexico holds great promise, considering that 226Ra activities observed in this study are some of the highest recorded to date for species to which the 210Pb/226Ra dating technique has been applied.

Accounting for 222Rn loss during oven drying for the immediate laboratory gamma-ray spectroscopy of collected soil samples

Drying soil samples in an oven to remove water alters the 222Rn emanation rate. Measurements of the oven drying 222Rn emanation rate from soil were made with a continuous radon monitor and the degree of 222Rn disequilibrium was quantified by laboratory gamma-ray spectroscopy. This paper presents a disequilibrium correction where the 226Ra activity in oven-dried soil samples is inferred from immediate laboratory gamma-ray spectroscopy of 214Bi before 222Rn and its decay progeny reach secular equilibrium.

Application of natural and artificial fallout radionuclides to determining sedimentation rates in New Zealand lakes

Sedimentation rates have been estimated for New Zealand Lakes Rotoiti, Taupo, Ngahewa, Tikitapu, Okareka, Pukaki, Rotorua, Waikaremoana, Wakatipu, Tekapo, and Ohau, and also the Falls Dam, Central Otago, the Pauatahanui inlet (Wellington), and Lake Vanda (Antarctica). Sedimentation rates were able to be calculated using the 210Pb and 210Po methods in 26 out of 28 cores. For the 137Cs method (shown equivalent to the previous methods) calculable rates were obtained in 13 out of 19 cores. An age estimation was always possible by one of the methods. Values of 147Pm and 144Ce showed that most surface layers were undisturbed, hence the hypothetical mixing often invoked to explain 137Cs profiles lacking the expected 1965 peak is incorrect for most of these cores. Deficits in surface layer 210Pb caused by 222Rn loss did not affect the estimates of sedimentation rate. One core contained a record dated back to 1800 A.D. Deposition rates of 210Pb depended on rainfall, not river input. Some local geothermal activity emits 222Rn and deposits more 210Pb than usual for New Zealand lakes, but otherwise absolute 210Pb levels are generally very low compared to other studies. Deposition rates varied from a low of 0.11 kg m‐2 yr‐1 (Lake Tikitapu) to a high of 28 kg m‐2 yr‐1 (one Lake Pukaki core).

222RN IN THE WATER DISTRIBUTION SYSTEM OF TUCSON, ARIZONA1

ABSTRACT: A study of 222Rn concentrations in the water distribution system of Tucson, Arizona, revealed levels of 60 to 1260 pCi/L in domestic waters. These measurements are comparable to levels of between 80 and 1400 pCi/l for 222Rn found in ground water samples in the North‐Central Tucson basin (Kalin et al., 1995). Estimated loss of 222Rn due to radioactive decay during travel from the well head to the home ranges from 8 to 50 percent.

Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

The accumulation rate of sediments in the Sabine-Neches estuary, near Beaumont, Texas, was studied in four sediment cores using radioisotopes 210Pb and 239,240Pu. Due to very low and variable activities of unsupported 210Pb in the sediments, the sedimentation rates could not be reliably established using the 210Pb dating method. However, 239,240Pu profiles showed distinct peaks, corresponding to the maximum fallout in 1963, and using these Pu peaks the average sedimentation rates were estimated at 4–5 mm yr −1 in the upper and lower estuary. Various assumptions that are commonly used in the 210Pb dating method were tested. It was found that the 226Ra activities were in equilibrium with 238U, and the possible loss of 222Rn was judged to be insignificant. The lack of an exponential profile of excess 210Pb with depth did not appear to be a direct result of sediment mixing, as the post-depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas. A significant positive correlation between the amount of < 63-μm fraction sediments and the total 210Pb activity was observed in the surface sediments. While the hydraulic residence time was relativelt short in this estuary (∼ 10 days), the average total residence time of 210Pb was ∼ 10 days. Between 50% and 94% of the total 210Pb in the water column was found in the < 0.5-μm fraction, resulting in low K d-values. The measured inventories of excess 210Pb and 239,240Pu in estuarine sediments were only ∼ 10–34% and ∼ 19–50%, respectively, of the total expected inventories. It appears that the estuarine retention and sediment inventory of these radionuclides, and by analogy, other particle reactive metals, are a function of the hydraulic residence time, grain size, and possibly complexation of these nuclides with DOC.

222Rn in Water

The U.S. Environmental Protection Agency has proposed a drinking water standard for 222Rn in public water supplies. When promulgated, operators of public water supplies will be required to determine water 222Rn concentrations. Most likely, water samples will be collected and mailed to laboratories for analyses. Additionally, it is probable that homeowners will test private well water in a similar manner by collecting water samples and mailing them to a laboratory for 222Rn analyses. In anticipation of these actions, this study was conducted to evaluate two methods of water sample collection and to evaluate the potential losses of 222Rn from water samples mailed to a laboratory. Thirdly, temporal variations in 222Rn concentrations in several groundwater supplies in North Carolina were examined. Water supplies at four sites in eastern North Carolina and five sites in western North Carolina were sampled over a 7-mo period beginning in the fall of 1993 and continuing through the spring of 1994. Samples were analyzed using a liquid scintillation method. This study showed that a "slow-flow" method is not only suitable for sample collection, but may be the preferable method for water sample collection since slow-flow collection resulted in less 222Rn loss than was observed during syringe collection, i.e., the water 222Rn concentrations in samples collected by the slow-flow method were generally higher. Further, based on this study, mailing water samples to a laboratory for analysis should not have a substantial effect on the measured 222Rn concentrations. Consequently, water samples can be collected by the slow-flow method by water supply operators, as well as homeowners, and mailed to laboratories for 222Rn analyses with reasonable assurance that the samples have not suffered significant 222Rn loss. Temporal variations in water 222Rn concentrations were observed in this study. One factor complicating the study of temporal variations in 222Rn concentrations was the striking influence that inconstant water usage and inadequate well purging prior to sample collection had on 222Rn loss from the water.

Groundwater geochemistry in the Koongarra ore deposit, Australia (II): Activity ratios and migration mechanisms of uranium series radionuclides.

The concentrations of uranium series radionuclides in groundwater were determined to investigate the migration behavior of radionuclides in the Koongarra ore deposit. Particular attention was given to 238U and alpha-emitting radionuclides in its decay chain, including 234U, 230Th, 226Ra, and 222Rn, and beta-emitting 210Pb. Disequilibrium between various members of the 238U decay chain in the Koongarra system arises from a combination of factors, including differences in solubility, surface affinity, the degree of weathering, diffusion of gaseous 222Rn, alpha-recoil effects and redox processes. Measured groundwater 234U/238U activity ratios were below unity in the surficial weathered zone (shallower than about 20 m depth), and greater than unity in the deeper unweathered zone (>30 m depth). These were attributed to various mechanisms related to the alpha-recoil process. Groundwater concentrations of 230Th, and also 230Th/238U ratios were extremely low, indicating that thorium is immobile in this system. Radium-226 was relatively immobile in groundwaters of the weathered zone, with lower 226Ra/238U ratios than deeper groundwaters. This was attributed to co-precipitation of radium together with manganese and ferric hydroxides at the base of the weathered zone, and also to the greater abundance of radium-sorbing minerals in the weathered zone. Large excess concentrations of 222Rn were found in most Koongarra groundwaters, indicating substantial loss of 222Rn from the solid phase despite its short half-life. The 210Pb/222Rn ratios were relatively constant and it was possible to compute an average scavenging residence time for 210Pb in the groundwater of about 6 days using a simple box model. The patterns of dispersion of uranium series radionuclides in Koongarra groundwaters also suggest that present-day migration is toward the south of the orebody. This conclusion is in agreement with the outcome of the geochemistry study.

Lead-210 in animal and human bone: A new analytical method

Lead-210 delivers the highest radiation dose to the skeleton of any naturally occurring radionuclide. A robust analytical method for the accurate determination of its concentration in bone was developed which minimizes the use of hazardous chemicals. Dry-ashing experiments showed that no substantial loss of 210Pb occurred at ≤700°C. Additional experiments showed that no loss of 222Rn occurred from dry-ashed bone. Ashed human-bone samples from three U.S. regional areas were analyzed for 210Pb and 226Ra using the new method.

Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system

Three flood-type lava flows from the Keweenaw Peninsula in upper Michigan have been precisely dated, using U Pb analyses of zircon and baddeleyite. Zircons were extracted from pegmatoid segregations within two basaltic flows. Ages from the Copper City Flow, near the base of the Portage Lake Volcanics, and the Greenstone Flow, near the top, are 1096.2 ± 1.8Ma and1094.0 ± 1.5Ma, respectively (95% confidence errors). U Pb analyses of baddeleyite from these samples are in general agreement with the zircon results although some fractions appear slightly older, possibly due to 222Rn loss. The age of an andesite flow from the Lake Shore Traps, 1000 m above the Portage Lake Volcanics and within the Copper Harbour Conglomerate, is 1087.2 ± 1.6Ma from analyses of zircon. This gives an age for sedimentation within the Copper Harbor Conglomerate. These ages, along with stratigraphic data, indicate a subsidence rate of 1.3 −0.6 +1.8 mm/y during eruption of the Portage Lake Volcanics and 0.2 ± 0.1mm/y (1σ errors) for the transitional period from flood basalt eruption to clastic sedimentation. Magma eruption rates were at least 0.02–0.05 km 3/y during emplacement of the Portage Lake Volcanics. Rates as high as 0.2–0.6 km 3/y are inferred from regional stratigraphic data and currently available precise dates from throughout the Lake Superior basin. These values are comparable to the high rates of magma production estimated for Phanerozoic continental flood basalt provinces. Analyses of three detrital zircon grains from a sandstone at the base of the Copper Harbor Conglomerate give ages from 1101 to 1106 Ma. A fraction consisting of 29 detrital zircon grains has an age of 1104 ± 2Ma indicating that sediments within the Copper Harbor Conglomerate were probably derived almost entirely from lower Keweenawan rocks. Since the Portage Lake Volcanics are paleomagnetically normal, the age of the Keweenawan paleomagnetic polarity reversal is now bracketed between 1096.2 ± 1.8Ma and1097.6 ± 3.7Ma, the age of a reversed flow within the Osler Group. From this and other studies it appears that apparent polar wander rates are different for reversed and normal poles and that normal and reversed paleomagnetic poles of comparable age have different geographic positions. This suggests that the asymmetry between reversed and normal Keweenawan poles for igneous rocks may be due to a non-geocentric dipole.

A model for 238U radioactive daughter loss from sediment-hosted pitchblende deposits and the Late permian-early triassic depositional U [sbnd]-Pb age of the Müllenbach uranium ore (Baden-Württemberg, F.R.G.)

Abnormally high radiogenic 207Pb/ 206Pb ratios in pitchblende from the Mullenbach uranium deposit (Baden-Württemberg, Federal Republic of Germany) result in calculated Pb Pb apparent ages which are older than the host Upper Carboniferous sediments ( > 320 Ma), suggesting disequilibrium in the 235U or 238U decay series. A theoretical model is proposed to explain this phenomenon based on a correlation between 206Ph loss relative to 238U in pitchblende and the apparent ages. The model shows the possibility of calculating the 238U r.d. (radioisotope daughters) /Pb loss ratio and consequently the true age of mineralization in U-disseminated deposits. Calculated losses of 237U r.d., chiefly 226Ra and/or 222Rn, are proportional to Pb radiogenic losses, in the percent ratio of 0.046. The proportionality is observed by the alignment of most samples in a U Pb concordia plot. The “isochron” gives the age of mineralization [239±6 Ma (2α)] at the upper intercept with the concordia. In the Mullenbach deposit, loss of 222Rn by emanation is more likely than diffusive loss of 226Ra because Ra is adsorbed on pitchblende. The high deficit of 238U r.d. in relation to Pb radiogenic loss (−54%) in the black shales with organic matter associated to the U ore is consistent with this hypothesis. The amount of calculated 222Rn leakage in the pitchblende (∼ 2.5%) is consistent with an α-recoil model for a 1-μ-diameter pitchblende sphere, the size observed under the scanning electron microscope.

The Use of In Situ222Rn Soil Measurements in the Characterisation of Soil Profiles

Abstract Recent in situ222Rn/214Bi soil profile measurements on hermetically sealed and weighed samples have been made using a new, experimental field method based on sequential measurements of the prompt decay of the 214Bi counting rate. At least two 214Bi count rate measurements are made in the first 11/2-2 h after sample collection and sealing, To. Two subsequent 214Bi count rate measurements are made: the first within 4-12 h, and the second approximately 900 h after To. These measurements yield the following information for each sample: (1) the extrapolated, To214Bi count rate and, indirectly, the Toin situ222Rn activity; (2) a minimum 214Bi count rate if 222Rn was lost in the sampling process; and (3) a final, constant and usually a maximum 214Bi count rate reflecting the establishment of secular radioactive equilibrium between the 226Ra, 222Rn and 214Bi in the sealed sample. This information provides the basis for: (1) assesing the state of 226Ra/222Rn radioactive equilibrium in the soil sample for the 2-3 h period immediately preceding sample collection; (2) characterising the potential loss of 222Rn from a soil profile; (3) identifying factors contributing to the mobility and transport of 222Rn; and (4) modelling the 222Rn surface flux values that might be expected under different soil moisture and atmospheric conditions.