Thoron Decay Products Research Articles

Radon and thoron decay product and 210Pb measurements at Jungfraujoch, Switzerland

During one year, the atmospheric activity concentrations of the short-lived “radon” ( 222Rn) and “thoron” ( 220Rn) decay products were measured at Jungfraujoch (3450 m a.s.l.). The measurements were performed using a modified epiphaniometer, with a time resolution of 1 h. In addition, also the long-lived radon decay product 210Pb was measured with a time resolution of one month. Strong seasonal variations of all activity concentrations were found, the average summer values being a factor of ten higher than the average winter values. During summer time the activity concentrations of the radon and thoron decay products showed. pronounced diurnal variations. During winter these diurnal variations were absent, and the activity concentrations were close to free tropospheric background values. From the ratio between the activity concentrations of the radon decay products 214Pb and 210Pb a mean residence time of aerosol particles in the atmosphere of about six days could be estimated. The average activity concentrations of the radon and thoron decay products at Jungfraujoch were found to be related to the regional soil temperatures at a near-by low altitude Swiss Midland site and to the static stability of the air mass between this Midland site and the Jungfraujoch. Furthermore, a significant correlation was found between the radon and thoron decay product activity concentrations and synoptic situations with higher than average activity concentrations for air flours from NE to SW and lower than average concentrations for air flows from W to N.

The Determination of Thorium Lung Content by Measurement of Thoron in Exhaled Air

A method is being developed for assessing intakes by inhalation of insoluble thorium by measurement of thoron (220Rn) in exhaled air. It has been shown that it is possible to collect the short-lived decay products of thoron, 216Po and 212Pb, electrostatically. The subsequent counting is done using a silicon surface barrier detector, which gives a high detection efficiency and good energy resolution. Experiments have been carried out using a calibrated thoron source to determine the effects of parameters such as flow rate, applied collection voltage and humidity on the collection efficiency of thoron daughters. The estimated minimum detectable activity of the prototype system for lung thorium activity is between 1 and 10 Bq for a 45 min. exhaled air collection period. With this method it will be possible to measure thorium levels which correspond to intakes of a fraction of an ALI (Annual Limit on Intake).

The Determination of Thorium Lung Content by Measurement of Thoron in Exhaled Air

A method is being developed for assessing intakes by inhalation of insoluble thorium by measurement of thoron ( 220 Rn) in exhaled air. It has been shown that it is possible to collect the short-lived decay products of thoron, 216 Po and 212 Pb, electrostatically. The subsequent counting is done using a silicon surface barrier detector, which gives a high detection efficiency and good energy resolution. Experiments have been carried out using a calibrated thoron source to determine the effects of parameters such as flow rate, applied collection voltage and humidity on the collection efficiency of thoron daughters. The estimated minimum detectable activity of the prototype system for lung thorium activity is between 1 and 10 Bq for a 45 min exhaled air collection period

Exposure to natural sources of radiation in Spain

Studies carried out by us over the last 3 yr have produced a map of natural radiation for spain. The map contains the respective contributions of outdoor and indoor terrestrial gamma-rays, of cosmic-rays and indoor radon, shown by administrative region. Terrestrial gamma-rays were measured outdoors in situ at more than 1000 locations. Data for indoor gamma-rays were derived from the radioactive content of the most typical builiding materials used in Spain and by in situ measurements in approximately 100 houses. The cosmic-ray component was calculated according to latitude and altitude. Values for indoor radon exposure were derived from a national survey, which covered more than 2000 individual measurements employing active and passive detectors. By taking account also of other exposures, the mean annual effective dose equivalent from these and the sources under study has been evaluated. Doses from thoron decay products and internal exposure are not dealt with in this evaluation.

An Online α-Impactor for Short-Lived Radon Daughters

A low-pressure cascade impactor was developed for continuous measurements of the size distributions of aerosol-attached radon and thoron decay products under environmental conditions. The emitted α-particles of the size fractionated activities are registered during air sampling with surface barrier detectors (active area = 2000 mm2) with an energy resolution < 6% (FWHM). The impactor consists of nine stages and back-up filters and operates at a flow rate of 5.1 m3 /h. The 50% cutoff diameters range between 65 and 15,530 nm. The detection limit for the short-lived radon daughters is ∼ 2 Bq/m3 for a sampling time of 1 h and a statistical error of 10%. To minimize energy losses of the ∼-particles and blow-off effects the sampling units are rotatable.

The Distribution of Radon and its Decay Products in Some UK Coal Mines

The distribution and variability of radon and its decay products in five UK coal mines have been studied. Repeated measurements have been made at intervals over periods up to 19 months, at locations representative of the ventilation circuits. The state of equilibrium between radon and its decay products has been determined and the concentrations of thoron decay products measured. The variability of concentrations in space and time has been considered in the light of the conditions prevailing during the surveys and the effects of various operational factors have been assessed. The suitability of a computer model to calculate concentrations of radon and its decay products in a mine atmosphere has been investigated and estimates made of the rates of radon exhalation from the mine strata.

Inspiratory deposition of ultrafine particles in human nasal replicate cast

The deposition of particles in replicate cast models of the human nasal cavity has been measured in three different laboratories for a range of particle sizes from 0.6 to 200 nm. The results of these measurements on four different casts can be fit by a single equation of the form η=1 − exp [−bQ − 1 8 D 1 2 ], where η is the fraction of particles deposited in the nasal cavity, Q is the volumetric flow rate (1 min −1), D is the particle diffusion coefficient (cm 2s −1), and b is found to be 12.65 ± 0.17. The measurements were conducted over a range of flow rates from 1.4 to 28.71 min −1 (501 min −1 for sizes from 4.6 to 200 nm) using radon and thoron decay product aerosols as well as larger ultrafine particles. These results thus represent a current best estimate of the diffusional deposition of ultrafine particles in the human nasal cavity.

Long-Lived Alpha-Particle Activity in Dust Samples from Hard-Rock Uranium Mines

The long-lived α-particle activity in dusty air samples taken in hard-rock underground uranium mines has been measured. Total respirable dust was estimated by conventional weighing techniques. Respirable quartz dust was measured using X-ray diffraction techniques. The long-lived α-particle activity in the samples was measured with α-particle counters (scalers) using a ZnS(Ag) screen optically coupled to a photomultiplier tube, and associated electronic circuitry, as the α-particle detector. Radioactivity measurements were made at least one month after collecting the dust samples to ensure complete radioactive decay of the airborne short-lived decay products of radon and thoron attached to dust particles. The long-lived α-particle activity was plotted against respirable quartz dust content in the samples. A linear relationship was obtained which can be expressed by the equation A=mWQ+b, where A and WQ represent the α-activity (mBq) and respirable quartz mass (mg) in the sample, respectively. Because measurements of airborne long-lived radioactivity are expensive and time consuming, the data presented here are relevant and useful as they enable the determination of airborne long-lived α-particle activity concentration, a potentially serious occupational hazard in uranium mines, from routine measurements of airborne respirable quartz dust concentration. Practical application of the results submitted here is limited to the mines studied. Extension of these results to other mines is possible, however.

Long-Lived Alpha-Particle Activity in Dust Samples from Hard-Rock Uranium Mines

The long-lived α-particle activity in dusty air samples taken in hard-rock underground uranium mines has been measured. Total respirable dust was estimated by conventional weighing techniques. Respirable quartz dust was measured using X-ray diffraction techniques. The long-lived α-particle activity in the samples was measured with α-particle counters (scalers) using a ZnS(Ag) screen optically coupled to a photomultiplier tube, and associated electronic circuitry, as the α-particle detector. Radioactivity measurements were made at least one month after collecting the dust samples to ensure complete radioactive decay of the airborne short-lived decay products of radon and thoron attached to dust particles. The long-lived α-particle activity was plotted against respirable quartz dust content in the samples. A linear relationship was obtained which can be expressed by the equation A=mWQ+b, where A and WQ represent the α-activity (mBq) and respirable quartz mass (mg) in the sample, respectively. Because measurements of airborne long-lived radioactivity are expensive and time consuming, the data presented here are relevant and useful as they enable the determination of airborne long-lived α-particle activity concentration, a potentially serious occupational hazard in uranium mines, from routine measurements of airborne respirable quartz dust concentration. Practical application of the results submitted here is limited to the mines studied. Extension of these results to other mines is possible, however.

Natural background radiation

The data presented show that the yearly background dose equals on the average about 2 mSv (200 mrem). Two thirds of the dose is attributable to internal radiation from natural radionuclides—primarily the decay products of radon and thoron, entering the human body with inhaled air. The external irradiation is due approximately equally to cosmic radiation and natural γ emitters such as40K and radionuclides of the thorium and uranium series.

An Assessment of the Estimated Thorium Lung Burdens of 130 Miners in A Rare-Earth and Iron Mine in China

The thorium lung burdens of 130 miners in a rare-earth and iron mine were estimated by electrostatic collection of the decay products of exhaled thoron. The free-emanating 224Ra equivalents at the mouth of 73 individuals were higher than 1.0 pCi. The weighted mean value of 224Ra equivalents at the mouth of the 130 miners was 2.1 ± 0.3pCi (±S.E.). The corresponding weighted mean of 10 inhabitants of the same district was 0.8 ± 0.2 pCi. On the basis of the thorium lung burdens estimated from these results, and the dose-risk relationship obtained from epidemiological studies of miners exposed to radon daughters, the number of excess lung cancer cases among 2072 dust inhaling miners between 1958 and 1993 was calculated to be very small. A relationship was, however, found between the estimated thorium lung burdens and different stages of pneumoconiosis.

Electrical Characteristics of the Short-Lived Decay Products of Thoron in Underground Uranium Mines

The thoron progeny charged fraction has been determined in underground uranium mines. Use has been made of a long hollow aluminum tube (108 cm long, 5.92 cm internal diameter, and 0.42 mm wall thickness) equipped with a coaxial conductor and a DC high voltage (HV) supply. One end of the tube was open-face for air intake; the other end was terminated with a filter holder, housing an appropriate filter where the activity not collected by the tube walls and coaxial conductor was deposited and measured when an HV was supplied. The charged fraction was calculated from the activity collected on the filter with and without an applied voltage. A value for the charged fraction of about 60% was obtained.

Electrical Characteristics of the Short-Lived Decay Products of Thoron in Underground Uranium Mines

The thoron progeny charged fraction has been determined in underground uranium mines. Use has been made of a long hollow aluminum tube (108 cm long, 5.92 cm internal diameter, and 0.42 mm wall thickness) equipped with a coaxial conductor and a DC high voltage (HV) supply. One end of the tube was open-face for air intake; the other end was terminated with a filter holder, housing an appropriate filter where the activity not collected by the tube walls and coaxial conductor was deposited and measured when an HV was supplied. The charged fraction was calculated from the activity collected on the filter with and without an applied voltage. A value for the charged fraction of about 60% was obtained.

Some observations on the concentrations of short‐lived decay products of radon and thoron in the monsoon rains of Bombay, India

The concentrations of radon decay products 214Pb(RaB) and 214Bi(RaC) and thoron daughter 212Pb(ThB) have been measured in rainfall at Bombay. The presence of short‐lived 218Po(RaA) is indicated in a few samples. The levels of 214Pb varied from 300 to 7000 pCi (11–260 Bq) per liter, while the activity ratios of 214Bi/214Pb are in the range of 0.5–1.5. From these values of the ratios a cloud drop lifetime, from nucleation to deposition, of 20–60 min is estimated. The concentrations of 212Pb varied from 2 to 30 pCi (0.075–1.1 Bq) per liter. The levels of radon and thoron daughter products are lower in Bombay summer rainfall compared with other areas due to their reduced concentrations in the maritime monsoon winds. Approximate estimates of scavenging ratios based on the surface air concentrations are 85±65 (1 standard deviation) and 30±20 (1 standard deviation) for radon and thoron daughters, respectively.

Radon daughter and thoron daughter deposition velocity and unattached fraction under laboratory-controlled conditions and in underground uranium mines

The deposition velocity and unattached fraction of the short-lived decay products of thoron and radon have been measured in a large (26 m 3) radon/thoron calibration facility and in underground uranium mines. Measurements were conducted under various environmental conditions of relative humidity, temperature, aerosol concentration, and air velocity (0–1.2 m s −1). Unattached fractions were determined using wire screens. Plate-out experiments, carried out to determine deposition velocities, were done using circular stainless steel plates attached to walls. The average RaA unattached fraction in the aerosol concentration range 3.5 × 10 2–1.5 × 10 3 cm −3 was about 0.5. Substantially lower values were obtained for RaB and RaC. Similarly, the average ThB unattached fraction in the same aerosol concentration range was approx. 0.1. Significantly lower values were found for ThC. The unattached fractions in underground uranium mines were considerably lower than the above values, i.e. < 0.01, an experimental result which is consistent with the much higher aerosol concentration in underground working areas. (Experimentally determined radon and thoron daughter deposition velocities were in the range 0.06–0.6 cm/s.) These values are substantially lower than theoretical estimates by Jacobi (0.5–1.5 cm/s) and values used by other authors in the past. Our radon daughter data are, however, in agreement with recent laboratory work by several authors. Deposition velocity and unattached fraction data are relevant for the calculation of daughter product plate-out on mine walls, and hence, for realistic determination of airborne radiation levels in underground uranium mines. This study is of particular interest for some Canadian mines, and other mines, where the presence of thoron and its daughter products is appreciable.

Design and development of a computer-based continuous monitor for the determination of the short-lived decay products of radon and thoron

A portable, rugged, monitor has been designed and built for measuring the short-lived decay products of radon and thoron. The monitor is computer-based and employs a continuous filter strip which can be advanced at programmable time intervals to allow unattended continuous operation with automatic sampling, analysis and recording of radiation levels. Radionuclide analysis is carried out by two silicon diffused-junction alpha-detectors and electronic circuitry with multichannel spectral analysis capabilities. Standard gross α-count methods and α-spectroscopy methods can easily be implemented. The built-in computer performs a variety of operations via a specially designed interface module, including control and data recording functions, and computations, program storage and display functions. Programs and data are stored in the built-in casette tape drive and the computer integrated CRT display and keyboard allow simple, prompted menu-type operation of standard software. Graphical presentation of α-spectra can be shown on the computer CRT and printed when required on the computer built-in thermal printer. In addition to implementing the specially developed radionuclide analysis software, the operator can interact and modify existing software, and program new ones, through BASIC language programming, or employ the computer in a totally unrelated, general purpose mode. Although the monitor is ideally suited for environmental radon (thoron) daughter monitoring, it could also be used in the determination of other airbone radionuclides provided adequate analytical procedures are developed or included in the already existing computer software.

Measurement of Potential Alpha Energy Concentration of Radon and Thoron Daughters Using an Electret Dosemeter

An electret dosemeter consisting of an electret loaded into a 60 ml chamber has been used earlier for quantitative measurement of gamma radiation. The chamber was modified to hold an air sampling filter paper at the bottom and air was sampled through it. It was shown that nearly 20% of the energy of alpha radiation emitting from the filter paper was dissipated in the volume of the chamber. This energy in turn produced ion pairs which were collected by the electret resulting in a measurable change in charge status of the electret. By taking an air sample of known volume and allowing the registration of alpha energy, not only during sampling but also for three hours after sampling, a direct measure of the potential alpha energy concentration of radon daughters could be calculated in easily interpretable working level units. By proper choice of the thickness of the electret and the total flow, it was possible to measure radon daughter concentrations down to milliworking level units. The field measurements carried out by this method compared well with those obtained by established methods. The procedure was particularly useful for measuring time-averaged concentrations needed for effective survey of a mine area or for personal dosimetry. For thoron decay products, it was shown that the alpha energy concentration measured during sampling and up to a period of 7.7 hours post sampling, time being reckoned from the mid-time of sampling, yielded 33% of the potential alpha energy concentration due to thoron daughters. An air sample collected and measured in this way registered all of radon daughter alpha potential energy and one third of thoron daughter alpha potential energy, thus providing an equivalent radon daughter concentration. This procedure is suitable for evaluating any mixture of daughter products without consideration of whether they originated from radon or thoron.

Passive measurement of radon and thoron using TLD or SSNTD on electrets.

An electret is an electrical analogue of a permanent magnet and it carries a permanent electric charge. Our previous work has shown that such electrets are suitable for collecting decay products of radon and thoron in passive chambers. In the present work, the decay products are directly collected on the surface of a TLD or SSNTD providing in situ registration of the radiation from the decay products of radon and thoron. A 101. chamber, the sides of which were covered with a layer of Whatman No. 1 (W-1) filter paper, showed the following responses: (i) SSNTD (CR-39) recorded 92 +/- 13 tracks per cm2 per pCi/l, hr for radon and 9 +/- 1.5 tracks per cm2 per pCi/l. hr for thoron; (ii) for similar levels TLD (CaF2(Dy)) chips recorded an equivalent of 1.35 +/- 0.16 mR for radon and 0.30 +/- jk0.09 mR for thoron. Taking advantage of the differential response of the two chambers (one covered with a layer of W-1 filter paper and the other with a 75 mm polyurethene foam), simultaneous measurement of radon and thoron could be achieved.

Collection of radon decay products — a uranium exploration technique

The spontaneous deposition of short-lived radon decay products onto solid surfaces (“collectors”) provides the basis for a simple and efficient way of prospecting for uranium. The alpha activity of two of the decay products, 218Po and 214Po, can be measured by conventional counting techniques following the exposure of a collector to a radon source. Laboratory studies have shown: (a) radon decay products can be collected on a wide variety of materials; (b) the number of radon decay products increases with the collector surface area; (c) a negative charge applied to the collectors enhances the number of decay products collected; (d) the shape of the collectors is relatively unimportant; and (e) reproducibility is about ± 5% of the measured value. Field tests on known uranium anomalies involved suspending collectors for an overnight exposure, in either covered holes or inverted containers buried in soil. Subsequent removal of the collectors was immediately followed by counting the alpha activity. Anomalies were readily detected at three different test sites. Replicate measurements at selected sites both on an hour-to-hour and on a day-to-day basis showed remarkably good agreement. Some anomalies were also outlined using activity measurements from collectors suspended above soil aliquots in sealed containers. Because the half-lives of two of the thoron ( 220Rn) decay products are much longer than those of the radon ( 222Rn) decay products, a thoron correction can be applied to the original activity measured from the collectors, thus discriminating against thoron in thorium-rich terrains. The counting of radon decay products deposited onto collectors is an effective method for delineating radon anomalies in uranium exploration. The field techniques are simple to apply, and results are available after very short exposure times (about 18 hours) and after short counting intervals (5 minutes).

Electret--a new tool for measuring concentrations of radon and thoron in air.

Electret is an electrical analogue of a permanent magnet and it carries permanent electrical charge. A negatively charged teflon electret is used for the collection of decay products of radon or thoron (positively charged) formed inside a 10-l. wire mesh chamber whose sides are covered with 25-mm thick polyurethene foam. Teflon electret provides an electric field inside the chamber equivalent to that provided by a battery of 2000–3000 V. In dry conditions, nearly 70% of all the decay products of radon and thoron formed in the chamber are collected on the surface of the electret. After a known sampling time the electret is subjected to programmed a-counting to compute concentrations of radon and thoron. With a 3-hr sampling and subsequent counting, minimum detectable limits work out to be 30 pCi/m3 for radon and 110 pCi/m3 for thoron. The method is ideally suited for measurement of levels normally encountered in the environment and in dwellings. The paper discusses the method of preparing and characterizing electrets; the dependence of the collection efficiency upon relative humidity, area of the electret and the charge on the electret. Field measurements made by this method and the standard double filter system are shown to agree well.