Simultaneous measurement of radon, radon progeny and thoron concentrations using Makrofol-DE detectors

In this study we describe the set-up of a new passive integrating system to measure simultaneously ²²²Rn, ²²²Rn progeny (²¹⁸Po and ²¹⁴Po) and ²²⁰Rn concentration indoors. It consists of four Makrofol-DE (polycarbonate) circular foils. Two are enclosed within two diffusion chambers—each one with a different filter membrane—to measure ²²²Rn and ²²²Rn+²²⁰Rn. The other two foils are kept in direct contact with air and are electrochemically etched at different conditions to obtain the ²²²Rn daughters. Theoretical sensitivities of each Makrofol-DE foil are calculated using Monte-Carlo technique. The calculations are performed taking into account: (1) the Bethe–Bloch's expression for the stopping power of heavy charged particles in a medium, (2) the properties and behaviour of ²²²Rn, ²²⁰Rn and their progeny in the open air and within the diffusion chamber and (3) the etching conditions used to visualise -particles tracks.     

System for calibration of track detectors used in gaseous and solid alpha radionuclides monitoring

A laboratory system for the calibration of track detectors and charcoal detectors used in monitoring of radon and its decay products/their aerosols in air, is proposed. The system consists of three main components: (i) the alpha exposure chambers, including alpha monitoring devices and the connection with the ²²⁶Ra radioactive source. The CR-39 track detectors are mounted in the monitoring devices pre-equipped or not with paper filter; (ii) the calibrated  ²²⁶Ra source. Among the three tested sources: ²²²Rn, ²²⁶Ra+²²²Rn and ²²⁶Ra, the ²²⁶Ra source is considered the most appropriate radon source for our calibration system. It is kept into an airtight flat bottom flask, the radionuclide ²²⁶Ra being always in the radioactive equilibrium with their descendants. In the alpha exposure chambers, the source assures the radon at a constant rate; (iii) the ALFAUURASE program for the computation of radioactive accumulation of the alpha ²²⁶Ra descendants. For any initial mass of the parent, the amount and the activity of each alpha decay product and of all the decay products can be calculated by this computation program. Each component of the calibration system is described in the paper. The use of the system for the calibration of CR-39 track detectors in radon measurements is tested.     

确定大气颗粒物平均滞留时间的新方法

从低层大气的基本属性出发,利用大气颗粒物中放射性核素的衰变特性与转移速率等相关信息, 建立了单位体积大气颗粒物中被吸附的放射性粒子数、衰变粒子数和转移粒子数的数学模型, 获得了确定低层大气颗粒物平均滞留时间的新方法.该方法解决了当前相关研究中出现的困惑, 说明了大气具有有限的自净功能,给出了大气颗粒物中同一衰变链上各核素达到活度平衡的条件 及降低平均滞留时间的途径,为大气环境的监测与治理提供了科学依据. 关键词： 大气颗粒物 平均滞留时间 氡子体 监测     

Rn concentrations and the radiation exposure levels in the Nerja Cave

²²²Rn concentrations in the air in one sampling point located in Nerja Cave (Spain) () have been measured over one year, since July 2003–June 2004. ²²²Rn levels were continuously monitored using Alpha-Guard (Genitron instruments) equipment, model PQ2000PRO. Firstly, the aim of this study was to perceive the daily, monthly and seasonal variations patterns of ²²²Rn concentrations. Diurnal cycles are presented by season showing that the ²²²Rn concentrations in spring–summer are higher than autumn–winter. The hourly variations are discussed in relation to various meteorological factors measured in the Cave and also on the outdoor. We have evaluated the radiation exposure levels for workers and tourists; those represent only a low percentage of the exposure guides for the general population ICRP, 1994. Protection against Radon-222 at home and at work. Pergamon Press, Oxford (publication 65).     

Exhalation of radon and its carrier gases in SW Taiwan

Gas compositions of mud volcanoes reveal multiple sources for gas exhalation in SW Taiwan. For comparison, two sites, Yan-chao (YC) and Chung-lun (CL), were chosen for measurements of soil Rn concentrations using a portable radon detector. The ²²²Rn concentrations at the YC site were ca. 5200 Bq/m³. However, the average ²²²Rn concentrations at the CL site exhibited higher value of ca. 16,800 Bq/m³. With the reference of the gas flux and compositions from the nearby mud pool, the soil ²²²Rn concentrations are largely controlled by the flux of carrier gases exhaled from deep reservoirs.     

Soil radon levels measured with SSNTD's and the soil radium content

The source of the radon gas ²²²Rn in the ground air is the soil and the bedrock underneath. The potential radon level in the ground is given by the content of ²²⁶Ra in the ground. The presence of ²²⁶Ra is in turn dependent on the amount of ²³⁸U in the ground, and these two isotopes are not always found to be in equilibrium in a sample of soil or bedrock. Especially if the soil is washed out, the radium content may be reduced. When the soil is the relevant source of the radon gas, it is interesting to look for a possible relation between the radon level and the radium content of the soil.

In this paper we report on measurements of soil radon level carried out with SSNTDs at several European sites. Soil samples were collected at these sites and analysed with gamma spectrometry to determine their radium content. A comparison of the different degree of disequilibrium of radon, defined as the ratio between the actual and the secular equilibrium-with-radium soil radon concentration, found at the different sites and depths is presented. The influence on the result of soil type and climate is briefly discussed.     

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age.

It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of ²³⁸U; and spontaneous fission of extinct, short-lived ²⁴⁴Pu present in significant quantities in the early solar system.

A great track density attributed to the extinct ²⁴⁴Pu testifies to the high fission-track age. The model fission-track ages of (4.31±0.02)×10⁹ yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.     

The Radamon radon detector and an example of application

The etched track type Radamon radon detector is presented. As an example of application representative indoor radon survey was performed in Gyergyóremete (Romania) to pilot a study that can be done in other places of the country. Gyergyóremete was selected, because it was built on volcanic rock, where a number of aligning mineral water springs indicates the existence of geological faulting, and therefore, it was suspected that the village could be on a radon prone area. Measurements were done in sleeping rooms at pillow level. Representativity of the sample was ensured by random pull of 120 houses from the stock (hypergeometric statistical model). From the results it can be stated that the percentages of houses expected to have annual average ²²²Rn activity concentration higher than 400 Bqm⁻³ is less than 1%.     

Measurement of indoor Rn using CR-39 under a thin film geometry

In this work an assembly for indoor ²²²Rn measurement is presented. This assembly is made up of two acrylic plates (14 cm×14 cm) separated by a distance of 4 mm. To prevent radon daughters from outside entering the assembly (and produce alpha particle tracks striking the CR-39 detector placed in the central region of one of these plates), the borders of these plates were progressively closed, leaving open only a rectangular aperture of 5 mm×4 mm. The size of this opening was determined experimentally, by exposing in two indoor environments assemblies with different apertures as follows: (i) all borders open; (ii) two borders closed; (iii) three borders closed; (iv) four borders closed but one containing a 6 cm×4 mm opening; (v) four borders closed but one containing a 2.5 cm×4 mm opening and (vi) four borders closed but one containing a 0.5 cm×4 mm opening. Track density shows a noticeable decrease between assembly (i) and assembly (iii), remaining constant for smaller openings. Only ²²²Rn, a noble gas, should enter the assembly independently of the opening size.     

Study of uranium, radium and radon exhalation rate in soil samples from some areas of Kangra district, Himachal Pradesh, India using solid-state nuclear track detectors

LR-115 plastic track detectors have been used for the measurement of radon exhalation rate and radium concentration in soil samples collected from some villages of Kangra district, Himachal Pradesh, India. Uranium concentration has also been determined in these soil samples using fission track technique. Radium concentration in soil samples has been found to vary from 11.54 to 26.71 BqKg⁻¹, whereas uranium concentration varies from 0.75 to 2.06 ppm. The radon exhalation rate in these samples has been found to vary from 15.16 to 35.11 mBqKg⁻¹ h⁻¹ (502.12 to 1162.64 mBqm⁻² h⁻¹).     

Comparison of radon exposure assessment results: Po surface activity on glass objects vs. contemporary air radon concentration

Radon exposure assessment in case–control studies on radon and lung cancer is generally based on contemporary radon concentration measurements, which can be affected by significant changes in the building structures or in living habits. Another method to estimate the radon exposure of the subjects is the recently developed retrospective dosimetry technique based on the ²¹⁰Po surface activity from glass objects. In order to compare the results obtained by the two methods, a study has been carried out in a sample of 26 dwellings in Rome, with radon concentration values ranging from 28 to 623 Bqm⁻³. Retrospective detectors based on CR-39 and LR 115 were exposed on 50 glass objects in bedrooms and living rooms. The correlation factor between the two sets of data, after removing six extreme values, is 0.67, which is similar to results obtained in other validation studies of similar sample size. The correlation increases to 0.83 if the 21 objects exposed in non-smoky dwellings are selected, while it vanishes to −0.01 for the 23 objects exposed in smoky dwellings, suggesting quite larger variations of plate-out in presence of environmental tobacco smoke.     

Experimental study on the aging process of the LR 115 cellulose nitrate radon detector

An experimental determination of the aging process of cellulose nitrate detector material was based on the examination of special properties of the LR 115 solid state nuclear track detectors (SSNTDs) of various ages up to 18 years. The examined relevant parameters are the bulk etching rate v_b and the track etching rate v_t. These parameters are responsible for the appearance, the size and the registration efficiency of tracks of -particles from radon gas in the detector. To find a correlation between these material parameters and the detector sensitivity an experimental calibration of indoor room and outdoor soil detector devices based on LR 115 took place at the Umweltforschungszentrum Leipzig-Halle (Germany). To avoid routine calibration work in external radon exposure facilities a correction of the age dependent calibration factors with the material parameters measured in one's own laboratory was targeted. In this study a general age dependence, however, was not found. The following statements for practical applications can be made. (i) the bulk etching rate v_b for detectors of the same batch has a depth dependence and this dependence is constant over 2 years (LR 115 September 1994). (ii) detectors of different batches older than 5 years and stored at room temperature show an odd v_b behaviour when v_b is used for describing track shapes. (iii) the calibration factor of detectors of different batches that were stored at about +4°C is constant over 5 years (LR 115 September 1994 and February 1999, Table 2).

The conclusion is that LR 115 detectors not older than 5 years and stored in a refrigerator at about +4°C should be preferred for radon measurements. Furthermore these detectors should be recalibrated every year and the microscope work of this calibrations should be performed by the same person who performs the measurements.

In addition, a phenomenon related to fundamental track formation mechanisms was found, that the time straggling of the time t_through when vertical tracks penetrate the 12 μm thick detector layer is independent of the age of the detectors and the energy of the -particle at the detector surface.     

Alpha-recoil track densities in mica and radiometric age determination

Recoiling daughters of -decaying U and Th impurities in mica and other minerals produce localised lattice damage: alpha-recoil tracks. The age of a sample can be calculated from the number of tracks per unit volume (N_ART). To this end, the mica is etched and the etch pits at the sites of recoil-tracks are counted under the optical microscope. Because the measured track densities increase with etching time, N_ART is calculated from the fitted regression line. A number of problems inherent in this approach are overcome by the etch–anneal–etch and the mirror-image methods. The track densities determined with these methods are independent of etching time. Although both methods need improvement, they hold the potential of a precise determination of N_ART from a single measurement in future recoil-track dating.     

Slow and fast neutron detection using bare CN-85 cellulose nitrate detectors

We report on the application of CN-85 cellulose nitrate track detectors to neutron dosimetry. CN-85 has the ability to register proton tracks and therefore can be used for neutron detection via neutron-induced recoil events within the detector matrix. The CN-85 detectors were used to study the distribution of the weakly moderated spallation neutrons, produced in the irradiation of an extended Pb-target with 1 GeV protons. The results obtained are compared with Monte Carlo simulations using the MCNPX-2.5.c code. The thermal and epithermal neutrons can also be detected by CN-85 detectors without the need for a neutron converter via the ¹⁴N(n, p)¹⁴C reaction, which has a positive Q-value of 626 keV.     

A photometry method for measuring tracks density on SSNTDs

The purpose of this work is to determine alpha track density on solid state nuclear track detectors (SSNTDs) such as CR-39 efficiently with minimal time. Samples of CR-39 were irradiated with different doses from an alpha emitter. These samples were chemically etched and the tracks counted using an optical microscope. The intensity of the transmitted light (λ=400–1200 nm) through irradiated and etched CR-39 was measured using a photomultiplier. For all wavelengths used in this study, it is found that the transmittance of light decreases as irradiation time increases. In addition, the calibration factor was calculated from the relation between track density on CR-39 and light transmittance. The procedure appeared to be a good and fast technique for determining radon concentration in the environment and consequently dose reading.     

Computation of etched track profiles in CR-39 and comparison with experimental results for light ions of different kinds and energies

Computation of etched track profiles needs the knowledge of the variable track etch rates along the ion trajectories. Using the depth-dependent track etch rates experimentally determined for perpendicularly incident protons, deuterons and alpha particles as well as ⁷Li, ¹¹B, ¹²C, ¹⁴N and ¹⁶O ions of different energies simulations of the track development were performed. Two models of track etching were applied for that purpose recently published in literature. Although the models are based on the same physical fundamentals the results are slightly different. The reasons of the discrepancies were found by analysing the algorithms in detail. Comparison of the calculated track profiles with those determined experimentally from longitudinal sections of the etch pits showed good agreement for non-overetched as well as overetched tracks. The consistency of the whole experimental data set was checked by analysing the correlation of the track etch rates with geometric track parameters for all kinds of ions and etching times covered by the experiments.     

A passive radon dosimeter based on the combination of a track etch detector and activated charcoal

The aim of this work is to test a combination of a Makrofol track detector with a new type of charcoal (Carboxen-564) to design a personal radon dosimeter. The intention is to use this dosimeter as a personal radon dosimeter to measure the monthly radon exposure in workplaces, especially when the occupancy is not ecactly known. The proposed combination was exposed to low and high concentrations of radon in a large range of relative humidity (RH). For the optimal layer thickness, a charcoal bed of 2.2 mm, a specific track density of 5.1 tracks cm⁻²/kBq h m⁻³ was obtained. For a monthly working exposure (170 h) at an average radon concentration of 100 Bq/m³, this means 87 tracks/cm² or 10 times the background of the Makrofol detector, with a statistical uncertainty of 15%.     

Estimation of indoor radon, thoron and their progeny using twin cup dosemeters with solid-state nuclear track detectors in Digboi of Upper Assam

The seasonal variations of indoor radon, thoron and their progeny levels were studied in 20 houses situated in the oil-rich areas of Digboi of Upper Assam. Solid-state nuclear track detectors LR-115 type II were exposed using plastic twin chamber dosemeters to record tracks formed by the two gases and their progeny. Inhalation doses were found out using extracted equilibrium factors and ventilation rates. The random (counting) errors in track densities were determined.