Estimate of etched tracks by optical method and spark counting

Solid State Nuclear Track Detectors (SSNTD) are commonly used for long term measurements of radon, thoron and progeny concentrations. In the present study, we compared the optical counting method and the spark counting technique for counting the alpha tracks on LR 115 track detector films. The paper discusses the various parameters that are innate in the process. More than 300 films were counted by both optical microscope and spark counter and the results are compared. The overall results show that the tracks obtained by spark counting are marginally less compared to the optical measurement. A linear fit of the data gives a slope less that one, which indicates that both the methods are almost in good agreement for counting the tracks when the track density is low. At higher track densities the spark counter gives an underestimation with respect to actual number of tracks formed, which could be corrected using a relation obtained between the tracks measured by optical method and spark counting.      

Experimental determination of CR-39 counting efficiency to α particles to design the holder of a new radon gas dosemeter

To design a new radon gas dosemeter, the knowledge of the SSNTD α counting efficiency is essential. It depends primarily on the energy and incidence angle into the detector, then on the etching conditions and finally on the track counting procedure. Therefore the elicited efficiency function represents all these factors. The detectors were exposed to α particles at different angles of incidence and energies, by using a ²⁴⁴Cm source; two alternative NaOH solution etching conditions were tested, 6.25 N 70°C at 6 and 12 h, respectively. The counting was performed with a light microscope, 267×, and with an automatic track analysis procedure. The critical incidence angle dependance on energy, in our standard condition (6 h), resulted θ_C(E)=380.0e^{(−0.286•(E−0.050))}(1−e^{(−0.186•(E−0.050))}), and it was used to design our new radon gas holder.     

Monte Carlo simulation of radon SSNT detectors

A Monte Carlo based software for the computation of the sensitivity of etched radon track detectors was developed. It can be applied to the measurement of radon and radon daughters in free air or inside of a measurement chamber. LR 115 and CR-39 detectors, with or without an attenuator, are specifically addressed. Various etching conditions and observation criteria for counting the track density may be specified. The latent track formation and the etching process are realistically modelled. The dependence of the etch-rate ratio on the energy is taken into account. The plate-out phenomenon is included in the model. An inhomogeneous source distribution in the detector cup can be considered.     

A new method for measuring tracks density in CR-39 detectors by compensating for overlapping tracks

CR-39 nuclear track detectors have been used as an efficient radon monitoring system. Exposing these detectors to radon would result in tracks where ever particles hit the surface of the detector. Natural radioactivity of alpha particles (radon concentration) is measured based on the count of these tracks on CR-39 detector. Counting these tracks by eye is not an easy task especially when the density of the tracks is high. Automated tracks counting systems are attractive solution for this problem. However, as the density of tracks increases the accuracy of these methods deteriorates. This is due to the fact that overlapped tracks are counted as one track during the segmentation process resulting in an underestimation of the real count of the tracks. This work proposes a new image processing based automated approach for tracks counting. This new automated approach solves the problem of counting overlapping tracks by approximating their number and then adjusting the final count of tracks by compensating for those uncounted tracks. The experimental results show that the proposed approach reduces the counting percentage error from 5.64% to 2.54% by solving the counting problem of the overlapped tracks.     

Investigation of vertical migration of alpha-emitting nuclides in soils for southern regions of the republic of Belarus

An investigation of vertical distribution of alpha-emitting nuclides for various types of soils using solid state nuclear track detectors (SSNTD) has been carried out. As SSNTD the DNC-film was used. Experimental system for measurement of total alpha activity in the samples of soils, including devices and equipment for selection of probes, exposition, chemical etching of detectors, automated accounting of tracks has been developed. The method of determination of specific activity of alpha emitting nuclides has been created. A comparison results of measurements with the method pointed out and radiochemical method as well as x-y-spectrometry method has been carried out.     

Distribution of indoor radon levels in mexico

Our laboratory has carried out a systematic monitoring and evaluation of indoor radon concentration levels in Mexico for ten years. The results of the distribution of indoor radon levels for practically the entire country are presented, together with information on geological characteristics, population density, socioeconomic levels of the population, and architectural styles of housing.

The measurements of the radon levels were made using the passive method of nuclear tracks in solids with the end-cup system. CR-39 was used as the detector material in combination with a one-step chemical etching procedure and an automatic digital-image counting system. Wherever a high level was measured, a confirming measurement was made using a dynamic method. The results are important for future health studies, including the eventual establishment of patterns for indoor radon concentration, as it has been done in the USA and Europe.     

Influence of an electret on the sensitivity of CR-39 nuclear track detector in diffusion chamber

Cylindrical diffusion chamber for radon measurement equipped with nuclear track detector (CR-39) and electretis considered in this work. Electret is electrostatic analogous of permanent magnet which preserves constant and strong electric field for long time period. Electrets were used to attract radon progeny formed in a cup where CR-39 was placed for radon measurements. In this way the sensitivity of detector could be increased by bringing progeny just in front of detector in more convenient measuring geometry. Influence of distance between electret and detector on sensitivity of such equipment has been investigated theoretically. Different geometrical setups of detector and electret were tested in order to optimize this kind of device. Some results are presented in this report.      

A fast method to measure the thickness of removed layer from etching of SSNTD based on EDXRF

Solid state nuclear track detectors are commonly used for measurements of concentrations of radon gas and/or radon progeny. All these measurements depend critically on the thickness of the removed layer during etching. However, the thickness of removed layer calculated using the etching period does not necessarily provide a sufficiently accurate measure of the thickness. For example, the bulk etch rate depends on the strength of stirring during etching for the LR 115 detector. We propose here to measure the thickness of the removed layer by using energy-dispersive X-ray fluorescence spectrometry. In the present work, a reference silver nitrate pellet is placed beneath the LR 115 detector, and the fluorescence X-ray intensity for silver is then measured. We have found a linear relationship between the X-ray intensity and the thickness of the removed layer for LR 115 detector. This provides a fast method to measure the thickness of removed layer from etching of LR 115 detector. However, this method was found to be inapplicable for the CR-39 detector. Therefore, alternative methods have yet to be explored for the CR-39 detector.     

Indoor radon measurement via Nuclear Track Methodology: A comparative study

Indoor radon concentration represents an important public health challenge, for simple and inexpensive measurement devices and methods, suitable for large-scale indoors radon measurements, are required. Nuclear Track Methodology, by using a closed-end cup device as a radon chamber is an attractive option for such large-scale indoor radon measurements. A comparative analysis of the detection efficiency of four different (one commercial and 3 specially designed) passive closed-end cup devices for the measurement of indoor radon concentrations is presented. CR-39 (Lantrack®) polycarbonate was the detector material. The four devices were simultaneously exposed to a mean radon concentration of 860 ± 60 Bq m⁻³ inside a closed room for periods of one, two and three months. An AlphaGUARD^® radon monitoring system was used to continuously monitor the radon concentration within the room. The chemical etching and reading procedures were carried out following a well-established protocol for indoor radon surveys. The detection efficiency and the exposure-time-response relationship of each of the devices were determined.     

Determination of the range of alpha particles in SSNTD by optical density method

A technique based on the optical density (D) measurement of the etched track is useful for charged particles spectroscopy using SSNTD. It was shown that the stopping power of alpha particles in CR-39 is proportional to D. We measured the optical density and derived an expression to estimate the range of alpha particles in CR-39 detector as a function of the bulk etching rate and etching time. The relation between the etching time, track parameters (depth, radius) and D for different alpha particles energy and etching conditions were studied. A relation describing D as a function of track size is proposed.     

Neutron dosimetry at low and high fluences with CR-39

In the Laboratory of Nuclear Microanalysis, we have developed two techniques for neutron dosimetry; the first for low fluence, the second for high fluence. These two techniques use a Solid State Nuclear Track Detector (SSNTD): the CR-39. The low fluence technique is based on the measurement of etched tracks resulting from a neutron-proton conversion. A Monte Carlo code performs a simulation of the (n,p) collision in the detector, and a numerical computer code of latent track etching allows the evaluation of the etched track parameters. The object of this is to obtain characteristics of a neutron fluence from the measured etched track parameters. When there is a high fluence and high doses, CR-39 is unsuited for optical microscopy analysis. This is because of track overlapping which makes it impossible to carry out counting and exploitation. We have therefore developed a new method permitting the reading of samples based on the measurement of the angular distribution of coherent light (He---Ne laser) transmitted through the irradiated etched SSNTD. We present these two techniques and our initial results.     

SHI induced nano track polymer filters and characterization

Swift heavy ion irradiation produces damage in polymers in the form of latent tracks. Latent tracks can be enlarged by etching it in a suitable etchant and thus nuclear track etch membrane can be formed for gas permeation / purification in particular for hydrogen where the molecular size is very small. By applying suitable and controlled etching conditions well defined tracks can be formed for specific applications of the membranes. After etching gas permeation method is used for characterizing the tracks. In the present work polycarbonate (PC) of various thickness were irradiated with energetic ion beam at Inter University Accelerator Centre (IUAC), New Delhi. Nuclear tracks were modified by etching the PC in 6N NaOH at 60 (±1) °C from both sides for different times to produce track etch membranes. At critical etch time the etched pits from both the sides meet a rapid increase in gas permeation was observed. Permeability of hydrogen and carbon dioxide has been measured in samples etched for different times. The latent tracks produced by SHI irradiation in the track etch membranes show enhancement of free volume of the polymer. Nano filters are separation devices for the mixture of gases, different ions in the solution and isotopes and isobars separations. The polymer thin films with controlled porosity finding it self as best choice. However, the permeability and selectivity of these polymer based membrane filters are very important at the nano scale separation. The Swift Heavy Ion (SHI) induced nuclear track etched polymeric films with controlled etching have been attempted and characterized as nano scale filters.      

Alternative procedure for LR 115 chemical etching and alpha tracks counting

An improved procedure for etching and analysis of alpha tracks induced in LR-115 detectors is proposed with the advantages of simplicity and its relatively low cost. A new type of detector holder was designed to etch and rinse efficiently up to 100 detectors. We develop a simple and reliable methodology with a semiautomatic track count using a Nikon digital camera coupled to a PC and employing software “SCION” freely available on the Internet. Track images are binarized prior the application of software “SCION” so that original track shapes are not distorted, space resolution is improved and track counting has low dependence on focus and illumination level. High discrimination for tracks is achieved when marks and rips perturb the detector surface. An image generator of nuclear tracks is included to study the effect of track overlapping effect on counting.     

In situ measurements of gamma-ray intensity from radon progeny in rainwater

The radon progeny concentration in raindrops as well as in cloud droplets has been determined based on a simplified rainout model from the in situ measurement of gamma-ray intensity using an HPGe detector. The radon concentration in cloud air was deduced from the radon progeny concentration in cloud droplets by applying the Junge's equation. From the measured radon progeny concentration in raindrops the additional exposure rate due to rainfall has been evaluated and compared with the HPIC monitoring data.     

Study of current drain during electrochemical etching of polycarbonate detectors

Polycarbonate (PC) detector is one of the common detectors for neutron and radon gas detection. Using this detector it is possible to measure the dose in mSv, by counting tracks/cm² on an etched surface. In this paper, a special procedure has been suggested to determine the dose based on current drain during the etching process. In these experiments the effects of voltage, frequency, effective etched area, PC detector's thickness, etched area (one side or two sides), etching solution temperature and dose absorbed by the PC foil have been studied.The results obtained show the current drain variation for a voltage of 200–1600 V, a frequency of 2–10 kHz, effective area with a diameter of 2–12 cm, PC thickness of 125–250–375– and a temperature of etching solution of 25–.Lexan PC foils were exposed to doses of of neutrons. The unexposed foils were considered as the background (BG) foils. Most of the experiments were performed at a voltage of 800 V, a frequency of 2 and 8 kHz, foil thickness of , diameter of effective etched area of foils of 2, 6 and 12 cm, temperatures of 25 and and the etching process from 0 up to overload stage. Overload stage occurs when the foil becomes so thin due to growth of the tracks that it leads to sparking between phase and null that makes a hole in the foil.Current drain curves versus the function of the etching time are absolutely different for various doses from zero (BG) to 10 rad (BG up to ). This is true especially for the time interval from 3 h of etching up to overload stage. In this way, it is possible to obtain a calibration of PC detector net current drain based on its absorbed dose.In this experiment, the number and diameter of tracks and their relation with drain current and PC foil residual thickness at overload stage have been studied.The same experiment has been performed for various concentrations of radon gas (Bq/m3) as well.     

Study of inhomogeneity in thickness of LR 115 detector with SEM and Form Talysurf

Long-term measurements of radon progeny concentrations using Solid-State nuclear tract detector are being actively explored. These measurements depend critically on the thickness of the removed layer during etching. Scanning electron microscope (SEM) observations have identified irregularities in etched LR 115 detectors, such as detachment of the active layer from the substrate and formation of air gaps in the substrate. After discarding these irregularities, by using “Form Talysurf” surface profile measurements, the thickness of the active layers for the LR 115 detector are found to be 11.8±0.2 and 5.0±0.4 μm before and after 2 h of etching, respectively. The coefficient of variation has thus risen from 1.7% to 8.0% on etching. The increased inhomogeneity is explained by the formation of track-like damages, which have been observed using Form Talysurf, SEM, optical microscope and atomic force microscope. With this relative large coefficient of variation, the thickness of the active layer in the LR 115 detector cannot be assumed to be homogeneous in general, and the associated uncertainties should be considered carefully when the detector is used for alpha spectroscopy.     

Indoor radon survey in Eastern Sicily

Inhalation of radon (Rn-222) and its progeny is one of the most significant sources of natural radiation exposure of the population. Nowadays, high radon exposures have been shown to cause lung cancer and many governments all over the world have therefore recommended that radon exposures in dwellings and indoor workplaces should be limited. Radon levels in buildings vary widely from area to area depending on local geology. This paper presents the results of a long-term survey of radon concentrations carried out from 2005 till 2010 in schools and dwellings of Eastern Sicily, using the solid-state nuclear track detector (SSNTD) technique. The investigated area shows medium-high indoor radon concentrations, higher than the Italian average of about 70 Bq/m³, with peaks of 500 Bq/m³ or more in buildings near active faults. Fortunately, only a small fraction of the measurements, about 1.5% of total, was found greater than EU and Italian action limits for indoor and workplaces.     

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.     

Verification of radioactive equilibrium to discriminate radon and progeny with LR 115 under a thin film geometry

A system of image analysis is applied to the track analysis of a plastic detector (LR 115). Based on track characteristics the system permits the separation of tracks produced by ²²²Rn and ²¹⁸Po from those produced by ²¹⁴Po. LR 115 was used inside a thin film geometry monitor and irradiated under very low ventilation conditions, where secular radioactive equilibrium between radon and progeny is expected. The monitor is used intending to exclude the entrance of external radon progeny in the sensitive volume of the plastic detector. The radioactive (secular) equilibrium is rapidly attained inside the monitor and equal numbers of particles from ²¹⁸Po, ²¹⁴Po and ²²²Rn are expected to reach the plastic. In order to guarantee good detection efficiency in the restricted volume, and to discriminate alpha energies, the LR 115 piece inside the monitor is covered with an aluminum foil of suitable thickness. The separation of tracks produced by alpha particles of different energies is made through the use of two track parameters related to area and opacity of the tracks. The ratio of tracks produced by (²¹⁸Po and ²²²Rn) and ²¹⁴Po—expected as two—is used to test the separation method in a series of controlled irradiations. Results point to the experimental attainment of radioactive equilibrium inside the monitor supporting the determination of ²¹⁴Po concentration alone, and points to an exclusive entrance of ²²²Rn inside the monitor, at low ventilation rates. This way, in situations where radioactive equilibrium inside the monitor may be considered, the exclusive measurement of radon concentration in the environment is possible.