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.     

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.     

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.     

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.     

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.     

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.     

Indoor radon monitoring in some buildings surrounding the national Hydroelectric Power Corporation (NHPC) project at upper Siang in Arunachal Pradesh, India

In the present study measurement of radon and its progeny concentration has been undertaken in the buildings constructed in the surroundings of National Hydroelectric Power Corporation (NHPC). LR-115 Type-II solid state nuclear track detectors fixed on a thick flat card were exposed in bare mode. Track etch technique has been used to estimate the radon concentration in the rooms of some buildings. Annual effective dose has been calculated from the radon concentration to carry out the assessment of the variability of expected radon exposure of the population due to radon and its progeny. The radon levels in these dwellings vary from 9±4 to 472±28 Bq m⁻³ with an average value of 158±14.9 Bq m⁻³ whereas annual effective dose varies from 0.1±0.04 to 7±0.4 mSv y⁻¹ with an average value of 2.3±0.2 mSv y⁻¹. These values are below the recommended action levels.     

Study of fission track registration efficiency of γ-irradiated polyester track detector in solution medium

The fission track registration efficiency (K _wet) of the Garware polyester track (GPF) detector in solution has been determined with respect to the Lexan detector whose K _wet is very well known in the literature. The K _wet is found to be (1.2 ± 0.1) 10⁻³ cm. The track registration efficiency of the GPF detector in solution is better than the efficiencies of other nuclear track detectors reported in the literature. This implies that the threshold energies for track registration are different for different detectors and that GPF detector has a lower threshold-energy value. The effects of gamma irradiation in the dose range of 5–51 Mrad on the fission track registration efficiency (K _wet) of this track detector from the solution medium are also investigated. The results show that the K _wet values for the gamma-irradiated detectors in the above dose range in 2M HNO₃ solution medium decreases by ∼7–15%. The changes induced in the gamma-irradiated detectors as a function of gamma dose have also been studied by bulk-etch rate measurements.     

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.     

Application of the ray tracing method in studying tracks in SSNTDs

The ray tracing method was applied to study etched tracks from α-particles in CR-39 solid-state nuclear track detectors. The transmission mode of a microscope operation was simulated. A track was considered as a set of small triangular elements, and the brightness of all elements was calculated systematically through the entire track to create the final image. Preliminary results are given for α-particle tracks in the CR-39 detector for initial energies of 4 and 4.5 MeV, and incident angles of 40 and 90°, respectively. Total reflection, as well as the slope of a surface element in the track wall, were identified as the main factors that affect the brightness of that element. At this stage, comparisons with experimental results can be made in terms of the average grey levels for the entire tracks.     

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.      

Measurement of bulk etch rate of LR115 detector with atomic force microscopy

Equations for calculating track parameters have been proposed, which invariably involve the track etch rate V_t and the bulk etch rate V_b. The present study measured V_b for the LR115 solid-state nuclear track detector using atomic force microscopy (AFM). The detectors were partially masked using rubber cement and then etched in 2.5 N NaOH solution at 60°C for time periods ranging from 5 to 40 min. The rubber cement was then peeled off and cross-sectional images of the LR115 detectors were obtained by AFM. V_b has been found to have different values below and beyond the etching time of about 13.5 min, with the values of 0.0555 and 0.0875 μm min⁻¹, respectively. The increase in V_b with the etching time can be explained by a diffusion-etch model, in which the additional damage of the detector material is due to those etchant ions diffused into the detector over time. Now that V_b has been determined, this can be combined with the track etch rate V_t to calculate track parameters.     

Seasonal variation of radon level and radon effective doses in the Catacomb of Kom EI-Shuqafa,Alexandria, Egypt

Inhalation of radon has been recognized as a health hazard. In the present work radon concentration was measured, in the atmosphere of the archaeological place, namely Catacomb of Kom El-Shuqafa, in Alexandria, Egypt, which is open to the public, using time-integrated passive radon dosimeters containing LR-115 solid-state nuclear track detector. The measurements were performed throughout winter and summer. Seasonal variation of radon concentration, with the maximum in summer ranging from 243 to 574 Bq m^− 3 and minimum in winter ranging from 64 to 255 Bq m^− 3 was observed. Because of the variations of the catacomb ventilation system, the equilibrium factor between radon and its progeny ranges from 0.14 to 0.48. The tour guides are exposed to an average estimated annual effective dose ranging from 0.21 to 0.52 mSv y^− 1 and the visitors from 0.88 to 2.28 μSv y^− 1. The effective doses the catacomb workers are exposed to ranged from 0.20 mSv y^− 1 in winter to 4.65 mSv y^− 1 in summer which exceeds the lower bound of the recommended level (3–10 mSv y^− 1) (ICRP, 1993).     

New radon and gamma device using TLD and SSNTD to carry out monitoring in the environment

The research program, that we are leading, concerns and takes into consideration the potential risk of the natural and artificial radio-elements in the environment and particularly to consider their migration from environment to the people and with special respect to radon effects. To begin these investigations, we developed a new passive detectors using two solid nuclear track detector techniques: thermo-luminescent detectors (TLDs) (gamma measuring) and solid state nuclear track detectors (SSNTDs) (alpha measuring). The characteristics of the cell provide for alpha (SSNTD - LR115) and gamma-radioactivity (TLD-CaSO₄:Dy) measuring in different soil depths. The used technologies are with a low priced. The instrumentation has been first validated in laboratory to quality precisely its response and then, we have performed the survey for one site in Bulgaria.     

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.      

Inverse Laplace transforms of osculatory and hyperosculatory interpolation polynomials

In the numerical calculation of f(t), the inverse Laplace transform of F(p), where f(′) = (1/2πi) °_c−i∞^c+i∞ e^pt F(p)dp, sufficient accuracy is usually obtainable when p³F(p), s > 0, is replaced by an interpolating polynomial in 1/p. From the values of F(p) with F′(p), or with F′(p) and F″(p), for p at points equally spaced on the real axis, an osculatory or hyperosculatory interpolation polynomial for p⁸F(p), namely L_2n−1(x) or L_3n−1(x), where x = 1/p, is obtained in barycentric form. Then f(t) is calculated by a Gaussian-type quadrature formula employing complex values of L_2n−1 or L_3n−1 and instead of p^sF(p) which may be unknown or more difficult to compute. For calculating L_2n−1 and L_3n−1, auxiliary coefficients, suitable for economical storage in the program, are given exactly for n = 2(1)11 and n = 2(1)7, furnishing up to 21st and 20th degree accuracy, respectively.     

Measurement of the Po concentration in air using makrofol-de detectors

The measurement of the ²¹⁴Po concentration in air with Makrofol-DE detectors is useful to estimate the long-term averaged equilibrium factors indoors. To differentiate α-particles emitted by ²¹⁴Po from those emitted by ²¹⁸Po and ²²²Rn, the detector must register only α-particles with energies between 6.2 and 7.5 MeV. The required energy response is obtained only if a removed layer of about 43 μm is achieved in a chemical etching of the detector. The methodology used to determine the etching conditions is described in this paper. The optimum conditions found are: a) chemical etching for 6 h at a temperature of 40°C, using 7.5 M KOH mixed with 50% ethanol as an etchant, and b) electrochemical etching for 1 h at a frequency of 3 kHz and an electric field strength of 34 kV cm⁻¹. Several dosimeters have exposed during 2 months in dwellings located in the Barcelona area, Spain. A ²¹⁴Po averaged concentration of (13.6 ± 8.6) Bq m⁻³ was obtained.     

Uranium, radium and radon exhalation studies in some soil samples using plastic track detectors

Radium concentration and radon exhalation rate have been measured in soil samples collected from some areas belonging to upper Siwaliks of Kala Amb, Nahan and Morni Hills of Haryana and Himachal Pradesh states, India using LR-115 type II plastic track detectors. Uranium concentration has also been determined in these soil samples using fission track registration technique. Radium concentration has been found to vary from 5.30 to 31.71 Bq.kg⁻¹, whereas uranium concentration varies from 33.21 to 76.26 Bq.kg⁻¹. The radon exhalation rate in these samples varies from 216.87 to 1298.00 mBq.m⁻²hr⁻¹ (6.15 to 36.80 mBq.kg⁻¹.hr⁻¹). Most of the samples have uranium concentration above the worldwide average concentration of 35 Bq.kg⁻¹. A good correlation (R ² = 0.76) has been observed between uranium concentration and radon exhalation rate in soil. The values of uranium, radium and radon exhalation rate in soil are compared with that from the adjoining areas of Punjab.     

Further studies on ageing and fading of CR39 PADC track detectors used as air radon concentration measurement devices

PADC detectors are commonly used both as radon detectors and as personal dosemeters for neutron radiation. In both cases the measurement in workplace fields are characterized by long term irradiations, lasting up to several months. During this period the detectors undergo to uncontrolled and unmonitored environmental conditions. In a recent work the same authors demonstrated that the environmental conditions, mainly temperature, can seriously affect the detector sensitivity. The temperature effect is to reduce the V ratio by decreasing the track etching velocity V_t. This causes a decrease in limit angle and efficiency.This paper describes the same ageing/fading test made with CR39 from a different supplier. In this case it seems that the temperature has a negligible effect, so no compensation algorithm is needed.