Background Track Density Research Articles

A pilot survey on indoor radon concentration in Beijing

ObjectiveTo explore the level of indoor radon concentration especially after recent 3 decades’ great economic development in metropolis Beijing, so as to describe the outline of indoor radon level in Beijing. MethodsA one-year integrated measurement on 800 dwellings was carried out from September 2018 to August 2019. Passive integrated radon cup monitor with CR-39 as an alpha-track detector was adopted for the survey. As a pilot study, measurement scheme was designed in detail. For measurement quality control, the background track density of CR-39 was strictly defined and controlled throughout the operation of the survey. ResultsThe annual geometric and arithmetic mean of radon concentration of the surveyed are (39.3 ​± ​12.9) and (42.0 ​± ​13.7) Bq/m3, respectively, varying from (12.1 ​± ​2.5) to (119.0 ​± ​7.8) Bq/m3. Radon concentration of the ground floor dwellings is obviously higher than that of other floors. No difference of radon level is found among the dwellings of other floors. Higher radon concentration in buildings built after 2010 is obtained compared with buildings built in 1980s, 1990s, and 2000s. ConclusionsThere is no big change of radon concentration level in dwellings in Beijing area observed in general compared with a previous survey performed more than ten years ago. For high-rise buildings, building materials are suggested to be the main source of indoor radon. For higher radon concentration in buildings built in latest ten years, the change on building materials and indoor ventilation rate are thought to be the causes, further study in detail is needed to clarify.

Configuration of the parameters for scanner-based track detector evaluation system

Abstract According to the new European Union Basic Safety Standards (EU-BSS), preparation of the National Radon Action Plan is obligatory for the Member States. One of the plan’s aims is to carry out an indoor radon survey to identify radon-prone areas. In the radon surveys, track detector methods are used. At the University of Pannonia (Veszprém, Hungary), a new scanner-based detector evaluation system has been developed. For the application of the new system, the selection of appropriate parameters is necessary. In this study, selection of the applied track detectors and setting of the etching conditions have been carried out. Two different types of allyl diglycol carbonate (ADC or CR-39) track detectors were investigated, taking into account the detector’s background and response during the exposure (determination of calibration factor). The Baryotrak’s background track density (0–1.5 tracks mm−2) was lower than that of the 0.8–4 tracks mm−2. The response of the Tastrak was higher, but the deviation of the calibration factor was much higher (1.2–5.3 × 10−3 tracks mm−2/(Bq day m−3)) than in the case of the Baryotrak (1.4–2.8 × 10−3 tracks mm−2/(Bq day m−3)). After the systematic review of the etching system, a new method was developed. For the determination of the optimal track diameter, the argon fluoride (ArF) laser was applied to create tracks with diameters in the range of 10–100 μm. The optimum track size was in the range of 40–60 μm. On this basis, new etching conditions were determined: 6.25 M NaOH solution, a temperature of 90°C, and time period of 8 hours.

Concentration and duration dependence of a new prototype polycarbonate/activated-carbon fabric individual and environmental radon monitor

The basic design of new multi-purpose polycarbonate track detector (PCTD)/activated-carbon-fabric (ACF) detector for radon monitoring in particular for parametric studies has been recently self-invented in our laboratory by enhancing PCTD's sensitivity by using the ACF adsorptive layer as external alpha radiator. The PCTD/ACF monitor basically consists of a PCTD; half bare and half covered with ACF in order to adsorb radon on its carbon active sites. From parameters studied, radon concentration and exposure duration were discovered to be the most important parameters affecting the monitor's sensitivity enhancement. An amplification factor (AF), defined as ratio of net PCTD/ACF to PCTD/bare alpha track density responses after background track density subtraction, was found being highly dependent on the two stated parameters. The AF versus duration response is flat with a value ∼1.5 constant up to 32 days in ∼180±20 Bq.m−3 radon environment and also flat with a constant value ∼7.5 up to 8 days in ∼25±1.1 kBq.m−3 radon environment after which the AF decreases down to ∼3.0 up to 32 days exposure. This phenomenon seems be due to full occupancy of radon atoms on the ACF active sites at high concentrations with an equilibrium between adsorption and desorption processes. The latter significantly dominates after 8 days in ∼25±1.1 kBq.m−3 environment leaving behind non-active sites requiring regeneration. The prototype monitor developed can be applied to normal level individual and/or environmental radon monitoring for an extended period of time but to high radon concentrations for short-term such as a week as a “grab sampler” with a high prospect for determination of radon equilibrium factor with its progeny in the radon environment under measurement. The actual radon monitor and its parametric studies are under further development.

Efficient alpha particle detection by CR-39 applying 50 Hz-HV electrochemical etching method

Alpha particles can be detected by CR-39 by applying either chemical etching (CE), electrochemical etching (ECE), or combined pre-etching and ECE usually through a multi-step HF-HV ECE process at temperatures much higher than room temperature. By applying pre-etching, characteristics responses of fast-neutron-induced recoil tracks in CR-39 by HF-HV ECE versus KOH normality (N) have shown two high-sensitivity peaks around 5–6 and 15–16 N and a large-diameter peak with a minimum sensitivity around 10–11 N at 25°C. On the other hand, 50 Hz-HV ECE method recently advanced in our laboratory detects alpha particles with high efficiency and broad registration energy range with small ECE tracks in polycarbonate (PC) detectors. By taking advantage of the CR-39 sensitivity to alpha particles, efficacy of 50 Hz-HV ECE method and CR-39 exotic responses under different KOH normalities, detection characteristics of 0.8 MeV alpha particle tracks were studied in 500 μm CR-39 for different fluences, ECE duration and KOH normality. Alpha registration efficiency increased as ECE duration increased to 90 ± 2% after 6–8 h beyond which plateaus are reached. Alpha track density versus fluence is linear up to 106 tracks cm−2. The efficiency and mean track diameter versus alpha fluence up to 106 alphas cm−2 decrease as the fluence increases. Background track density and minimum detection limit are linear functions of ECE duration and increase as normality increases. The CR-39 processed for the first time in this study by 50 Hz-HV ECE method proved to provide a simple, efficient and practical alpha detection method at room temperature.

BACKGROUND TRACK DENSITY REDUCTION OF 50-HZ-HV ECE-PROCESSED THICK POLYCARBONATE DETECTORS TO IMPROVE LOWER DETECTION LIMIT.

A recent novel development of 50-Hz-HV ECE of 1-mm-thick and 250-µm-thick polycarbonate track detectors (PCTDs) has proved some promising results for some health physics, dosimetry and ion-beam-related applications. The method while proved having some good characteristics for some applications provided a relatively higher background track density (BGTD) in particular when very high voltages are applied to the PCTDs. In order to decrease the minimum detection limit (MDL) of the PCTDs and to further promote its applications for low dose, the BGTD was reduced by applying a layer removal methodology applying ethylendiamine (EDA). The effects of EDA concentrations (50, 60, 65, 70, 75, 80, 85 and 90 %) in water at room temperature (26°C) and soaking durations up to 100 min at different EDA concentration on BGTD reduction were studied. The thickness of the layer removed from the surface of a PCTD highly depends on the soaking time and EDA concentration; it increases as the EDA concentration increases up to for example 700 µm after 2 h of soaking in the EDA solution. After ∼10 min of soaking duration at any of the above-stated concentrations, the BGTD reaches its minimum value, a value which differs from concentration to concentration. An EDA concentration of 85 % in water provided the lowest BGTD of 64.06 ± 3.12 tracks cm- 2; ∼6 times lower than that of its original value. It is shown that the layer removal process does not change the registration characteristics of the PCTD and its appearance significantly. The MDL of the PCTDs depends strongly on the BGTD. The MDL values for a desired confidence level were also studied by three calculation methods. The results of the BGTD and the MDL studies under different conditions applied are presented and discussed.

Combined effects of frequency and layer removal on background track characteristics of ECE polycarbonate detectors

Abstract Polycarbonate track detectors (PCTD) when electrochemically etched (ECE) provide excellent characteristics for registering relatively lower-LET charged particles (e.g. alphas, fast-neutron-induced recoils) for many health physics and ion detection applications.The layer removal method of PCTDs by ethylenediamine (EDA) developed in our laboratory reduces the background track (BGT) density significantly. The frequency of the applied electric field strongly affects the BGT density and diameter and thus affects the minimum detection limit (MDL). In order to study the combined effects of the frequency and layer removal on the BGT density and thus on the MDL, this research was conducted. The BGT density versus the layer thickness removed at frequencies up to 12 kHz decrease rapidly to about 10–20 µm above which they reach a minimum constant level, while the mean BGT diameter verses layer removed at all frequencies are constant with flat responses. On the other hand the BGT density and diameter versus frequency at different layers removed up to ~50 µm increase till 4 kHz above which they reach plateaus. The PCTDs with ~20 µm layer removal at frequencies up 1 to 2 kHz showed the lowest MDL. The results are presented and discussed.

Broadening alpha registration energy range of 250 μm polycarbonate detectors by a single 50 Hz–HV ECE method

Our recent studies on efficient detection of nitrogen and helium ions in a plasma focus device and alpha particles in 1 mm thick polycarbonate track detectors (PCTD) by 50 Hz–HV ECE method in PEW (potassium hydroxide, ethanol, water mixture) solution at 26 °C for 10 h proved having promising results. In order to further study the characteristics of the method and extend the applications, studies were performed by using 250 μm PCTDs by 50 Hz–HV method instead of commonly applied 2 kHz–800 V or similar conditions. Alpha detection efficiency, mean track diameter, detection energy range, lower and upper registration energy thresholds and track shapes as well as background track density and diameters were studied by varying field conditions, ECE durations and alpha energies. The efficiency, meantrack diameter and background in general increased as high voltage and ECE duration increased. The optimum ECE conditions were determined to be 50 Hz–2kV in PEW solution at 26 °C for 2–3 h. The 50Hz–2kV method provides some unique characteristics compared to 2 kHz–800 V in terms of having a higher efficiency in a broader efficient registration energy range ∼300 keV to ∼4.5 MeV, smaller track diameters and thus capability to detect higher fluencies of charged particle at the cost of a relatively higher background. Further research is in progress on alpha registration studies.

A new method for detection of alpha particles in 1 mm thick polycarbonate detectors using 50 Hz – HV ECE method

Recent studies in our laboratory have proved that electrochemical etching (ECE) of polycarbonate track detectors (PCTD) under 50 Hz – high voltage (HV) field conditions has potentials for time-integrated heavy charged particle detection and dosimetry applications. The rationale in the study is the ECE process of alpha particle tracks in 1 mm thick PCTDs by a 50 Hz – HV generator at optimized ECE conditions. Tracks of 3.2 MeV alpha particles from a collimated beam of an 241Am source degraded in air and background tracks were registered. The effects of HV and ECE duration on alpha track registration efficiency and track diameters were studied for 3 sets of 50 Hz – 4, 5 and 6 kV field conditions in a PEW solution (potassium hydroxide, ethanol and water) at 26 °C. The optimized ECE conditions obtained at this stage of development for 1 mm thick PCTDs are 50 Hz – 4 kV in PEW solution at 26 °C for 10 h. Alpha track registration efficiency at 3.2 MeV is about 30% with 37 ± 6 μm mean track diameter. The mean background track density at the above stated optimized conditions is about 571 ± 16 tracks.cm−2 with a mean diameter of 65 ± 5 μm. All tracks are observable by the unaided eyes. The mean background track diameter is near two times larger than that of alpha particle tracks at the optimum conditions applied; they are easily distinguished against each other. This high background track density while at this stage of development seems a drawback for low dose and low fluence particle applications, it has minimal effects on high fluence ion detection applications. The simple 50 Hz – HV generator used proved to be convenient for efficient alpha track amplifications. Studies are underway for improvement of the method in particular for reducing background track density.

Effect of Thermal Annealing on the Sensitivity of Makrofol-DE Polycarbonate

Solid state nuclear track detectors have been applied in di erent studies including nuclear physics, particle physics, archaeology etc. Makrofol, a particular group of thermoplastic polymers, is used for radon detection. High background track numbers in the foils cause big uncertainties for places where the radon concentrations are quite low. Therefore, it is of great importance to lower the background track signal as much as possible, without reducing the sensitivity of the foils. The present study reports track density and sensitivity of Makrofol-DE after thermal annealing. The unexposed foils were heated at temperatures ranging from 110 ◦C to 140 ◦C for 10 min with the aim of observing changes in the number of background tracks. As a result a reduction in the background track density was observed. In order to observe the change in the sensitivity, pre-heated and unheated foils were exposed to alpha radiation in a radon chamber. At the temperatures of 110, 120, and 130 ◦C, the track density in the heated and unheated exposed foils is quite similar. An increase was observed in the track density in heated exposed foils at 140 ◦C.

Practical consequences for the use of a personal dosemeter for fast neutrons at high-energy accelerators based on PADC detectors exposed up to one year

Abstract Since 1998, Paul Scherrer Institut has employed the routine use of a personal neutron dosimetry system based on chemically etched PADC (poly allyl diglycol carbonate) detectors and automatic track counting. In 2004, a new concept for individual monitoring at high-energy accelerators was implemented. In this concept the photon dosemeter of a combined photon/neutron dosemeter is evaluated monthly. The neutron dosemeter is only evaluated if the personal photon dose exceeds 2 mSv, or if the exposition period of the neutron dosemeter exceeds one year. Significant consequences for the evaluation of the neutron dosemeter in the dosimetry service were evidenced in the new concept. The wearing period of the neutron dosemeter can range from 1 to 12 months, potentially longer. Therefore, the long-term behavior of background track density and variation of response to Am–Be within 17 months was studied. The effects of “fading” and “aging” that influence the response of PADC detectors are determined. The application of an algorithm for neutron dose calculation takes into account long-term behavior and is described. Furthermore, repeated field calibrations were performed at the CERN-EU high-energy reference field (CERF) facility.

Progress report of the CR-39 neutron personal monitoring service at PSI

At the Paul Scherrer Institute a personal neutron dosimetry system based on chemically etched CR-39 detectors and automatic track counting is in routine use since the beginning of 1998. The quality of the CR-39 detectors has always been a crucial aspect to maintain a trustable personal neutron dosimetry system. This paper summarises the 7 y experience in routine use. The effect of detector material defects which could lead to false positive neutron doses is described. The potentiality of improving the background statistics by extending the pre-etch time is investigated and involves as a drawback a quite lower sensitivity to thermal neutrons. Furthermore, the impact of small changes in the production process of the detectors on the response to fast and thermal neutrons is shown. For the personal dosimetry at CERN, a new dosimetry concept was launched by combining a CR-39 neutron dosemeter with a Direct-Ion Storage (DIS) dosemeter for photon and beta radiation. The usage period of the CR-39 dosemeters is prolonged now from 3 months up to 12 months. In this context, the long-term behaviour over 1 y of the background track density and the response to Am-Be are described.

CR-39 acceptance test and optimisation for fast neutron dosimetry applications.

The ENEA fast neutron dosemeter is based on a planar PADC (poly allyl diglycol carbonate) placed in a polyethylene holder. The CR-39 (registered trademark of PPG Industries Inc.) material, produced by Intercast Europe S.p.A., has been used in the routine of the Individual Monitoring Service (IMS) since 1998. Since then, acceptance tests on average sheet background track density and sheet neutron sensitivity have been made on new batches as a quality control within a quality assurance programme of the IMS of ENEA-Institute for Radiation Protection (IRP). Dosemeters were irradiated with a 241Am-Be source at ENEA-IRP and processed through a chemical etching procedure (pre-etching with 40% KOH water solution 6.25 N and 60% ethyl alcohol at 70 degrees C followed by 12 h of etching in 6.25 N KOH water solution). In this paper we present the analysis of acceptance testing data for more than 30 sheets of CR-39 plastic produced in 1998, 1999 and 2000. Moreover, we compare the performance of sheets of CR-39 of standard composition with that of sheets of CR-39 with the addition of DOP (dioctylphthalate), in different concentrations, on the hasis of average background density, neutron sensitivity and background fluctuation that limit the lower detectable dose. This study demonstrates the need for acceptance tests to assure the quality of the dosimetric performance of these dosemeters, which is considerably dependent on the quality of the CR-39 plastic.

Spin coating technique for the preparation of thin nitrocellulose films for solid state nuclear track detection

Thin and uniform nitrocellulose (CN) films were prepared over a supporting polyester film by the spin coating technique. The paper describes the first systematic approach to the application of the spin coating technique for the preparation of thin nitrocellulose films of required thickness and discusses the effects of solvent, concentration and viscosity of the nitrocellulose solution on the characteristics of the film. The method was used to prepare composite nitrocellulose-polyester films which can be used as solid state nuclear track detectors (SSNTDs). The etching conditions were optimized and the films were found to be comparable with the commercially available LR 115 Type II films with respect to background track density and the alpha track registration efficiency.

Ten Years On: The NRPB PADC Neutron Personal Monitoring Service

Development of the NRPB PADC neutron dosimetry system was carried out during the years 1982 to 1985. In 1985 the monitoring service was set up, and following 9 months of operational trials, was given approval by the UK Health and Safety Executive in April 1986. This paper traces the developments, and outlines the present situation. At an early stage it was judged that the most critical component of the system was likely to be the reliable supply of plastic of suitable quality. However, the 'quality' of plastic is very dependent on the processing and read procedures chosen. A commercial manufacturer of PADC was selected, and a two stage etch (chemical, electrochemical) plus a chemical post-etch, was developed which gave adequate dosemeter response characteristics, combined with a low background track density. Rigorous quality control procedures needed to be applied to the sheets of PADC in order to satisfy an operational criterion of a minimum detectable dose of 200 µSv. The choice of reader was made largely on the basis of known reliability. Procedures were kept as simple as possible with the further aim of maximising commonality with other NRPB monitoring systems. The intention was to optimise the system from an operational point of view. This meant, for example, the choice of etch parameters which did not necessarily give the greatest values of sensitivity, but gave values which were less dependent on small changes in plastic properties or etch parameters.

Measurement methodology for indoor radon using passive track detectors

A passive radon monitoring methodology based on CR-39 polycarbonate, alpha trace detectors (ATDs), is described for use in radon research and routine indoor measurements in Mexico. The monitoring device consists of a small exposure cup that contains the CR-39 chip. Unexposed ATDs are given an etching pretreatment which reduces the subsequent background track density and improves reproducibility. After exposure is complete, the ATD is sequentially etched, chemically and then electrochemically, for easy visualization and counting of etched tracks. At a radon activity concentration of 70 Bq/m 3 the accumulation rate is 10 tracks/cm 2-day; an integration time of 28 days produces a signal-to-noise ratio of 10. The background over that time is 30 tracks/cm 2. A simple calibration chamber is described. The data are intercompared with a calibrated dynamic detector system. Measurement of radon exposure inside dwellings gains added significance in Mexico City because of the increased potential for lung cancer caused by the combined effects of radon, ozone and extremely heavy, chemical air pollution.

Improvements in Passive Etched Track Radon Detectors

Although etched track radon detectors are generally reliable, there is scope for improvement in their performance. In order to ensure that the detecting elements are rapidly and unambiguously identified with the place of measurement, the elements may have bar codes engraved on them and be placed in transparent holders to allow the code to be read when the element is in the holder. To allow for variations in background track density across a sheet of detector material, an area on each element may be covered to prevent exposure to radon, allowing a measurement of the background on each element. Changes in the sensitivity may be detected by irradiating a small area of each element with alpha particles from a standard source. Although etched track detectors have a very wide dynamic range, the overall range of radon levels in homes is even wider. This problem may be overcome by using a holder with two detection chambers with different sensitivities to radon.

A Comparison of the Neutron Response of CR-39 Made by Different Manufacturers

CR-39 obtained from American Acrylics and Plastics, Inc. (A.A.), N.E. Technology, Ltd. (N.E.), and Tech/Ops Landauer, Inc. (LT) were exposed to radioisotopic neutron source at SLAC, and moderated {sup 252}Cf at ORNL. The A.A. and N.E. detectors (0.06 cm thick) were electrochemically etched (a pre-etch for 1 hour and 45 minutes in 6. 5 N KOH at 60{degree}C, a 5 hour etch at 3000 V and 60 Hz, a 23 minute blow-up step at 2 kHz and post-etch for 15 minutes). The LT detectors were chemically etched in 5.5 N NaOH at 70{degree}C for 15.5 hours and some A.A., N.E. and LT detectors in 6.25 NaOH at 70{degree}C for 6 hours. A pre-etch step in 60% methanol and 40% NaOH at 70{degree}C for 1 hour was added for some N.E. detectors. The results of the background track density and neutron dose equivalent response are reported in this paper.

Indigenous Development of a Track Etch Detector

Solid state nuclear track detectors (SSNTDs) have been recognised by IAEA as a standard method for estimation of radon, thoron and their daughter products in the environment. The detectors that are commonly used in environmental monitoring are generally made from cellulose nitrate (LR-115) and polycarbonates (CR-39). In view of the non-availability of these detectors in India, need was felt to develop them indigenously. Accordingly, an attempt has been made to develop cellulose nitrate films for their use in SSNTD. Cellulose nitrate with a particular nitrogen content was used for preparing these films by a cast method. This films were annealed, evaluated and then compared with imported films. The background track density and alpha track density after exposure to 150 nCi of /sup 241/Am source at 2.5 cm distance were found to be comparable with those of imported films.

The two-step electrochemical etching technique applied for polycarbonate track etched detectors

The two-step ECE technique was optimized by varying the electrical field strength and applied for Makrofol DE for neutron dosimetry and radon monitoring using an electric field strength of 26.7 kV⋯cm −1 . In comparison with the previously applied combination of chemical and electrochemical etching, the neutron response was increased above a threshold of about 1.5 MeV by a factor of 3 to a value of 52 tracks⋯cm −1 ⋯mSv −1 . The background track density and its standard deviation of (6±2) cm −2 allows to indicate already 0.1 mSv. The alpha energy range was extended from an alpha window of about 1.5 MeV to an alpha energy range from 0.5 to 4 MeV.

Neutron sensitivity of CR-39 SSNTD

Abstract The SSNTD material CR-39 is successful used in different fields of research on account of its low registration threshold, good optical and mechanical parameters. It is produced by several firms in various quality. For application of CR-39 in neutron induced autoradiography a good lateral homogeneity and low neutron induced background track density are necessary. In this paper are presented results of testing different kinds of CR-39 and their properties for neutron registration.