A NEW GENERATION OF PASSIVE RADON MONITORS: THE FILM-BADGES FOR OCCUPATIONAL EXPOSURES.

The passive radon monitors are typically formed by a radon-diffusion chamber, enclosing a track detector or an electret. Recently, new passive Rn monitors have been developed, which are similar to the neutron film badges. These badges present unique characteristics for the assessment of the occupational exposure to radon, such as compactness, fast-time response and any desired response sensitivity. Finally, these badges make it very easy to stop and to start the measurements, as required for assessment of the occupational exposures, which operations are very difficult, if not impossible, to achieve with existing monitors. These radon badges are based on the radon sorption by solid materials, namely plastics for radon absorption and activated carbon cloths for radon adsorption. Plastics may have a rubber-like (e.g. silicone) or a glass-like morphology (e.g. polycarbonate). The most interesting materials for these applications are the glass-like solids, the properties of which are expected to be little dependent on temperature. If radon badges with a large response sensitivity are desired, then adsorptive radiators formed by thin layers of activated carbon tissues are used. The key strategy, adopted for the radon film badges, was to use radiators into which radon diffuses rapidly, in order to ensure a fast-time response for radon monitoring. All the Rn film-badges, listed above, are formed by the same very compact device and may have response sensitivities, which may differ by several orders of magnitude.

AN INTERNATIONAL COOPERATION BY USING AN ALL-ENCOMPASSING PASSIVE RADON MONITOR.

The recently developed radon film-badge makes it possible to measure radon indoors, in soil, in water and/or in aqueous media (e.g. mud). As a result of its wide response linearity, this monitor has been successfully used to measure radon in-water with concentrations from 10 to ~10 000 Bq/L. By exploiting the unique characteristics of this badge, a mini-survey has been carried out by Health Canada in which radon in water was measured from 12 private wells, as well as in tap water originating from the Ottawa River. Due to the widespread interest of different laboratories in using these passive monitors, laboratories were provided with plastic films to construct their own badges by using in-house CR-39 detectors. Monitors were then irradiated by a known radon concentration at the National Institute of Radiation Metrology (ENEA)'s radon chamber and sent back to each laboratory for processing and counting. Even though these laboratories have been using different etching- and counting-procedures, the film-badge responses varied only within ~12%.

Surface-deposition and distribution of the radon-decay products indoors.

The exposure to radon-decay products is of great concern both in dwellings and workplaces. The model to estimate the lung dose refers to the deposition mechanisms and particle sizes. Unfortunately, most of the dose data available are based on the measurement of radon concentration and the concentration of radon decay products. These combined measurements are widely used in spite of the fact that accurate dose assessments require information on the particle deposition mechanisms and the spatial distribution of radon decay products indoors. Most of the airborne particles and/or radon decay products are deposited onto indoor surfaces, which deposition makes the radon decay products unavailable for inhalation. These deposition processes, if properly known, could be successfully exploited to reduce the exposure to radon decay products. In spite of the importance of the surface deposition of the radon decay products, both for the correct evaluation of the dose and for reducing the exposure; little or no efforts have been made to investigate these deposition processes. Recently, two parallel investigations have been carried out in Rome and at Universidad Nacional Autónoma de México (UNAM) in Mexico City respectively, which address the issue of the surface-deposited radon decay products. Even though these investigations have been carried independently, they complement one another. It is with these considerations in mind that it was decided to report both investigations in the same paper.

Mimicking lung dose by wire-mesh-capped deposition sensors: a new dosimetric strategy of radon (thoron) decay products.

The dose conversion factor (DCF) of radon decay products may vary by a factor of ∼40 within the particle size range from ∼0.5 nm to tens of micrometres. An ideal detector should have a response, which closely mimics the strong dependence of the DCF on the particle size. This dependence is essentially determined by the different deposition rates of the particles with different sizes on the trachea-bronchial tree and alveoli. These deposition rates versus the particle sizes are similar to those of the decay products onto indoor surfaces. These conclusions are conducive to a new strategy for the dosimetry of radon (thoron) decay products, which is simply based on the detection of decay products deposited on flat surfaces. The dependence of the deposition rate of radon decay products onto flat surfaces versus the particle size is necessarily different from that of the deposition rate on the trachea-bronchial region, especially for particle sizes smaller than a few nanometres and larger than a few micrometres. In the present work, in order to obtain a better mimic between the measurement of flat-surface-deposited radon (thoron) decay products and the DCF at any given particle size, a suitable screen is placed against the surrogate surface, used for the assessment of the radon (thoron) decay products deposition.

Four passive sampling elements (quatrefoil)--II. Film badges for monitoring radon and its progeny.

The four passive samplers (quatrefoil) already described in a parallel paper, make it possible to obtain thin radiation sources, useful for alpha and beta counting by any passive and real-time detector. In the present paper, the applications of this quatrefoil for measuring radon gas by etch-track detectors will be described. In the case of radon measurements, different solids have been identified, with radon-sorption partition coefficients related to air from 1 to 2000. Uniquely compact radon badges can be obtained by using a layer of these solids facing an alpha track-etch detector. These radon badges make it possible to overcome most of the shortcomings of existing passive monitors. Moreover, these badges show promise for studying the radon solubility of polymer films.

Four passive sampling elements (quatrefoil)--I. Monitoring radon and its progeny by surface-contamination monitors.

Four passive sampling elements (quatrefoil) have been recently developed, which transform airborne radionuclides into surface-bound radionuclides. These samplers, once exposed, result in thin radiation sources that can be detected by any real-time or passive detector. In particular, by using a large collecting-area sampler with a low surface density (g cm(-2)), it is possible to measure radon and its decay products by beta surface-contamination monitors, which are rarely used for these applications. The results obtained to date prove that it is finally possible to carry out the measurements of radon (and its decay products) indoors, in soil and in water simply by a Pancake Geiger-Muller counter. Emphasis will be given to those measurements, which are difficult, if not impossible, to carry out with existing technologies.

The measurements of thoron, radon and their decay products thanks to Pinocchio, Tengu and Trolls.

In the present paper, the long noses of Pinocchio, Tengu and Trolls are used to measure, respectively, radon, thoron, and their decay products both by track-etch detectors and by Geiger-Müller (G-M) counters. Just recently, four new passive samplers (termed quatrefoil) have been developed which greatly simplify the detection of all airborne radionuclides by using either passive or real-time detectors. In particular, surface-deposited radon (thoron) decay products are sampled by films with large area and small surface density (0.1-1 mg cm(-2)). Once exposed, these films are stacked together for their detection by a pancake G-M counter. For the measurements of radon and thoron in soil, 25-cm-long tubes with sampling films along their internal surfaces can be successfully used. Once exposed, these films can be counted by a pancake G-M for the selective measurement of radon and thoron.

Thoron: its metrology, health effects and implications for radon epidemiology: a summary of roundtable discussions.

A roundtable discussion was made at the end of the workshop. All the presentations were summarised in this discussion. It involved measurement techniques, quality assurance and dose assessment and health effects of thoron and its progeny. In particular, major epidemiological studies may be affected by thoron interference in radon measurements. Since their data are not sufficient when compared with that of radon, further efforts in thoron studies will be needed.

Advanced passive detectors for neutron dosimetry and spectrometry.

Different neutron detectors have been developed in the past which exploit electrical and electrochemical processes in plastic foils and thin-film capacitors (namely metal-oxide-silicon devices) to trigger avalanche processes, which greatly facilitate the detection of neutron-induced charged particles. These detectors are: (i) spark-replica counter of neutron-induced fission-fragment holes in plastic films, thin-film breakdown counter of neutron-induced fission fragments, and electrochemically etched detectors of neutron-induced recoils in plastic foils. The major shortcomings of damage-track detectors for the measurement of low neutron fluencies, such as those of cosmic ray neutrons at civil aviation altitudes, are their large and unpredictable background and their small signal-to-noise ratio. These shortcomings have been overcome respectively by using long exposure times and large detector areas and counting coincidence-track events on matched pairs of detectors even for a few-micron-long tracks such as those of neutron recoils. The responses of all these detectors have been analysed both with neutrons with energy up to approximately 200 MeV and protons up to tens of gigaelectron volts. Applications of these detectors for the cosmic ray neutron dosimetry and/or spectrometry will be mentioned.

Hardness ratios of different neutron spectra.

Extensive data have been gathered in the past on the response of different detectors, based on the registration of neutron-induced fissions in bismuth, gold, tantalum and thorium by the spark-replica counter and the thin film breakdown counter. These detectors make it possible to exploit the excellent characteristics of the fission reactions for the measurements of high-energy neutrons. Most of the investigations have been carried out at the quasi-monoenergetic neutron beam facility at The Svedberg Laboratory-TSL of the Uppsala University in cooperation with the Khlopin Radium Institute (KRI). The responses of different fission detectors in the neutron energy range 35-180 MeV have been evaluated: a region where the predictive power of available nuclear reaction models and codes is not reliable yet. For neutron energy >200 MeV, the fission-detector responses have been derived from the data of the proton fission cross sections. By using the ratio of the responses of these detectors, a simple and accurate way to evaluate the spectrum hardness can be obtained, thus providing a tool to obtain spectral information needed for neutron dosimetry without the need to know the entire spectrum. Extensive data have been already obtained for the high-energy neutron spectrum from the CERN concrete facility. In the present paper, the measured values of the response ratios for different fissile detectors exposed at the CERN facility are compared with those calculated for the spectra from the same facility and from different altitudes in the atmosphere, respectively.

Exposure of aircraft crew to cosmic radiation: on-board intercomparison of various dosemeters.

Owing to their professional activity, flight crews may receive a dose of some millisieverts within a year; airline passengers may also be concerned. The effective dose is to be estimated using various experimental and calculation tools. The European project DOSMAX (Dosimetry of Aircrew Exposure during Solar Maximum) was initiated in 2000 extending to 2004 to complete studies over the current solar cycle during the solar maximum phase. To compare various dosemeters in real conditions simultaneously in the same radiation field, an intercomparison was organised aboard a Paris-Tokyo round-trip flight. Both passive and active detectors were used. Good agreement was observed for instruments determining the different components of the radiation field; the mean ambient dose equivalent for the round trip was 129 +/- 10 microSv. The agreement of values obtained for the total dose obtained by measurements and by calculations is very satisfying.

Detectors/Dosemeters of galactic and solar cosmic rays.

Different passive multidetector stacks have been developed at the Italian National Agency for Environmental Protection (ANPA-stack), which makes it possible to measure directly ionising radiations, low-energy and high-energy neutrons, and high-energy charged (HZE) particles. The stack consists of several types of passive devices, namely recoil-track and fission-track detectors, bubble detectors, thermoluminescence dosemeters and an electronic personal dosemeter. Most of these detectors have been used on earth for the assessment of the occupational exposure, or in outer space for cosmic ray physics and/or for the assessment of the dose received by astronauts. A great deal of efforts and new developments have been required to make these detectors useful for in-flight measurements. As outcome of these extensive efforts, different new detectors have been developed, which exploit some of the most successful principles of radiation detection, such as the use of avalanche processes to facilitate the registration of nuclear tracks and the use of coincidence-counting to increase the signal-to-noise ratio. On the basis of these new detectors, different systems (generally referred to as ANPA-stack) have been obtained, which have been successfully applied for a variety of different measurements of cosmic ray radiation fields and doses.

A passive radon dosemeter suitable for workplaces.

The results obtained in different international intercomparisons on passive radon monitors have been analysed with the aim of identifying a suitable radon monitoring device for workplaces. From this analysis, the passive radon device, first developed for personal dosimetry in mines by the National Radiation Protection Board, UK (NRPB), has shown the most suitable set of characteristics. This radon monitor consists of a diffusion chamber, made of conductive plastic with less than 2 cm height, containing a CR-39 film (Columbia Resin 1939), as track detector. Radon detectors in workplaces may be exposed only during the working hours, thus requiring the storage of the detectors in low-radon zones when not exposed. This paper describes how this problem can be solved. Since track detectors are also efficient neutron dosemeters, care should be taken when radon monitors are used in workplaces, where they may he exposed to neutrons, such as on high altitude mountains, in the surroundings of high energy X ray facilities (where neutrons are produced by (gamma, n) reactions) or around high energy particle accelerators. To this end, the response of these passive radon monitors to high energy neutron fields has been investigated.

Recent studies on the exposure of aircrew to cosmic and solar radiation.

Investigations of the impact of cosmic and solar radiation on aircrew involve many challenges. The great variety of primary and secondary ionising and non-ionising radiation, the wide range of energies involved and the role played by the Earth's atmosphere and magnetic field and the Sun combine to produce a very complicated scenario. These factors are reflected in conditions on aviation routes where exposure to radiation varies with altitude, latitude and stage of solar cycle. The great increase in air travel and consequent rise in numbers of aircrew whose occupation requires them to work in this environment has prompted new concern about exposure risks at aviation altitudes. The situation has also been highlighted by the tendency for aircraft to fly at higher altitudes in recent years and by the 1990 recommendations of the ICRP that exposure of civil aircrew be considered as being occupational. These have recently been translated into a legal requirement in the European Union. Several studies have been completed using a very wide range of detectors on subsonic and supersonic routes and new investigations are underway. With the completion of the DOSMAX project in another three years or so. world data for a whole solar cycle will be more complete than ever before. Results indicate that for most routes investigated during solar minimum, aircrew are unlikely to receive doses in excess of 6 mSv.y(-1).

The Italian national survey of aircrew exposure: I. Characterisation of advanced instrumentation.

The survey of aircrew exposure required the characterisation and/or the development of a diverse array of both passive and active instruments, which were not available at any one laboratory. To ensure the availability of the most advanced dosimetric systems and the relevant calibration facilities needed for the survey, an international collaboration was formed, which was facilitated by multinational research contracts promoted by the Commission of the European Communities. Close cooperation among scientists with long-term experience in different disciplines, such as cosmic ray measurement in space and radiation protection dosimetry, made it possible to exploit successfully damage track detectors for the accurate evaluation of very low fluences of particles with high energy and high charges. For a long time, the major concern for the assessment of aircrew exposure has been the accurate evaluation of the cosmic ray neutron dose. In this paper, four different dosimetric systems are considered, the response of which is sensitive to both low and high energy neutrons. All these dosimetric systems have provided consistent results when exposed together to the high energy beam facility at CERN, which is considered to approximate the cosmic ray field.

The Italian national survey of aircrew exposure: II. On-board measurements and results.

The Italian survey of aircrew exposure has been carried out with different advanced dosimetric systems, as described in part I of this paper. The key strategy of the survey was to obtain on-board comparison of measurements for both passive and real-time detectors flown together with passengers. The survey has been carried out in the period of solar minimum (1995-1997), in which the exposure to galactic cosmic rays reaches its maximum value. Even though carrying out the survey in the period of solar minimum was entirely coincidental, this circumstance has been used to good advantage to obtain a comprehensive set of data of the galactic cosmic radiation with little or no disturbance by the solar activity modulation. This comprehensive set of data covers flight routes between -20 degrees and 75 degrees geographic latitude at different civil aviation altitudes. The survey obtained with different advanced dosimetric systems has been supplemented by a large variety of data gathered with passive stacks on different short-range and long-range flights at supersonic and subsonic altitudes. Some of the most important conclusions which can be drawn from the survey are: (i) aircrew of civil aviation receive annual doses within the range of 1 mSv to 6 mSv; (ii) data from different periods of solar minimum agree well, since the discrepancies encountered seem mainly due to the different dosimetric systems used; (iii) repeated measurements on the same route are highly consistent.

Damage track detectors in radioprotection dosimetry: a novel approach.

Limited sensitivity and unpredictable background are the major drawbacks of damage track detectors in the assessment of low neutron doses and low concentrations of alpha emitters in biological and environmental samples. The simplest way to increase the sensitivity of the damage track detectors is to increase both the exposure time and the detector area. However, the strong variability of the background may make this task often impossible. This background problem has been finally solved by a new registration method based on counting coincidence spots in geometrically matched pair of detectors. By using spark counting and electrochemical etching, both of which produce spots visible at low magnification, coincidences induced in two matched detector-surfaces by a few-microns-long tracks can be easily seen. This novel counting approach can be considered just the converse of those used in the past with Bi-fission detectors and cosmic ray stacks.

Cosmic rays and dosimetry at aviation altitudes.

Recent concerns regarding the effects of the cosmic radiation field at aircraft altitudes on aircrew have resulted in a renewed interest in detailed measurements of the neutral and charged particle components in the atmosphere. CR-39 nuclear track detectors have been employed on a number of subsonic and supersonic aircraft to measure charge spectra and LET spectra at aircraft altitudes. These detectors are ideal for long term exposures required for these studies and their passive nature makes them suitable for an environment where interference with flight instrumentation could be a problem. We report here on measurements and analysis of short range tracks which were produced by high LET particles generated mainly by neutron interactions at aviation altitudes. In order to test the overall validity of the technique measurements were also carried out at the CERN-CEC field which simulates the radiation field at aviation altitudes and good agreement was found with dose values obtained using mainly heavy ion calibration.

In-flight measurements of radiation fields and doses.

The cosmic ray field at civil aviation altitudes consists of low-ionising radiations and neutrons with different energies. At the meeting on radiation exposure of civil air-crew held in Luxembourg in 1991, it was pointed out that there was a need to obtain a better evaluation of the dose with special regard to that of high energy neutrons. This problem has been tackled within the multinational research programmes of the Commission of the European Communities. These research activities made it possible to set up and calibrate the most advanced systems for the assessment of the occupational exposure of aircraft crew. In particular, two advanced neutron spectrometers have been developed, which cover the entire energy range of neutrons. Under the CEC contracts, a large variety of data has been gathered on many longhaul flights, using different dosimetric systems. Since most of the data have been gathered during the period of solar minimum (1994-1997) they represent an upper limit of the dose due to galactic cosmic rays.