Track-etched detector

Several papers present reviews of measurement methods or improvements in existing methods. Yamashita et al. (1987) present the description of a portable liquid scintillation system that can be used for thoron (Rn-220) as well as radon (Rn-222) in water samples. Thoron measurements have not been made for houses where radon in water may be a significant source. Such an instrument could be useful in making such determinations as well as in studying geochemical problems as described in this report. A review of measurement methods by Shimo et al. (1987) and of development and calibration of track-etch detectors (Yonehara et al., 1987) are also included. Samuelsson... 

In 47 houses, Rn-222 measurements were performed by Track Etch detectors from Terradex Inc. during two periods, November-May and June-October. From these measurements, the normal seasonal variations were assessed. In Fig. 2 a plot of the cumulative distribution of mean Rn-222 concentrations during summer and winter are shewn. From this figure, the mean concentration during sunnier is about 50 % of the mean concentration during the winter months. 

M kel inen, I., Experiences with track etch detectors. Proc. of the 13th International Conference on Solid State Nuclear Track Detectors, Rome (September 1985) (to be published). 

Urban, M. and Piesch, E., Low Level Environmental Radon Dosimetry with a Passive Track Etch Detector Device, Rad. Prot. Dos. 1 97-109 (1981). 

Measurements were made using two types of passive track-etch alpha dosimeters. One of them was the bare detector of CR-39. After exposure these dosimeters were etched by 30 % NaOH at 70°C for 5 hours. The number of pits was scored under a microscope with a television camera in Shiga University of Medical Science. Methods of calibration and adjustment for deposition of radon daughters introduced by Yonehara (Yonehara et al., 1986) were adopted. The second detectors were Terradex type SF (Alter and Price, 1972). These detectors consist of a plastic cup, covered by a filter to allow entry only of gases, with a track-etch detector inside. The reading of results was carried out by Terradex Corp. in Walnut Creek, California, U.S.A.. The measurements of radon concentration were carried out by both methods in each location, except for Hokkaido where the measurements were done only by Terradex. However, the data obtained by CR-39 detectors will be mainly presented in this paper, because the two methods did not give identical results as separately reported in this proceedings by Yonehara et al. (Yonehara et al., 1986). 

In order to assess the accuracy of the present method, we compared it with two other methods. One was the Track Etch detector manufactured by the Terradex Corp. (type SF). Simultaneous measurements with our detectors and the Terradex detectors in 207 locations were made over 10 months. The correlation coefficient between radon concentrations derived from these methods was 0.875, but the mean value by the Terradex method was about twice that by our detectors. The other method used was the passive integrated detector using activated charcoal which is in a canister (Iwata, 1986). After 24 hour exposure, the amount of radon absorbed in the charcoal was measured with Nal (Tl) scintillation counter. The method was calibrated with the grab sampling method using activated charcoal in the coolant and cross-calibrated with other methods. Measurements for comparison with the bare track detector were made in 57 indoor locations. The correlation coefficient between the results by the two methods was 0.323. In the case of comparisons in five locations where frequent measurements with the charcoal method were made or where the radon concentration was approximately constant, the correlation coefficient was 0.996 and mean value by the charcoal method was higher by only 12% than that by the present method. 

Urban, M. Piesch, E. (1981) Low level environmental radon dosimetry with a passive track etch detector device. Radiation Protection Dosimetry, 1, 97-109. 

There are various kinds of plastic material suitable for use as track-etch detector, of which LR 115 (cellulose nitrate), MAKROFOL (polycarbonate) and CR-39 (allyl-diglycol poly carbonate) are the most popular. These materials are described by Fantini and Richard (1981) and Cartwright et al. (1978). 

All decay products, with their relatively short half-lives resulting in correspondingly high specific radioactivity (i.e., high numbers of decays per second and mass unit), are best identified and measured by alpha-and gamma-spectrometry (IAEA 2000). Very low time-averaged concentrations of airborne Rn and its short-lived decay products can be determined using track etch detectors (Urban and Piesch 1981). 

Large-scale surveys for the assessment of the population exposure require a multitude of different detectors to be deployed in the primary living spaces (namely bedrooms) for year-long integrated measurements in several thousands dwellings. The most suitable detectors for these large-scale surveys are track-etch detectors. In contrast, short-term measurements are appropriate for screening surveys. 

In addition to NAA, neutrons are widely used in prompt radiation analysis for the determination of concentration and spatial distribution of elements in different matrices. For example, a track-etched detector (LR-115, Makrofol KG, CR 39, mica) placed on the polished surface of a sample is irradiated with fast or thermal neutrons then etched with a suitable chemical to deduce the concentration profiles from the track distributions. This method can also be used for the detection of suspended and dissolved U, Th, and Pu in water by (n,f) reactions N in polymers by the N(n,p) C reaction B and Li in semiconductors or glasses by the B(n,a) Li and Li(n,ot) H reactions, respectively. For the detection of fission fragments the use of mica is recommended. 

Two types of track-etch monitor occur, open and closed types. In the open type, the SSNTD is not contained in a volume and is exposed to the air as a bare foil. This detector will register the alpha radiation from the Rn and RnD in the air, and the track density on the foil represents the sum of these activities. However, the Rn signal will be much larger than the signal from the RnD, except at very high levels of RnD (high F factor), and the track density has to be interpreted in terms of this ratio, which is typically unknown. In close monitors the SSNTD is enclosed in a closed container into which Rn diffuses through a filter. This prevents the entry of RnD and dust particles into the chamber, and the foil is then sensitive only to the alpha radiation from Rn and RnD formed in the container. There is a repeatable equilibrium between the isotopes in the container, and calibration provides the relationship between the Rn concentration and the track density on the foil. A typical track-etch radon monitor of the closed type is shown in Fig. 9.27. 

FIGURE 17.28 Home radon detectors Long-term track etch (leftj and short-term charcoal canister (rightj. 

Some other detectors in common use either measure radiation in terms of exposure or by tracks. Among the former are ion chambers, dosimeters of various kinds, and photographic film. Tracks can be observed in other types of photographic film, cloud chambers, and track-etch films. 

In alpha-track detectors the carbonate etch method was found to be 20 times more sensitive than the nitrate track etch. The instruments that measured instantaneous radon did not correlate well with the pos.sibly more reliable methods based on a 30-day sample time. All the radon-measuring techniques correlated poorly with the radiometric equivalent uranium in the soil, which led to the conclusion that the radon was coming from below the surface. 

The mean radon concentrations determined by the passive detectors are based on calibrations using NBS standard radium-226 solutions and also from participation in the OECD (Nuclear Energy Agency)/CEC radon dosimeter intercomparisons (Commission of the European Communities, 1986) held at the U.K. National Radiological Protection Board (NRPB). Recent calibrations of the new LR-115 based detectors, in terms of response to radon concentrations and F factors have been carried out at the NRPB, which assistance is greatly appreciated. For the etching and track counting procedures used the inner LR-115 piece in the detectors has a mean sensitivity of approximately 1.6 tracks cm 2 kBq l m hr l. 

The accuracy of the measurement of radon concentrations with bare track detectors was found to be unsatisfactory due mainly to the changes of the deposition rate of radon progeny onto the detector as a result of air turbulence. In this work, therefore, a method was developed which can correct the contributions of the deposition to the track densities by classifying the etched tracks according to their appearance, i.e. round or wedge shaped. Using this method, about 30% improvement in the error of measurements was achieved. The calibration coefficient ob tained by experiment was 0.00424 tracks/cm /h/(Bq/m ), which agreed well with the calculated value. Comparison was also made of the present method with other passive methods, charcoal and Terradex, as to their performance under the same atmosphere. 

The detectors were 5 x 2.5 cm pieces of CR-39 (Solar Optical Japan, Osaka, Japan) which were fixed on cardboard. They were set up on the wall or in the other places in the dwellings to be investigated. Following exposure, the CR-39 pieces were etched in 30% solution of NaOH at 70°C for 5 hours. The tracks were scored at total magnification of xl210 us ng T.V. assisted optical microscopy. The counting area was 3 cm at most, corresponding to 750 microscope fields. 

Irradiation of heavy ions like An and Kr deposits a heavy damage along the ion track in polymer films. When some polymer films are irradiated with heavy ions and dipped in an alkaline aqueous solution, micropores are produced along the ion track. Since the shape and size of pores of polymers can be controlled by etching conditions and by the species of heavy ions, these polymer films are expected to be used for high-performance filters and ion detectors [92]. The development of the polymer filters and ion detectors are described as follows. 

To meet the need to monitor levels of 222Rn in houses, passive samplers have been developed which measure average concentrations over long periods and do not need power suplies. In the Karlsruhe dosimeter (Urban Piesch, 1981), a polycarbonate nuclear track detector foil is mounted inside a plastic cup. The mouth of the cup is closed with a filter to allow radon to enter but to exclude decay products. After exposure, the detector foil is etched and the tracks counted optically. This is a... 

In a passive detector developed by the National Radiological Protection Board (Wrixon et al., 1988), the etched pits in the detectors are filled with scintillator fluid. After exposure to radon, the detector is irradiated with an alpha source, and the resulting scintillations counted with a photo-multiplier tube. In this way, track density over 1 cm2 of detector can be measured in a few seconds. Passive detectors used in the UK National Survey were sensitive down to 20 kBq m-3 h of accumulated exposure, equivalent to a radon concentration of 5 Bq m-3 measured over 4000 h exposure. 

Examinations of the same and of other lead-bearing samples for spontaneous fission events with large proportional counters in Dubna seemed to confirm these findings, but further measurements [37] of thin samples sandwiched between two plastic fission-track detectors showed that the events were background caused by cosmic-ray induced reactions of lead. Other groups [38] found no evidence for spontaneous fission activities in lead and other samples at a lower detection limit of 10" 3 g/g achieved with the sandwich technique. Even lower limits down to 10"17 g/g can be reached by etching... 

When charged particles, e.g. a particles, impinge on certain types of plastic materials like polycarbonate or cellulose nitrate, they cause radiation damage tracks in the material. The tracks can be made visually detectable through chemical or electrochemical etching procedures. The visible tracks can be counted using a microscope, microfilm reader or automatic image analyzers. The number of tracks is used to calculate the total amount of radiation to which the detector material was exposed. 

A solid-state nuclear track detector is a piece of special plastic which is exposed as the sensitive element in a radon monitor. The alpha radiation from Rn and RnD, which penetrates the surface of the plastic, causes radiation damage along the entrance path, as shown in the schematic in Fig. 9.26. Chemical etching of the plastic after exposure... 

When the alpha particle strikes matter such as a cellulose nitrate film, it damages the surface by leaving a trail of broken molecular bonds in its wake. This damaged microtrack is attacked more readily than the undamaged surface by etching fluids, such as NaOH, and eventually becomes visible under the microscope as a track or dot. These are called etched track detectors. 

The fact that mica (and fused silica) can serve as solid state track detector (SSTD) of fission fragments was reported shortly before the final stage of development of the method for element 104 [12-14], In the dielectric solids, fission fragments produce tiny tracks visible by electron microscopy. Mica and silica are very resistant to active chemical reagents and elevated temperatures. The tracks proved to stay in hostile conditions of real experiments for a reasonably long time. Thanks to this, after the end of bombardment (EOB), the mica sheets could be etched with hydrofluoric acid to enlarge the tracks to micrometer size they were distinct in appearance and were searched out by scanning the surface of the detectors with an... 

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