Track density

The track density, that is, the number of tracks per unit surface area... 

The track density can be easily determined on a newly cut, polished, and etched surface by counting, under an optical microscope, the number of etched tracks in a measured area of the solid. The uranium concentration can be determined by a number of analytical techniques. Following these... 

Some of the 220 detectors recently recovered have been analysed not only for radon exposure but also to determine the value of F (the equilibrium factor) in the houses. A preliminary set of such F factor results, obtained by analysing the inner and outer LR- 115 track densities of each detector, are presented in Table III for 12 houses with mean indoor radon concentrations greater than 200 Bq/nP. In Table III are also presented radon daughter doses estimated using the individually determined equilibrium factor values F together with the doses estimated on the basis of an assumed mean F factor value of 0.45. 

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 sources of a-rays which produce the tracks on the bare CR-39 detectors are divided into airborne activity and activity deposited on the surface of the detectors. The relationship between time-averaged radon concentration (Cq) and the track density (T) on the bare track detector is represented by... 

It can be seen from this figure that whether a round track or a wedge-shaped track is produced is related to the incidence angle and the residual range of the a -ray at the incidence point on the surface of detector. It can be shown that for the tracks produced by the deposited Po-218 and Po-214, the ratios of the round track density and the wedge-shaped track density to the total track density are 0.60 and 0.40, respectively. If the respective ratios for airborne activities are denoted as a and b, the round track density (Tr) and the wedge-shaped track density (T ) are represented by... 

However, as regards the conventional method, a linearity exists only in the case when the fan was off, and the track densities T in the turbulent atmosphere were higher than those in the still atmosphere. The calibration coefficients K and K were calculated with a multi-step method of linear regression analysis (Skinner and Nyberg,... 

Figure 4. Relationships between the time-averaged radon concentration and the track densities per unit time. Error bars represent standard error of track counting.

Turning now to alpha radiation, the track density is such that almost any overlap with a macromolecule results in multiple hits. For 2-m.e.v. alpha particles, a dose of r rads requires a flux of only 4 X 104 r particles per sq. cm., and with a track radius a = 15A., the total area covered is only 2.8 X 10"9 r per sq. cm. For 50% of target molecules to be hit at the doses envisaged each must have a larger area. 

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. 

Fig. 7. CH4 as carbon (ng/g) in total sample versus percentage of mineral grains with track densities > 108 cm-2...

The track density (number of fission tracks per cm ) in a mineral is a function of the concentration of U and the age of the mineral. For the purpose of dating, a sufficient number of tracks must be counted, which means that the concentration of U or the age (or both) should be relatively high. Usually, first the fission track density due to spontaneous fission of is counted, then the sample is irradiated at a thermal neutron flux density in order to determine the concentration of U in the sample by counting the fission track density due to neutron-induced fission of The age t of the mineral is calculated by the formula... 

Fission tracks are influenced by recrystallization processes in the solids (ageing), the rate of which increases appreciably with temperature. At higher temperatures, fission tracks may disappear quantitatively, but recrystallization processes may also be effective at normal temperatures, e.g. under the influence of pressure or of water. By comparing the ages calculated on the basis of fission track densities with those obtained by other methods, information can be obtained about the temperature to which the minerals have been exposed in the course of time. 

Unusually high fission track densities are found in the vicinity of nuclear explosions and at the natural reactors at Oklo. 

Figure 2 Darkfield transmission electron micrographs, (a) Solar-wind sputtered rim on exterior surface plus implanted solar flare tracks in chondritic IDP U220A19 (from Bradley and Brownlee, 1986). (b) Solar flare tracks in a forsterite crystal in chondritic IDP U220B11 (from Bradley et al., 1984a). The track densities in both IDPs are...

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. 

Two Other factors unique to these extraterrestrial samples must also be accounted for. The first is that tracks from heavy cosmic ray particles (mainly Fe-group nuclei) and other nuclear interactions with cosmic rays may also be present, in addition to fission tracks. The simplest method for correcting for the cosmic ray background is to measure the track density in adjacent silicate mineral grains, such as feldspar and pyroxene, which do not contain uranium and are therefore free of fission tracks. The second factor is that the samples are so old as to contain tracks from the spontaneous fission of now-extinct transuranic elements, particularly with a half-life of 82 Myr. Such Pu tracks will only be present in samples older than about 3.9 Gyr (Crozaz and Tasker 1981). 

The fission track age, t, is then calculated from the ratio of spontaneous (ps) to induced (pi) track densities according to the standard fission track age equation (Fleischer and Price 1964, Naeser 1967) ... 

Measurement of a fission track age following Equation (3) requires the determination of three different track densities, ps, Pi and pd. The various different experimental strategies involved have been elaborated by Naeser (1979a), Gleadow (1981), Hurford and Green (1982) and Wagner and Van den Haute(1992) and are summarized in Figure 6. Of the five main alternatives, only the Population (PM) and... 

---