1. Introduction
Radon, a natural radioactive and lung carcinogen gas [1] can be detected by many techniques [2] [3] [4]. The SSNTD are the most widely used detectors in this way by many researchers around the world [5] [6] and LR 115 is the best SSNTD for radon [7]. Alpha particles emitted by radon and its progeny hit the detector and left latent tracks in it. Several different techniques of track revelation are known [8] [9] [10], but, the chemical etching technique is the most frequently used. Many authors had established that the removed active layer of Solid State Nuclear Track Detector during etching is one of the main factors influencing the track parameters or shape characteristics [11] [12] [13]. Ion-track growth in SSNTD has been suggested to base on two parameters, Vt and Vb [14], where Vt is the track etch rate and Vb the bulk etch rate which is one of the most important parameters that control track formation and development and with Vt, are needed to simulate track growth and to calculate track parameters [15] [16]. The bulk etch rate is strongly related to the removed thickness of the active layer and it has been shown that Vb depends on many factors like the purity of the basic substances, the molecular structures of polymers, conditions of polymerization, environmental conditions during the irradiation and finally on etching conditions [17]. Yip and contributors [18] showed that the bulk etch rate of LR 115 could not be controlled by temperature and etchant concentration only, and was also affected by the amount of stirring. Therefore, actual monitoring of the active layer thickness is necessary when using this detector. Various techniques were used for the determination of the bulk etch rate [18] [19] [20] [21]. Our main concern in this study is to determine in the standard etching conditions, the bulk etch rate of LR 115 type 2 by measuring the thickness of its active layer before and after chemical etching using an electronic comparator.
2. Materials and Methods
Commercially available, LR-115 type 2 strippable films manufactured by DOSIRAD-France, consist of an active layer or cellulose nitrate on 100 µm clear polyester base. 10 pieces of this detector film (size 2 × 2 cm) were taken. The active layer of each sample was peeled off with a lancet and its thickness (initial thickness) measured on 10 different points covering its surface using an electronic comparator (Figure 1). We then deduced from the 100 measurements, the arithmetical mean of the initial thickness of LR 115 type 2 active layer.
36 other samples with the same size were irradiated with a Pu-239 radioactive source which α particle emission rate in 2π sr geometry is 3055 α/s, through a collimator of 21.79 mm height (Figure 2) during 5 minutes. Then, these samples were introduced in an etching bath without stirring of NaOH solution with the concentration 2.5 mol/ L at the constant temperature of 60˚C. After 80 minutes, three samples were taken out from the bath, rinsed off with tap water, then put in distilled water before been dried in dry air. Five (5) minutes later, three other samples were submitted to the same process. And so on every 5 minutes until 135 minutes. After been dried, each sample was peeled off with a lancet, its residual thickness measured as for the 10 previous before etching and its removed thickness calculated using the relation:
(1)
where
e is the removed active layer thickness (μm);
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Figure 2. Schematic of irradiation system.
em, the mean of its initial thickness (μm);
and er, its residual thickness (μm).
Chemical etching of an irradiated SSNTD foil works on the principle that the solution preferentially attacks the damaged core of the latent track and penetrates along its length with a velocity Vt or track etch rate while, the undamaged area of the foil is attacked at a lower rate with a velocity Vb also called bulk etch rate. Vb is generally constant for given etching conditions but not Vt which depends on the amount of damage present in the region of the core. Finally the track leads to the formation of a cone with semi-cone angle δ also known as the critical angle of etching and given mathematically by:
(2)
For the present experiment, we deduced the bulk etch rate of LR 115 type 2 using the formula:
(3)
where
Vb is the bulk etch rate (μm/h);
and t the etching time (h).
3. Results and Discussion
The arithmetical mean of the initial thickness of LR 115 type 2 active layer obtained from all the measurements is (12.45 ± 0.34) µm.
Our other experimental results are reported in the Table 1 below. Using these data, two graphs are drawn in Figure 3 and Figure 4.
In Figure 3, the thickness of the removed active layer of LR 115 type 2 increases linearly with the etching time.
According to our results, under the standard etching conditions the removed thickness obtained for 120 min of etching is 6.68 µm, and the corresponding residual thickness of the active layer is 5.77 µm. This last value fits well the important criteria for an optimum alpha track counting on strippable LR 115 type 2 film using a spark counter which is that the residual thickness of its active layer must range between 5.5 and 6.5 μm [22].
Figure 4 shows that for etching times between 80 and 135 minutes a constant bulk etch rate was attained considering the bar errors. Its calculated arithmetical mean is (3.21 ± 0.21) µm/h. This value agrees with those obtained by other authors [23] [24]. Therefore, it’s possible to evaluate, with this constant value of Vb, the removed active layer thickness at any etching time using the previous equation (3).
Our calculated bulk etch rate is a little different from the value of, 3.61 ± 0.14 µm/h, obtained by Yip and contributors [18], both in the same etching conditions. The difference may be due to the fact that the initial thickness of LR 115 type 2 active layer is not exactly 12 µm, as usually taken for granted.
Now that Vb was determined, it can be combined with the track etch rate Vt to give the V function (V = Vt/Vb), which is required for calculating track parameters [25] [26] [27]. For example, with a normally incident track, we have the following relation:
(4)
where
D is the major axe of the track opening;
d its minor axe;
and t the etching time.
In fact, the track opening being the intersection of a cone with a plane surface, the resulting shape is a conic section which is an ellipse with major axis D and minor axis d.
For a normally incident track, D = d is simply called diameter of track opening. Then, in the case of plastics like LR 115 where Vt is very much greater than Vb so that V is very much greater than 1, the Equation (4) gives approximately.
(5)
The values of track opening diameters calculated using Equation (5) are reported in Table 2 below.
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Table 1. Removed thickness and bulk etch rate as a function of etching time.
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Table 2. Track opening diameter as a function of etching time for a normally incident track.
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Figure 3. Removed thickness as a function of etching time.
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Figure 4. Bulk etch rate as a function of etching time.
4. Conclusion
Bulk etch rate is one of the most important factors controlling track developments in SSNTD. So, that requires precise measurement of this parameter. The result on the bulk etch rate of LR 115 type 2 film obtained under standard etching conditions without stirring during an etching time ranging between 80 and 135 minutes and using an electronic comparator is (3.21 ± 0.21) µm/h. This value agrees well with those reported by other authors. Moreover, the measured thickness of the residual active layer at the etching time of 120 minutes fits well the spark counter use criteria for an optimum alpha track counting on strippable samples of this detector as it was the case for those exposed to radon gas in Côte d’Ivoire.
Acknowledgements
The authors would like to thank the International Atomic Energy Agency (IAEA) which financed this work in the frame of a Technical Cooperation among Developing Countries (TCDC) through an AFRA/RAF 0038 project entitled radon gas monitoring in Côte d’Ivoire. For their precious help, particular thanks to Dr. Z. L. Mokrani and Dr. M. Aitziane from the Department of Dosimetry and Ionizing Radiations located at the Nuclear Research Centre /Nuclear Energy Commission of Algiers (Centre de Recherche Nucléaire/Commissariat à l’Energie Atomique d’Alger (CRNA/COMENA)) where this study has been carried out.