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In this study, we used strippable LR 115 type 2 which is a Solid State Nuclear Track Detector (SSNTD) widely known for radon gas detection and measurement. The removed thickness of the active layer of samples of this SSNTD, were determined by measuring the average initial thickness (before etching) and residual thickness after 80 to 135 minutes chemical etching in the standard conditions, using an electronic comparator. These results allowed the calculation of the bulk etch rate of this detector in a simple way. The mean value obtained is (3.21 ± 0.21) μm/h. This value is in close agreement with those reported by different authors. It is an important parameter for alpha track counting on the sensitive surface of this polymeric detector after chemical etching because track density depends extremely on its removed layer. This SSNTD was then used for environmental radon gas monitoring in Côte d’Ivoire.

Radon, a natural radioactive and lung carcinogen gas [_{t} and V_{b} [_{t} is the track etch rate and V_{b} the bulk etch rate which is one of the most important parameters that control track formation and development and with V_{t}, are needed to simulate track growth and to calculate track parameters [_{b} 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 [

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 (

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 (

e = e m − e r (1)

where

e is the removed active layer thickness (μm);

e_{m}, the mean of its initial thickness (μm);

and e_{r}, 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 V_{t} or track etch rate while, the undamaged area of the foil is attacked at a lower rate with a velocity V_{b} also called bulk etch rate. V_{b} is generally constant for given etching conditions but not V_{t} 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:

sin δ = V b V t (2)

For the present experiment, we deduced the bulk etch rate of LR 115 type 2 using the formula:

V b = e t (3)

where

V_{b} is the bulk etch rate (μm/h);

and t the etching time (h).

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

In

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 [

_{b}, 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 [

Now that V_{b} was determined, it can be combined with the track etch rate V_{t} to give the V function (V = V_{t}/V_{b}), which is required for calculating track parameters [

D = d = 2 V b t V − 1 V + 1 (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 V_{t} is very much greater than V_{b} so that V is very much greater than 1, the Equation (4) gives approximately.

D = d = 2 V b t (5)

The values of track opening diameters calculated using Equation (5) are reported in

Etching time (min) | Removed thickness (μm) | Bulk etch rate (μm/h) |
---|---|---|

80 | 3.85 ± 0.23 | 2.85 ± 0.17 |

85 | 4.22 ± 0.30 | 2.97 ± 0.21 |

90 | 4.37 ± 0.29 | 2.91 ± 0.19 |

95 | 5.16 ± 0.33 | 3.27 ± 0.21 |

100 | 5.17 ± 0.38 | 3.10 ± 0.23 |

105 | 5.88 ± 0.44 | 3.36 ± 0.25 |

110 | 6.22 ± 0.40 | 3.39 ± 0.22 |

115 | 6.80 ± 0.44 | 3.54 ± 0.23 |

120 | 6.68 ± 0.42 | 3.34 ± 0.21 |

125 | 6.78 ± 0.44 | 3.26 ± 0.21 |

130 | 7.12 ± 0.54 | 3.28 ± 0.25 |

135 | 7.39 ± 0.47 | 3.28 ± 0.21 |

Etching time (min) | Track opening diameter (μm) |
---|---|

80 | 7.60 ± 0.45 |

85 | 8.42 ± 0.60 |

90 | 8.73 ± 0.57 |

95 | 10.36 ± 0.67 |

100 | 10.33 ± 0.77 |

105 | 11.76 ± 0.88 |

110 | 12.43 ± 0.81 |

115 | 13.57 ± 0.88 |

120 | 13.36 ± 0.84 |

125 | 13.58 ± 0.88 |

130 | 14.21 ± 1.08 |

135 | 14.76 ± 0.95 |

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.

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.

The authors declare no conflicts of interest regarding the publication of this paper.

Agba, D.S.I., Djagouri, K., Gogon, B.D.L.H. and Koua, A.A. (2021) Bulk Etch Rate of LR 115 Polymeric Radon Detector. Detection, 8, 1-8. https://doi.org/10.4236/detection.2021.81001