Suitability testing of polyethylene terephthalate ﬁlm as a solid-state nuclear track detector for using in radon studies

The present work describes the procedure for testing the suitability of a polyethylene terephthalate ﬁlm (after irradiation with heavy ions it is widely used in various ﬁelds) as an alpha-track detector. The test was carried out in order to determine the prospects for the use of this ﬁlm in studies evaluating the indoor and outdoor radon concentrations. The study was conducted using a radionuclide source of alpha particles Am-241. Irradiated ﬁlm samples were chemically treated. In order to further compare the results, unirradiated ﬁlm samples were also chemically etched under the same conditions. To determine thepresenceoftracks, compareandanalyzetheirparameters, allsampleswereexaminedusingascanning electron microscope. SEM images of the investigated samples are shown. The results obtained showed the unsuitability of using these ﬁlms for registration of alpha particles. Currently, studies of the indoor and outdoor radon concentrations are being carried out using solid-state nuclear detector LR-115 based on cellulose nitrate.


Introduction
The heavy radioactive gas radon makes the largest contribution to the average dose to the public from all sources of natural radiation. According to the International Commission on Radiological Protection (ICRP), radon-222 is a recognized cause of lung cancer, the second after smoking [1].
Various methods are used to study the concentration of radon. The method based on the use of solid-state nuclear track detectors (SSNTD) is widely used in radon measurements. The main advantage of this method is the possibility of simultaneous exposure of several detectors and their centralized chemical treatment.
A heavy charged particle passing through the detector causes intense ionization of the material. As a result, narrow lesions appear at the level of molecules along their trajectory. The sizes of latent tracks formed by heavy ions are very small (only about 10 nm in diameter) [2].
For several decades, polyethylene terephthalate film (PET) has been widely used in the creation of track membranes for use in various fields: in plasmapheresis of blood, filtration of water and biological media, etc. [12,13]. Along with this, the film irradiated with heavy ions of PET is successfully used as a template for the synthesis of metal nanostructures. This method allows one to obtain nanostructures of various shapes and sizes [14][15][16].
The study consists of several main stages: -preparation of detectors; -exposure of detectors at the source of alpha particles; -chemical etching of irradiated and non-irradiated detectors; -research of detectors on a microscope; Further, in case of detection of tracks on the detector: -determination of the density of tracks on the detector; -calculation of the calibration coefficient for this type of detector.

Materials and methods
For the experiment, polyethylene terephthalate films of the Hostaphan type manufactured by Mitsubishi Polyester Film (Germany) 12 µm thick were used.
As a source of alpha radiation, Americium-241 with an activity of 340 Bq produced by the Scientific and Technical Center "RADEK" was used. The energy of the intense alpha particle is 5.486 MeV, the half-life is 432.17 years.
Film samples were cut into 3 × 3 cm 2 squares and irradiated with alpha particles emitted from a 241 Am point source, applying a detector to the source in 180 • geometry relative to the source.
The exposure time of the detectors varies (17,23, and 30 days) to obtain different doses of alpha particles. The dose rate to which the detectors are exposed was calculated by the Rad Pro calculator version 3.26 program, designed to calculate the dose rate of radiation from radioisotopes by entering its activity and the distance between the source and the detector. Doses for each time are shown in Table 1. After irradiation, the detectors were chemically etched in a 2.2 M sodium hydroxide (NaOH) solution at a temperature of 87 ± 0.1 • C. Chemical etching is the most widely used method of many other methods for fixing and enlarging hidden tracks created in solid state track detectors.
Chemical etching conditions depend on many parameters, such as etching time, type, molar mass and the temperature of the etching solution.
The etchant solution is obtained using the following equation [17]: where: W : weight of NaOH needed to prepare the solution, mg W eq : equivalent weight of NaOH = 40. N : molar mass = 2.2 M. V : volume of distilled water = 1000 ml. NaOH pellets are dissolved in distilled water using a stirrer to obtain a homogeneous solution, then a volumetric flask (1000 ml) is filled with the solution and the detectors are immersed in it. A volumetric flask containing an etching solution and detectors are placed in a tank with a water bath at a temperature of 87 ± 0.1 • C for various time intervals.
A water bath is a laboratory device made of a container filled with heated distilled water and equipped with a thermostat to heat the water to the desired temperature ( Figure 1).
After removing the detectors from the etching solution, they must be processed in neutralization solutions and dried. To do this, they were immersed in a 1% solution of acetic acid, then washed with distilled water, and then wiped with special paper to clean the film samples from the etching solution well and to obtain good microscopic images and calculation of alpha particle tracks.

Results and discussion
The processed detectors based on polyethylene terephthalate were investigated using a JEOL-7500F scanning electron microscope (SEM). The microscope is There are no visible structural changes on the surface of the initial samples. All changes are due to the degradation of the film itself under the influence of the chemical etching of the samples. Table 2 shows the time of exposure to alpha source and chemical etching time of film samples.

Conclusion
From the obtained results it was found that particle tracks were not detected on the film samples, and from the comparative analysis between the unirradiated and irradiated samples, no changes were observed in the latter. Visible defects on the surface of the films are a consequence of the destruction of the structure of the surface of the film during chemical etching. In this regard, we can conclude that the PET film for the detection of alpha particles of low activity (and hence natural radon) is not applicable.
Currently, indoor and outdoor radon concentration measurements using a solid-state nuclear track detector based on the LR-115 film in the process.