Nuclear Track Detectors (NTDs) have often been the detectors of choice for detecting highly ionizing rare particles (strangelet, magnetic monopole) in cosmic rays [1, 2] or in accelerators [3, 4] . Some of the most important advantages of NTDs are low cost, ease of use, and the existence of natural thresholds of detection [5] . NTDs offer excellent charge( 1 e, where e is the electronic charge) and position resolution (∼ 1µm) [6, 7] . In addition, being passive in the nature of action (as they do not require electrical power during their operation), NTDs are often the best choice for the experiments which require detector arrays to be set up in remote locations for cosmic ray observation [8, 1] . Previous studies show that the responses of the NTDs (e.g., bulk etch-rate, sensitivity, etc.) are largely affected both in open-air and in accelerator by the exposure due to ambient electromagnetic radiations [9, 10, 11] . To investigate this particular issue, we pursued a systematic study on the attenuation of electromagnetic radiation over a wide range of photon energies for three commercially available NTDs, namely -PET (Desmat, India), Makrofol (Covestro AG, Germany) and CR-39 (TASL, England). The attenuation of gamma-ray, X-ray, UV-visible, and infrared radion in NTDs is studied using NaI(Tl) detector, Gas Electron Multiplier (GEM) detector, UV-Vis spectrophotometer, and FTIR spectrophotometer respectively. The values of some important parameters (e.g., optical depth, attenuation coefficient, etc.) of different NTDs at the relevant region of the electromagnetic spectrum is determined.
Electromagnetic radiation interacts with matter through three processes: (i) photoelectric effect, (ii) Compton scattering, and (iii) pair production. While passing through the transparent material, the number of transmitted photons get reduced by scattering and absorption due to the processes mentioned above. It can be shown that the intensity of the photons I 0 , after passing through a distance x inside the absorber reduces to
where µ, the attenuation coefficient, is a characteristic of the absorbing material and the energy of the photon [12] .
The details of the measurement techniques, experimental setup, and the results of the study of the attenuation of electromagnetic radiation (gamma-ray, X-ray, UV ray, visible ray, and infrared ray in particular)
while passing through the NTDs, are presented in the next few sections.
Nuclear Track Detectors (NTDs) are optically transparent, dielectric solids comprising of long polymer chains. The chemical composition [13] and thickness of the different plastic NTDs used in these experiments are given in Table 1 . The detailed experimental procedures in different electromagnetic regions are described below.
A NaI(Tl) scintillator detector [14] coupled with a Photo Multiplier Tube (PMT) is used to detect the gamma radiation from a 60 Co source. A high voltage of 600 V is applied to the PMT. The signal from the PMT is fed to a charge sensitive pre-amplifier which is integrated with the base of the PMT. The pre-amplifier signal is fed to a spectroscopy amplifier (ORTEC-671) with a coarse gain of 500. The amplified signal is then connected to a PC controlled Multi Channel Analyser (MCA) (Ortec EASY-MCA-8k) to obtain the energy spectrum. The attenuation of γ rays is studied by placing the NTDs in-between the source and the detector, as shown in Fig. 1 (a). The energy spectrum in the given range (300 − 3000) of the ADC channel number is plotted in Fig. 1 (b). The first peak is due to 60 Co source of energy 1.173 MeV and is fitted with a Gaussian function.
Total number of counts within one standard deviation (±1σ) from the mean (µ); i.e., between ADC channel number 1555 to 1718 is plotted in a histogram ( Fig. 2(b) ). The NTDs are placed one by one before the scintillation counter. From Fig. 2 (b) the attenuation coefficient of different NTDs in the energy range 1.2 ± 0.8 MeV can be calculated and the values are presented in Table 2 .
In this experimental setup, X-ray radiation is detected by a Gas Electron Multiplier (GEM) detector [15, 16, 17] . Triple GEM detector is a gaseous ionization detector; it consists of three GEM foils cascaded inside a chamber filled with gas. A pre-mixed gas mixture of Ar:CO 2 in 70:30 volume ratio is used at a flow rate of then collected from the readout anode pad placed after the third GEM foil. The experimental arrangement is shown in Fig. 3(a) . X-rays from 55 Fe source get partially attenuated by the NTD placed in-between the source and the detector. The signal from the GEM detector is fed to a charge sensitive pre-amplifier (VV50-2) [18] . The analog signal is then fed to a four output linear Fan-in-Fan-out (linear FIFO) module in a NIM based data acquisition system. One of the outputs is connected to a MCA to obtain the energy spectrum. One such spectrum of 55 Fe on GEM detector is shown in Fig. 3(b) where the noise peak (below channel number 150), argon escape peak (channel number 570, energy 2.9 keV), and main 5.9 keV X-ray peak (channel number 1190) from 55 Fe source are clearly visible. Another output from the linear FIFO is fed to a Single Channel Analyzer (SCA), the threshold of which is set at 0.1 V to reject the noise. The total count of the incident particles is measured by a NIM scaler, which is connected to the SCA via a TTL-NIM adapter. The findings of this study are presented in the results and discussions section.
To check the transmittance of different NTD samples (Table 1) in Ultra-Violet (UV) and visible radiation, they are scanned by a Perkin-Elmer Lambda 25 UV/Vis spectrometer (Fig. 4(a) ), covering wavelength ranging from 200 nm to 1000 nm with a resolution of 1 nm. Fig. 4(b) shows the transmission profile of different NTDs in the visible and UV region of electromagnetic radiation. spectroscopy [19] (wavenumber ranging from 450 to 4000 cm −1 ) using Perkin-Elmer Spectrum 100 FT-IR spectrometer ( Fig. 5(a) ). The infrared transmission spectrum of the samples is shown in Fig. 5(b) . Fig. 4(b) shows that the transmittance near the visible region of electromagnetic radiation is more than 85% for all NTDs, which is helpful to use them in transmitted light optical microscopy. Optical depths (τ )
corresponding to the wavelength 560 nm (yellow light) are calculated using the relation
where φ i and φ t are the radiant flux received and transmitted by the material at that wavelength. For PET,
Makrofol and CR-39 the values of τ (at wavelength 560 nm) are given in Table 3 .
The optical energy gap (E g ) is related to the absorption coefficient (α) through the relation [21] ,
where α is calculated from α = 2.303 × A/t (A and t are the absorbance and thickness of the material respectively); B is a constant. By plotting (αhν) 2 as a function of photon energy (hν) [Fig. 6 ] and extrapolating the linear part of the curve to hν-axis at zero absorption, optical energy gap for different NTDs are computed ( [22] ) and the corresponding values are given in Table 3 . Fig. 4(b) shows that the value of trans- mittance is negligibly small (≃ 0) below the wavelengths 310 nm, 280 nm, and 230 nm for PET, Makrofol and CR-39 respectively (Table 3 ). This shows that PET film blocks UV radiation more effectively than the other two. Thus, while using stacks of NTDs, keeping PET film in the top layer during open-air exposures, the effect of UV rays on altering the etch-rate may be prevented in the lower layers of NTDs [11] . The fact that PET can block UV radiation can have other practical uses as well. For example, UV radiation is effective in disinfecting SARS-CoV-2 surface contamination [23] . But prolonged exposure to UV can pose a health hazard to humans. So PET films can offer a cost-effective way to shield humans from the harmful effects of UV.
Results from the FTIR spectroscopy ( Fig. 5(b) ) is useful in the identification of polymers (i.e., their chemical composition), by identifying the peaks due to different functional groups [24] . However, as we are interested here in the attenuation of electromagnetic radiation in NTDs, these results are not considered for further study.
We have estimated some of the relevant physical quantities (like attenuation coefficient, optical depth) of previously unexposed NTDs over a wide range of the electromagnetic spectrum. These results will serve as a reference in the study of the response (i.e., bulk etch-rate, sensitivity) of NTDs exposed to electromagnetic radiation over a long period of time. In this work, it is also shown that a single PET film of thickness 90 µm can block the UV radiation almost completely. So, when a stack of PET films is given open-air exposure, the topmost PET film can prevent the change of etch-rate due to UV related damage in all subsequent layers.
The ability of PET to block UV rays can find applications in other fields of research and industry.