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| ORIGINAL ARTICLE |
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| Year : 2022 | Volume
: 45
| Issue : 3 | Page : 138-152 |
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Theoretical evaluation of calibration factor for LR-115 cellulose nitrate solid state nuclear track detectors: A model for determining alpha radioactivity level in natural water
Biswajit Das1, Argha Deb2
1 Department of Physics, Vidyasagar University, Midnapore; School of Studies in Environmental Radiation and Archaeological Sciences, Jadavpur University, Kolkata, West Bengal, India
2 School of Studies in Environmental Radiation and Archaeological Sciences, Jadavpur University; Department of Physics, Jadavpur University, Kolkata, West Bengal, India
| Date of Submission | 01-Oct-2022 |
| Date of Decision | 21-Dec-2022 |
| Date of Acceptance | 03-Nov-2023 |
| Date of Web Publication | 18-May-2023 |
Correspondence Address:
Biswajit Das
Department of Physics, Vidyasagar University, Midnapore - 721 102, Paschim Medinipur, West Bengal
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/rpe.rpe_26_22
Alpha-sensitive solid-state nuclear track detectors (SSNTDs) are used successively for decades for determining the alpha-radiation level in various environmental materials. A model for measurement of total alpha-radioactivity in natural water by using alpha-sensitive LR-115 type II cellulose nitrate (CN) film SSNTDs has been developed in the present work. The LR-115 CN films in bare mode are assumed to be immersed into a water sample for alpha-exposure. During the exposure time period, the alpha-particles that are emitted from the alpha-emitters present in the water sample will interact with the CN film and create their latent trails in the film. These latent trails will be manifested as pits of alpha-tracks after chemical etching of the exposed film. For alpha-radioactivity measurement, calibration factor (CF) for LR-115 type II SSNTDs has been evaluated theoretically by using the range of alpha-particles of residual energies situated between 1.6 MeV and 4.7 MeV in natural water and the actual geometry of experimental setup for alpha-exposure. The evaluated CF is found to be 204.93 Bq.l − 1.(tracks. cm − 2.h − 1)−1. The evaluated CF and experimental procedure can reliably be utilized in monitoring the alpha-radioactivity of natural water and in other radioactivity field measurements.
Keywords: Alpha particles range, alpha radioactivity measurement, calibration factor, etching condition, LR-115 type II CN-film, natural water
How to cite this article:
Das B, Deb A. Theoretical evaluation of calibration factor for LR-115 cellulose nitrate solid state nuclear track detectors: A model for determining alpha radioactivity level in natural water. Radiat Prot Environ 2022;45:138-52 |
How to cite this URL:
Das B, Deb A. Theoretical evaluation of calibration factor for LR-115 cellulose nitrate solid state nuclear track detectors: A model for determining alpha radioactivity level in natural water. Radiat Prot Environ [serial online] 2022 [cited 2023 May 28];45:138-52. Available from: https://www.rpe.org.in/text.asp?2022/45/3/138/377232 |
| Introduction | |  |
Alpha-sensitive CR-39 and LR-115 solid-state nuclear track detectors (SSNTDs) are very useful and convenient tools for detecting the levels of alpha-particle radiation in different environmental materials. These two types of SSNTDs have been used successfully for decades in different track-etch techniques to monitor routinely the alpha-radioactivity level and to determine the activity concentrations of alpha-emitting radionuclides in different environmental materials such as water, air, soil, sand, gravel, coal, fly ash, fertilizer, foodstuff, and building materials.[1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14]
Generally, the SSNTDs need to be calibrated first before use in the measurements made with the SSNTDs. In our earlier work,[14] a new track-etch technique by using CR-39 SSNTDs for measuring the total alpha-radioactivity concentration in natural water of different sources has been developed and evaluated also the calibration factor (CF) of CR-39 detectors for this measurement. The total alpha-radioactivity of water consists of two parts, one is the gross alpha-activity of all solid alpha-emitting radionuclides of 238U and 232Th decay series present in water, and the other is the alpha-activity due to the only radioactive gas radon (an alpha-emitter) present in water. In the present study, a model for the track-etch technique using LR-115 type II cellulose nitrate (CN) film SSNTDs has been developed to measure the total alpha-radioactivity level in any natural water samples. For such measurement, the calibration of LR-115 type II SSNTDs has been done theoretically by considering the range of alpha-particles in natural water and taking into account the actual geometry of experimental setup for alpha-exposure. The detector LR-115 type II has been chosen in the present study, because the LR-115 CN film is one of the most sensitive nuclear track detectors[15] and it is very sensitive to alpha-particles; additionally, many measurements regarding the alpha-radioactivity of water have been carried out with the LR-115 type II detectors.[1],[8],[13],[16],[17],[18],[19],[20],[21],[22] In the present technique, the LR-115 type II detectors in bare mode are assumed to be immersed into a sample of natural water for a certain time period for exposure to alpha-particles that would be emitted by the decay processes of alpha-emitting radionuclides present in the water sample. During the exposure period of time, the emitted alpha-particles will interact with the CN film of the detectors and create latent trails of alpha-particles. The latent trails will appear as pits of alpha on chemical etching of the exposed film. The pits of alpha are called alpha-tracks which are a measure for the alpha-radioactivity level in the water sample.
The main objective of the present study is to develop a simple model for the track-etch technique by using LR-115 type II CN film SSNTDs to determine the total alpha-radioactivity level in water of different natural sources. A theoretical method for CF evaluation for LR-115 type II film and an experimental procedure for the track-etch technique using this film both are reported in details in the present paper. The new thing presented in this study is the way to calculate the effective volume of water around the CN film and the method to evaluate the CF for the CN film.
| Theory for Evaluating the Calibration Factor for Lr-115 CN Film Solid State Nuclear Track Detectors | | |
Calibration of a solid state nuclear track detector (SSNTD) is an experimental part of a track-etch technique. The calibration factor (CF) for an alpha-sensitive SSNTD is a parameter and it can be used to convert the track density rate registered on the SSNTD into the alpha-radioactivity of a sample material. Its value should be evaluated accurately to get actual results in the measurements made with the SSNTDs.
The CF for a SSNTD can be defined by different ways depending upon the measurement techniques used and the measurement parameters investigated in a study. In case of LR-115 SSNTDs immersed into a water sample for measuring the alpha-radioactivity of the water sample, the CF for the LR-115 SSNTD is defined by the alpha-radioactivity of the water sample due to a single alpha-track registered per unit area of the SSNTD during an exposure period of unit time. If the CF in unit of Bq.l-1.(track.cm-2.h-1)-1 for a SSNTD is multiplied by the rate of alpha-track density (in unit of track.cm-2.h-1) registered on the SSNTD, then its value is simply the alpha-radioactivity concentration (in unit of Bq.l-1) in the investigating water sample.[14] Here, the volume of water is a little part of the total volume of water sample and it is located in front of the alpha-sensitive face (CN film) of the LR-115 detector [Figure 1]. This little volume of water is known as the effective volume of water and it is representative for measuring the alpha-radioactivity concentration in the given water samples. To estimate the CF for the LR-115 SSNTD, it is therefore required to estimate the track density rate (in unit of track.cm-2.h-1) registered on the LR-115 detector and the effective volume of water (in unit of liter) around the alpha-sensitive CN film of the LR-115 detector. A complete experimental procedure for estimating the track density rate registered on the LR-115 detector has been described in details in the section “Materials and Method for measurement of total alpha-radioactivity in natural water by using LR-115 type II SSNTDs”. Actual geometry or the shape and size of the effective water volume are depending upon (i) the dimension of the LR-115 film used for alpha-exposure; (ii) the range of alpha-particles in natural water; (iii) the critical angle for alpha-particle detection, or the critical angle of etching for alpha-particle registration in the LR-115 film; (iv) the residual energy limits (lower and upper) of alpha-particles before reaching the LR-115 film, or the energy window of LR-115 film for alpha-track formation in it; and (v) the etching condition for registering the alpha-tracks in the LR-115 film.
During the time period of exposure, the alpha-particles will be emitted naturally by the natural decay processes of alpha-emitters present in the water sample. When the emitted alpha-particles are coming from the region of effective volume of water, some of them will be able to reach the detector's surface (CN film) and interact with it. Obviously, these interacting alpha-particles have a detection probability larger than zero. However, the alpha-particles that are not coming from the effective volume cannot be detected by the LR-115 film, i.e., they have zero detection probability on the film.[23] The interacting alpha-particles will be detected by the CN film as latent and damage trails of alpha if their residual energies onto the film are situated within a certain range, called the energy window, or the alpha-particle detection limits of the LR-115 film. It is observed that the LR-115 track detectors have a limited energy window ranging from 0.06 MeV to 6 MeV.[15] The lower and upper energy limits of the energy window of the LR-115 film are depending upon the critical angle of etching and the chemical etching conditions utilized for registering the latent trails of alpha as bright track holes permanently in the LR-115 film. The critical angle of etching is the angle between the direction of incident alpha-particles on the film and the surface of the film, and below the critical angle, no alpha-particles of any energy are able to register as visible etched tracks in the film.[14],[24] The critical angle of etching depends on the alpha-particle energy and etching condition for LR-115 film.[23]
It is important to note that the enlarged profiles of the damaged alpha-trails formed in the LR-115 SSNTDs will be revealed as alpha-tracks by using proper chemical etching process. An optimal chemical etching condition utilizing in the SSNTDs provides maximum detection efficiency and reasonably high track contrast in the SSNTDs.[25] It is mentioned in various studies that the experimental etching condition of 2.5 N NaOH aqueous solution at temperature of 60°C for etching time of 2 h is the optimal etching condition for the LR-115 type II SSNTDs and it is utilized most frequently for etching the LR-115 type II SSNTDs.[26],[27],[28],[29] During this chemical etching process, the detectors recorded a good number of alpha-tracks with 100% efficiency.[30],[31] Under this optimal etching condition, the residual thickness of the LR-115 type II film is 5 μm which corresponds to the lower energy limit (say, Emin) of 1.6 MeV and upper energy limit (say, Emax) 4.7 MeV of for registration of alpha-tracks in the LR-115 type II film.[3],[8],[11],[21] It is observed that the energy limits between 1.6 MeV and 4.7 MeV correspond to an average critical angle of 40° for alpha-track registration as bright track holes in the LR-115 type II film.[32] The average critical angle of etching 40° also validates for other energy windows and etching conditions for the LR-115 type II film.[9],[16],[25],[31],[33],[34],[35],[36] Therefore, the alpha-particles that incident on the CN film of a LR-115 type II SSNTD with a critical angle equal or greater than to 40° and hit the CN film with their residual energies situated between 1.6 MeV and 4.7 MeV are only to be registered as bright track holes of alpha in the CN film after etching with the optimal etching condition: 2.5 N NaOH solution at temperature of 60°C for a period of 2h. These observations emphasize to take into account the energy window of 1.6 MeV and 4.7 MeV, the critical angle of etching 40°and this optimal etching condition in the CF evaluation method for LR-115 type II SSNTDs.
Generally, the alpha-particles have minimum energy typically above 4 MeV which is greater than to Emin=1.6 MeV. Hence, under the above mentioned optimal etching condition, the CN film of the LR-115 type II detectors does not detect the alpha-particles that are emitted from the alpha-emitters deposited onto the CN film itself.[15] Let Rmin and Rmax are, respectively, the ranges of alpha-particles of energies Emin and Emax in natural water. If an alpha-particle is coming from a distance greater than either Rmin or Rmax from the LR-115 surface AB, as shown in [Figure 2], it will be slowed down too much in the sample material and then interact with the CN film with an energy either lower than Emax or greater than Emin. On the other hand, if the alpha-particle is coming from a distance smaller than Rmin from the LR-115 surface AB, it will not sufficiently be slowed down too much with energy between Emin and Emax in the materials, and hence, it will not be able to interact with the CN film [Figure 2]. Therefore, the alpha-particles that are coming from a location within a region covered by distances Rmin and Rmax with energies (or residual energies) situated within the range Emin=1.6 MeV and Emax=4.7 MeV will strike the CN film, and then generate their perforated tracks in the film. It has been reported in an article that the alpha-particles that are emitted between Rmin and Rmax can only generate their perforated tracks in the LR-115 detectors.[23] These characteristics for alpha-particle detection in the LR-115 type II track detectors indicate that the effective volume of water in front of the track detector cannot reach the detector's surface (CN film) due to the existence of an energy window (Emin=1.6 MeV and Emax=4.7 MeV) of the LR-115 detectors. That means the effective volume is separated or distanced from the detectors, and its lower surface (EFGH) and upper surface (IJKL) are, respectively, situated at distances of Rmin and Rmax from the detector's surface ABCD (CN layer), as shown in [Figure 3]. The area of the lower surface EFGH of this effective volume is equal to the area of the detector's surface ABCD. The separation (say, △Rw) between the lower and upper surfaces of the effective water volume is: | Figure 2: Range of alpha-particles (Rmin and Rmax corresponds to energy window (Emin and Emax) of LR-115 SSNTD. SSNTD: Solid state nuclear track detector
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 | Figure 3: Pictorial view of the geometrical shape of the effective volume of water around a LR-115 track detector ABCD for 2π-geometry alpha-exposure
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△Rw= Rmax -Rmin
Where Rw is denoted for the range of alpha-particles in natural water.
The range of alpha-particles of energies 1.6 MeV and 4.7 MeV in natural water is now calculated using the Bragg-Kleeman formula:[14]
Where Ra=range of alpha-particle in air, κa=density of air ≈ 1.293 ×10-3 g.cm-3, ρsub>w =density of water ≈ 1 g.cm-3, and the atomic masses of air and water are, respectively, Aa and Aw. The range of alpha-particles in air (Ra) in unit of cm is calculated using the Geiger formula:[14]
Where Eα = the energy in unit of MeV of the alpha-particle. Therefore,
Since, the ratio of two molar masses of any two compounds is numerically equal to the ratio of two atomic masses of these two compounds.[14] Therefore, the ratio of molar masses of water and air is used instead of the ratio of atomic masses of water and air in Formula (3). The molar masses of water and air are, respectively, 18.015 g.mol-1 and 28.97 g.mol-1 obtained from Wikipedia for molar mass (http://en.wikipedia.org/wiki/Molar_mass).[14]
After putting the above mentioned values of ρsub>a, ρsub>w, Aa, Aw, and Ra in Formula (3), the expression for Rw in unit of cm becomes:
For Eα= 1.6 MeV, the range Rw=Rmin≈6.69×10-4 cm =6.69 μm; and for Eα= 4.7 MeV, Rw=Rmax≈33.67×10-4 cm =33.67 μm. Hence, the separation between Rmax and Rmin is:
△Rw=(33.67-6.69) μm=26.98 μm
The two values (i.e., Rmin =6.69 μm and Rmax=33.67 μm) of the range of alpha-particles in natural water and a rectangular shape of the LR-115 SSNTD of sides a and b are now used in calculating the effective water volume (say,Vw) around a small piece of the CN film of LR-115 detector. For estimating the value of Vw, let a piece of LR-115 SSNTD, say CN 1, is immersed into a water sample which is kept inside a cylindrical PVC container so that two faces of the CN 1 are parallel to two circular faces S1 and S2 of the PVC container [Figure 1]. The alpha-sensitive face CN layer of the detector is, therefore, exposed to 2π-geometry.
The actual geometrical shape of the effective water volume Vw is now drawn around the rectangular shape of LR-115 film of sides a and b for 2π-geometry of alpha-exposure and its pictorial view is shown in [Figure 3]. All the straight lines drawn from or to 4 sides of the film are equal to Rmax. Here, it is assumed that the alpha-particle emission is isotropic. The direction of the incoming alpha-particles to the film is shown by arrow lines. As shown in [Figure 3], the effective volume of water around the alpha-sensitive CN film of the LR-115 SSNTDs is divided into three regions, namely Region I, Region II, and Region III. The two-dimensional presentation (i.e., cross-section with one vertical plane) of Region I and Region II is shown in [Figure 4], and Region III is shown in [Figure 5]. A pictorial view of the geometrical shapes of Region I, Region II, and Region III is shown, respectively, in [Figure 1]a, [Figure 1]b, [Figure 1]c in Appendix A of the Supplementary Material.
Region I, as shown in [Figure 3], is the vertical parallelepiped -shaped water column of EFGHLIJK, located in the space covered by vertical distances Rmin and Rmax from the detector's surface ABCD. The water column EFGHLIJK is a part of the ABCDLIJK parallelepiped-shaped water column which is located vertically on the detector's surface ABCD. When the emitted alpha-particles are moving from upper to lower surfaces through the water column EFGHLIJK, they may be able to interact upright with the detector's surface ABCD, because the lower and upper surfaces of the parallelepiped are located, respectively, at Rmin and Rmax from the surface ABCD of the CN film. These interacting alpha-particles are able to produce visible alpha-tracks in the CN film. Volume of this water column, say VI, is simply the algebraic subtraction between the volume of ABCDLIJK parallelepiped and the volume of ABCDHEFG parallelepiped, i.e.,
VI= a.b. (Rmax-Rmin)= a.b.(△Rw)
The volume VI is a part of the effective volume of water Vw.
Region II is the 4 horizontal 1/4th tube-shaped water columns. It is located just around at 4 outer faces of the ABCDLIJK parallelepiped-shaped water column [Figure 3]. Each of these 4 tube-shaped water columns is a thick and annular tube of radii Rmin and Rmax (i.e., its thickness △Rw). As shown in [Figure 3], the length of such 4 oppositely faced water columns is either a or b. To realize this, a thick and annular tube-shaped water column IJBONMIA of length b and thickness △Rw is drawn at around the face ABJI of the water column ABCDLIJK such that its axis is collinear to the side b of the detector. The emitted alpha-particles from the 4 thick and annular tube-shaped water columns are incident obliquely on the CN film ABCD and then interact with it. Some of these interacting particles may be registered as visible alpha-tracks in the film depending upon the critical angle of registration (say, θc). The total length of the 4 tube-shaped water around LR-115 film is simply the periphery or circumference of the film, i.e., 2(a+b), and hence, the total volume of these 4 tube-shaped waters is equal to π (△Rw)2. 2(a+b). However, it is evident that 1/4th the volume of these 4 tube-shaped waters, i.e., ¼ π (△Rw)2. 2(a+b), is located just at above the surface ABCD of the film. By taking into account the critical angle for registration θc (=40°) in estimating the volume of tube-shaped water column IJBONMIA, the total volume of such four 1/4th water columns (say VII) will be
The volume VII is a part of the effective water volume Vw. The factor  is multiplied in estimating the total volume of the four tube-shaped water columns, because <IAN= <JBO =90°, <MAN= TBO= θc and hence <IAM=<JBT=90°-θc= 50°.
Region III, as shown in [Figure 3], is the four 1/8th spherical-shaped water columns which are located just at outside the 4 diagonal edges of the parallelepiped-shaped water column of ABCDLIJK. Such a spherical-shaped water column is JTOD′VUJB, and located just at a vertical edge JB (of height Rmax) of the ABCDLIJK water column. The 4 spherical-shaped water columns are the 4 thick spherical shells, and each of these has radii of Rmin and Rmax and hence thickness △Rw. The axis of the JTOD′VUJB water column is collinear to the vertical edge JB [Figure 3] and [Figure 5]. The alpha-particles emitted from these 4 spherical-shaped water columns are incident obliquely on the CN film (ABCD) and then interact with it [Figure 3]. Some of these emitted alpha-particles may produce their visible alpha-tracks in the CN film depending upon the critical angle of registration θc. The volume of each 1/8th spherical-shaped water is  By taking into account the critical registration angle θc (=40°) in estimating the volume of each 1/8th spherical-shaped water column, the total volume of the four 1/8th spherical-shaped water columns (say VIII) will be
The volume VIII is a part of the effective water volume Vw.
Hence, the effective volume of water Vw, where all the alpha-particles are present before reaching the detector's surface ABCD, is simply the algebraic sum of VI, VII and VIII, i.e.,
Vw= VI+VII+VIII
To simplify the calculation for Vw, let a = b = 1 cm, so the dimension of each LR-115 detector is 1 cm × 1 cm, i.e., its surface area ab = 1 cm2 which is also equal to the cross-sectional area ab of the vertical parallelepiped-shaped water column (EFGHLIJK). Thus, the final expression for Vw is:
Using △Rw = 26.98×10-4 cm =26.98 μm, the value of Vw is found to be about 2.711×10-3 cm3=2.711 μl. This is the required effective volume of water around the CN film of LR-115 type II SSNTD of surface area 1 cm2. Now, Vw = 2.711 μl is used in evaluating the CF for LR-115 type II SSNTDs.
To evaluate the CF, let N is the number of alpha-tracks registered on the alpha-sensitive face CN layer of the LR-115 type II SSNTD of area 1 cm2 during the exposure period of t sec due to the alpha-emitters present in the water sample only. Hence, the track density rate in unit of tracks.cm-2.s-1 is equal to N/t. Obviously, the rate of track density (N/t) gives the total alpha-radioactivity (in unit of Bq) of the water sample of volume equal to Vw. The concentration of total alpha-radioactivity (in unit of Bq. cm-3), say Aw, of the water sample can be measured using Formula (5):[14]
Since, the alpha-particles that would be interacting with the CN film of the LR-115 SSNTDs are only coming from the water volume Vw. Thus the CN film is exposed to 2π-geometry only. However, in the actual case, each detector is placed and fully immersed into the water sample. Additionally, the emission of alpha-particles in water is assumed to be isotropic. These procedures provide a 4π-geometry exposure to alpha-irradiations. Therefore, to get 4π geometry exposure, the alpha-track density should be accounted twice, i.e., 2N instead of N in Formula (5). This means the alpha-particles which are traveling toward the sensitive side of the films are detected and the alpha-particles which are moving in the opposite directions are not recorded in the films. Hence, we are missing half of the total alpha-particles emitted within the effective volume assuming that the emissions are isotropic in nature. For this reason, one particle registered is equivalent to two particles actually emitted. That is why the experimental track density N is multiplied by a factor 2 to get the actual total number of alpha-particles emitted within the effective volume. This is how we take into account the 4π-geometry exposure. The formula for estimating the alpha-radioactivity concentration (Aw) is, therefore, given by:
i.e., the CF is found to be 737.74 kBq l-1.(tracks.cm-2.s-1)-1 or 204.93 Bq.l-1.(tracks.cm-2.h-1)-1. To establish the validity of the evaluated CF, the value of the evaluated CF is required to compare with the CF obtained in other published studies. This comparison is presented in [Table 1]. It is observed from this table that the theoretical value of CF 204.93 Bq.l-1.(tracks.cm-2.h-1)-1 evaluated in the present study is almost consistent with the CF value of 215.82 Bq.l-1.(tracks.cm-2.h-1)-1 estimated in a published study.[8] This consistency in the results may validate the evaluated CF value in the present study. To confirm the validity of the evaluated CF for LR-115 type II SSNTDs in the present study, some samples of natural drinking water of alpha-radioactivity previously measured using CR-39 SSNTDs and reported in our published study[14] have been collected from 18 different locations of some railway stations, universities, and some rural and urban areas of West Bengal (WB) state of India. In the present study, the alpha-radioactivity of these collected samples has been measured using the LR-115 type II detectors. The evaluated CF 204.93 Bq.l-1.(tracks.cm-2.h-1)-1 for LR-115 type II detectors has been utilized in Formula (6) given below for measuring the alpha-radioactivity of the collected water samples. Details of the alpha-radioactivity measurement method have been described in the section “Materials and Method for measurement of total alpha-radioactivity in natural water by using LR-115 type II SSNTDs”. The reported (using CR-39 SSNTDs) and measured (using LR-115 SSNTDs) alpha-radioactivity values have been presented in different columns of [Table 2]. It is observed from this table that both the reported and measured values of alpha-radioactivity are almost equal. Hence, this observation confirms the validity of the evaluated CF for LR-115 type II SSNTDs in the present study. Details of the alpha-radioactivity measurement in the collected water samples by using LR-115 type II SSNTDs are presented in [Table 1]A in Appendix B of the Supplementary Material.{Table }
As the detectors are fully immersed within the water samples, the present method does not suffer from some drawbacks such as effects of misclassification phenomenon, quenching problem, and self-absorption, which are usually observed in some instruments or counters,[14] for examples, United States Environmental Protection Agency analytical chemical procedures of Method 900.0, gas flow proportional counters, alpha-spectrometry, and liquid scintillation counters.
The formula for measuring the total alpha-radioactivity concentration (in unit of Bq.l-1) in a natural water sample is now rewritten as:[14]
Here, N=the track density in unit of tracks.cm-2 registered on the CN film of the LR-115 type II detector during the exposure period of t h. In the present work, the exposure time (t) is taken to be 30 days, i.e., 720 h. If N= 1 tracks.cm-2 registered on the LR-115 film during the period of 720 h, then minimum detectable alpha-radioactivity (MDAR) of a natural water sample is found to be about 284.63 mBq.l-1.
| Materials and Method for Measurement of Total Alpha-Radioactivity in Natural Water by Using LR-115 type II SSNTDS | | |
In this section, a new model for a track etch method using LR-115 type II SSNTDs is described in details to measure the total alpha-radioactivity level in any natural water sample. The materials, apparatus and experimental setup required for utilizing this new method all are similar to the method developed very recently in our earlier research work,[14] except, type of detector's materials and etching conditions as well as exposure geometry. The exposure geometry for CR-39 and LR-115 SSNTDs is different because of the critical angle and energy window of these two types of SSNTDs are different. In our earlier work, we have used CR-39 detectors, but in the present study, we have used LR-115 type II detectors. The LR-115 is a commercial name of CN detector; and it is made by Kodak-Pathe, or DOSIRAD, France. A LR-115 type II track detector consists of two parts: (i) an etchable active layer (or film) of red-dyed CN of thickness ranging from 12 μm to 13 μm and (ii) a nonetchable polyester base substrate of thickness of 100 μm.[30],[31] The CN film is coated on the polyester base substrate. This CN film has a chemical composition of C6H8O9N2 and it is sensitive to alpha-particles only. The density, the ratio of atomic mass to atomic number, and the ionization potential of the CN film are, respectively, 1.4 g.cm-3, 1.939, and 81.1 eV.[15] The polyester base substrate of LR-115 is clear and transparent, and it is not sensitive to the alpha-particles or other charged particles.
Experimental setup for alpha-exposure
For making an experimental setup, three small pieces of LR-115 type II CN film SSNTD have to be fixed separately at three positions on a pre-labeled narrow glass tube with high-quality adhesive tapes. This is shown in [Figure 1]. The dimension of each SSNTD is about 1 cm × 1 cm. As shown in [Figure 1], two pieces, 1st (CN 1) and 3rd (CN 3), of the LR-115 detector are attached at two positions on a glass tube, and 2nd piece (CN 2) of this detector is attached between the positions of CN 1 and CN 2 on the glass tube. One end of the glass tube along with CN 1, CN 2, and CN 3 is now to be placed and then fixed with high-quality adhesive at inside bottom surface of a cylindrical PVC container so that the axes of both glass tube and container are collinear, and two surfaces of each detector are parallel to two faces S1 and S2 of the container.[14] The length of the glass tube should be equal to the internal height of the container. After that, the open face of the container must be closed tightly by its lid. This is the procedure to make an experimental setup that can be used for the exposure of LR-115 detector to the alpha-radiations of a water sample. An experimental setup must be used at once for a particular sample only.
Sample collection and alpha-exposure techniques
The experimental setup must carry carefully to the respective sampling site for collecting the water sample. A sample of water is usually taken into the container of an experimental setup so that all the LR-115 type II SSNTDs are fully immersed into the water sample [Figure 1]. The container must be kept fulfilled with the water sample so that no air bubbles could be present inside the container before closing the container's lid. After that, the open surface of the container must be covered tightly by its lid, and then well sealed by a high-quality adhesive tape such that volatile radionuclides could not escape from the water sample and the ambient air could not enter the container.[14]
Thereafter, the entire experimental setup with water sample is usually left undisturbed in a safe place for a period of 30 days for alpha-exposure.[14] During this exposure time period, the decay products of the radionuclides of natural decay series of 238U and 232Th are produced within the water samples and reach secular equilibrium with their respective parent nuclei.[17] Additionally, the alpha-particles that would be emitted by the decay processes of the alpha-radioactive nuclides present in the water sample will interact with the CN film of the LR-115 detectors, and then leaving their latent image of alpha-tracks along their trajectories in the CN film. The latent images will manifest themselves as pits of alpha during chemical etching of the exposed LR-115 detectors.
Chemical etching process and alpha-radioactivity measurement method
For chemical etching with the optimal etching condition, all the exposed LR-115 type II detectors are usually immersed into 2.5 N NaOH solution at 60°C temperature in an etching bath for a period of 2h.[3],[10],[11],[12],[26],[27],[28],[29] To keep temperature constant, the closed etching equipment must be used for etching and also takes into account the effect of stirring.
The etched LR-115 detectors are now to be rinsed through distillated water for 15-20 min and dried then under normal temperature in a dust-free open-air space. Subsequently, each etched detector is usually to be scanned under a high-resolution optical microscope provided with image analysis system for counting the number of alpha-tracks registered in the detector, and then estimating the track density ρ/i> in unit of tracks.cm-2. To estimate actual alpha-track density (N) due to the alpha-particles emitted from the water sample only, background (BG) count for alpha-tracks registered on the detectors must be subtracted from the estimated track density ρ/i>. The BG can be estimated by placing few LR-115 detectors inside few empty (without samples) well-sealed PVC containers and left for 30 days for alpha-exposure. After etching these exposed detectors, counting the number of alpha-tracks that might be registered on these etched detectors due to the alpha-emitters present within these empty containers and then estimating the average value of the alpha-track densities (tracks.cm-2). This average value is the BG count which must be subtracted from the measured track density χ to obtain the actual track density N for the sample.[14] Therefore,
N = χ-BG
To estimate the total alpha-radioactivity concentration (Bq l-1) in a given water sample, the track density rate registered in each LR-115 type II detector must be estimated accurately. The average value of the estimated track density rates N/t in unit of tracks.cm-2.h-1 and the evaluated CF of 204.93 Bq.l -1.(tracks.cm-2.h-1)-1 for this detector can be used in Formula (6) for measuring the total alpha-radioactivity concentration (in unit of Bq.l-1) in the investigating water sample.
| Results and Discussions | | |
A new model for track-etch method using the LR-115 type II CN film solid-state track detectors (SSNTDs) has been developed in the present study for measuring directly the total alpha-radioactivity level in any natural water sample. The measuring method is based on (i) placing small strips of the LR-115 track detectors in bare exposure mode into the water samples and (ii) geometry of the experimental setup for alpha-exposure. The emitted alpha-particles from the alpha-emitters present in the water sample will interact with the CN film and create latent tracks of alpha, which are a measure of the alpha-radioactivity of the investigating water sample.
In order to express the alpha-radioactivity concentration in unit of Bq l-1 in the water samples, the CF in unit of Bq.l-1.(tracks.cm-2.h-1)-1 for the CN film of LR-115 type II SSNTDs has been evaluated theoretically. The method for evaluating the CF is based upon the range of alpha-particles in natural water, energy window of CN film, critical angle of etching for CN film, optimal etching condition for CN film, and effective volume of water around the CN film. The effective water volume around the CN film of LR-115 track detectors from which alpha-particles can reach to the CN film has been estimated accurately to evaluate the CF. This small volume is contributing toward the alpha-track development in the CN film. Actual geometrical view of the effective volume of water has been presented clearly in the present paper. The effective volume of water is found to be 2.711 μl. This volume of water is also representative for the CF evaluation and alpha-radioactivity measurement methods. The theory and method for evaluating the CF have been described in details in the present paper.
The evaluated CF in the present work for LR-115 type II SSNTDs is found to be 204.93 Bq.l-1.(tracks.cm-2.h-1)-1. The theoretical value of this CF is consistent well with the estimated CF in another study.[8] This consistency in results of CF may establish the validity of the CF evaluated in the present study. Some samples of natural drinking waters (tap water and tube-well water) with alpha-radioactivity reported in our earlier published study[14] have also been measured by the LR-115 type II SSNTDs using the evaluated CF in the present study. These radioactivity values are tabulated in [Table 2]. It is observed from this table that the alpha-radioactivity values measured in the present study and the alpha-radioactivity values reported in our earlier published study both are almost equal. The consistencies in the measured and reported alpha-radioactivity values also confirm the validity of the evaluated CF in the present study. Hence, the evaluated CF for LR-115 type II SSNTDs can reliably be utilized to measure the total alpha-radioactivity of any natural water sample. The evaluated CF is new and unique for such measurement.
In the present study, the total alpha-radioactivity concentration (in unit of Bq l-1) in some samples of natural drinking waters of 18 different locations in six districts of Paschim Medinipur, Purba Medinipur, Howrah, Kolkata, Bankura, and Purba Bardhaman in WB state, India, has been measured using Formula (6) and evaluated CF. The experimental method for alpha-radioactivity measurement is similar to that described in the earlier section. The measured alpha-radioactivity values of the investigated water samples are presented in [Table 2]. The values of arithmetic mean with standard error of the estimated alpha-track densities have been used in calculating the alpha-radioactivity of investigated water samples for each water point.
It is observed from [Table 2] in the present study that the measured alpha-radioactivity concentration in the investigated waters has been found in the range of 15.37 ± 0.58 Bq l-1 to 36.43 ± 3.22 Bq l-1, of which the alpha-radioactivity values vary from 15.37 ± 0.58 Bq l-1 to 34.16 ± 0.20 Bq l-1 in case of tap waters and 20.78 ± 2.43 Bq l-1 to 36.43 ± 3.22 Bq l-1 in case of tube-well waters. Relatively low alpha-activity concentration has been found in most of the tap water samples and high alpha-activity concentration has been found in most of the tube-well water samples. Both minimum and maximum values of alpha-radioactivity of the tap water samples are also lower than that of the tube-well water samples. The low alpha-radioactivity level in tap waters may be due to the decreasing of the activity level of alpha-emitters, mainly short-lived radon and its alpha-decay products, when these emitters are migrated through water flow from the water source points to the water reservoirs and then reservoirs to the water supplying pipes.[14]
The above mentioned radioactivity data indicate that the measured values of alpha-radioactivity of the water samples are varying from location to location. This variation may be depending upon the local geological formation and the origin and mobility of alpha-emitters present in bedrocks and soils in the sampling sites. The local geological characteristics and geochemistry of radionuclides both may control the distribution of radionuclides and hence their activity concentration in the natural water of the locations.[14] The radioactivity data reported in the present study may serve as a good baseline.
However, the new track-etch method developed in the present study offers us an effective and accurate way of determining the level of alpha-radioactivity in water of all natural sources. This method is much simpler, portable, reliable and cost-effective. To utilize this method, it is not necessity the use of any reference radioactive source for the calibration of LR-115 SSNTDs.
Not only this model is suitable for determining the alpha-radioactivity of natural water, but also it can be used for observing and monitoring the radioactivity contamination level in relation to the alpha-emitting radionuclides in natural water. Additionally, the method developed in the present study can be helpful for researchers working in the radioactivity field, and it can also be useful for determining the population exposure to alpha-radiations by consumption of natural drinking water. The new thing in the present study is the way to calculate the effective volume of water around the CN film of the LR-115 type II track detector and the evaluated CF for this detector. The CF evaluated and the experimental method developed in the present study can reliably be utilized in research with radiological physics and technology to assess the radioactivity contamination risk for the health of consumers of alpha-radioactivity contaminated water.
| Conclusions | | |
A new model of a track-etch method has been developed for measuring the total alpha-radioactivity concentration in water of different natural sources by using LR-115 type II SSNTDs. This method has the advantages of being much simpler, reliable, portable, and nonexpensive, and it offers an effective and accurate way of measuring the total alpha-radioactivity of natural water.
The present study is the first that developed a new theory and methodology for evaluating the CF for the LR-115 type II SSNTDs to monitor reliably the total alpha-radioactivity of natural water. The evaluated CF value is new and unique, and its value is found to be 204.93 Bq.l-1.(tracks.cm-2.h-1)-1. The evaluated CF can be utilized in the alpha-radiation detection and measurement as well as in other works relevant to the radioactivity field measurements.
Utilizing the present model developed in this study, the alpha-radioactivity concentration in some drinking waters of WB state in India has been measured. The measured alpha-radioactivity values have been found in the range of 15.37 ± 0.58 Bq l-1 (tap water) to 36.43 ± 3.22 Bq l-1 (tube-well water). The variation of alpha-radioactivity may be attributed to the local geological formation of the locations. The alpha-radioactivity results revealed that the relatively high alpha-radioactivity is present in tube-well waters and low alpha-radioactivity is present in tap waters of a location. The obtained alpha-radioactivity values in the present study agree well with the reported alpha-radioactivity values in our earlier study.
Novelty of the present research work relies on the method for CF evaluation and the model developed for alpha-radioactivity measurement in natural water. The present work is also helpful in describing the methods and procedures for the radioactivity contamination risk assessment. Moreover, the present work can be deemed as useful for public health and safety.
Acknowledgments
The authors are thankful to Emeritus Professor Dipak Ghosh, Department of Physics, Jadavpur University, Kolkata – 700032, West Bengal, for his academic support in preparing this manuscript.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]
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