Comparative study of two mixture dyes based on PVA for industrial use as gamma radiation dosimeter

New film dosimeters PVA/(DCP/CR) and PVA/(DCP/BPR) have been prepared through the casting method. The colorimetric properties of both films changed from violet to pink and from green to yellow with exposures to gamma rays between 0–80 and 0–40 kGy, respectively. The color response was accelerated with chloral hydrate to half of the radiation dose between 0–40 and 0–20 kGy for both systems, respectively. The UV–visible spectrophotometer shows outstanding stability of the prepared dosimetric systems. Relative humidity was studied and the response indicated that both films possess very good stability during irradiation.


Introduction
Radiation indicators are applied for the un-irradiated and irradiated products identification, in quality control processing of food and medical product sterilization. Gamma colorimetric dosimetry is based on a color change induced by exposure to ionizing radiation such as gamma radiation that causes chemical and physical changes in the physical and chemical properties by which the absorbed dose can be measured [1], which in turn affects its response towards various factors such as (light, dose rate, temperature, humidity, etc.) [2]. plastic dosimeter films are the most applied in radiation processing owing to their favorable characteristics [3]. Many researchers applied polymers as hosting the material in film preparation as it is portable detectors, lightweight, have long shelf-life, are easily placed anywhere, its ruggedness is stable, easy to control, and have adequate spectrophotometric analysis [4,5]. The polymeric film is made of different polymeric materials like vinyl acetate [6], polyvinyl chloride, polyvinyl alcohol [7,8], polystyrene [9], cellulose acetate [10] polyvinyl butyral [11] are applied as dosimeters and it was found that the resulting change occurred in the polymeric matrix of this films depended on the hosting polymer type and the dye used either radio-chromic [12] or pH-indicator dye [13]. Polyvinyl alcohol has the advantage of complete biodegradability with non-carcinogenic and harmless effects [14], has great mechanical properties [15], high thermal stability, and excellent physical and chemical properties due to the presence of hydroxyl group which makes the PVA suitable as hosting material for dosimeter Polyvinyl alcohol is also characterized as being transparent, flexible, easy to handle, and cheap. It also causes stability of the radiometer over time [16,17]. Polymer doping with dye or organic compound induces new desirable characteristics that to widen gamma radiation detection, measurement, and bio-sensors application [18]. Many mixtures of the dyed polymeric film were prepared from two dyes having different responses to gamma radiation which lead to the discovery of their new sensitivity and dose range scale [19]. 2, 6 Di-ChlorophenolindoPhenol (DCP) / PNP (4-Nitro Phenol) mixture dyed PVA film had a color change from green to yellow firstly then bleached when exposed to high doses. The applicable dose range for using this film as a label ranged from 1 to 50 kGy (from green to yellow color) and from 5 to 150 kGy in case of bleaching occurs to the yellow color [20]. (DCP) with Tetra Bromo Phenol blue (TBPB) mixture dyed PVA films were studied at various concentrations of chloral hydrate. These films changed from blue to yellow color passing through green thedose range was from 1 up to15 kGy. The decomposition of chloral leads to the release of positive hydrogen ions, which act on the acidity of the medium by changing the pH, which leads to a faster decomposition of the dyes used with lower radioactive doses [21]. Congo Red (CR) dyed Polyvinyl alcohol films was applied for electron radiation and gamma dosimetric applicaion. The color of these films bleaches upon irradiation [22] that showed excellent pre and post irradiation stability [23]. poly (Vinyl Butyral) PVB or poly (Vinyl Alcohol) PVA films dyed with sodium salt of 2.6-Dichloro Phenol-indophenol(DCP) applicable in high-dose routine applications. These films were bleached after gamma radiation exposure and the response depended on both concentration of the dye and polymer used. Chlorophenol red (CPR) and Quinaldine Red (QR) dyed PVA films were studied as gamma radiation dosimeters in the dose range from 2 up to 30 kGy. The color film changed from dark red to yellow after gamma irradiation. Different concentrations of Chloral hydrate were studied to investigate the acceleration that occurred in the dye degradation upon gamma radiation exposure [24]. Our work aims for preparing new dosimeters in the form of dyed mixture PVA films through mixing and casting techniques and studies their optical response upon radiation exposure, effect of additive on the prepared films (chloral hydrate), sensitivity and stability of the prepared films with the time.

Material
Poly (vinyl alcohol) (PVA), average molecular wt. 125.000 was purchased from El-Goumhouria Co. Egypt. CongoRed, BromoPhenolRed, and 2,6-Di Chloro phenol indo Phenol have been purchased from A quality product of Qualikems, India. Whereas Chloral Hydrates, with a molecular weight of 165.4 has purchased from Merck, Germany.

Preparation of PVA/(DCP/CR) film dosimeter
The solution mixture of CR and DCP was prepared by adding 0.04 gm of Congo Red Dye equivalent to 1.148 mol/ Land 0.04gm of DCP Dye equivalent to 2.98 mol/L in 50 ml of distilled water. The film solution was prepared by adding 9 ml of the prepared dye solution mixture equivalent to 0.05 mol/liter for Congo Red and 0.1341 mol/L for DCP dyes to 100 ml of PVA solution (5%), which was dissolved at 60 °C. The prepared solution was allowed under stirring for 2 h to obtain complete homogeneity between the dyes and the hosting polymer. It was divided into four portions, Three of these were used to study the film behavior with and without chloral hydrate at different concentrations (0.06, 0.08, and 0.1) gm per 20 ml of PVA which is equivalent to (6, 8, and 10) phr. The solutions were kept well stirred at room temperature for about 3 h to obtain complete homogeneity of the mixtures. The prepared solutions were poured on a horizontal glass plate 15 × 15 cm 3 , and the specially designed box was put over the casted film in order to protect the film from dust, and impurities and dried at room temperature for about 48 h. After drying, the films were stripped from the glass plate, then cut into small pieces 1 × 1 cm 2 and stored for further investigation. The thickness of the obtained films was found to be 0.08 ± 0.005 mm.

Preparation of PVA/(DCP/BPR) film dosimeter
The solutions mixture of DCP and BPR was prepared by adding 0.04 gm of DCP dye equivalent to 2.98 mol/L and 0.04 gm of BPR dye equivalent to1.562 mol/L in 50 ml of distilled water under stirring for 2 h, adding 9 ml of dye solution mixture equivalent to 0.1341 mol/L for DCP and 0.07 mol/L for BPR dyes to 100 ml of 5% PVA solution which was dissolved at 60 °C dividing the solution prepared into four parts. Three of these were used in order to study chloral hydrate different concentrations of (0.06, 0.08, and 0.1) gm per 20 ml of PVA dyed solution equivalent to (6, 8, and 10) phr. After preparing the four solutions were kept well stirred at room temperature for about 3 h in order to obtain a uniformly mixed solution. Each solution was poured on a 15 × 15 cm 3 horizontal glass plate, the specially designed box was put over the casted film in order to protect the film from dust, and impurities and dried at room temperature for about 48 h. After drying, the films were stripped from the glass plate, then cut into 1 × 1 cm 2 pieces and stored for further investigation. The thickness of the obtained films was found to be 0.07 ± 0.005 mm.

Characterization
The absorption spectra of the irradiated and unirradiated films were measured using a UVKON 860 spectrophotometer with a wavelength range (of 200-800 nm). The film thickness was measured using a Digitrix-Mark thickness Gauge (precision ≠ 1 um; 1δ). Gamma irradiation was carried out in 60 Co gamma chamber 4000A irradiation facility (product of India). The absorbed dose rate was measured to be 1.42 kGy/h, using alanine dosimeters as a reference.

Results and characterization
Absorption spectra of PVA/(DCP/CR) and PVA/(DCP/ BPR) dosimeter films The data of spectral UV analysis for the unirradiated and irradiated two dosimetric films Show an absorption spectrum in the region of visible color 400-800 nm. The first system consists of PVA/ (DCP/CR) mixture dyed films and the second one consists of PVA/ (DCP/BPR) mixture dyed films. It was affected by gamma radiation [25] and that's observed from the color change of the prepared film dosimeters [26] from violet to pink as represented in Table 1 and from green to yellow for the second system as shown in the previous table. From (DCP/CR)/PVA film absorption spectra Fig. 1a, it appeared that the band amplitude gradually decreases with the increase of the gamma-ray photons. The maximum absorbance λ max at 640 and 539 nm characteristic of blue and orange color (appeared violet). These prepared films are applicable for high-dose dosimetric applications with a dose range from 0 up to 80 kGy. For the second system, the band appears at λ max 640 and 450 nm for green and yellow color (appeared green), and also its intensity decreases by increasing radiation dose but differs in sensitivity as shown in Fig. 1b, presenting much more sensitive to radiation than the first system, the applied dose range extended from 0 up to 40 kGy.The fast degradation of PVA/ (DCP/BPR) film than (DCP/CR)/PVA film owing to the complex structure of congo red having two azo groups (N=N), with high molecular weight than that of bromophenol red [27]. The absorption spectra indicate the outer electron's excitation (π-π* transition) with increasing radiation Table 1 Effect of gamma irradiation on PVA/(DCP/CR) and PVA/(DCP/BPR) mixture dyed film at different irradiation doses dose, upon increasing radiation, the hosing polymer (PVA) and the dyes mixture presented suffer structure damage with free radical libration, inducing the dye degradation as shown in Fig. 1a, b the absorbance curves amplitude at 640 nm decreases gradually until the degradation of the most of dye molecules at which the band bleaches (at 80 kGy for PVA/ (DCP/CR) and 40 kGy for PVA/(DCP/BPR) dyed film [28,29].

Effect of chloral hydrate addition on the film response to gamma rays
The two prepared films were studied with chloral hydrate addition after and before radiation exposure as shown in Fig. 2a, b. The amplitude of the two systems bands at 640 nm decreases much faster by the addition of chloral hydrate with the increase of absorbed gamma doses, in the case of PVA/ (DCP/CR) films the dose range was from (0-30) kGy instead of (0-80) kGy. Whereas in the case of PVA/(DCP/BPR) films the dose range was from (0-20) kGy instead of (0-40) kGy. Chloral hydrate addition accelerates the reaction indicating the formation of the strong acid [25]. With increasing radiation dose, a large number of chlorine ions dissociate from the carbon chain of chloral hydrate and then combine into an acidic form as chloral hydrate lowers the pH of dyed films during gamma irradiation [30]. Chloral films apply at lower doses than the films prepared without chloral hydrate.

Response curves of PVA/(DCP/CR) and PVA/(DCP/ BPR) film dosimeters
The dose-response curves were established in terms of change in absorbance measured ΔA = A 0 -A i , against the absorbed dose at 640 nm, for un-irradiated and irradiated three films samples containing three chloral hydrate concentrations (6, 8, and 10) phr for each system, where A i and A 0 are values of optical absorbencies for the irradiated and un-irradiated film. Both two systems show a linear behavior up to 15 kGy as shown in Fig. 3 (a and b). All curves in Fig. 3a, b show similar trends but have different slope values (slope of the initial linear part of curves). The slope increases with the increase of chloral hydrate concentrations with the ratio of 20% per 2 phr increase of chloral concentration for both two systems of the studied films.

Sensitivity measurements
The radiation sensitivity of the dyed film samples toward gamma radiation was defined as the slope of the dose-response curves [31] containing different chloral hydrate concentrations. For both two systems from Fig. 4 both two systems have the same trend with increasing chloral hydrate concentration. From the obtained results, the sensitivity increases in the shape of a polynomial manner curve Fig. 5.

Relative humidity of PVA/(DCP/CR) and PVA/(DCP/ BPR) film dosimeters
The effect of relative humidity (RH) during irradiation on the response was investigated [32]. The films were suspended over various solutions of saturated salt in closed vials, except for the 0% RH which was suspended over dried silica gel.
Relative humidity effect on the performance of PVA/(DCP/ CR) and PVA/(DCP/BPR) films at (92 76, 54, 33, 12, and 0%) relative humidity were studied for three days to establish the equilibrium conditions. The films were exposed to radiation in the same vials to 20 kGy. The absorbance variation of films irradiated to 20 kGy was normalized relative to the value measured at 33% RH. From Fig. 5 we concluded that the dose-response with relative humidity change (less than 78%) has low change indicating that (DCP/CR) and also (DCP/BPR) dyed films possess great stability in a different range of relative humidities.

Irradiation stability of PVA/(DCP/CR) and PVA/(DCP/ BPR) film dosimeters
The stability experiment aimed to investigate the effects of different storage conditions on the absorbance of pre-irradiated and post-irradiated storage films applied by putting un-irradiated and irradiated film samples in various storage conditions. One group of samples was stored at room temperature under Fluorescent laboratory lighting, while the second group was stored at room temperature in the dark. plotted for (pre or post-irradiated film is the relation between relative stability (measured absorbance reading relative to the first day measured absorbance) and time per day for the two films stored in dark and light.

Pre-irradiation stability of PVA/(DCP/CR) and PVA/(DCP/ BPR) film dosimeters
The pre-irradiation stability of PVA/(DCP/CR) and PVA/ (DCP/BPR) films before irradiation in dark and light were studied in Fig. 6a, b at different storage conditions of dark and light at room temperature [33]. The film's absorbance was measured at 640 nm, during the storage period of 30 days. There is a 30% decrease in absorption during the first 5 days, then the film is stable for 30 days, which is the duration of the study.
The PVA/(DCP/CR) film stored in the dark shows slightly higher absorbance than that stored in the light which indicates that the dyed films are slightly light sensitive as the dye mixture shows tiny degradation by light action [34]. While in the case of PVA/(DCP/CR) the absorbance varies between dark and light conditions.

Post-irradiation stability of PVA/(DCP/CR) and PVA/(DCP/ BPR) film dosimeters
The color stability of PVA/(DCP/CR) and PVA/(DCP/CR) films after irradiation to 20 kGy was studied in Fig. 7a, b in different conditions of dark and light at room temperature. The film's absorbance was measured at 640 nm during a storage period of 30 days. The films show good stability after the first five days of storage. The post-irradiated PVA/(DCP/CR) film stored in the dark shows higher absorbances than that stored in light. As in the dark, the film was protected from small light degradation action. While in the case of post-irradiated PVA/ (DCP/BPR) films, the film stored in light shows higher absorbances than that stored in the dark as the remaining small numbers of free radicals generated by radiation action are excited upon light exposure, leading to a slight increase in absorbance reading [35].

Conclusion
In this study, a new gamma irradiation dosimeter based on two mixture dyes (DCP-CR) and (DCP-BPR) was investigated. Radiation dosimeter films of PVA/(DCP-CR) and PVA/(DCP-BPR) have been prepared by casting method. The color of PVA/(DCP-CR) films changes when exposed to gamma radiation doses from violet to pink in the range (0-80) kGy and this range decrease when chloral is added to fall within the radiation dose range of (0-40) kGy, while the second PVA/( DCP-BPR) dyed film, the color changes from green to yellow in the dose ranged from 0 to 40 kGy without chloral addition which accelerates the radiation response as it liberates HCl upon radiation exposure the dose range was from 0 to 20 kGy. Both two prepared films show good stability before and after irradiation (in dark light) and ease of handling. Relative humidity response changes slowly (less than 78% indicating that (DCP-CR) and also (DCP-BPR) dyed films possess very good stability in this applicable range of relative humidity during irradiation. These films reflect their suitability for use as a radiation dosimeter for food sterilization and pharmaceutical applications.
Acknowledgements All persons who have made significant contributions to the work mentioned in the manuscript (eg, technical assistance, writing and editing assistance, general support) are the working authors and we thank the National Centre of Radiation Research and Technology, Egyptian Atomic Energy Authority for the support needed to complete this work Author contributions All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, Methodology, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the journal of Radioanalytical and Nuclear Chemistry.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.
Data availability Data are available under the request.

Conflict of interest
The authors declare that they have no conflict of interest.
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