To Determine Dose Response Curves of Dyed Polyvinyl Alcohol Films Irradiated with Gamma-Rays

The Polyvinyl Alcohol (PVA) films dyed with different colors such as methylene blue (MB), methyl orange (MO), methyl red (MR) and crystal violet (CV) were investigated. The dyed PVA films were irradiated by with Co γ-source in dose range of 1 150 kGy. The optical density change in these films at pre and post irradiation was studied by spectrophotometer. The gamma dose response curves of the dyed PVA films were described by saturated exponential function of the energy transfer model with high correlative coefficient. The color sensitivities on the dyed PVA films irradiated by source were different. The PVA films dyed with MB had the highest sensitivity.


Introduction
PVA is an excellent matrix for radiochromic centres and widely used for lowand high-dose dosimetry purposes.For example, the colored polyvinyl alcohol films can be used in dosimetry for gamma rays and neutrons in nuclear reactors [1].Using dosimeters for high doses control of radiation processing is extremely important in the field of radiation technology.The PVA is a tissue-equivalent organic material that occupies a very small place in space.So the PVA material is highly suitable to use in gamma dose measurements [2] [3] [4] [5] [6].Some researchers in the world have used different colors which have radiation sensitive properties to add to the PVA films for high dose control.The dyed PVA film containing a mixing of ethyl violet and bromophenol blue [7], and the dyed PVA Open Journal of Safety Science and Technology film containing methyl red [8] are useful as routine high-dose dosimeter with Co 60 γ-source in the 1 -30 kGy range.The film dosimeters made from PVA films dyed with methylene blue [9] and methyl orange [10] were studied for high-dose control with Cs 137 γ-source in dose range of 100 -200 kGy.
Most of these studies have carefully surveyed the factors that affect the discoloration of films such as pH, color concentration, film thickness, pre-irradiation and post-irradiation stabilities.Apart from that, the researchers usually pay attention to investigate discoloration of post-irradiation films for different dose ranges [7] [8] [9] [10].However, they did not clearly define the dose response curves for the different dose ranges that they surveyed.
The current works are to define the dose response curves of the dyed PVA films.Several dyed PVA films containing difference colors such as crystal violet, methyl red, methylence blue and methyl orange were strongly changing the original colors under Co 60 γ-source and became stable radiochromic thin films which can be used as dosimeter for measurement of radiation doses in different dose ranges.

Materials and Methods
We prepared four types of film with four different colors.From the results of optical density values on the pre and post-irradiation films, we determined the gamma dose response curves and investigated the different color sensitivity of four film types with four different colors.

Absorption Spectra
The optical absorption spectra of unirradiated and irradiated films were measured in the wavelength range of 540 -660 nm and maximum at 599 nm with CV-PVA, 400 -600 nm and maximum at 520 nm with MR-PVA, 500 -750 nm and maximum at 668 nm MB-PVA and 300 -600 nm and maximum at 440 nm with MO-PVA as shown in the Figures 2-5    The amplitude of these absorption bands decreased gradually with the increase of dose of γ-ray without changing λ max .The change in optical density at λ max per unit thickness increased when γ-ray dose increased.

Response Curves
The response curves have been built by characteristic functions of radioactive energy transfer from γ-source to material made dosimeters.The energy transfer process is mainly performed by δ-electrons.They get enough energy and leave their orbits in the atom to become free electrons.According to the energy transfer model, the response curves of the dyed PVA films can be expressed as the saturated exponential function of the optical density values n(D) as follows: ( ) where D is dose; n 0 and n s are respectively number of the activated element at D = 0 and D = ∞; k is the total probability that one radiation sensitive element of the system will be activated per unit of time.In fact, k consists of two components: where p is the probability that one sensitive element will be activated per unit of time; q is the probability that one activated element experiences reactivation per unit of time [11] [12].
Optical densities of the dyed PVA films decreased when they irradiated gamma source with increasing dose.The absorbance reductions of films were fitted in formula (1) giving the response curves (as the dose characteristic function) which depicted in Figure 6 and Table 1.

Sensitivity of the Dyed PVA Films
To assess the color variability of the dyed PVA films which were irradiated by the gamma source, we determined the color sensitivity of a dosimeter by the formula: where n 0 and n s are respectively number of the activated element at D = 0 and D = ∞.From expression (3) it is clear that s value (the color sensitivity) is big, the discolorment of the film is big.The sensitivities of different dyed films were shown in Table 1.These results showed that The MB-PVA dyed films were the best dosimeters, while MR-PVA dyed films were the bad dosimeters.

Conclusion
Study on the optical density change of the dyed PVA films at pre and post irradiation by Cs 137 γ-source in dose range of 1 -150 kGy shows that the γ-ray absorptions of these films were described by saturated exponential function of the energy transfer model.They are appropriate to make dosimeters for routine irradiation processes.The sensitivity results indicate that the MB-PVA dyed films are the best dosimeter.

Figure 2 .
Figure 2. The absorption spectra of unirradiated and irradiated CV-PVA films measured at wavelength range 540 -650 nm for difference doses.

Figure 3 .
Figure 3.The absorption spectra of unirradiated and irradiated MR-PVA films measured at wavelength range 400 -600 nm for difference doses.

Figure 4 .
Figure 4.The absorption spectra of unirradiated and irradiated MB-PVA films measured at wavelength range 500 -750 nm for difference doses.

Figure 5 .
Figure 5.The absorption spectra of unirradiated and irradiated MO-PVA films measured at wavelength range 300 -600 nm for difference doses.

Figure 6 .
Figure 6.The response curves of the dyed PVA films.

Table 1 .
The coefficients of the gamma dose response curves.