Nghiên cứu phát triển liều kế màng PVA nhuộm để đo liều gamma và neutron

Radiation dosimetry is closely associated with the sustainable development of nuclear physics, nuclear technology, radiation technology and other related fields such as industry, agriculture, medicine, etc. Dosimetry methods and techniques are generally based on the evaluation of energy absorption in which radiation is directly transmitted to matter by increasing heat, or based on the estimation of energy absorption via secondary operations working together in matter like ionization, structural changes, chemical and biological reactions, discoloration of materials, etc.

       Polyvinyl Alcohol (PVA) is a tissue-equivalent organic material that occupies a very small place in space. Dyed PVA film dosimeters are thus highly suitable for gamma dose measurement, particularly for high-dose control of radiation processing. They may be also used in dosimetry of neutron and other types of radiation. Less attention, however, has been paid to these dosimetry purposes due to difficulties in estimating the sensitivity of dose ranges. The trouble has happened since the dose response curves for the different dose ranges have not been defined clearly. The purpose of this research is to determine characteristic functions and the sensitivity of the dyed PVA film dosimeters for gamma and neutron dosimetry by measuring a wide range of absorbed doses and using suitable mathematical functions as well as dyes. 

Fig.  Evolution of the optical density  versus the thermal neutron dose of the dyed PVA films at 668nm wavelength

        The dyed PVA films containing methylene blue and different masses of boric acid undergo a big change in their color when they are irradiated with gamma-rays. In order to investigate the discoloration of the dyed PVA films, in this research, the samples were irradiated with 60Co γ-source in the dose range of 0 to 150 kGy. The UV-VIS 2450 spectrophotometer was used to measure the optical absorption spectra of the irradiated films at 668 nm wavelength. The gamma dose response curves of the dyed PVA films were described by saturated exponential function of the energy transfer model with a high correlation coefficient. Besides the optical density changes, fading effect of the dyed post-irradiation films was investigated carefully.

       On the basis of optical density values of the pre and post-irradiated films, the research team established the gamma dose response curves, evaluated the discoloration of post-irradiated films at different dose ranges, and investigated the color sensitivity of different film types with different colors. The research results showed that the dyed PVA films with methylene blue had the highest color sensitivity in comparison with other dyes in the dose range of 0 to 150 kGy. The team also investigated roles and influences of PVA solution concentration on the films. The team found that the films dyed with 2.94% of PVA content had the highest value of optical density reduction, which indicated that these films have the best performance.

       In order to increase radiation sensitivity of the films, a suitable amount of boric acid was added into the films. This has contributed to a considerable increase in optical density value of the pre-irradiated films and in optical density reduction of the post- irradiated films. Thus, the gamma radiation sensitivity of the films has been improved significantly.

       For neutron irradiation, the PVA films dyed with methylene blue were decolorized after having been irradiated in durations from 1 hour to 24 hours in the dose range of 0.059 to 1.411 Gy with thermal neutron source at the end of the channel No. 2 of the Dalat nuclear research reactor. The thermal neutron flux at an outer position of beam line at the channel No. 2 was 1.6×106 n/cm2/s. The UV-VIS 1249 spectrophotometer was used to estimate the optical density of the pre and post-irradiated films at 668 nm wavelength. A suitable amount of boric acid was also added into the dyed PVA films to improve physical properties of the films. The results indicated that the optical density value of the films decreased when the duration of irradiation increased. The neutron dose-response curves of those films were described by the saturated exponential function of the energy transfer model.

The research results have been published in international journals, including:

1. Anh Thi Vo, Nghiep Dai Tran, Giap Van Trinh, Diep Bang Tran, Binh Van Nguyen (2018). Investigation characteristics of polyvinyl alcohol films dyed Methylene blue as a radiation dosimeter, Journal of Radioanalytical and Nuclear Chemistry, https://doi,org/10,1007/s10967-018-5921-2.

2. Anh Thi Vo, Nghiep Dai Tran, Giap Van Trinh, Son Ngoc Pham (2018). Effect of thermal neutron irradiation at No. 2 channel of Dalat Nuclear Research Reactor on dyed polyvinyl, Journal of Radioanalytical and Nuclear Chemistry, https://doi.org/10.1007/s10967-018-6177-6.

Vo Thi Anh, INST