ESR Analysis Method And Performance Stability Study Of Carbonate Hydroxyapatite Dosimeter

The ionizing radiation has been widely applied in the fields of industrial production,scientific research and medical care,so ionizing radiation dosimeters with stable development performance and suitable for different applications are particularly important in order to ensure the accuracy and reliability of dose monitoring.In recent years,the ESR dosimetry method,which uses electron spin resonance(ESR)to detect free radicals and other paramagnetic components induced by ionizing radiation,has aroused extensively research and widely attention.Some methods have been developed into secondary standard dosimetry method and used in practice,such as alanine/ ESR dosimetry.Hydroxyapatite(molecular formula is Ca10(PO4)6(OH)2,abbreviated as HAP),one of biological active materials,is similar to enamel on composition and structure and especially has high performance of bioequivalence and produces a dose-dependent stable CO2-free radical postirradiation.Therefore,using different doping methods to transform the structure of hydroxyapatite has become an important way to obtain a new dosimeter material.As a dosimeter material,stability performance under various environmental conditions is an important factor in evaluating and determining its practicability.The purpose of this paper is to develop a promising type of radiation dosimeter based on doped hydroxyapatite and to in-depth study on the stability of dosimetric characteristics meanwhile establishing a shaping method to form tablet dosimeter as well as setting up ESR quantitative analysis method,therefore to lay the foundation for the later process optimization and the control of sample storage and transport conditions in the future application.The main research contents,methods and results are as follows:1?Making pellet dosimeter pieces.The carbonate hydroxyapatite(CHAP)synthesized powder samples in our lab-made was selected in the work.In order to prepare sample piece suitable for this work,the shaping method based on tablet compressing techniques was established and the optimal size of the pellet piece and suitable adhesion agent accessory material and reasonable ratio of mixing were determined by systemic experiments.The set up approach was as each sample had a thickness of 4.2 mm,a diameter of 2.7 mm and a mass of about 30 mg.The actual measurement process should contain two pellets for each measurment.The experimental results showed that pellet pressing has no significant influence on the ESR signal intensity,but this way can improve the measurement dispersibility through comparing the ESR signal characteristics between pellet pressing and powdered samples after radiation.The result suggested that to simplifier the measuring process,powder sample was practical for measurement in small-scale experiments or in high-dose applications and pellet pressing technology would be more beneficial in the case of long-term storage or low-dose experiment in order to improve the controllability of the dosimeter measurement.2?Establishing practical ESR measure ment method.The tablet of CHAP dosimeter was taken as research object.The ESR quantitative analysis was conducted by ensuring the high signal-to-noise ratio and signal resolution at the same time.Furthermore in order to balance the signal range caused by the doses and the signal saturated,experiments were mainly focused on the effects of ESR measurement parameters such as the modulation amplitude,microwave power,scanning time and time constant.We selected weak pitch as the calibrating standard sample in the whole measurement process to overcome the fluctuation in environmental conditions and the unstability of the instrument.The optimal measurement conditions were determined as following: modulation frequency 100 kHz,gain 4.9×104,sweep width 200 Gs,microwave power 10 mW,modulation amplitude 2Gs,scan time 60 seconds and time constant 20 ms.3?Researching on the stability characteristics of CHAP dosimeter material..This research set up different storage conditions respectively of different temperature,humidity and illumination conditions to store the CHAP sample after 60Co-? ray radiation And then ESR technique was used to quantitatively measure the radiation-induced free radical signals in each specific spans.The results showed that signal intensity of each condition was not significantly attenuated but different storage conditions have some influences on the signal measurement process and signal dispersion.The maximum relative shifting away from the mean value was taken as evaluation index of the measurement.Within the three months of the experiment,the signal change was less than 3% when the samples were preserved at 4? or room temperature.At 40?,the signal changed about 13%.At room temperature with the humidity less than 36%,the signal only changed less than 2% but the change rose to about 8% when humidity was 76%.However,no significant decay of signal strength occurred at relatively high temperature and high humidity conditions.When samples were stored under conditions of average illumination of 1600 lux or in light-resistant container,the signal changes were less than 3.8% or 3.4% respectively.Therefore,The free radical signal was not influenced drastically by relatively strong light exposure.Long term stability inspection for more than 1 year(380 days)at room temperature suggested a signal change within 4.8%.We tried to evaluate the signal attenuation according to the decaying time constant with the fitting curve of exponential function and got a decaying time constant only as small as 3× 10-6.The result further suggests that the dosimetric properties of the material would not change significantly under natural environment at room temperature and exhibited good stability over long term storage.Conclusion: This research explored the shaping method of the CHAP dosimeter and determined the optimal size and suitable adhesion agent accessory material and reasonable ratio of mixing.The established method can stabilize the quality of samples,decrease the difference between sample groups,enhance operability,reduce the influence of operative error and guarantee the uniformity of the mechanical strength and size reproducibility,therefore the measurement accuracy and efficiency could get remarkable improved and furthermore it will contribute to lower the detectable dose limit and will be more suitable in low-dose radiation measurement applications.The research studies systematically dose-related ESR signal characteristics based on self-synthesized CHAP after irradiation under various experimental environment conditions within 3 months(91 days)observation.The preliminary results showed that the radiation-induced free radical signal intensity was not significantly attenuated when the dosimeter material was stored at room temperature or below 40?.Although the signal fluctuation increased at a humidity of 76%(the simulation of the high humidity environment)it didn't show significant signal attenuation.It can be suggested to place the sample in a dry environment for a certain period of time or sealed before the measurement in order to reduce the possible impact of high humidity and the measurement error.Besides,the free radical signal was not influenced drastically by relatively strong light exposure.Long term stability of more than one year at room temperature did not show remarkable decay.The above stability characteristics suggest that the developed CHAP dosimetric material can meet the requirement of routine dose inspection and radiation quality guarantee in the application of radiation processing and medical radiation in the aspect of dosimeter features as well as storing and transporting conditions.However,the high temperature or high humidity environment may have some influences on measurement process,which should be taken into consideration in further applications.It will give reference for material structure and construction technologies,based on preliminary study results of the stability of the dosimeter.