Methodology Of Using Accelerator Mass Spectrometer To Measure Non-exchangeable Organically Bound Tritium In Tree Rings

The number of nuclear facilities has gradually increased with the rapid development of nuclear technology.Tritium is one of the main radionuclides emitted by nuclear facilities.Tritium,as an isotope of hydrogen,may enter all structures containing hydrogen atoms along the hydrogen cycle and food chain in the biosphere.Therefore,the demand for monitoring and researching the migration and transformation of tritium has generated.Α-cellulose in tree rings is the framework of the annual tree ring cells.So it is clearly corresponded to the growth period of the tree.The non-exchangeable organically bound tritium contained inα-cellulose does not exchange hydrogen with the tritiated water and tissue free water tritium in the plant,and can accurately reflects the historical tritium exposure in the ambient environment of the tree.It is possible to reconstruct the historical tritium exposure of the ambient environment during the tree rings’growth period by measuring the concentration of non-exchangeable organically bound tritium in the tree rings.Currently,environmental tritium measurement methods measure the decayed product during beta decay of tritium,such as liquid scintillation counting method and~3He in-growth method.In order to obtain enough decay amounts,the traditional tritium measurement methods require long measurement time and large sample size.For example,tritium measurement using liquid scintillation counter needs ten grams of tree rings samples and dozens of hours measurement time.These methods are not suitable for large-volume samples,and also time-consuming and laborious.Accelerator mass spectrometer measures particles directly.Therefore it can rapidly measure tritium concentrations from milligrams of samples.Also taking the tree ring as an example,the accelerator mass spectrometer tritium measurement requires only about 100 mg of tree rings sample and 200 s to 15 min measurement time.This research set up a new method for the measurement of non-exchangeable organically bond tritium in tree rings with accelerator mass spectrometry,and base on which the level of non-exchangeable organically bond tritium in tree rings around Qinshan nuclear power plant were measured preliminarily.Work of present study mainly includes following aspects:1)A standard sampling and pretreatment method for accelerator mass spectrometry was established,including the pretreatment of log samples and the synthesis of powder of titanium hydride.Through modification of the extraction methods for cellulose,the purity of cellulose was raised to 83.9%.With the two-step method developed,the cellulose was transformed into titanium hydride which can be measured using accelerator mass spectrometry directly.In addition,the isotopic fraction effect during sample pretreatment was depressed and stabilized.2)By accelerator mass spectrometry,the level of tritium around nuclear power plant was measured with solid sample for the first time in China.Moreover,the parameters involved in the measurement of tritium with accelerator mass spectrometry were improved and calibrated,and the feasibility and reliability of the method was justified with standard water sample.The isotope abundance ratio was measured in between 105-107 TU,which agrees well with the results measured by the method of liquid scintillation.3)Base on the developed new method,the non-exchangeable organically bond tritium in tree rings of a tree around the Xiajia Wan monitoring station around Qinshan nuclear power plant was measured preliminarily.The measurement of non-exchangeable organically bond tritium with accelerator mass spectrometry requires only 10mg of cellulous and 200s of measurement period.Furthermore,the variation of non-exchangeable organically bond tritium level share similar trends with the tritium concentration in atmosphere and rain water,hence the historical missing data of tritium in the environment could be restored.