Study On AMS Measurement Of Ultra Trace Nuclides In Rare Event Detector Material

Detector materials with untra-high purity are increasingly required inexperiment of modem particle physics. The signals from extremely rareprocesses such as neutrino interaction and oscillation, double beta decay, anddark matter interaction are often drowned out by common terrestrialbackgrounds nuclides, such as uranium, thorium, and potassium-40, whichhave long half-lives and remain abundant in the earth's crust. The decayproducts of these nuclei and their daughters can often have the samelow-energy signals of the rare processes studied. To enable experimentsprobing rare phenomena, a key point is to select detector materials with veryhigh radionuclide purity.The goal of this work is to develop a measurement method with AcceleratorMass Spectrometry for the measurement of radionuclide in rare event detectormaterials. An organic liquid scintillator (LS) and an inorganic scintillator (CsI)were selected for studies since they are among the most commonly useddetector materials in various kinds of rare event detections. The main topicsof the research presented in this thesis are the measurement of ¹²⁹I and ⁴⁰K inLS and CsI.The main contributions of the author are sample preparation, nuclidesmeasurement and result analysis in this thesis. During the period of the thesisresearch, a series of standards of ¹²⁹I and ⁴⁰K was prepared, chemicalextraction procedures for ¹²⁹I and ⁴⁰K from LS were established, the AMSmeasurement method for ⁴⁰K was developed, and the contents of ¹²⁹I and ⁴⁰Kin both LS and CsI were obtained.The CsI powder sample was directly used in AMS measurement without chemical treatment. However, the organic scintillator sample must be treatedand transformed into the form suitable for AMS measurement. For this reason,two chemical procedures were established for the extraction of ¹²⁹I and ⁴⁰Kfrom the LS sample. The recoveries of more than 90% were obtained.The standards are the key to AMS measurement. A series of standardsamples of ¹²⁹I and ⁴⁰K were prepared by the dilution from stock standardsolutions. The linearity of the calibration curves was verified by SSMS (SparkSource Mass Spectrometry) and AMS method.AMS measurement of ⁴⁰K in both materials is one of the most importantand difficult tasks in this work, because this nuclide is ubiquitous in natureand constitutes a persistent background. In this work, ⁴⁰K was determined byAMS measurement of ³⁹K combined with the constant ratio of ⁴⁰K/³⁹K innature. ³⁷Cl was used to simulate ³⁹K transmission to optimize the relevantparameters of AMS system. There is no interference, so, the ³⁹K may bedirectly measured by a semiconductor.A new method has been developed for AMS measurement of ¹²⁹I and ⁴⁰K inrare event detector materials of LS and CsI with detection limits of 3.2×10^-17g/g and 8.6×10^-18g/g for ¹²⁹I and ⁴⁰K in LS, respectively.The method is applicable not only for ¹²⁹I and ⁴⁰K, but also for other traceradionuclides in the analyses of rare event detector materials.