A Quench Index for ⁹⁰Y and Its New Applications in ⁹⁰Sr Analysis

Based on the customized quench index q_x for ⁹⁰Y and the modified 2-window approach, a simple method has been developed to analyze ⁹⁰Sr using a liquid scintillation spectrometer, sometimes supplemented by γ spectrometer. The liquid scintillation counting(LSC) spectrum of ⁹⁰Y exhibits good linearity in the high-energy part, on the basis of which q_x is defined. The key point of the modified 2-window approach is to obtain the ratio of the count rates of two windows(also known as region of interest, or ROI), CR_H/CR_W, where CR_W is the count rate in ROI_W, i.e. the whole region of interest(usually taken as channels 1-1024); CR_H is the count rate in ROI_H, i.e. the high-energy region of interest. The right endpoint of ROI_H is usually fixed at channel 1024, and the left endpoint varies with the quenching level of the sample. This paper provides a method to select the left endpoint of ROI_H and to keep the value of CR_H/CR_W about 0.2779, with the relative standard deviation of 0.84%. There are two prerequisites for using this method: (1) the quenching level of the sample is relatively low, in order that the counting efficiency of ⁹⁰Sr or ⁹⁰Y can be approximately considered as 100% in ROI_W; (2) there is no interference from other nuclides than ⁹⁰Y in ROI_H. Three types of real samples from the hot test of high-level radioactive waste separation have been analyzed to show the specific steps of using this method. Because of the radioactive equilibrium of ⁹⁰Sr/⁹⁰Y in these samples, three ways of calculating the count rate of ⁹⁰Sr have been adopted to verify each other. The first one is to obtain the count rate of ⁹⁰Sr by subtracting the count rate of ⁹⁰Y from the total count rate of ⁹⁰Sr/⁹⁰Y. The second one is just to let the count rate of ⁹⁰Sr be equal to the count rate of ⁹⁰Y. The third one is to divide the total count rate of ⁹⁰Sr/⁹⁰Y by 2 to obtain the count rate of ⁹⁰Sr. The results indicate that the count rate of ⁹⁰Sr obtained by this method is accurate and reliable. This method only requires linear fitting of the high-energy part of the LSC spectra. The method is simple and easy to use. With the application suggestions, users can choose appropriate analysis strategies according to the characteristics and analysis requirements of the sample. For analyzing ⁹⁰Sr in samples of ⁹⁰Sr/⁹⁰Y with or without ¹³⁷Cs, neither chemical separation nor waiting for the radioactive equilibrium of ⁹⁰Sr/⁹⁰Y is required, if the count rate of ⁹⁰Sr is not too low.