基于CeBr₃闪烁探测器的铀含量快速测定方法研究

Research on a Rapid Quantitative Determination Method for Uranium Content Based on a CeBr₃ Scintillation Detector

摘要: 为了满足天然矿样中铀含量快速、无损测定的要求，本文建立了以CeBr₃闪烁探测器为基础的γ能谱检测系统，并提出了用234 mPa 1001 keV特征γ射线对铀进行快速定量的方法。该系统由CeBr₃晶体、高灵敏光电倍增管、前端模拟电子学模块、模数转换单元和FPGA数字处理模块组成，可以将入射γ射线产生的闪烁光信号有效地转化为电脉冲，并形成能谱，实现天然矿样快速γ能谱采集和实时分析。系统经过¹³⁷Cs、⁶⁰Co标准源标定之后，在¹³⁷Cs 662keV处得到平均能量分辨率为4.42%，在609~2614 keV范围内具有较好的能量线性，可以保证低计数率下目标峰的可靠识别。针对天然矿样中²³⁸U、²²⁶Ra、²³²Th和⁴⁰K等多种天然放射性核素混合导致的谱干扰问题，本文以²³⁴mPa 1001 keV峰为定量目标峰，结合1764 keV和2614 keV峰作为辅助约束，对²²⁶Ra与²³²Th干扰贡献进行剥离，并建立了²³⁸U、²²⁶Ra、²³²Th和⁴⁰K标准谱数据库。采用高斯峰叠加线性本底模型对目标峰和辅助峰进行拟合，消除干扰后得到²³⁸U活度和铀质量分数的准确反演结果。该方法对低品位矿样具有较好的峰识别能力以及干扰抑制作用，即使目标峰的计数率很低，误差仍然控制在12%以内。用国家一级放射性标准物质GBW04112、GBW04127a对方法进行验证。结果表明，GBW04112样品²³⁸U活度测定值为9.20 Bq/g，与标称值8.35 Bq/g相比，相对误差为10.21%；GBW04127a测定值为11.03 Bq/g，标称值10.56 Bq/g，相对误差为4.47%。进一步对三种低品位天然矿样(铀含量约0.005%)进行应用验证，样品A、B、C的γ能谱法测定值分别为0.00556%、0.00481%和0.00601%，与化学法结果相比，相对误差分别为8.79%、9.31%和11.68%。结果表明，该方法不仅可以对低品位天然矿样中的铀含量进行快速无损测定，在现场初筛、大批量矿样快速分级方面也具有较好的工程适用性以及推广前景，未来在环境监测、核废料管理等领域也有着广阔的应用前景。

Abstract: To find a better solution for determining the quantity of uranium in nature rocks quickly and without destruction, we built up a γ ray spectrometer with CeBr₃ scintillator. Then put forward to use the special 1001 keV radiation gamma ray from 234 mPa effectively to measure its amount. The system has a CeBr₃ Crystal, high sensitive photomultiplier tube and Analog Front - End Electronic Components, A/D transformation part and FPGA Digital Processing parts. It turns the scintillations generated when incident γ-rays strike it into pulse currents efficiently to make energy spectrums; thus, γ-spectra can be acquired from natural ore samples fast, and they can be analyzed straightaway. Calibrate with ¹³⁷CS and ⁶⁰Co standards sourceto get the calibration factor, it is obtained at an averageof 4.42% for the 662 keV (¹³⁷CS) energy level and have a good linear relationship on 609~2614 KeVwhich ensure that we can findthe correctpeak easily evenwhen working on high backgroundcountrate scenarios. To avoid spectrum interference due to the coexistence of more than two kinds of natural radionuclide such as ²³⁸U, ²²⁶Ra, ²³²Th, ⁴⁰K in natural ore sample, we choose the 1001 keV peak of ²³⁴mPa as main quantification peak. The 1764 keV and 2614 keV peaks are included in order to help separate the interfering contributions of radium-226 and thorium-232. A standard spectral database has been formed for ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K. For my target, as well as for the auxiliary peak I have applied the Gaussian with linear background fit. The accurate inversion of the ²³⁸U activity and the uranium mass fraction after interference subtraction. It is good at recognizing the peaks and suppressing interferes on the low grade ore sample with an overall error of < 12% though there are very few. Validated with the national first - class certified reference material: GBW04112, GBW04127a the GBW04112’s measured ²³⁸U activity is 9.20 Bq/g and the relative error between its activity and the certified value of 8.35 Bq/g is 10.21%. For GBW04127a, the value is 11.03 Bq/g and its certified value is 10.56 Bq/g; thus, there is an error of 4.47%. Moreover, it was also employed for 3 low grade nature ore sample which have about 0.005% of uranium the results of the γ spectrometry on samples A, B and C are: 0.00556%, 0.00481%, 0.00601% with corresponding relative error compared to chemically analyzed result is: 8.79%; 9.31%; 11.68%. Above all, indicates the method we put forward can quickly determine uranium content on low level nature material without destroying it and has huge potential for use in on-site sorting of ore with first category and so on. It can be applied in the environment monitoring as well, like nuclear waste disposal.