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首页> 外文会议>International Conference on Advances in Destructive and Non-Destructive Analysis for Environmental Monitoring and Nuclear Forensics; 20021021-20021023; Karlsruhe; DE >IMPROVED ENVIRONMENTAL AND FORENSICS MEASUREMENTS USING MULTIPLE ION COUNTERS IN ISOTOPE RATIO MASS SPECTROMETRY
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IMPROVED ENVIRONMENTAL AND FORENSICS MEASUREMENTS USING MULTIPLE ION COUNTERS IN ISOTOPE RATIO MASS SPECTROMETRY

机译：在同位素比重质谱中使用多个离子计数器改进的环境和法医学测量

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A new detector system designed for isotope ratio mass spectrometers provides improved precision on measurements of samples with very low amounts (<10~(11) g) of analyte. An array of continuous dynode electron multipliers has been installed on a new ThermoFinnigan MAT Triton thermal ionization mass spectrometer acquired by the New Brunswick Laboratory. These ion counters are modifications of miniaturized, commercially available continuous dynode electron multipliers. They can be readily installed to replace individual Faraday cups in a multidetector mass spectrometer or bundled together and located along the detector plane with a set of Faraday cups. On the New Brunswick Laboratory mass spectrometer, nine Faraday cups, one conventional discrete dynode electron multiplier and seven miniaturized ion counters were installed. The size of the miniaturized ion counters is identical to the standard Faraday cups, and thus the ion counters can be aligned with unit mass spacing for the measurement of high mass elements such as uranium and plutonium. Six of the small ion counters were bundled together and positioned on the low mass side of the Low 4 Faraday cup. One additional ion counter was positioned on the high mass side of the Low 4 Faraday cup. This arrangement allows simultaneous measurement of either all uranium (including ~(233)U) or plutonium (including ~(244)Pu) isotopes, also a combination of the two, and allows the measurement of larger ~(238)U intensities on the Faraday cup, if needed. Compensation for small mass dispersion differences at unit mass spacing of uranium, plutonium or other actinides with slightly different masses is readily achieved by the use of a mass dispersion zoom lens. The advantage of multiple ion counting is the simultaneous collection of isotopes. It overcomes many of the problems such as transient signal variation in sample emission and ionization, which would significantly reduce the attainable precision of a single collector measurement. For a given sample, multiple ion counting generates a greater number of counts for each isotope relative to single detector ion counting and provides improved counting statistics by a factor of two or more. In addition, multiple ion counting eliminates the need for drift correction algorithms that are required in single collector measurements and thus eliminates a potential source of systematic error. Initial tests indicate that the multiple ion counters exhibit high counting efficiency, a dark noise of less than 10 counts per minute and typically less than 1 count per minute, and show linear response characteristics over four orders of magnitude using standard deadtime correction algorithms. For higher count rates in excess of about 100 000 counts per second, additional correction schemes such as the RLR algorithm can be used. The stability of the detectors at high count rates as a function of time is being investigated, and likely will require the incorporation of this parameter into a correction algorithm. The miniaturized continuous dynode electron multipliers have sufficient stability to yield a repeatability of 0.1% RSD on the ~(235)U/~(238)U ratio in a 10 x 10~(-12) g uranium samples of CRM U500. These data were obtained by measuring in the total evaporation mode. Further test results are presented.
机译：一种专为同位素比率质谱仪设计的新型检测器系统，可提高分析物含量极低（<10〜（11）g）的样品的测量精度。新不伦瑞克实验室（New Brunswick Laboratory）购置的新型ThermoFinnigan MAT Triton热电离质谱仪上已安装了一系列连续打拿极电子倍增器。这些离子计数器是小型化的，可商购的连续倍增极电子倍增器的修改。它们可以很容易地安装，以代替多探测器质谱仪中的单个法拉第杯，也可以捆绑在一起，并用一组法拉第杯沿探测器平面放置。在新不伦瑞克实验室的质谱仪上，安装了九个法拉第杯，一个常规的倍增打拿极电子倍增器和七个微型离子计数器。小型化的离子计数器的尺寸与标准法拉第杯相同，因此可以将离子计数器与单位质量间距对齐，以测量诸如铀和mass的高质量元素。将六个小型离子计数器捆绑在一起，并放置在Low 4 Faraday杯的低质量侧。在Low 4法拉第杯的高质量侧放置了一个额外的离子计数器。这种布置可以同时测量所有铀（包括〜（233）U）或p（包括〜（244）Pu）同位素，也可以同时测量两者的同位素，并且可以测量更大的〜（238）U强度。法拉第杯，如果需要的话。通过使用质量分散变焦镜头，可以轻松实现铀，p或其他质量稍有不同的act系元素在单位质量间距处的较小质量分散差异的补偿。多个离子计数的优点是可以同时收集同位素。它克服了许多问题，例如样品发射和电离过程中的瞬态信号变化，这将大大降低单个收集器测量的可达到精度。对于给定的样品，相对于单个检测器离子计数，多次离子计数为每个同位素生成更多数量的计数，并提供两倍或更多倍的改进计数统计量。此外，多次离子计数消除了单收集器测量中所需的漂移校正算法的需要，因此消除了潜在的系统误差源。初步测试表明，多个离子计数器具有很高的计数效率，每分钟少于10个计数，通常每分钟少于1个计数的暗噪声，并使用标准的停滞时间校正算法显示出四个数量级的线性响应特性。对于每秒超过约100000个计数的较高计数率，可以使用其他校正方案，例如RLR算法。目前正在研究探测器在高计数率下作为时间的函数的稳定性，并且很可能需要将此参数纳入校正算法中。微型连续倍增极电子倍增器具有足够的稳定性，可在10 x 10〜（-12）g CRM U500铀样品中，在〜（235）U /〜（238）U比上产生0.1％RSD的重复性。这些数据是通过在总蒸发模式下测量获得的。给出了进一步的测试结果。

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