Graduate Thesis Or Dissertation
 

Peak reconstruction of single particle mass spectrometry transient signals to recapture ion count and improve SNM ratio measurements

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3n2046821

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  • Quick and accurate measurements of nanoparticles have important applications in biology, environmental and non-proliferation areas. Calculation of isotopic ratios, particularly in special nuclear materials, has relied on integration of nanoparticle counts and limited the capability of counting minor isotopes. Rapid transient detection method of nanoparticles has demonstrated the effectiveness of counting individual ions using a time-to-digital converter (TDC). Non-linear responses occurred when gold nanoparticle sizes exceeded 20 nm. Detector failure due to dead-time was the main contributor to reduction in total measured counts. Peak reconstruction was used to model the failures and recapture the lost ion counts. An exponential modified Gaussian (EMG) was assumed to match the ideal measured count rate, and the integrated EMG to match the cumulative ion counts. The ideal EMG was corrected using two different failure models. The first is a full model that accounts for non-paralyzing and paralyzing dead-times along with RC decay times. The second is the cut model that removes the middle portion of the measurement so only data that has not entered a ``failure" mode are considered. Data and models were fed to a non-linear fitting package in Python called LMFIT. Reconstruction of ion counts improved the linear response of gold nanoparticle measurements from 20 nm to 150 nm, where such reconstructed amplitudes demonstrated a 0.4\% detector response. NIST 612 uranium glass measurements were reconstructed to improve the U-235 to U-238 isotopic ratio. Reported standard ratio was 0.002391. Poisson (R3) and multiplicative errors (R4) estimators were used to determine the slope of the data. Raw data had an estimation of 0.00604 and 0.00412, whereas the cut model reconstructed the amplitudes for an estimation of 0.00354 and 0.00284, and the full model had reconstructed amplitude estimations of 0.00264 and 0.00280 for R3 and R4, respectively. Finally, four sets of plutonium data were analyzed for reconstruction. One to five particles for each data set were reconstructed. Lower ion count data in the plutonium data set does not exhibit dead-time failures seen in previous nanoparticle measurements due to the detection system not being over-saturated and causing dead-time. The linear response of these particles was used to check the reconstructed particles. Each plutonium data set had reconstructed values that restored the failed particle(s) back to a linear response compared to the non-failed particles.
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