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Factorial Design Techniques Applied to Optimization of AMS Graphite Target Preparation

Published online by Cambridge University Press:  18 July 2016

R. Michael Verkouteren  and
George A. Klouda
R. Michael Verkouteren
Affiliation:
Surface and Microanalysis Science Division, National Institute of Standards and Technology Gaithersburg, Maryland 20899 USA
George A. Klouda
Affiliation:
Surface and Microanalysis Science Division, National Institute of Standards and Technology Gaithersburg, Maryland 20899 USA
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Abstract

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Many factors influence the preparation and quality of graphite targets for 14C accelerator mass spectrometry (AMS). We identified four factors (sample size, H2 pressure, catalyst temperature and pretreatment time) as potentially critical, and investigated their effects on two particular characteristics: the integrated rates of CO2 reduction (to graphite) and methane production. We used a 2-level fractional factorial experimental design and determined chemical reduction yield rates through manometry and partial pressure monitoring of residual gases by mass spectrometry.

Chemical reduction yield rates ranged from 0.2% to 6.2% per hour. With respect to their influence on percent yield rate, the factors we studied were ordered as: sample size > level of hydrogen > pretreatment of the catalyst. The temperature of the catalyst, and the sample size × hydrogen (2-factor) interaction, were only marginally influential. Other interactions did not appear to be significantly important. We estimated uncertainty in the order of influence and magnitudes of the effects by the Monte Carlo method of error propagation.

We observed significant methane production in only one experiment, which suggests that methane originates from indigenous carbon in untreated iron catalyst only in the presence of hydrogen and only at thermodynamically favorable temperatures. This exploratory investigation indicates that factorial design techniques are a useful means to investigate multivariate effects on the preparation and quality of AMS graphite targets.

Type
I. Sample Preparation and Measurement Techniques
Information
Radiocarbon , Volume 34 , Issue 3: Proceedings of the 14th International Radiocarbon Conference , 1992 , pp. 335 - 343
Copyright
Copyright © The American Journal of Science 

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