Predetermining acceptable noise limits in EXAFS spectra in the limit of stochastic noise
EXAFS measurements are used to probe a variety of experimental systems, but excel at elucidating local structure in samples which have slight disorder or no long-range crystalline order. Of special interest to the authors is the use of EXAFS in understanding the molecular-level binding structure and characteristics of actinides on the surface of environmental minerals and model mineral analogs. In environmental systems the element of interest can be on the order of 10-7% by weight of the total sample. Obviously such samples would be impossible to measure using EXAFS techniques. It is therefore essential to increase the concentration of the element of interest while still preserving a sample's ability to represent environmental conditions. Under such low concentration limits it is expected that the collected data is countrate, or stochastically limited. This condition occurs as we approach the signal-to-noise (S/N) limit of the technique where the random noise of the measurement process dominates over possible systematic errors. When stochastic error is expected to dominate systematic error, it is possible to predict, with the use of simulations, the ability of model fits to tolerate a certain level of stochastic noise. Elsewhere in these proceedings, we discuss how to tell when systematic errors dominate in measured EXAFS spectrum. Here, we outline a technique for determining the number of EXAFS scans necessary to test the relevance of a given structural model. Appropriate stochastic noise levels are determined for each point in r-space by collecting data on a real system. These noise levels are then applied to EXAFS simulations using a test model. In this way, all significant systematic error sources are eliminated in the simulated data. The structural model is then fit to the simulated data, decreasing the noise and increasing the k-range of the fit until the veracity of the model passes an F-test. This paper outlines a method of testing model systems in EXAFS fitting before measurements are conducted to determine the quality of measured data required for fitting of a particular model system with statistical confidence. It is important to reiterate that the calculated {alpha}surface in figure 1 is only applicable to the particular model presented in this paper. Furthermore, this procedure only takes into account stochastic noise; consequentially any confidence levels calculated should be viewed as upper limits to the confidence levels in systems which also contain a significant amount of systematic noise.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Chemical Sciences Division; Nuclear Science Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 980741
- Report Number(s):
- LBNL-2889E; TRN: US201015%%2114
- Resource Relation:
- Conference: 14TH INTERNATIONAL CONFERENCE ON X-RAY ABSORPTION FINE STRUCTURE (XAFS14)
- Country of Publication:
- United States
- Language:
- English
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