Vadim G Palshin schrieb:
Hi everyone, I have the following question: I have a set of spectra collected in transmission with 3 ionization chambers and a standard between 2nd and 3rd ion chambers for calibration, so I know my spectra are nicely calibrated relative to each other. Should I then force Athena(/autobk) to use the same E0 for all of them (e.g. use the atomic value) or let it find E0 independently for each sample? Thanks! Vadim.
Hello, I wrote the message below a few days ago and when I wanted to post it to the mailing list I noticed that Vadim already posed a related question. Sorry for not supplying an answer but to extend the list of questions :( ... Recently I stumled across a behaviour in Athena I did not understand while discussing with a collegue how to choose the right E0 for background subtraction in Athena. I would be glad if I could benefit from the expertise of the members of this mailing-list. The initial question was how to choose a value for E0 when comparing samples with mixtures and the measurements of the pure compounds (e.g. iron, iron-oxide and a sample containing a mixture of both). Usually in this case I set E0 to the atomic value for all samples (Assuming I can be sure that the monochromator was calibrated to return a correct absolute energy. Otherwise I do an appropriate energy-calibration with reference to a standard measured at the same time before entering Athena). I do that to ensure a uniform background subtraction for all samples. However, for a system of e.g. Se, RuSe2 and a mixture of both, the atomic value E0 can be quite far away from the inflection point of the RuSe2- and mixture-samples. So the first question would be: is it better to use the same E0 for all samples or to somehow adjust the E0 to the inflection point of the specific sample? In this context my idea of the algorithm of background subtraction was that a shift of E0 could be completely compensated for by appropriatly shifting all other parameters in the backward direction. E.g. when shifting E0 by 4 eV I should end with the same FT when shifting the values for pre-edge range, normalization range, spline range and k-range by the same amount. However, a test revealed that only the pre-edge lines and post-edge lines are identical. The background splines are different and the FTs differ quite significantly from each other. Is my general assumption wrong? Another question (more cosmetic) concerns the use of 'E0' and 'E shift' as it is irrelevant which parameter to adjust to achieve an energy shift. Is it a good idea to set E0 always to the atomic value and use 'E shift' for entering the offset? Actually this is the way I prefer to do it although Athena initially does not set 'E shift'. I also have a kind of feature request in this context and would be interested if others would also benefit from it: for some samples the initial value Athena picks for E0 is obviously inadequat, e.g. when it chooses a prepeak as inflection point. As a consequence the parameters for background subtraction are also inadequate. After choosing a right value for E0 a new feature could be: "automatically adjust background parameters with regard to the current E0", so one could still benefit from Athenas auto-function. Usually I adjust these parameters anyway but it would simplify doing a quick first look at the data. Thanks for any help, Gerrit -- Gerrit Schmithals Hahn-Meitner-Institute Berlin