Hi Irmi, This is not a programming error -- the cross sections in the various tables that Hephaestus can use do vary, sometimes by as much as 10%. That reflects the current (or perhaps "historic") uncertainty in the absolute values of the actual cross-sections. For EXAFS (and even XANES), the absolute values of the absorption cross-sections rarely matters. We're more interested in the variation with energy over short energy scales, and are almost always making a differential measurement anyway. We "never" (more later) take into account the airpaths, thicknesses and compositions of x-ray windows, and so on. So for XAFS needs, knowing the absolute absorption to 10% or so is fine. If there are discrepancies between the tables, I would not know which ones are more correct. I believe the general wisdom is (where "inaccurate" means ~10%): - the Henke tables tend to be more suited for low energy, and may become inaccurate at higher energies (and end at 30 keV). - the Cromer-Libermann tables weren't really designed for mu, but do an OK job. - the McMaster tables use a single parameterized function for all cross-sections, and is also fairly old. They are efficient, but possibly inaccurate. - the Chantler and Elam tables are probably the most accurate in general. My understanding is that Chis Chantler (U.Melbourne) is actively working on absolute measurements of absorption cross-sections, but that not many other groups are. Perhaps your group is? Now on to your questions:

To make it short, I think I have found an error in the tool. In the tool "formulas" I took Kr, 30000 eV and get: the Absorbtion length: 15.031 cm (Resources Henke) : 14.431 cm (Resources Elam) which I also found in NIST Database

then use the tool "Ion Chamber" put in Kr (only one gas), 10cm pathlength, 760 Torr, 30000 eV I got for Percentage absorbed: 48.58% (Resources Henke) which is correct I think: 39.22% (Resources Elam) which is wrong I think In my eyes the correct result for 14.431 cm Absorbtion length has to be: 49.99% absorbed photons.

Why do think one is correct and another wrong? Have you measured it? The measurement is not as easy as some might think.... It would seem strange to me that the Henke tables are more accurate at its highest energy point than all the other tables: Kr at 30 keV Table barns/atom Elam 2578.2 McMaster 2601.7 Henke 2475.3 Chantler 2560.4 C-L 2490.1 The range is ~5%, with Elam and Chantler in good agreement and Henke the lowest value.

It is critical for me, since the correct flux measured with the ionisation chamber goes directly in my results.

Then using tabularized data is something you'll want to avoid! You may want to think about how your measurements might be immune to inaccuracies in the absolute flux....

We investigated the detector response on monochromatic X-rays at HASYLAB of a widely used flat-panel detector in medicine.

And I have also a question to you: Why do you use 30 eV/e- as ionization energy for the gases? Is there a reference for that?

I believe the ionization potentials are in the CRC, and also here (from a quick google search): http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/ionize.html When I do these calculations, I typically use 32eV for all gases, so that it nicely cancels the electron charge in coulombs. Then again, absolute accuracies that are better than 50% is not important to me. And again, to be much more accurate, I think you'd have to be careful about other factors (gas purity, voltage dependence of total current, space charge effects, harmonics, barometric pressure, and so on).

In addition, don´t we have to take the room temperature into account not only the pressure, that means: Density(Temperature) = (273,15K/293,15K)*0.003749g/cm^3(at 273,15K)

That's right, the temperature is assumed to be "standard". Again, if the goal is high precision, one would need to take many factors into account. --Matt

On Friday 01 June 2007, I.Reitz wrote: A couple more comments:

And I have also a question to you: Why do you use 30 eV/e- as ionization energy for the gases? Is there a reference for that?

That's not very defensible. 32 eV is probably a better number. 30=32 isn't an awful approximation ;-)

In addition, don´t we have to take the room temperature into account not only the pressure, that means: Density(Temperature) = (273,15K/293,15K)*0.003749g/cm^3(at 273,15K)

Of course you do. Hephaestus is really supposed to be a quick-n-dirty tool to help you out at the beamline and is targeted at the XAS user. If you want to absorb aproximately 10 percent of the photons in I0, it doesn't really seem necessary to make the user measure the ambient temperature in the hutch. Or, to put it another way, having controls for temperature or density or whatever adds complexity to Hephaestus's interface without adding much value to its intended user. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/