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<div class="" style="word-wrap:break-word; line-break:after-white-space">Dear Thierry,<br class="">
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<div>Comments and answers follow in-line below. </div>
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<div class="">On 13 May 2020, at 16.23, Peter Kjær Willendrup <<a href="mailto:pkwi@fysik.dtu.dk" class="">pkwi@fysik.dtu.dk</a>> wrote:</div>
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<div class=""><span class="" style="font-family:Verdana; font-size:small">As I try to get advantage of these special conditions to try and use McStas, I have a beginner's question:</span></div>
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One of my aims is to be sure to have correct orders of magnitude for absolute flux.<br class="">
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I started to use the components Source_simple and Source_gen.<br class="">
I assume that for the simplest case, the parameters "flux" in Source_simple and "I1" in Source_gen play the same role.<br class="">
My expectation is that the flux measured by a monitor at a certain distance from the source follows Equation 3.1 in the "McStas 2.6 Component Manual of 24/01/2020", which links the source brilliance with source area, solid angle and wavelength interval.<br class="">
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This is the case with both sources if I set the parameter to 1: with 1 Ang bins, I get a flux on my monitor which scales according to this equation.<br class="">
Following the component help, this behaviour should be observed by setting the parameter to 0. This in not the case, with 0 as input the calculation gives different flux values, which I am not able to link to anything physical.<br class="">
The documentation is a bit misleading, because it recommends to put the value to 0 and the default is 1.<br class="">
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in Source_simple:<br class="">
name: flux <br class="">
unit: 1/(s*cm**2*st*energy unit) <br class="">
description: flux per energy unit, Angs or meV if flux=0, the source emits 1 in 4*PI whole space.
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default: 1<br class="">
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in Source_gen:<br class="">
name: I1<br class="">
unit: 1/(cm**2*sr)<br class="">
descrption: Source flux per solid angle, area and Angstrom if I1=0, the source emits 1 in 4*PI whole space.<br class="">
</font></font><font size="-1" class=""><font face="Verdana" class=""><font size="-1" class=""><font face="Verdana" class="">default: 1<br class="">
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My feeling is that:<br class="">
- the unit should be in both case as in Source_simple,<br class="">
- the same sentences would work with "flux=1" and "I1=1", which would be consistent with the default values. I have no idea if the 0 value is useful for some case.<br class="">
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<div>I agree, there is clearly a documentation issue here. :-)</div>
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<div>Documentation aside, as far as I can see, the two components behave numerically exactly the same using flux and I1 - see attached instrument where once can select the used source and the intensity as input parameters.</div>
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<div class="">I believe the right documentation sentence should be along the lines of</div>
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<div class="">“Source flux per source area (specified by e.g. xwidth x yheight), per solid angle (specified by dist+ focus_wx x focus_yh), per second (McStas “intensities” always are assumed to be) and per used wavelength/energy interval.”</div>
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<div class="">I think that perhaps the special flux=0 / I1=0 case at least originally was envisioned correspond to setting the flux-unit to unity and “not apply focusing”, but as the documentation is written and how the components in practice works, that special
case seems to make no sense. :-)</div>
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<div class="">I will define a GitHub issue to ensure this is rectified in a future release. :-)</div>
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<div class="">Best and thanks for the input, </div>
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<div class="">Peter</div>
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Peter Kjær Willendrup<br class="">
Forskningsingeniør, Specialkonsulent<br class="">
Næstformand for DTU Fysik LSU<br class="">
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DTU Physics<br class="">
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