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Dear Developers, Colleagues, and Friends,</div>
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Hi, I am a Ph.D. student working on neutron scattering at Johns Hopkins Univerisity. This is my first post concerning a question that has confused me for a while, and would really appreciate any thoughts and advice about it.</div>
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Concerning the secondary extinction correction of Type I, I have read the code in "SCExtinction(ref,im,phfx,hfx,pfx,calcControls,parmDict,varyList)" starting at line 2275 of the script "GSASIIstrMath.py" to understand how GSAS-II evaluate the extinction correction
and found that the treatment for neutron scatterings with constant wavelength (SNC) and time of flight neutron scatterings <span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">(SNT)<span style="margin: 0px"> </span></span><span> </span>are
quite different: </div>
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In lines 2292-2295 (<span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">for </span><span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">SNT</span>), we have <span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">the
definition "PL=SQ ", SQ is defined in the line 2282 so it means <span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">PL=1/4 ref[4]^2=1/(4D^2); </span></span></div>
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<span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important"><span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">In lines 2296-2299 (for SNC), we instead have the definition "PL=np.sqrt(1.0-cos2T**2)/parmDict[hfx+'Lam']",
which means "PL=sin(2*theta)/lambda". </span></span></div>
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For both cases, AV and PLZ are defined in a similar fashion. The variable PL appears later in the line 2316 for the secondary extinction of type I, as "PSIG = parmDict[phfx+'Eg']/PL". For the case of SNC, the following equation of extinctions seem to agree
with those in Becker & Coppens (1973). However,<b><span> </span>I do not understand how the PL defined for SNT can also be correct. PL defined for SNT (1/(4D^2)) even has a different dimension compared to <span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">PL
defined for SNC (<span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">sin(2*theta)/lambda</span>). Is there any crucial difference between the extinction correction of these two cases (eg: dimensions of F^2, Eg, other
factors) other than that the fact that for SNT we have different neutron wavelengths for different peaks?</span></b></div>
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<span style="margin: 0px; background-color: rgb(255, 255, 255); display: inline !important">Another question I have is why we need a<span> </span><b>1e7 in lines 2293 and 2297 for the definition of AV?</b><span> </span>Is that for some unit transformation,
I am under the impression that if we have FcTsq in unit of barn, volume of unit cell in unit of Ang^3 , wavelength and D in units of Ang and tbar in unit of cm we do not need any unit conversion in the evaluation of extinction.</span></div>
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I apologize in advance if this is a silly question, any clarification you can provide will be much appreciated.</div>
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Best regards,</div>
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Yi Luo</div>
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