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emission_profile_modelling [2012/07/20 22:09]
alancoelho
emission_profile_modelling [2012/10/12 21:40]
johnsoevans
Line 46: Line 46:
 Cheers Cheers
 Alan Alan
 +
 +====== Empirical Profile Modelling: Split Peaks in LaB6 ======
 +
 + ​======
 +
 +From the Rietveld mailing list 5/10/2012:
 +
 +Yaroslav
 +Thank you for the MYTHEN data.
 +
 +And thank you Lubo for also sending the data and for pointing out that the splitting increases at high angles and hence the opposite effect to a capillary.
 +
 +I took the liberty of trying to fit to the data in a purely empirical manner; it's a little naive of course as many have no doubt spent a lot of time looking at the MYTHEN detector in detail. I myself would favour alignment such that splitting does not occur as Francois explained.
 +
 +FWIW however and when desperate a '​perfect'​ empirical fit is possible with four Gaussians for an emission profile with an Rwp of 3.52% for a structural fit and 3.80% for a Pawley fit. It also seems that a Gaussian convolution that is constant with 2Th is also necessary. The main components of the peak shape are:
 +
 +<code topas>​macro LL { min 1e-5 max 1 val_on_continue = Rand(.001, .1); } prm xx  0.58476` min .3 max 1.5 macro Fn(x) { / Tan(x)^xx }
 +prm w1  0.00017` min -.01 max .01 val_on_continue = Val + Rand(-1, 1) 0.0001;
 +prm w2 -0.00047` val_on_continue = Val + Rand(-1, 1) 0.0001;
 +
 +prm w3 -0.00038` min -.01 max .01 val_on_continue = Val + Rand(-1, 1) 0.0001;
 +prm w4  0.00040` val_on_continue = Val + Rand(-1, 1) 0.0001;
 +
 +prm w5  0.00000` min -.01 max .01 val_on_continue = Val + Rand(-1, 1) 0.0001;
 +prm w6 -0.00022` val_on_continue = Val + Rand(-1, 1) 0.0001;
 +lam
 + ymin_on_ymax 0.001 
 + la 1 
 + lo  0.82257
 + lg @  0.21693` LL
 + lo_ref
 +
 + la  @  0.17840` min .1 max 10
 + lo =  0.82257 + w1 + w2 Fn(Th); ​
 + lg @  0.19588` LL
 + la  @  0.28193` min .1 max 10
 + lo =  0.82257 + w3 + w4 Fn(Th); ​
 + lg @  0.07650` LL
 +
 + la  @  1.24781` min .1 max 10
 + lo =  0.82257 + w5 + w6 Fn(Th); ​
 + lg @  0.17791` LL
 +
 +gauss_fwhm @  0.0150418688` min 1e-5</​code>​
 +
 +I apologize if the TOPAS script is not understandable to some. The w2, w4 and w6 parameters offsets emission profile lines as a function of 1/Tan(Th) which then offsets the emission profile lines in 2Th space proportional to 2Th. The xx parameter if set to 1 increases Rwp by around 1%. 
 +
 +In any case if desperate then an empirical fit is possible using an emission profile comprising 4 Gaussians. ​
 +
  
  

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