Changes between Initial Version and Version 1 of Ticket #789, comment 4


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Timestamp:
Nov 27, 2016 10:02:30 AM (8 years ago)
Author:
butler
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  • Ticket #789, comment 4

    initial v1  
    33* The documentation (both original IGOR and !SasView) make clear that the normalization N is the number density of **individual disks** not of stacks.  However both the IGOR code and the !SasView code normalize by the stack number density.  Thus by multiplying by the number of disks in a stack converts number density of stacks to number density of disks and indeed doing so the cylinder model is now identical to the stacked disk model (assuming core and layer have the same SLD and use a sigma of nearest neighbor spacing of zero) 
    44 
    5 * Both the IGOR and !SasView documentation state that the exponent in the S(Q) term is q*cos(alpha) but both codes use q*d*cos(alpha).  However it is not obvious what the effect of this term is.  Some tests using both ways seem to give exactly the same result. 
     5* Both the IGOR and !SasView documentation state that the exponent in the S(Q) term is q*cos(alpha) but both codes use q*d*cos(alpha).  However it is not obvious what the effect of this term is since there is nothing I can thing of to compare to once we allow a non zero sigma_d (and if it is zero that term becomes a multiply by one so has no effect).  However d*cos(alpha) is just the projection of d into q,,parallel,,. Thus cos(alpha) alone makes no sense so assume the code is correct in this case and documentation is wrong. 
    66 
    7 * !SasView documentation also shows sigma_d in that same exponent term to not be squared while the IGOR documentation shows it squared.  Both sets of code do use the square term so probably fair to assume it should be squared 
     7* !SasView documentation also shows sigma_d in that same exponent term to not be squared while the IGOR documentation shows it squared.  This is effectively a Debye-Waller term (exp[-q^2^ x^2^]).  And x must be d*cos(alpha)*sigma_d assuming sigma_d is given as a fraction of x and of course x should be x in the direction of q.  Hence the square shoudl be correct and moreover is another reason that the previous point should be d*cos(alpha) not just cos(alpha) Further both sets of code do use the square term.