Dear CCP4BBers, I believe the answer to this question is that the correct unit for the scattering factor is actually length (the square root of the scattering cross section), i.e. it is strictly the scattering length. In the dim and distant past I did some neutron diffraction, and scattering factors here are typically expressed in cm (not very "SI" I know). In ND the factors vary oddly with atomic number so you have to use the correct units.
In X-ray diffraction it goes with the number of electrons (which are all the same after all) so it was convenient to define scattering factors as a ratio by dividing by the scattering length of hydrogen, so f for hydrogen (i.e. one electron) becomes one, rather than a length in cm. f, f' etc. then become dimensionless quantities, and the maps come out effectively in e/A**2 (whereas they are really in scattering density). In ND, of course, you cannot do this and the maps are in units of scattering density. Simon Phillips --------------------------------------------------------------------------- | Simon E.V. Phillips | --------------------------------------------------------------------------- | Director, Research Complex at Harwell (RCaH) | | Diamond Light Source Ltd | | Diamond House | | Chilton | | Didcot | | Oxon OX11 0DE | | United Kingdom | | Email: simon.phill...@diamond.ac.uk | | Tel: +44 (0)1235 778946 | | +44 (0)1235 778431 (sec) | | +44 (0)7884 436011 (mobile) | | www.mrc.ac.uk/OurResearch/ResourcesforScientists/ResearchComplex | --------------------------------------------------------------------------- | Astbury Centre for Structural Molecular Biology | | Institute of Molecular and Cellular Biology | | University of LEEDS | | LEEDS LS2 9JT | | United Kingdom | | Email: s.e.v.phill...@leeds.ac.uk | | Tel: +44 (0)113 343 3027 | | WWW: http://www.astbury.leeds.ac.uk/People/staffpage.php?StaffID=SEVP | --------------------------------------------------------------------------- -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Bernhard Rupp Sent: 26 February 2010 08:46 To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] units of f0, f', f'' Depends on in what units you want to get your electron density in, or what scattering objects (electrons) you integrate over for the SF formula. Since the exponent is dimensionless in the SF formula, and the FT commonly is electron density, electrons (not negative charge) has to be somewhere in the SF formula. If fo is in electrons, then f' and f" have to be units of electrons as well. The f' component reduces the real part scattering, it is negative (in electron units, again not in charge). BR -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Tim Gruene Sent: Thursday, February 25, 2010 11:25 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] units of f0, f', f'' Dear all, I just stumbled across the question about what is the unit of f' and f''. The first couple of hits from ixquick.com claim it was e^-. Since e^- is not a unit but symbolises an elemtary particle (of which fractions are considered non-existent), I was wondering whether the unit of f, f', and f'' is actually e (a positive charge!) and the value of f^0 of Fe at its K-edge was actually 26e or -26e - see e.g. Table 1 in http://www.ccp4.ac.uk/courses/proceedings/1997/j_smith/main.html Cheers, Tim -- Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A
