I think that you should suggest a new unit of 10^(-11) m, a JHm perhaps. If it is convenient to have B in A^2 then u^2 should be in JHm^2.
Adam On Thu, 19 Nov 2009, James Holton wrote: > Many textbooks describe the B factor as having units of square Angstrom > (A^2), but then again, so does the mean square atomic displacement u^2, > and B = 8*pi^2*u^2. This can become confusing if one starts to look at > derived units that have started to come out of the radiation damage > field like A^2/MGy, which relates how much the B factor of a crystal > changes after absorbing a given dose. Or is it the atomic displacement > after a given dose? Depends on which paper you are looking at. > > It seems to me that the units of "B" and "u^2" cannot both be A^2 any > more than 1 radian can be equated to 1 degree. You need a scale > factor. Kind of like trying to express something in terms of "1/100 > cm^2" without the benefit of mm^2. Yes, mm^2 have the "dimensions" of > cm^2, but you can't just say 1 cm^2 when you really mean 1 mm^2! That > would be silly. However, we often say B = 80 A^2", when we really mean > is 1 A^2 of square atomic displacements. > > The "B units", which are ~1/80th of a A^2, do not have a name. So, I > think we have a "new" unit? It is "A^2/(8pi^2)" and it is the units of > the "B factor" that we all know and love. What should we call it? I > nominate the "Born" after Max Born who did so much fundamental and > far-reaching work on the nature of disorder in crystal lattices. The > unit then has the symbol "B", which will make it easy to say that the B > factor was "80 B". This might be very handy indeed if, say, you had an > editor who insists that all reported values have units? > > Anyone disagree or have a better name? > > -James Holton > MAD Scientist >
