unit prefix

[Les Slaback <lester.slaback@nist.gov>]

It was noted .....
"y       yocto   1E-24
 The last prefix is very useful for describing the size of a barn."

This raises the question of usage.  A barn is 10^-24 cm^2.
So do you write  ycm^2, .0001 ym^2,  (.01 pm)^2, .0001 (pm)^2 ???

Perhaps life is simpler using barns, although not SI correct.
Disclaimer:  the above are the personal musings of the author, and do not
represent any past, present, or future position of NIST, the U.S. government,
or anyone else who might think that they are in a position of authority. 
Lester Slaback, Jr.  [Lester.Slaback@NIST.GOV] 
NBSR Health Physics 
Center for Neutron Research 
NIST 
Gaithersburg, MD  20899 
301 975-5810