Help with backscatter equation, please

[Robert.Morris@rfets.gov (Robert Morris)]

     I need to calculate beta backscatter factors for various different 
     substrates.  I am attempting to use equations shown in Table 9.16 of 
     the third edition of the Handbook of Health Physics and Radiological 
     Health, but I am having trouble.  The equations are from Tabata, Ito, 
     and Okabe, "An empirical equation for the backscattering coefficient 
     of electrons" Nuclear Instruments and Methods 94:509-513;1971.  It 
     seems like something is missing from the equations (maybe electron 
     energy?).  I would appreciate some help.
     
     Question 1.  If there is something missing handbook version of the 
     equations, what is the correct version of the equations?
     
     Question 2.  Is this an appropriate set of equations to calculate the 
     backscatter of a beta source on a substrate of a given Z?
     
     Question 3.  If not, what is?
     
     As always, thanks in advance for the help
     
     Robert Morris, CHP, CIH
     B&W Services
     SSOC
     Rocky Flats Environmental Technology Site
     Golden, CO
     Robert.Morris@RFETS.gov