ctXai.f

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!         test ctXai
!   program testcsynkTXai
!   implicit none
!   real*8 x, ctXai, y
!
!   x = 1.e-2
!   do while (x .lt. 4000.) 
!       y = ctXai(x)
!       write(*, *) sngl(x),sngl(y)
!           x = x * 10.**0.1
!   enddo   
!   end
!       *************************************************************
!
!           ctXai: T(X)
!
!         magnetic pair creation function.
!         This is a polynomial fit to the T(x)=0.16/x *K1/3(2/3x/)**2 
!          (Eq.3.3d) of Erber.
!        Note that Table VI seems half of this value.
!
!       *************************************************************
    real*8 function ctxai(x)
    implicit none
    real*8 x      ! input.  x = Xai = hv/m * H/Hc/2
    
    real*8 z

    if(x .lt. 0.25) then
        ctxai = 0.46 * exp(-4.d0/3.d0/x)
        elseif(x .lt. 1000.) then   
           z = log(x)
           ctxai =exp( (((((((((-0.1055848e-06*z + 0.3326052e-05)*z 
     *            -0.4298832e-04)*z +  0.3164778e-03 )*z
     *            -0.1791347e-02)*z +  0.1028788e-01 )*z 
     *            -0.5477092e-01)*z +  0.2251815 )*z    
     *            -0.6962186)*z     + 1.227734)*z -2.443217)       
    
    else
       ctxai = 0.60 * x**(-0.3333333333)
    endif   
    end