.. index:: ThermalNeutronBk2BkExpAlpha
ThermalNeutronBk2BkExpAlpha is a function to calculate \alpha(d_h) of :ref:`ThermalNeutronBk2BkExpConvPVoigt <func-ThermalNeutronBk2BkExpConvPVoigt>`. It is defined as
\alpha(d_h) = \left(n\alpha_0 + \alpha_1 d_h + (1-n)\alpha_0^t - \frac{\alpha_1^t}{d_h}\right)^{-1}
where
n = \frac{1}{2} \mathit{erfc}(\text{Width}(t_\text{cross} - d^{-1}))
and
erfc(x) = 1-erf(x) = 1-\frac{2}{\sqrt{\pi}}\int_0^xe^{-u^2}du.
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