  Pieces of $n$-type silicon with 3.5 k$\Omega \cdot $cm resistivity have been
irradiated by reactor neutrons to fluences of (1, 5 and 10) $\times 10^{16}$
cm$^{-2}$. Using light-transmission measurements, the absorption coefficients
have been determined for photon energies, $E_\gamma $, between 0.62 and 1.30 eV
for the samples as irradiated and after 15 min isochronal annealing with
temperatures between 80{\deg}C and 330{\deg}C. The radiation-induced absorption
coefficient, $\alpha_\mathit{irr}$, has been obtained by subtracting the
absorption coefficient for non-irradiated silicon. The $E_\gamma $-dependence
of $\alpha_\mathit{irr}$ shows a resonance peak, which is ascribed to the
neutral divacancy, V$_2^0$, sitting on a background, and $\alpha_\mathit{irr}
(E_\gamma )$ is fitted by a Breit-Wigner line shape on a parameterized
background. It is found that at an annealing temperature of 210{\deg}C the
V$_2^0$ intensity is reduced by a factor 2, and that at the meV level, the
position and the width of the fitted Breit-Wigner do not change with
irradiation dose and annealing.
