  A family $A_\alpha$ of differential operators depending on a real parameter
$\alpha\ge 0$ is considered. This family was suggested by Smilansky as a model
of an irreversible quantum system. We find the absolutely continuous spectrum
$\sigma_{a.c.}$ of the operator $A_\alpha$ and its multiplicity for all values
of the parameter. The spectrum of $A_0$ is purely a.c. and admits an explicit
description. It turns out that for $\alpha<\sqrt 2$ one has
$\sigma_{a.c.}(A_\alpha)= \sigma_{a.c.}(A_0)$, including the multiplicity. For
$\alpha\ge\sqrt2$ an additional branch of absolutely continuous spectrum
arises, its source is an auxiliary Jacobi matrix which is related to the
operator $A_\alpha$. This birth of an extra-branch of a.c. spectrum is the
exact mathematical expression of the effect which was interpreted by Smilansky
as irreversibility.
