  The Bessel-Gaussian modal spectrum, generated in spontaneous parametric
down-conversion of a Gaussian pump beam, is considered. This is done by first
deriving a general expression for the true probability of detecting specific
transverse spatial modes for the pairs of photons generated in a spontaneous
parametric down-conversion process. These expressions are applied for
Bessel-Gaussian modes in type I phase matching with collinear, degenerate
down-converted beams. The result shows that a broad orbital angular momentum
spectrum can be obtained for Bessel-Gaussian modes under certain conditions.
