With a Petawatt class CPA laser of the LLNL Livermore
or the proposed GSI (Darmstadt) type laser, interactions
with matter can be studied in the upper 1020
W/cm2 regime. With such a laser focused into
an underdense plasma, strong electron bursts with energies
up to several 100 MeV are ejected in a forward direction.
This leads to a comparable burst of bremsstrahlung radiation
in the presence of high-Z material. Here, we discuss
the corresponding γ-induced nuclear reactions including
secondary particle production, including pions. Due to
the threshold behavior in the production, and the advantage
of delayed detection, we propose to employ these reactions
for probing the initial plasma conditions.