In the last decade the intensities of light fields which can be produced in a laser focus increased by four orders of magnitude from 10~(16) to 10~(20) W/cm~2. Intensities exceeding 10~(18) W/cm~2 allow for the production of relativistic laser plasmas, that is the quiver energy of plasma electrons reaches the electron rest mass. These plasmas are sources of a whole spectrum of energetic particles, such as highly relativistic electrons, hard bremsstrahlung, protons with energies up to a few hundred MeV, neutrons and deuterons These particles can be used to induce nuclear reactions like photo-fission (γ,f), neutron generation by (γ,n)-(p,n)- or (d,n)-reactions, neutron capture or fusion. Up to now, nuclear reactions have mainly been used for measuring the temperatures of laser produced bremsstrahlung photons and electrons. Nevertheless, various applications of laser induced nuclear reactions such as production of radioisotopes for medical applications increasingly come to the fore. A very interesting application of laser generated bremsstrahlung is the investigation of photo-induced reaction cross-sections for (γ,n) and (γ,f)-reactions of long-lived actinides and fission products, which are both hazardous components of nuclear waste. Recently we have shown that - up to now unknown -(γ,n)-reaction cross sections of long lived fission products can be determined from measurements with a laser as γ-source. This topic is of particular interest in the context of transmutation of nuclear waste. Considering photo induced fission (γ,f) of ~(232)Th and ~(238)U as well as the (γ,n)-reaction of ~(181)Ta we show how nuclear reactions are used for the characterization of the bremsstrahlung spectrum from laser generated relativistic plasmas. With help of these results we determine the (γ,n)-cross section of ~(129)I.
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