Fluka Calculations of Gamma Spectra at BESSY

Since 22nd October 2012 the synchrotron light source BESSY is operated in top-up mode. Losses of electrons during injection cause an electromagnetic cascade, that consists of high energetic photons of the bremsstrahlung, and secondary electrons and positrons from the pair creations. The bremsstrahlung spectrum has a maximum at 1.022 MeV owing to pair creations. The spectrum has a high energetic tail, that reaches up to the electron energy of 1.7 GeV at BESSY. The low energy part of the electromagnetic cascade is produced by compton scattering or the photo effect. Due to the opened beamshutters during top-up injections, the low energetic part of the bremsstrahlung spectrum can reach the experimental hall. We used the particle interaction and transport code FLUKA for the calculations of both the fluence and the dose distribution. We calculated the gamma spectra of the radiation through the shielding walls and through the front-ends. We discussed the question whether additional safety measures are necessary for top-up operation due to the low energy part of the spectrum. From our calculations we determined the correction factors for our ionisation chambers of the ambient dose measurement system.