X ray spectrum measurement up to 1600 KEV from absorption data

In order to measure the X ray spectrum of 2 flash X ray machines (a 600 keV generator and the 1.6 MeV machine ANGELIX), we developed a multimaterials absorption system. To be able to reconstruct the initial spectrum from dose measurements behind the materials, we have to solve an ill posed problem. As we dimensioned the absorption system, we focused on the choice of masses per unit area for the materials providing the minimum conditioning of the system kernel matrix, in order to minimize the noise amplification due to the system inversion. Moreover, for the spectrum reconstruction, the problem was regularized by the use of a smoothing constraint on the solution and the regularization parameter was determined by the Cross Validation method. The response of the experimental device submitted to various theoretic spectra was simulated using the Monte Carlo code MCNR We were able to reconstruct most of the incident spectra from the simulated transmitted fluency, even if artificial noise is added to the results. The first experimental results will be presented at the ICOPS conference.