limited to the laser focal spot which can be as small as a few micrometers the electrons 48 penetrate the target in a spot whose size is comparable to the laser spot size the generated hot electron distribution f e is maximilian more specifically f e ae 1 2 exp u2212 e kt h where t h is the hot electron temperature and k is planck constant a judicious choice of target geometry and composition of the laser pulse duration polarization and or intensity allows shaping the x ray spectrum and to maintain and control the hot electron temperature between 10 and 50 kev the conversion efficiency is drastically reduced for hot electron temperatures lower than 10 kev it has also been found that hot electron temperatures higher than 50 kev induce a degradation of imaging contrast due to the high energy tail in the x ray spectrum the emitted x ray spectrum is composed of continuum bremsstrahlung and discrete emission lines the k u03b1 lines radiation is dominant over bremsstrahlung and can be used with appropriate filtering for imaging as it will be explained hereinbelow in more detail the laser 14 is configured for providing sufficiently high contrast sub picosecond pulses with peak and average power optimized for the imaging task i e breast thickness composition and imaging time according to the present invention the radiation pressure of the pulse produced by the laser 14 is used to balance the thermal plasma pressure occurring on the target 16 during the laser pulse this allows the thermal and hydrodynamic mechanisms tending to spread the.