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the scientific requirements and the advanced design of simbol  x @xcite imply the necessity of great care in the minimization of the background radiation at high energy .
this is canonically achieved with a detailed characterization of the expected background events @xcite , which will also be a key input for the simbol - x instrument scientific calibration @xcite . in this context , the background events can be broadly divided into two main categories : ( a ) the ones due to diffuse cxb photons , and ( b ) the hadronic component originated by prompt and delayed events caused by high energy particles .
the minimization of the photonic component is generally achieved by shielding the detector aperture to the unfocused photons through an high z material .
+ in this work a quantitative trade - off analysis of the passive shielding on board the simbol  x mirror spacecraft ( msc ) is addressed considering both its geometry and composition , with the aim of evaluating the impact of the required mass budget and design on the shielding efficiency and photonic background level .
since the formation flight architecture avoids the possibility of using the canonical telescope `` tube '' connecting the mirror to the focal plane unit , simbol  x passive shielding system consists of two main parts @xcite : the collimator tube placed on top of the focal plane and an hexagonal passive shield ( sky screen ) around the mirror module ( mm ) .     t lateral dithering .
( _ right _ ) : sky screen diameter in function of the collimator height for a perfect shielding.,title="fig : " ]   t lateral dithering .
( _ right _ ) : sky screen diameter in function of the collimator height for a perfect shielding.,title="fig : " ]    the passive shielding design must account for the formation flight relative motion , since a lateral dithering between the spacecrafts results in both a collimator walls vignetting effect and a detector opening angle to the unfocused cxb photons . if we assume a collimator height of 2.14 m and a @xmath05 mm dithering , the sky screen diameter required for a perfect shielding is 2.32 m ( see fig .
[ fig : per_shield ] ) .
+ the msc design is characterized by an hexagonal platform ( fig .
[ fig : thales ] , left panel ) , with the mm placed inside the thrust cylinder ( fig .
[ fig : thales ] , right panel ) .
the adapter , the cone trunk structure , is the interface between the mm and the thrust cylinder .
the presence of the thrust cylinder around the optics hampers a complete shielding around the mm .
this region is shielded by coating , using the same passive material of the sky screen , the adapter except for the holes , the contribution of which to the background level must be evaluated .
the msc passive shielding geometry will thus foresee the sky screen ( an hexagonal plate covering the region around the thrust cylinder ) and the adapter cover ( on the dsc side ) . since the sky screen flat - to - flat diameter is 2.38 m , the simbol 
x focal plane is shielded for a dithering up to @xmath05 mm .
+ the selected materials for the msc passive shielding main absorber are pb , w and ta . since the main absorber fluorescence lines fall within simbol  x energy range , a grading ( sn + cu + al + c ) is added .
in order to evaluate the photonic residual background level ( due to leaking or fluorescence photons ) on simbol  x focal plane , a geant4 @xcite based monte carlo simulation of the msc passive shielding interaction with the cxb flux @xcite has been implemented .
the absorption effect of the msc is simulated with a 3.5 mm thick al layer on the backside of the shielding while the thermal blankets are represented by a 1 mm thick c layer .
+ the spectrum of the msc passive shielding residual background ( fig .
[ fig : res ] ) is evaluated for a 15 kg total mass budget and a ta main absorber ( with and without the grading layers ) .
+ the horizontal dashed - dotted line shows the maximum accepted residual background level ( @xmath1 cts @xmath2 s@xmath3 kev@xmath3 ) , given by the simbol  x scientific requirements . + the resulting background spectra indicate that the fluorescence emission for an only main absorber composition is well above the limit and the grading layers are required .
the main result of the present work is that a 15 kg total mass budget allows for an average residual background flux within the requirement , with the thrust cylinder leakage component being efficiently absorbed by the payload and thermal covering .
+ although the grading layers absorb about 90% of the main absorber fluorescence emission , k@xmath4 and k@xmath5 lines are still present in the residual background spectrum .
these might be minimized by a further grading layers optimization .
the present work shows a quantitative trade - off analysis of the simbol - x mirror spacecraft ( msc ) passive shielding , in the phase space of the various parameters : mass budget , dimension , geometry and composition . 
 a simplified physical ( and geometrical ) model of the sky screen , implemented by means of a geant4 simulation , has been developed to perform a performance - driven mass optimization and evaluate the residual background level on simbol  x focal plane .     address = inaf / iasf  bologna , via gobetti 101 , 40129 bologna , italy , altaddress = dipartimento di astronomia , universit di bologna , via ranzani 1 , 40127 bologna , italy     address = inaf / iasf  bologna , via gobetti 101 , 40129 bologna , italy ,     address = inaf / iasf  bologna , via gobetti 101 , 40129 bologna , italy ,     address = dipartimento di astronomia , universit di bologna , via ranzani 1 , 40127 bologna , italy     address = thales alenia space , corso marche 41 , 10146 torino , italy     address = thales alenia space , corso marche 41 , 10146 torino , italy