  Multi-MeV flash radiography is often used as the primary diagnostic technique
for high energy and density (HED) physics experiments. Primary X-ray which is
attenuated by the object offers density information of the object. For a thick
metal object with area density as high as 150 g/cm2, the rest part of primary
X-ray which passes through the object may drowned in scattered X-ray fog. It
seriously limits accuracy of density quantification. In this research, an
online scatter estimation method is newly designed which can be easily arranged
by putting an additional slit collimator downstream of the general X-ray
radiography layout. The basic ideal of this method is that the proportion of
scatter and primary x-ray will be changed a lot when x-ray passes through a
slit like collimation, then scatter component is solvable with known the
collimation performance of the silt on scatter and primary x-ray. Monte Carlo
simulation shows that, with this method, evaluation error of average scatter is
less than 2% when object area density is as high as 200 g/cm2. In addition to
the average scatter, an accelerated Monte Carlo method is developed to obtain
scatter distribution in iterative reconstruction. By employing the Genetic
algorithm as an optimizer, reconstruction can be done by searching a density
which projection with scatter best matches experimental result. This
reconstruction method requires neither a priori-knowledge such as mass
restriction nor regularization. Simulations show that for France Test Object
(FTO), the error of reconstructed density is less than 2%, and uncertainty
basically covers the real density.
