#include "G4EmCalculator.hh"
#include "G4ParticleDefinition.hh"
#include "G4Gamma.hh"
#include "globals.hh"
#include "G4UnitsTable.hh"




//G4Gamma aGamma = G4Gamma::G4Gamma();
G4EmCalculator emCalculator; 
G4Material* material = G4NistManager::Instance()->FindOrBuildMaterial("G4_WATER"); 
G4double massSigma = emCalculator.ComputeCrossSectionPerVolume(20*CLHEP::keV,G4Gamma::GammaDefinition(),"compt",material);  
//G4cout << G4BestUnit(massSigma, "Surface/Volume") << G4endl;
G4cout << "cs: " << massSigma/CLHEP::cm2 << " cm^2 " << G4endl;