double radiator_length = 500.*mm;
double prism_length = 200*mm;

 G4ThreeVector prism_pos = G4ThreeVector(-179.5*mm, radiator_length/2+prism_length/2, 0.*mm);
 prism_pos_arr[k][i] = prism_pos+G4ThreeVector((33.+3*prism_width)*k+(1.+prism_width)*i,0.,0.);
 k 0...4
 i 0...2