Created attachment 801 [details]
Sample program to reproduce problem

If I define a material using its isotopes composition the physics results change depending on the order in which I add the isotopes to the material.
I have attached an applications where neutrons are shot against a Fe target.
When run with run1.mac it produces root histograms with the abundance of activation isotopes as a function of time. Each histogram is for a different activation isotope as shown in its title. The abundance of 54Mn created varies by orders of magnitude depending on whether the Fe material is defined as in case A or case B below in the file DetectorConstruction.cc.


*************** case A ***************************
G4Material* Fe = nist->FindOrBuildMaterial("G4_Fe");
  // define an Element from isotopes, by relative abundance
  G4Isotope* Fe54 = new G4Isotope("Fe54", 26, 54,53.94*g/mole);
  G4Isotope* Fe56 = new G4Isotope("Fe56", 26, 56,55.93*g/mole);
  G4Isotope* Fe57 = new G4Isotope("Fe57", 26, 57,56.94*g/mole);
  G4Isotope* Fe58 = new G4Isotope("Fe58", 26, 58,57.93*g/mole);

  G4Element* elFe  = new G4Element("natural iron","Fe", 4);
  elFe->AddIsotope(Fe56,  91.754*perCent);
  elFe->AddIsotope(Fe54, 5.845*perCent);
  elFe->AddIsotope(Fe57,  2.119*perCent);
  elFe->AddIsotope(Fe58,  0.282*perCent);

  G4Material* Femat = new G4Material("Femat",7.874*g/cm3,1);

*************** case B ***************************
G4Material* Fe = nist->FindOrBuildMaterial("G4_Fe");
  // define an Element from isotopes, by relative abundance
  G4Isotope* Fe54 = new G4Isotope("Fe54", 26, 54,53.94*g/mole);
  G4Isotope* Fe56 = new G4Isotope("Fe56", 26, 56,55.93*g/mole);
  G4Isotope* Fe57 = new G4Isotope("Fe57", 26, 57,56.94*g/mole);
  G4Isotope* Fe58 = new G4Isotope("Fe58", 26, 58,57.93*g/mole);

  G4Element* elFe  = new G4Element("natural iron","Fe", 4);
  elFe->AddIsotope(Fe54, 5.845*perCent);
  elFe->AddIsotope(Fe56,  91.754*perCent);
  elFe->AddIsotope(Fe57,  2.119*perCent);
  elFe->AddIsotope(Fe58,  0.282*perCent);

  G4Material* Femat = new G4Material("Femat",7.874*g/cm3,1);