diff --git a/include/musrPrimaryGeneratorAction.hh b/include/musrPrimaryGeneratorAction.hh
index 4965535..0906a86 100644
--- a/include/musrPrimaryGeneratorAction.hh
+++ b/include/musrPrimaryGeneratorAction.hh
@@ -65,6 +65,7 @@ class musrPrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
     void SetTilt(G4ThreeVector v)           {xangle0=v[0]; yangle0=v[1];}
     void SetSigmaTilt(G4ThreeVector v)      {xangleSigma=v[0]; yangleSigma=v[1];}
     void SetPitch(G4double val)             {pitch=val;}
+    void SetBeamDirection(G4ThreeVector vIniDir);
     void SetInitialMuonPolariz(G4ThreeVector vIniPol);
     void SetInitialPolarizFraction(G4double val) {
       if ((val>1.)||(val<-1.)) {
@@ -111,6 +112,7 @@ class musrPrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
     G4double xangle0, yangle0, xangleSigma, yangleSigma, pitch;
     G4bool   UnpolarisedMuonBeam, TransversalyUnpolarisedMuonBeam;
     G4double xPolarisIni, yPolarisIni, zPolarisIni;
+    G4double xDirection, yDirection, zDirection;
     G4double polarisFraction;
     G4double muonDecayTimeMin;
     G4double muonDecayTimeMax;
diff --git a/include/musrPrimaryGeneratorMessenger.hh b/include/musrPrimaryGeneratorMessenger.hh
index ae7af0e..92ac594 100644
--- a/include/musrPrimaryGeneratorMessenger.hh
+++ b/include/musrPrimaryGeneratorMessenger.hh
@@ -65,6 +65,7 @@ class musrPrimaryGeneratorMessenger: public G4UImessenger
     G4UIcmdWith3VectorAndUnit* setSigmaTiltAngleCmd;
     G4UIcmdWithADoubleAndUnit* setPitchCmd;
     G4UIcmdWith3Vector*        setMuonPolarizCmd;
+    G4UIcmdWith3Vector*        setDirectionCmd;
     G4UIcmdWithADouble*        setMuonPolarizFractionCmd;
     G4UIcmdWith3VectorAndUnit* setMuonDecayTimeCmd;
     G4UIcmdWithAString*        setTurtleCmd;
diff --git a/src/musrPrimaryGeneratorAction.cc b/src/musrPrimaryGeneratorAction.cc
index 6c066d9..6d79c57 100644
--- a/src/musrPrimaryGeneratorAction.cc
+++ b/src/musrPrimaryGeneratorAction.cc
@@ -271,8 +271,8 @@ void musrPrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
   G4double px, py, pz;
   px = p*sin(xangle);
   py = p*sin(yangle);
-  pz = std::sqrt(p*p - px*px - py*py);
-
+  // works for beam along +-z only for now
+  pz = zDirection * std::sqrt(p*p - px*px - py*py);
 
   // Assign spin 
   G4double xpolaris=0, ypolaris=0, zpolaris=0;
@@ -381,6 +381,16 @@ void musrPrimaryGeneratorAction::SetInitialMuonPolariz(G4ThreeVector vIniPol)
   }
 }
 
+//===============================================================================
+void musrPrimaryGeneratorAction::SetBeamDirection(G4ThreeVector vIniDir)
+{
+  G4double magnitude=vIniDir.mag();
+  xDirection=vIniDir(0)/magnitude;
+  yDirection=vIniDir(1)/magnitude;
+  zDirection=vIniDir(2)/magnitude;
+  G4cout<< "Initial Beam Direction set to ("<<xDirection<<","<<yDirection<<","<<zDirection<<")"<<G4endl;
+}
+
 //===============================================================================
 void musrPrimaryGeneratorAction::SetMuonDecayTimeLimits(G4ThreeVector decayTimeLimits) {
   muonDecayTimeMin = decayTimeLimits[0];
diff --git a/src/musrPrimaryGeneratorMessenger.cc b/src/musrPrimaryGeneratorMessenger.cc
index 200c0a0..4372b47 100644
--- a/src/musrPrimaryGeneratorMessenger.cc
+++ b/src/musrPrimaryGeneratorMessenger.cc
@@ -126,10 +126,15 @@ musrPrimaryGeneratorMessenger::musrPrimaryGeneratorMessenger(musrPrimaryGenerato
   //  setMuonPolarizCmd->SetDefaultValue(0) ;
 
   setMuonPolarizCmd = new G4UIcmdWith3Vector("/gun/muonPolarizVector",this);
-  setMuonPolarizCmd->SetGuidance("Set initial mu polarisation as a vector (without unit)");
+  setMuonPolarizCmd->SetGuidance("Set initial mu polarisation as a vector (without units)");
   setMuonPolarizCmd->SetGuidance("    The vector does not have to be normalised to 1");
   setMuonPolarizCmd->SetParameterName("mes_polarisX","mes_polarisY","mes_polarisZ",true,true);
 
+  setDirectionCmd = new G4UIcmdWith3Vector("/gun/direction",this);
+  setDirectionCmd->SetGuidance("Set initial mu beam direction as a vector (without units)");
+  setDirectionCmd->SetGuidance("    The vector does not have to be normalised to 1");
+  setDirectionCmd->SetParameterName("DirectionX","DirectionY","DirectionZ",true,true);
+
   setMuonPolarizFractionCmd = new G4UIcmdWithADouble("/gun/muonPolarizFraction",this);
   setMuonPolarizFractionCmd->SetGuidance(" Set the fraction of the muon polarisation (in the range of -1 to 1),");
   setMuonPolarizFractionCmd->SetGuidance(" where fraction = (N_up_spin - N_down_spin) / (N_up_spin + N_down_spin)");
@@ -196,6 +201,7 @@ musrPrimaryGeneratorMessenger::~musrPrimaryGeneratorMessenger()
   delete setSigmaTiltAngleCmd;
   delete setPitchCmd;
   delete setMuonPolarizCmd;
+  delete setDirectionCmd;
   delete setMuonPolarizFractionCmd;
   delete setMuonDecayTimeCmd;
   delete setTurtleCmd;
@@ -245,6 +251,8 @@ void musrPrimaryGeneratorMessenger::SetNewValue(G4UIcommand * command,G4String n
     { musrAction->SetPitch(setPitchCmd->GetNewDoubleValue(newValue));}
   if( command == setMuonPolarizCmd)
     { musrAction->SetInitialMuonPolariz(setMuonPolarizCmd->GetNew3VectorValue(newValue));}
+  if( command == setDirectionCmd)
+    { musrAction->SetBeamDirection(setDirectionCmd->GetNew3VectorValue(newValue));}
   if( command == setMuonPolarizFractionCmd)
     { musrAction->SetInitialPolarizFraction(setMuonPolarizFractionCmd->GetNewDoubleValue(newValue));}
   if( command == setMuonDecayTimeCmd)