diff --git a/Simulation/G4Extensions/G4ExternalDecay/CMakeLists.txt b/Simulation/G4Extensions/G4ExternalDecay/CMakeLists.txt
index 181c1a740d5cd5fa2fd3cb55806e2f2766c5d47a..619384a27e8db65a1a0d2f99e3eccd954cf1bf7a 100644
--- a/Simulation/G4Extensions/G4ExternalDecay/CMakeLists.txt
+++ b/Simulation/G4Extensions/G4ExternalDecay/CMakeLists.txt
@@ -1,6 +1,4 @@
-################################################################################
-# Package: G4ExternalDecay
-################################################################################
+# Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 
 # Declare the package name:
 atlas_subdir( G4ExternalDecay )
@@ -37,5 +35,5 @@ atlas_add_library( G4ExternalDecay
                    INCLUDE_DIRS ${XERCESC_INCLUDE_DIRS} ${CLHEP_INCLUDE_DIRS} ${LHAPDF_INCLUDE_DIRS} ${GEANT4_INCLUDE_DIRS} ${PYTHIA8_INCLUDE_DIRS} ${extra_includes}
                    PRIVATE_INCLUDE_DIRS 
                    DEFINITIONS ${CLHEP_DEFINITIONS}
-                   LINK_LIBRARIES ${XERCESC_LIBRARIES} ${CLHEP_LIBRARIES} ${PYTHIA8_LIBRARIES} ${LHAPDF_LIBRARIES} ${GEANT4_LIBRARIES} Pythia8_iLib ${extra_libs}
+                   LINK_LIBRARIES ${XERCESC_LIBRARIES} ${CLHEP_LIBRARIES} ${PYTHIA8_LIBRARIES} ${LHAPDF_LIBRARIES} ${GEANT4_LIBRARIES} CxxUtils Pythia8_iLib ${extra_libs}
                    PRIVATE_LINK_LIBRARIES AtlasHepMCLib )
diff --git a/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/ATLAS_CHECK_THREAD_SAFETY b/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/ATLAS_CHECK_THREAD_SAFETY
new file mode 100644
index 0000000000000000000000000000000000000000..9271bccd5d6b2cff4e135fb7533d9f1d5d6c0a2e
--- /dev/null
+++ b/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/ATLAS_CHECK_THREAD_SAFETY
@@ -0,0 +1 @@
+Simulation/G4Extensions/G4ExternalDecay
diff --git a/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/Pythia8ForDecays.h b/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/Pythia8ForDecays.h
index a782ed4ef76e98df701dc9bfe01bc1098e787ab4..d9e5c6148e090ae8bdb9ecfae093125dafec8f15 100644
--- a/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/Pythia8ForDecays.h
+++ b/Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/Pythia8ForDecays.h
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 // Abused from Geant4 version of Pythai6.hh from extended examples
@@ -15,9 +15,10 @@
 #include <utility>
 // For all the various Pythia8 classes used here
 #include "Pythia8_i/Pythia8_i.h"
-// For unique_ptr and once flag
+// For unique_ptr
 #include <memory>
-#include <mutex>
+// For ATLAS THREAD macros
+#include "CxxUtils/checker_macros.h"
 
 class G4DynamicParticle;
 class G4ParticleDefinition;
@@ -28,7 +29,7 @@ class Pythia8ForDecays
 
    virtual ~Pythia8ForDecays() = default;
 
-   static Pythia8ForDecays *Instance();
+   static Pythia8ForDecays *Instance ATLAS_NOT_THREAD_SAFE ();
 
    /// Function that decays the RHadron; returns products in G4 format
    void Py1ent(const G4Track&, std::vector<G4DynamicParticle*> &);
@@ -49,10 +50,6 @@ class Pythia8ForDecays
    std::pair<int,int> fromIdWithSquark( int idRHad) const;
    bool isGluinoRHadron(int pdgId) const;
 
-   /// My own class; singleton pattern; thread safe for future-proofing
-   static std::unique_ptr<Pythia8ForDecays> s_instance;
-   static std::once_flag m_onceFlag;
-
    /// The instance of Pythia8 that will do the work
    std::unique_ptr<Pythia8::Pythia> m_pythia;
 };
diff --git a/Simulation/G4Extensions/G4ExternalDecay/src/Pythia8ForDecays.cxx b/Simulation/G4Extensions/G4ExternalDecay/src/Pythia8ForDecays.cxx
index c5173dcb754dd5d7b1fa06e61256355d82569f5c..fa7eba76cb61de3044c2ffe630545ab6fdd15d85 100644
--- a/Simulation/G4Extensions/G4ExternalDecay/src/Pythia8ForDecays.cxx
+++ b/Simulation/G4Extensions/G4ExternalDecay/src/Pythia8ForDecays.cxx
@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 // Abused from Pythia6.cc in Geant4 extended examples
@@ -30,15 +30,10 @@
 #include <cmath>
 #include <fstream>
 
-std::unique_ptr<Pythia8ForDecays> Pythia8ForDecays::s_instance(nullptr);
-std::once_flag Pythia8ForDecays::m_onceFlag;
 
-Pythia8ForDecays* Pythia8ForDecays::Instance() {
-  std::call_once(m_onceFlag,
-        [] {
-            s_instance.reset(new Pythia8ForDecays);
-    });
-  return s_instance.get();
+Pythia8ForDecays* Pythia8ForDecays::Instance ATLAS_NOT_THREAD_SAFE () {
+  static Pythia8ForDecays instance;
+  return &instance;
 }
 
 Pythia8ForDecays::Pythia8ForDecays()