Neutrons not produced (NeutronSource example)

kitu.utkarsh · April 7, 2026, 12:59am

Hi all,

I am not seeing any neutron production following a 7Li(t,n) reaction.

I just modified the NeutronSource example to modify the absorber material as `G4_LITHIUM_FLUORIDE` and my particle is a triton of energy 1 MeV (I tried other energies as well).

I’m using the same physics list as in the original example, except I am using the below physics constructor for Hadron Elastic collisions

fHadronElastic = new HadronElasticPhysicsHP(verb);
// fHadronElastic = new G4HadronElasticPhysicsXS(verb); //This was the default

However, I am not seeing any neutrons being produced. From the below verbosity, I think there is no nuclear reaction happening; only the tritium is decaying. through beta decay.

Region <DefaultRegionForTheWorld> --  -- appears in <World> world volume
 This region is in the mass world.
 Root logical volume(s) : World 
 Pointers : G4VUserRegionInformation[0x0], G4UserLimits[0x0], G4FastSimulationManager[0x0], G4UserSteppingAction[0x0]
 Materials : Air Stainless-Steel G4_LITHIUM_FLUORIDE 
 Production cuts :   gamma 10 km      e- 10 km      e+ 10 km  proton 0 fm 

Region <DefaultRegionForParallelWorld> --  -- is not associated to any world.
 Root logical volume(s) : 
 Pointers : G4VUserRegionInformation[0x0], G4UserLimits[0x0], G4FastSimulationManager[0x0], G4UserSteppingAction[0x0]
 Materials : 
 Production cuts :   gamma 10 km      e- 10 km      e+ 10 km  proton 0 fm 

========= Table of registered couples ============================

Index : 0     used in the geometry : Yes
 Material : Air
 Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
 Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
 Region(s) which use this couple : 
    DefaultRegionForTheWorld

Index : 1     used in the geometry : Yes
 Material : Stainless-Steel
 Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
 Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
 Region(s) which use this couple : 
    DefaultRegionForTheWorld

Index : 2     used in the geometry : Yes
 Material : G4_LITHIUM_FLUORIDE
 Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
 Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
 Region(s) which use this couple : 
    DefaultRegionForTheWorld

==================================================================

### Run 3 starts.

========================================================================================
--> G4TaskRunManager::CreateAndStartWorkers() --> Creating 1 tasks with 1 events/task...
========================================================================================

G4WT4 > 
G4WT4 > ========= Table of registered couples ============================
G4WT4 > 
G4WT4 > Index : 0     used in the geometry : Yes
G4WT4 >  Material : Air
G4WT4 >  Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
G4WT4 >  Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
G4WT4 >  Region(s) which use this couple : 
G4WT4 >     DefaultRegionForTheWorld
G4WT4 > 
G4WT4 > Index : 1     used in the geometry : Yes
G4WT4 >  Material : Stainless-Steel
G4WT4 >  Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
G4WT4 >  Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
G4WT4 >  Region(s) which use this couple : 
G4WT4 >     DefaultRegionForTheWorld
G4WT4 > 
G4WT4 > Index : 2     used in the geometry : Yes
G4WT4 >  Material : G4_LITHIUM_FLUORIDE
G4WT4 >  Range cuts        :  gamma  10 km     e-  10 km     e+  10 km  proton 0 fm 
G4WT4 >  Energy thresholds :  gamma  9.54965 GeV    e-  9.54965 GeV    e+  9.54965 GeV proton 0 eV 
G4WT4 >  Region(s) which use this couple : 
G4WT4 >     DefaultRegionForTheWorld
G4WT4 > 
G4WT4 > ==================================================================
G4WT4 > 
G4WT4 > ### Run 0 starts on worker thread 4.
G4WT4 > 
G4WT4 > *********************************************************************************************************
G4WT4 > * G4Track Information:   Particle = triton,   Track ID = 1,   Parent ID = 0
G4WT4 > *********************************************************************************************************
G4WT4 > 
G4WT4 > Step#       X          Y          Z         KineE         dEStep    StepLeng   TrakLeng    Volume     Process
G4WT4 >     0   7.624 mm  -8.246 mm    -2.1 cm       0 eV           0 eV       0 fm       0 fm G4_LITHIUM_FLUORIDE   initStep
G4WT4 >     1   7.624 mm  -8.246 mm    -2.1 cm       0 eV           0 eV       0 fm       0 fm G4_LITHIUM_FLUORIDE       Decay
G4WT4 > [thread 4] Thread-local run terminated.
G4WT4 > [thread 4] Run Summary
G4WT4 > [thread 4]   Number of events processed : 1
G4WT4 > [thread 4]   User=0.000000s Real=0.001551s Sys=0.000000s [Cpu=0.0%]
 Run terminated.
Run Summary
  Number of events processed : 1
  User=0.050000s Real=0.012727s Sys=0.010000s [Cpu=471.4%]

 The run is 1 triton of 0 eV  within G4_LITHIUM_FLUORIDE (D =  3 cm  L = 6 cm )

 Process calls frequency :
                Decay=      1 

 List of generated particles (with meanLife != 0) :

 Mean energy deposit per event = 0 eV ;  rms = 0 eV 
 Mean energy flow per event    = 0 eV ;  rms = 0 eV 

 List of particles emerging from the container :
There are histograms that can be viewed with visualization:
  14 h1 histograms(s)
  List them with "/analysis/list".
  View them immediately with "/vis/plot" or "/vis/reviewPlots".
  But...there are no entries. To make your histograms available for
  plotting in this UI session, use CloseFile(false) in your
  EndOfRunAction and Reset() in your BeginOfRunAction.
1 event has been kept for refreshing and/or reviewing.
  "/vis/reviewKeptEvents" to review one by one.
  To see accumulated, "/vis/enable", then "/vis/viewer/flush" or "/vis/viewer/rebuild".

Any insights into what I am missing will be highly appreciated.

Thanks a lot in advance.

Edit 1: I try to diagnose with the Hadr03 and this is the list of reactions I got, I don’t see any 7Li(t,n)9Be reaction. The output is attached in the test.out.

test.out.txt (29.2 KB)

jrellin · April 7, 2026, 11:52am

Tritium, unlike a neutron, is charged. 1 MeV is not sufficient to cross the coulomb barrier between Li7 and Tritium.

Edit - Using this calculator, I get about 1.07 MeV for the barrier. Generally you want to be a bit higher than this.

David-Batchelor · April 7, 2026, 5:08pm

Dear K,

I built Hadr06 and used the attached mac file. I do not see tracks my vis.mac is not setup for this also the tritium appears to have a very high cross section. I see production for 1MeV and attach the two pdfs for 1MeV and 2MeV- I did the dummy with LiF replaced by Galactic and just see Tritium.

So I See plenty of thermalized neutrons. Can someone explain this for me as, for me, it’s not a typical moderation route.
Davi

triitium.pdf (19.8 KB)

1Mev.pdf (19.0 KB)

maire1 · April 7, 2026, 5:11pm

If you have G4TENDL1.4 installed,

in PhysicsList use G4IonPhysicsPHP() constructor

Hadr03 output :

kitu.out.txt (2.4 KB)

David-Batchelor · April 7, 2026, 5:12pm

trial.mac.txt (683 Bytes). I had a problem wit the Mac file

David-Batchelor · April 7, 2026, 5:15pm

or provide me a link to literature

MathieuBoissier · April 7, 2026, 5:20pm

I think the reason of the thermalized spectra of neutrons is that you score neutrons emerging from the sphere which is water by default in Hadr06

kitu.utkarsh · April 7, 2026, 5:35pm

Hi,

Thank you for the suggestion.

I tried to use TENDL dataset but not sure if I did that correctly as i get segmentation fault.

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : hadhp01
      issued by : G4ParticleHPName::Init(...)
Data for Z=3 A=7 M=0 does not exist in G4NEUTRONHPDATA

*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

G4ParticleHPNames: There are no data available for some isotopes in this material 
G4ParticleHPNames: nor are there data for nearby isotopes.
G4ParticleHPNames: Please make sure G4NEUTRONHPDATA points to the directory 
G4ParticleHPNames: in which the neutron scattering data are located.
G4ParticleHPNames: The material was A = 7, Z = 3

If it helps, here is what I did to include the TENDL dataset.

1.) Downloaded and extracted the dataset in the Geant4/install/data/ directory.

2.) Set the environment variable in the geant4.sh file

export G4PARTICLEHPDATA="`cd $geant4make_root/../data/G4TENDL1.4 > /dev/null ; pwd`" #Added separately

Do I also have to do something for the G4NEUTRONHPDATA variable ?

Thanks

David-Batchelor · April 7, 2026, 9:33pm

Thanks. But I changed the World to “G4_Galactic”.

p.s. triton is of course not tritium. And this may be part of the reason it’s charged and presumably pretty big. Certainly changing it to a neutron was not so successful :-). I changed the vis.mac to make triton pink and also put a hole in the sphere and so could see the start.
triton doesn’t make it into the rest of the sphere except when it is “G4_Galactic”

maire1 · April 8, 2026, 10:38am

I am not too much familiar with environment variables. I hope that somebody else will help you more efficiently.

Here, my geant4.csh file that I “source” each time I open a new terminal screen. Around end of file, you can see that I do nothing concerning G4NEUTRONHPDATA

geant4.csh.txt (3.8 KB)