Isomeric transition gammas from Lead neutron activation

I am interested in simulating natural lead activation with 14 MeV neutrons. The specific reactions I care about are Pb207(n,n’)Pb207m and Pb208(n,2n)Pb207m. The Pb207m undergoes Isomeric Transition with 0.806 sec half-life time constant, emitting two gammas with energies 569.698 keV and 1063.656 keV.

The output I want to model is those delayed gamma, i.e., I want to correctly model the gammas’ emission time and energy. But I have not been able to get that from Geant4 yet.

To study this, I set the lead volume to have only one isotope type at a time, and this is what I’ve found so far:

1-Gamma emission energy: individual gammas emitted from neutron inelastic processes seem to follow the decay scheme for most isotopes. I think these are mostly (n,n’) reactions.

However, for Pb208, the individually emitted gammas form “boxes” in the spectrum around the decay scheme gamma lines. See the attached plot. These would include the Pb208(n,2n) reaction gammas. When summing the total energy from all neutron inelastic processes in the event, the Pb208 spectrum boxes are “washed out” into rolling bumps.

My question: Is there a reason to expect that the inelastic gammas emitted from Pb207(n,n’)Pb207m to be modeled more accurately than those from Pb208(n,2n)Pb207m, even though they have the same final excited state.

Note: It does not seem that Geant4 is actually modeling Pb207m as the final state.

2-Gamma emission time: the Pb207m isomeric transition to the ground state has a 0.806 sec half-life time constant. However, Geant4 is not producing delayed gammas with that time signature.

My question: is the isomeric transition time signature modeled in Geant4? If so, how is that turned on?




what Physics List do you use? I would suggest to try out QGSP_BIC_HP.


Hi Vladimir,
Thank you very much for your reply, and I apologize for delaying mine.
FYI, for the results above were obtained with geant4.10.05.p01 and the QGSP_BERT_HP Physics List. As you suggested, I run with QGSP_BIC_HP and got the same results.

The results do not seem to depend on any of these parameters (which I will just list below assuming you are familiar with all of them, but let me know if more detail is useful):


deex->SetMaxLifeTime, sec




NeutronHP uses PhotonEvaporation data set to modelise the deexcitation of excited states, but do not create explicitly such states.
Since Geant4 10.5, BinaryCascade generates explicitly excited ions, which can be passed to radioactiveDecay for deexcitation.
To see the difference, you can run Hadr03 with attached macro (becepe1.mac) with QGSP_BIC (or QBBC) physics constructor instead of QGSP_BIC_HP.

I have run example Activation (in extended/radioactivedecay) with macro becepe2.mac and QBBC physics constructor. On the plots, you can see that Pb207m is correctly handled.

PS: in becepe2.mac, the inactivate commands are only for simplicity; they are not essentials here.
becepe1.mac.txt (202 Bytes)
becepe2.mac.txt (628 Bytes)

Here the macro and the plots for Pb208 activation

becepe3.mac.txt (629 Bytes)

Hi maire, thank you very much for taking interest in this post. I ran the example Activation, and verified the results. It is great news! Thanks!

I had selected the “HP” physics list since I also care about the rate of those neutron activations, and assumed that the “HP” list would be the appropriate one to make sure cross section data are used for neutron transport and neutron inelastic interactions (n,n’) and (n,2n). That said, it is not clear to me what 208Pb(n,2n)207mPb cross section data would Geant4 use since the only data I found in was EAF-2010, which I believe is not one of the datasets “HP” lists use.

I will run a test to check the rate of the 208Pb(n,2n)207mPb and 207Pb(n,n’)207mPb interactions. But I am curious as to what is your input on this.

Thanks again!

From Hadr03 output, we can read : neutron inelastic cross section for neutron (14 MeV) on Pb207 : ~ 2.50 barn. Similar value for Pb208.
Also, similar results with G4NeutronInelasticXS data set (eg. BIC) and and G4NeutronHPInelasticXS (eg. HP).
Last year, Vladimir Ivantchenko did substantial enhancements in G4NeutronInelasticXS.
I hope he would comment himself.
Since BinaryCascade generates explicitly excited states, we can extract the contribution of the sub-channel Pb207m : ~0.3% for Pb207, 17.7% for Pb208. This cannot be done with neutronHP.
About example Activation, I admit that the output is too detailed. I suggest to modify line 75 of :
if ((track->GetTrackID() > 1) && (meanLife != 0.)) …