Xe-137 gamma lines between version 11.1.1 and 11.2.1

Hi! I noticed a difference in the gamma lines produced by the decay of Xe-137 between version 11.1.1 and 11.2.1. In particular, it seems that a line disappears. Checking in the nudat page, that line seems to be spurious. The bins at low energy are also different. I attach the plot of the energy of the gammas produced in a simulation of Xe-137 decays.

Are you aware of any issues regarding this?



I rerun the simulation and now the two versions match.

There must have been something wrong before, sorry for the spam!
This thread can be closed.

Hi Paola,

Actually I was able to reproduce the missing peak at 661 keV in the latest Geant4 version [1,2]. I would say this peak comes from the dexcitation of 137Ba (populated from 137Cs decay). I used the example geant4/share/Geant4/examples/extended/radioactivedecay/rdecay01, and the following macro file as input to reproduce the issue

# macro for rdecay01
/control/cout/ignoreThreadsExcept 0
/control/verbose 2
/run/verbose 1
/gun/particle ion
/gun/ion 54 137
/tracking/verbose 2
/run/beamOn 1
/tracking/verbose 0
/analysis/setFileName Xe137
/analysis/h1/set 1  150  0. 1500 keV	#e+ e-
/analysis/h1/set 2  150  0. 1500 keV	#neutrino
/analysis/h1/set 3  150  0. 1500 keV	#gamma
/analysis/h1/set 6  100  0. 2500 keV	#EkinTot (Q)
/analysis/h1/set 7  150  0. 15e3 keV	#P balance
/analysis/h1/set 8  100  0. 100. year	#time of life
/analysis/h1/set 9  100  1. 3. MeV  	#EvisTot
/run/printProgress 100000  
/run/beamOn 100000

Could it be possible that the changes in the first post might be caused by changes in the Geant4 data library? Did you use the latest G4 data for the second comparison?

You may want to take a look to the beta spectrum as well… in the latest patch a bug related to the shape of the beta spectrum was fixed (I can see the changes as well)

I do not see anything below 0.2 MeV in the gamma ray spectrum produced by the example, so I can not guess by your post what is the cause of those counts.


[1] My old local version: geant4-11-01-patch-02 [MT] (15-June-2023)
[2] My newer version: geant4-11-02 [MT] (8-December-2023)

Thanks, Álvaro! Given your input, I think that my confusion comes from the fact that the first time I may have not stopped the decay to the xenon’s daughter, while the second time I used

/process/had/rdm/nucleusLimits 137 137 54 54.

That would make sense! I will double check that I am being consistent now!
Thanks again



From release notes of version 11.2:

The default value of the time threshold for radioactive decays of ions - above which these decays are ignored - has been changed, from 10^27 ns (i.e. about twice the age of the universe) to 1 year. For applications where radioactive decays of ions do play an important role, it is recommended to increase the default time threshold of these decays to a very high value, e.g. 1.0e+60 years. This can be done in either one of the following three ways:

Via UI command, e.g. /process/had/rdm/thresholdForVeryLongDecayTime 1.0e+60 year, (command to be used after /run/initialization);
Via C++ interface, e.g. G4HadronicParameters::Instance()->SetTimeThresholdForRadioactiveDecay( 1.0e+60*CLHEP::year ) (to be placed in the main program before run initialization);
Via the second parameter of the constructor of the class G4RadioactiveDecay (for analogue mode only) or G4Radioactivation (for both analogue or biased mode), e.g. G4RadioactiveDecay( "RadioactiveDecay", 1.0e+60*CLHEP::year ) or G4Radioactivation( "Radioactivation", 1.0e+60*CLHEP::year ) (this is for custom physics lists, before run initialization).

If the threshold is extended to longer value than the 137Cs half life (as it was in previous versions) the peak of 137Cs happens again.


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