Dear all,
Recently I’m trying to simulate the decay characteristics of two extremely low-lying isomers, 229mTh (7.6 eV) in Geant4.
I found that in version 4.9.6, there seems to be an “energy threshold” for simulatable energy levels. Following example rdecay02, I tried to add user-defined LevelGamma data for 229mTh but it end up only being able to produce gamma-ray energy above 100 keV.
Then I moved to version 4.10.6, where a “level tolerance (for energy)” can be set from G4NuclideTable and a “minimum excitation” can be set from G4DeexPrecoParameters. I tried both ways but the result for the gamma decay of 229mTh is very strange when using the default LevelGamma data (see the figure below). Firstly, the gamma rays of energies above 10 keV are emitted even though the level energy is only 7.6 eV. Secondly, the half-life of the isomer remains about 10^-10 s regardless of any change in half-life settings in the user-defined LevelGamma data.
Your help and suggestions are greatly appreciated.
Regards,
Lan
unfortunately, you need to wait for the end of June. In the new version of Geant4 11.2beta this case better addressed. Please, retry your simulation when this version will be out.
According PhotonEvaporation5.7, excited state Th229[7.6 eV] should go on ground state Th229. His mean life is zero.
Instead, it decays by alpha emission, although this decay is not present in RadioactiveDecay5.6, for this excited state.
I use latest G4 development version: ref-05, 31 May 2023
Where is the improvement ? Th229.mac.txt (230 Bytes)
As far as I know, the decay characteristics of this isomer are rather tricky. Since its level energy is higher than the ionization energy of the thorium atom(~6 eV), it decays predominately by emitting internal conversion electrons, with a short half-life of about 7.6 μs. The direct decay by gamma-ray emission is less likely, with an internal conversion coefficient of ~10^9 and an “effective” half-life of ~10^4 s. Consequently, the decay channels would heavily depend on the ionic state of the atom. For instance, a 229mTh^2+ ion would only decay by direct gamma transition since the internal conversion channel is suppressed.
BTW, I don’t know if such dependence of decay channel on ionic states will be considered in the new release. Maybe we can consider this effect in a user-defined way in the future.
