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None Radioactive decay of Ba-133 

Keywords: Radioactive decay
Forum: Hadronic Processes
Date: 12 Jul, 2007
From: Luciano Pandola <Luciano Pandola>

A few weeks ago, a problem in the concerning branching ratios of gamma lines emitted in the decay of Ba-133 has been reported. A bug report (#952) was open.

The primary particle is a Ba-133 nucleus at rest, and Geant4 RDM takes care of its decay.

The branching ratios were ok with Geant4 7.0 but wrong with Geant4 8.2.p01. The effect is relevant for low-energy lines (<100 keV) while no difference is seen at higher energy. For instance:

 79.6 keV       ToI: 2.62%      G4 7.0: 2.53%   G4 8.2: 2.96%
 81.99 keV      Toi:34.06%      G4 7.0:33.71%   G4 8.2:42.79% (!)
 302 keV        ToI:18.33%      G4 7.0:18.01%   G4 8.2:18.37%
 356 keV        ToI:62.05%      G4 7.0:59.96%   G4 8.2:59.86%
 L x-rays       ToI:14.45%      G4 7.0:10.51%   G4 8.2: 6.62%
 K x-rays       ToI:121.4%      G4 7.0:120.1%   G4 8.2: 85.6% (!)

The effect was not dependent on the database version in use, both for G4RADIOACTIVEDATA and for G4LEVELGAMMADATA.

I think that the problem comes (at least partially) from line #279 of processes/hadronic/models/de_excitation/photon_evaporation/src/G4VGammaDeexcitation.cc, which was changed between the versions 7.2 and 8.0:

if ( _vSN != -1) (dynamic_cast <G4DiscreteGammaTransition*> (_transition))->SetICM(false);

This line prevents more than one electron conversion in the same de-excitation cascade.

This yields to the over-estimation of the branching of some gammas. The effect is visible in those nuclei (as Ba-133) in whose cascades the probability of having more than one electron conversion is not negligible (Ba-133 may have a cascade 79 keV-81 keV)

I don't know the reason why this restriction had been implemented in the past (maybe for fixing some other problems). Probably, because the AtomicDeexcitation (as implemented in G4PhotonEvaporation) is not able to handle more than one vacancy.

If line #279 of G4VGammaDeexcitation.cc is commented, branching ratio for the 81-keV gamma ray emitted in Ba-133 decays is consistent with version 7.0 and with the Table of Isotopes. I don't know if commenting it has some unwanted side-effects in other parts of the code.

The branching ratio of K x-rays is still too low. Maybe the origin is different.

Luciano Pandola INFN-LNGS

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