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Warning Alpha energies in Radioactive Decay are wrong! 

Forum: Hadronic Processes
Date: 02 Jul, 2014
From: Kseniia Rezynkina <Kseniia Rezynkina>

Hello,

It appears that all of the alpha energies in the "radioactive decay" are calculated incorrectly.

This was noticed for a handful of isotopes.
As an example, I've been trying to model the decay of 221Th. The kinetic energy of an alpha particle in g.s.->g.s. alpha decay should be 8470 keV.
To check this number, we took the atomic masses from AME2003:
M_a(221Th) = 205877.112 MeV
M_a(217Ra) = 202140.085 MeV
M_(alpha) = 3727.379 MeV
And derived the Q-value:
Q = 205877.112e3 - 202140.085e3 - 3727.379e3 - 2*511 = 8626 keV =>
E_alpha = 8626/(1+4./217) = 8470 keV
This agrees to the measured alpha energy (see, for example, https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html - 8470(5) keV)

Still, Geant4 generates alpha particles of 8509 keV for this transition.

We've tried to reverse-injeneer what's happening in Geant4 in order to find what's wrong.
Judging by the fact that G4 always returns the masses of bare nuclei when asked for the total energy of a nucleus at rest, we supposed that the NUCLEAR masses (atomic mass - electrons + binding energy) may have been used for the calculation of the alpha energy INSTEAD OF ATOMIC.
We have used the following approximation formula to deduce the M_n
http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/07/02/06.58-56403-AME2012.pdf
Which gives:
M_n(217Ra) = 202095.797 MeV
M_n(221Th) = 205831.843 MeV
So, if indeed the nuclear masses were by mistake taken instead of the atomic ones, and the whole lot of the electrons was ignored:
"Q" = 205831.843e3 - 202095.797e3 - 3727.379e3 = 8667 keV =>
"E_alpha" = 8510 keV

As Geant4 proposes alpha-energy of 8509 keV instead of 8470, I assume that indeed there is a mistake and an atomic mass is replaced by the nuclear mass. Same calculations can be carried out for various nuclides.

It is also wrong to use the nuclear masses, because practically none of them were measured directly, but are derived from the estimates similar to the one I've attached. It's the atomic masses that are measured.

Also, it is wrong that an object "ion" has a mass such as a bare nucleus. That's not what's happening in real life, they are atoms…! Fully stripping a nucleus like this is a challenge.

I failed to find where the masses are tabulated in Geant4. Could you tell me where is this? I guess, one could modify those files?
Are the tabulated masses the nuclear ones or the atomic ones (such as from AME)? If latter, where is the function that converts them?

Kind regards,
Kseniia

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1 None: Re: Alpha energies in Radioactive Decay are wrong!   (Laurent Desorgher - 10 Jul, 2014)
2 None: Re: Alpha energies in Radioactive Decay are wrong!   (Laurent Desorgher - 10 Jul, 2014)
3 None: Re: Alpha energies in Radioactive Decay are wrong!   (Laurent Desorgher - 10 Jul, 2014)
4 None: Re: Alpha energies in Radioactive Decay are wrong!   (Laurent Desorgher - 18 Jul, 2014)
1 None: Re: Alpha energies in Radioactive Decay are wrong!   (Laurent Desorgher - 18 Jul, 2014)
2 None: Re: Alpha energies in Radioactive Decay are wrong!   (Karl Hauschild - 09 Oct, 2014)
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