[mesa-users] beta decays and electron captures

[Evan O'Connor]

Hi Kelly,

1. spectrum of neutrinos: Unless you are doing neutrino transport and/or neutrino detection the actual spectrum doesn’t matter, so as Josiah said, for MESA the first two moments (lepton number loss rate, energy loss rate) are sufficient. However, if you want to spectra for one or both of these reasons, you will have to do some extra work.  

Most of these rate calculations start of with a set of transitions between the parent and the daughter nucleus, if you could get a hold of these individual transitions (Q values and strengths for each transition) you could reconstruct the emission spectra of neutrinos at some density/temperature by summing up the individual transitions.  These transitions often come from theoretical models (like the shell model) and are not really experimental data, so rounding that up wold be a chore I think, but the Langanke+ reference you cite would be a good start. 

Apart for doing that, you already said the procedure: assume a spectra shape ‘fit’ it with the tabulated lepton number and energy loss rate to get an effective q.  If there is one dominant transition this will be a good approximation, if there are two strong transitions with different energy level differences, the actual neutrino emission spectra will look double peaked. This effective q will not reproduce that, but it will reproduce the lepton and energy emission rate, and therefore the average neutrino energy.  

MESA doesn’t need these so I doubt they are defined in the code (either the individual transitions or the effective q value)

For neutrino transport we need the spectra.  We use this effective q formalism (plug: Sullivan et al. (2015) arxiv:1508.07348).  Of interest to you would be the routines we use for solving for the effective q (hence the spectrum) from the tabulated rates are in NuLib (www.nulib.org), we can talk off-list about how to use it if you are interested/have any questions. In principle, with very little work, NuLib can return a spectrum of neutrino emission from electron capture on any nucleus at any electron density and temperature  

2. beta-decays:  One reason why beta-decay rates are ignored often is the application is core collapse. There, beta-decays are quite blocked by the high electron chemical potential.  However, they are important pre-collapse when the electron fractions are higher and densities are lower.  One relavent piece in the literature is http://adsabs.harvard.edu/abs/2000ApJS..126..493M <http://adsabs.harvard.edu/abs/2000ApJS..126..493M> , where they state that beta-decays are in fact dominant during a particular phase of evolution.

Cheers,
Evan



> On Sep 3, 2015, at 6:11 AM, Kelly Patton <kpatton5 at asu.edu> wrote:
> 
> Hi-
> 
> I’m working on a project that’s attempting to calculate neutrino spectra from various processes in the late stages of a large star’s evolution.  Right now, I’m trying to work out some of the finer points of beta decay/electron capture.  This brings me to my questions.
> 
> I know MESA calculates the energy output from these nuclear reactions, and I’m wondering how and where the Q-values for the beta decay and electron capture reactions are defined in MESA.  Are all possible initial and final states accounted for separately, or is there some kind of effective Q value (eg. Langanke and Martinez-Pinedo arXiv:nucl-th/0101039v1 defines an effective Q value as one that accurately reproduces the average neutrino energy)?  
> 
> I’ve read a few papers that disregard beta decays altogether and only worry about the electron/positron captures (that same Langanke and Martinez-Pinedo paper). Does MESA calculate both decay and capture?
> 
> Any help or clarification would be appreciated.
> 
> Thanks,
> Kelly Patton
> 
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