[mesa-users] ni56 beta decay rate is function of temperature? and goes to 0 for logT < 8?
Schatz, Hendrik
schatz at nscl.msu.edu
Wed Oct 23 17:11:42 EDT 2013
Hi Bill,
For neutron deficient isotopes on can have electron capture and beta+ decay. Terrestrial 56Ni decays (from the ground state of 56Ni) occur primarily by electron capture. The beta+ branch from the ground state is very small, I think experimentally there is only an upper limit of a half-life of 30,000 years. This is a forbidden transition. The -99.000 in the table is probably too small, the reason probably being that forbidden transitions are not included in the theory that was used to make the table. The higher beta+ rates at high temperature may come from allowed decays from thermally excited states in 56Ni.
However, the beta+ should only be relevant if one does not have any electron capture. I think this only applies to environments with no electrons around, such as cosmic rays (or storage rings). Looking at the Fuller Fowler Newman tables (I don't have the Langanke ones ready) it seems that at all densities down to 10 g/cm^3 (the lowest in the table) and all temperatures electron capture dominates over beta+ decay. The code should use the sum of the two. So the relevant rate determining how long 56NI sticks around should be the electron capture rate. What is the electron capture rate entry in MESA for the conditions you are interested in?
Hendrik
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Hendrik Schatz
National Superconducting Cyclotron Laboratory
Michigan State University
640 S. Shaw Lane
East Lansing, MI 48824-1321
USA
E-mail: schatz at nscl.msu.edu<mailto:schatz at nscl.msu.edu>
Web : http://www.nscl.msu.edu/~schatz
Phone : +1 (517) 333 6397
Fax : +1 (517) 353 5967
see also Joint Institute for Nuclear Astrophysics
http://www.jinaweb.org<http://www.jinaweb.org/>
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On Oct 23, 2013, at 1:37 PM, Bill Paxton wrote:
Hi,
My turn to ask a question -- I have a model that manages to get ni56 up to regions of lower temperature compared to the usual places where it lives, i.e., where logT < 8 rather than the usual logT > 9. I was surprised to see the ni56 sticking around for a years when logT < 8.0, so I took a look at the weakreactions.tables in mesa/data/weaklib_data. You can find the table for ni56 beta decay by searching for "lbeta+ positron emission; ni56 co56". The values in the table are log10 of decay rate. The columns are temperatures in T9 = T*10^-9. Up at the high T9 end the rates are very large, while down at the low end, including my case of T9 = 0.1, they effectively go to zero.
ni56 co56 by positron emission and electron capture; LMP
T9 0.01 0.10 0.20 0.40 0.70 1.00 1.50 2.00 3.00 5.00 10.00 30.00
lYeRho lbeta+ positron emission; ni56 co56
1 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.021 -3.557 -1.373 -0.071
2 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.021 -3.557 -1.373 -0.071
3 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.021 -3.557 -1.373 -0.071
4 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.021 -3.557 -1.373 -0.071
5 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.021 -3.557 -1.373 -0.071
6 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.020 -3.556 -1.373 -0.071
7 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.019 -3.553 -1.370 -0.071
8 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.019 -3.551 -1.358 -0.068
9 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.019 -3.551 -1.354 -0.049
10 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.019 -3.551 -1.354 -0.028
11 -99.999 -99.999 -69.740 -35.714 -21.132 -15.299 -10.760 -8.472 -6.019 -3.551 -1.354 -0.027
I had expected to find something more like what I find for mg20 beta decay where the rates are (almost) independent of T.
mg20 na20 by positron emission and electron capture; OHMT
T9 0.01 0.10 0.20 0.40 0.70 1.00 1.50 2.00 3.00 5.00 10.00 30.00
lYeRho lbeta+ positron emission; mg20 na20
1 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.765 0.766 0.799 1.382
2 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.765 0.766 0.799 1.382
3 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.765 0.766 0.799 1.382
4 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.765 0.766 0.799 1.382
5 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.799 1.382
6 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.800 1.382
7 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.768 0.802 1.382
8 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.769 0.811 1.385
9 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.769 0.814 1.406
10 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.769 0.814 1.429
I thought this might be a bug in the preprocessor that builds the weakreactions.tables file from the separate sources. The ni56 => co56 reaction rates come from "LMP",
LMP: K. Langanke, G. Martínez-Pinedo / Nuclear Physics A 673 (2000) 481–508
so I checked that input file (mesa/weaklib/preprocessor/input_data/lmp.data). But I found the "-99.999" entries for beta decay rates there too.
neg. daughter ni56 z=28 n=28 a=56 q= -1.6240
pos. daughter co56 z=27 n=29 a=56 q= 1.6240
+++ ni56 --> co56 +++ --- co56 --> ni56 ---
t9 lrho uf lbeta+ leps- lrnu lbeta- leps+ lrnubar
0.01 1.0 -0.003 -99.999 -8.684 -9.066 -99.999 -99.999 -99.999
0.10 1.0 -0.058 -99.999 -9.164 -9.537 -99.999 -99.999 -99.999
0.20 1.0 -0.134 -69.740 -9.291 -9.652 -69.759 -74.128 -70.327
0.40 1.0 -0.306 -35.714 -9.387 -9.722 -37.930 -38.276 -38.162
0.70 1.0 -0.503 -21.132 -8.777 -9.063 -24.300 -23.078 -23.182
1.00 1.0 -0.511 -15.299 -7.439 -7.669 -18.824 -17.809 -17.866
What's the story here? Is ni56 stable to beta decay at logT < 8? ;p Curiouser and curiouser.
Thanks,
Bill
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