According to literature 55Fe decays via electron capture to the 7/2- level of 55Mn (100%). The subsequent gamma decay at 126 keV has an intensity of only 1.3×10-7. However, the vacancy in the atomic shell of 55Fe produces fluorescence gammas with intensities which are several orders of magnitude higher, e.g. Kα1 x ray at 5.898 keV, I(x ray): 16.2. After electron capture we have at the same time an excited 55Mn and a vacancy in the atomic shell of 55Fe. Why the intensities are so different ? Is it due the internal conversion ?
Ciao Jörg
Your literature is wrong! Following NNDC, the 3/2- gs of 55Fe decays via the "allowed" decay to the 5/2- gs of 55Mn. Only a small 10-7 fraction goes via the "forbidden" transition to the 7/2- excited state which decays via the 126 keV transition (which also will have a small electron conversion) to the gs of 55Mn. I'm not sure how the "allowed"/"forbidden" arguments correlated with angular momentum hold true for EC, but the captured electron will be preferably a s-electron with l=0 and the neutrino will still have preferrably l=0 too.
Cheers
Thorsten
http://www.nndc.bnl.gov/useroutput/AR_3893854085A2E4956D45AFC330C3F86F_2.html
According to the decay scheme of NNDC the level 7/2- level at 126-keV is populated only 1.3e-7% of the time and not 100% as you indicate.
Almost 100% of the 55Fe decays directly populate the ground state of 55Mn. So, one would not expect to see many 126-keV transitions.
As an aside the total internal conversion coefficient for a pure M1 of 126 keV in Mn is 0.01652 (see http://bricc.anu.edu.au/index.php) so it does not affect the gamma-ray intensity by much. The lack of 126-keV transitions is simply because the level is not
populated.
Thank you very much. I've misread the level scheme but it didn't came to my mind when looking at the intensities. Are there known X ray sources where nuclear and atomic transitions have approx. the same transition probabilites ?
Any EC would do the job if the mother and the gs of the daughter differ by more than 1 in spin. Even-even to odd-odd or vice versa are good candidates to search for. E.g. the well known 152Eu has 3- (odd-odd), the daughter 152Sm is even-even, so the EC will feed only excited states which decay by gammas or CE. But these X rays are too hard for your application?
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