Discussion in 'Chemistry' started by Cat_with_no_eyes, Dec 6, 2010.
Any ideas would be appreciated.
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Iodine-123 (123I or I-123) is a radioactive isotope of iodine used in nuclear medicine imaging, including single photon emission computed tomography (SPECT). The isotope's half-life is 13.22 hours; the decay by electron capture to tellurium-123 emits gamma radiation with predominant energies of 159 keV (this is the gamma primarily used for imaging) and 127 keV.
Wikipedia is your friend.
Interesting. I did not know that part now bold. Can very pure mass of I-123 be charged positively to prolong it half life?
OR does the nucleus capture one of the S-electrons which in classical terms passes thru the nucleus? I suspect this s-electron capture is the case, so charging positive would have no effect. As all elements have S-electrons, why is their similar decay not more common?
Electron capture (EC) isotopes are the ones which are notable exceptions to the half-life being a constant. There are numerous EC isotopes that have significant variations (several %) in the half-life depending upon their chemical state. These variations have been noted for many decades. Of course, the k-shell orbital electrons are the 'closest' to the nucleus, but the outer orbitals also come near to the nucleus, and thus changing their orbital chemistry does have a change in the ability to capture the electron, and thus changes the half-life depending upon the chemical state. PM me if you want more detail.
Thanks for confirming the non-constancy of EC type decay. I noted that the it would almost always be the s-electrons that were captured.
For those not familiar with the spectroscopic origin and now quantum mechanical designations, S P D ... are for orbiting electrons with 0,1, 2... units of angular momentum. Thus, in classical terms, ALL "s-electrons" have orbits that pass directly thru the nucleus. I think any shell could have its s-electron be the one which is captured but the intermost, (k shell) dominates as its electron's wave function density at the nucleus is much higher than any of the outter shell's s-electrons. (That density is directly related to probability it is "there" in some classical sense.)
BTW, as I recall, "S" comes from fact that most spectral lines with transitions terminating (or originating?) here are singlets and most originating from the D shell are Doublets (I forget the original German terms) The most famous of the doublets is probably the two yellow lines of Sodium (a few Angstrums separated as I recall). They are called the Sodium D lines.
Wikipedia and other sources suggests less than 1% is the norm for terrestrial chemistry like you might find naturally in rocks, but if you were to create a fully-ionized plasma, one would expect EC to drop to next to nothing.
K-shell refers to the 1s electrons -- the only electrons with principle quantum number 1.
The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7;
GE "Chart of the Nuclides", 12th Edition (Revised 1977) by William Walker, George Kirouac and Francis Rourke, Knolls Atomic Power Laboratory, Schenectady, NY:
There are a few nuclides for which measurable changes in the half-life, or disintegration constant, have been produced by chemical changes that alter the electron density near the nucleus. These effects are generally small, being a 0.27% greater half-life for Tc2S7compared with KTcO4 and a 0.08% greater half-life for BeF2 compared with Be metal. The 6.02 hour Tc-99m decays be a weak isomeric transition, the decay taking place primarily by emission of M- and N- conversion electrons; the 53.28-day Be-7 decays by K-electron capture. Other nuclides for which measurable variability in half-life have been found are 18.8-second Nb-90m1 and 26.1-minute U-235m. The effect found for Nb90m1 was a 3.6% greater decay rae for Nb metal compared with the niobium pentafluoride complex, the largest effect observed to date. ...
A useful reference on this topic is 'Survey on the Rate Perturbation of Nuclear Decay' by H.P. Hahn et al., which appeared in Radiochimica Acta Vol. 23, pages 23-37 (1970)."
This was reprinted in its entirety 12 years later in the 14th edition. I'm not certain if there is more recent information.
Nb-90m1 in some other references is Nb-90m or Nb-90m2. because a VERY short lifetime metastable state with halflife near 60 µs was found at a slightly lower excited state.
But regardless of what you call the metastable state, the decay is one of Isomeric transition (i.e. gamma rays, that don't change Z or A) and not electron capture.
Yes, not only k-electron capture, but some isomeric transitions are also effected by the electron density. There are likely a large number of other EC isotopes not referenced that would be effected by the electron density; as well as other isomeric transition isotopes.
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