On Mon, Jan 18, 2016 at 9:02 AM, Bob Higgins <rj.bob.higg...@gmail.com> wrote:
How do we determine that an element's nucleus is an isomer or is in its > ground state? Chemically they would behave the same. We cannot > conveniently distill the atoms and look at the spectra of the total energy > of the nucleus very easily. > In general the spin-parity assignments between a nuclear isomer and the ground state will be different. So 97Tc, in the ground state, has a spin-parity of 9/2+, while 97mTc, an isomer with a half-life of 91.4 days and a decay energy of 96.5 keV, has a spin-parity of 1/2-. There are various ways to discriminate between two nuclides with different spin-parities, using things like angular correlation measurements. So if the spin-parities are different, presumably they would have been found. (This might be a bad assumption.) Perhaps it is possible to have some long-lived isomers with the same spin-parity as the ground state. I do not know how these isomers would be detected apart from any associated gamma cascades that follow upon their decays. They would have different masses, but would the difference be detectable? Let me make a proposition (and please tell me if this is easily > falsifyable): > > - > *Many elements have large fractions of their nuclei in an > isomeric/non-ground state that is highly stable. * > > I'm guessing that for this situation to be hard to detect, we'd need (a) a very long half-life, (b) a decay straight to the ground state, with no competing transitions to a lower level above the ground state, and (c) the same spin-parity as the ground state. I only have a concrete justification for (a) and (c). Could the the primary branch of such an isomeric transition be low energy > gamma that is predominantly absorbed in the apparatus? > Someone else will know this better, but either the transition energy will have to be very small indeed for the gamma not to be penetrating (less than 20 keV?), or there'd need to be unexplored physics to explain why the gamma decay branch is overwhelmed by another branch such as internal conversion, which is perhaps a question unrelated to the isomerism by itself, although it might be related to whatever could trigger the transition. Eric
