On Sun, Feb 1, 2015 at 12:49 PM, <mix...@bigpond.com> wrote: It must be one of the thousands that I deleted unread, however I wouldn't > expect > that sort of thing to affect gamma radiation.
Maybe. But consider for a moment the decay of a [dd]* compound nucleus, which normally follows one of the two strong-interaction branches, where it breaks up, and very occasionally follows the EM branch, in which a gamma is emitted after a long period of time. Typically, I believe, such decays are measured in ion bombardment experiments or in dusty plasmas and the branching ratios are inferred from results obtained in such contexts. In the ion bombardment experiments, I assume the incoming d+ ion encounters the d atom embedded within the metal, but in a region of little charge density, and you get the usual branching ratios. (Or perhaps experimentalists work backwards from their results, assuming the normal branching ratios.) Suppose for a moment that the electron charge density had an effect on the branching ratios. If the charge density is high, the supposition is that the EM transition is heavily favored for [dd]* decay, but the momentum is shared with one or more electrons, so that you do not get a gamma, but instead one or more energetic electrons. A problem with this thought experiment is that it does not explain why gammas are seen in the decays of radioisotopes with gamma branches; presumably if electron charge density had an effect, you would not see sharp gammas peaks for such radioisotopes but instead energetic electrons and associated continuum radiation. Here a counterargument to the electron charge density hypothesis is that if charge density was a factor, you might expect to see a volume/surface effect. The more surface area, presumably the lower the charge density at the surface, and hence more gamma activity from the radioisotope. The argument is that this kind of volume versus surface effect is not observed, so the hypothesis needs to be revisited. The thought that I had to add to this discussion is that there need not be a surface-volume effect for the charge density hypothesis to remain a possibility. Even if the gamma emitting radioisotope is embedded deep within a solid, I assume the net charge around the nucleons will be positive. By contrast, if a [dd]* compound nucleus were decaying within the dense electron cloud of a metal, it might be straightforward for the surrounding electrons to overwhelm the 2+ charge from the two protons, leading to a net negative charge density, even within the field of the [dd]* nucleus. Eric
