The discovery of a new isotope was announced last week in Physical Review Letters (paper here, but it seems no open-access version exists, even on the arXiv). Oxygen–11, aka 11O, has 8 protons (because it's oxygen) and 3 neutrons (to give it overall mass number 11). That's a pretty extreme form of Oxygen, whose lightest stable isotope has 8 protons and 8 neutrons.
To make it, the experimenters from the NSCL (National Superconducting Cyclotron Laboratory at Michigan State University) started from a beam of stable oxygen–16 nuclei which they collided on a beryllium–9 (4 protons, 5 neutrons) target. This bombarding produced a range of nuclei lighter than the oxygen–16 beam from which a magnetic separator was used to focus the oxygen–13 component of the debris into a secondary beam. This was then sent to another beryllium–9 target. Some of the reactions between the oxygen–13 and beryllium–9 nuclei caused two neutrons to be knocked out of the oxygen–13 to give oxygen–11. Oxygen–11 quickly decays to carbon–9 (6 protons, 3 neutrons) and two protons. These decay products were detected; their coincident detection and the ability to reconstruct from the detection the properties of the parent oxygen–11 nuclei, repeated through thousands of events enabled the research team to confirm that indeed oxygen–11 had been produced.
Oxygen–11 is so unstable that it decays not by one of the three traditional radioactive decay mechanisms (alpha, beta, or gamma) but by losing two of its protons, and hence undergoing "two proton radioactivity". This puts it on the borderline of actually existing as a nucleus at all. The nucleus is so short-lived and unstable that it does not have a well-defined mass (or equivalently its ground state is not a stationary state of the nuclear Hamiltonian). The experimental values of the observed mass of 11O is seen to have a spread of values ranging over about 3 MeV/c2.
The picture above is taken from Ed Simpson's excellent Colourful Nuclear Chart. It currently has a blank space where oxygen–11 will no doubt soon go, just above nitrogen–10, and to the left of oxygen–12. The black tramlines in the plot show the so–called magic numbers which are numbers of protons and/or neutrons which confer extra stability to the nucleus. These cross each other at oxygen–10. I doubt if oxygen–10 will really turn out to be noticeably stable compared to its neighbours. There is already enough evidence that the magic numbers don't apply in light nuclei so far from stability. I wonder if the odd stray oxygen–10 nucleus was made in the experiment at NSCL, in too little a quantity to get a good measurement of.
The picture above is taken from Ed Simpson's excellent Colourful Nuclear Chart. It currently has a blank space where oxygen–11 will no doubt soon go, just above nitrogen–10, and to the left of oxygen–12. The black tramlines in the plot show the so–called magic numbers which are numbers of protons and/or neutrons which confer extra stability to the nucleus. These cross each other at oxygen–10. I doubt if oxygen–10 will really turn out to be noticeably stable compared to its neighbours. There is already enough evidence that the magic numbers don't apply in light nuclei so far from stability. I wonder if the odd stray oxygen–10 nucleus was made in the experiment at NSCL, in too little a quantity to get a good measurement of.
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