Suppressing a selection of excited states in carbon reveals others

In 2009 a paper was published on results from experiments done at iThemba LABS on the K600 spectrometer. It was shown that a long sought after excited state in the most abundant carbon isotope was obscured by another well-known state that is very strongly excited. This carbon isotope has six protons and six neutrons and can be excited into a state which looks like three bound helium nuclei, each having two protons and two neutrons. This state, and the excited state identified in 2009, plays a vital role in the formation of carbon and all elements heavier than carbon.

An experiment has just been completed on the K600 spectrometer where a heavier carbon isotope with two extra neutrons in the nucleus was used a target. This experiment used 100 MeV protons (hydrogen with no neutrons in the nucleus) as the beam and selected hydrogen with two neutrons in the nucleus as the reaction particle through the spectrometer. An advantage of this reaction is that it suppresses the excitation of some excited states. One of these states that was not excited is what is known as the Giant Monopole Resonance, a mode of excitation where the nucleus alternately increases and shrinks in size. To our surprise it also suppresses the excitation of the very strong state which was obscuring the excited state we found in 2009. Both these pieces of information will be important in puzzling out new information on the structure of carbon.