Hydrogen-Seven

An experiment at the GANIL facility in France is the first to make, observe, identify, and characterize the heaviest isotope yet of hydrogen, H-7, consisting of a lone proton and 6 neutrons. (An earlier experiment saw some inconclusive evidence for this state-see Korsheninnikov et al., Physical Review Letters, 8 Feb 2003.)

All of the lighter isotopes of hydrogen have previously been seen: H-1 (ordinary hydrogen), H-2 (deuterium), H-3 (tritium), and H-4 up to H-6. Technically speaking, the H-7 state (like H-4, H-5, and H-6) is not a fully bound nucleus. It is considered a resonance since (besides being very short lived) energy is required to force the extra neutron to adhere to the other nucleons.

In a proper nucleus energy is required to remove a neutron. In the GANIL experiment, a beam of helium-8 ions (themselves quite rare) is smashed into a carbon-12 nucleus residing in a gas of butane (see figure at http://www.aip.org/png/2007/283.htm). In a few rare occurrences, the He-8 gives one of its protons to the C-12, producing H-7 and N-13, respectively. The H-7 flies apart almost immediately into H-3 and 4 separate neutrons.

Meanwhile the N-13 is observed in the active-target MAYA detector (named after a cartoon character, Maya the Bee, whose honeycomb hive resembles the hexagonal cathode pads in the experiment), a device much like a bubble chamber, allowing its energy and trajectory to be deduced.

By taking the conservation of momentum and energy into account, the fleeting existence of the H-7 is extracted from the N-13 data (see the figure at www.aip.org/png). A total of 7 H-7 events was observed. A rough lifetime for H-7 of less than 10^-21 seconds can be inferred. The helium-8 nucleus (2 protons plus 6 neutrons) used to make the H-7 is interesting all by itself since it is believed to consist of a nuclear core with two “halo” neutrons orbiting outside.

This radioactive species must carefully be gathered up from carbon-carbon collisions (in a separate step) and then accelerated to participating in the H-7 experiment. One of the GANIL researchers, Manuel Caamaño Fresco (caamano@ganil.fr, 33-231-45-4435), says that one of the chief reasons for looking at H-7 is to get a better handle on exotic nuclear matter.

The H-7 nucleus, during its brief existence, might consist of a H-3 core and plus two 2-neutron outriders, or maybe even a single 4-neutron blob outside. Larger still hydrogen isotopes, such as H-8 or H-9, might be observable. (Caamaño et al., Physical Review Letters, upcoming article; PhD thesis at http://www.usc.es/genp/maya/)