When atomic nuclei get too fat
Old ladies are said to like to overfeed their lapdogs. Similarly, physicists fatten the atoms in their particle accelerators. With the difference that the scientists then shoot their fat nuclei against a wall with force.
In a he althy state, lithium with its three protons and three or four neutrons in its core is unobtrusively slim. The element only gets talked about occasionally when the discussion about a future nuclear fusion reactor gets down to the atomic level. In it, lithium-6, which has three neutrons in this isotope variant, is to decay under neutron bombardment into helium and the coveted tritium. In other words, a kind of canned tritium that awaits a great future as a fuel supplier for fusion power plants.
Lithium can also look different. Quasi as a result of the affluent society - because only such a society can afford to produce artificial atomic nuclei - it is increasing in mass and size. And neatly. Obese lithium-11 is so thick that its core is the size of lead cores. It's overloaded with the eight neutrons beyond what's physically acceptable and can't even get all of the particles properly housed inside. Two of the neutrons swirl around the actual nucleus, where they form something akin to an "atmosphere," which is why physicists call it a halo nucleus.
Now Lithium-11 isn't the only known halo core, but it is the largest and most fragile and thus the preferred object of curious scientists who would like to know what keeps the ugly lifebuoy attached to the chubby. Not an easy task, because two neutrons and a nucleus add up to three particles - and theoretical physicists have a hard time with the three-body problem, if only in principle. It doesn't matter whether it's planets, small children or atomic fragments - the behavior of three bodies that influence each other cannot be predicted analytically. Parents and other jugglers know this.
If paper and a multi-core workstation don't work, physicists are forced to resort to silly experiments, and in the case of atoms that means: we slam the thing against a wall and see what happens to the debris. Not a bad thought, but again lithium-11 gets in the way. The bundle of joy is obviously sensitive and sometimes falls apart before the goal. Under such circumstances, measurements that have so far been of little use and reveal something about the dynamics at the subatomic level.
Until a Japanese team led by Takashi Nakamura of the Tokyo Institute of Technology took on the Dickman. The researchers introduced two improvements in their experiments in particular: they set up two instead of one phalanx of neutron detectors so that no neutrons that had blown off escaped them. And they made sure that a single electron couldn't trigger two signals in weird ways.
The reward for her effort was a clear spike on her measurement curve. With an absorbed energy of 0.6 megaelectron volts, the first pieces of lithium-11 were already flying away. No normal atomic nucleus would break under these circumstances, only halo nuclei are so sensitive. This was the first time that this value was determined with such precision that it sufficed for theoretical calculations. With their help, the suspicions of some particle physicists were confirmed: there is a strong interaction between the two halo neutrons. This is one of the fundamental forces of nature that only holds together core particles. And the stronger the further they are apart. Like a rubber band, the force increases with increasing distance - until it eventually breaks.
The mysteries of the overweight halo cores are by no means all explained by these results. But at least a way has been found to practically circumvent the theoretically unsolvable three-body problem for these exotic species. Even if this is of course not a satisfactory answer to the question: Why on earth do you even make such thick cores?