The nucleus of a xenon-133 atom lives only a few days before it decays. Making a thin film target out of it and bombarding it with particles is out of the question: the film would vanish faster than it could be produced. So physicists did the opposite: they accelerated xenon-132 nuclei and aimed them at a target made of deuterium (heavy hydrogen). During the collision, a xenon nucleus would capture a proton and momentarily become an excited xenon-133 nucleus, which immediately emitted gamma rays.

Detectors surrounding the collision point caught those gamma rays and the outgoing protons. From their energies and emission angles, the researchers reconstructed the ladder of energy levels inside the nucleus, like taking fingerprints of an invisible object. For the first time for this xenon isotope, they measured the level density below 6 MeV (mega-electronvolts)—an energy millions of times greater than that of visible light, but for a nucleus it corresponds to the "lower floors" of its internal staircase.

The resulting level density and gamma-transition strength allowed the team to calculate the probability that a xenon-132 nucleus captures a neutron and becomes xenon-133. Such reactions occur in reactors and explosions, and precise numbers are needed for nuclear materials control and for understanding how much xenon will accumulate in fuel. Moreover, the data constrain the rate of a rare process: the nucleus can become excited by capturing an electron from its own inner shell — an effect that is hard to catch, but now its contribution can be estimated.

Thus the inverse method, where the target flies and the projectile stays put, opens access to nuclei that cannot be touched. Next could be other short-lived isotopes of noble gases, and each will tell its own story about the forces that hold protons and neutrons together.