Size isn't everything--at least in nuclear physics. But if atoms had egos, a few lithium atoms would be embarrassed right now, as scientists have shrunk their nuclei by about a fifth. The experimental results, published in the 5 March Physical Review Letters, may help produce a theory that can explain nuclear interactions of all varieties.
"Shrinkage of about 20% is very surprising," says Hirokazu Tamura, a physicist at Tohoku University in Sendai, Japan. That's because compressing a nucleus is very difficult. So instead of trying to squeeze it, Tamura and colleagues from Japan, China, Korea, and the United States set out to shrink a lithium-7 nucleus from within.
The physicists substituted a so-called strange quark for a down quark, turning one of the atom's neutrons into a particle called lambda, or L, that resembles the neutron, but is somewhat heavier. The quark substitution turned lithium-7 into lithium-6-L, a so-called "hypernucleus" with subtly different properties.
The difference stems from the Pauli exclusion principle, the quantum-mechanical rule that forbids certain particles from having the same quantum state. Given the chance, a neutron in a nucleus will occupy the lowest possible energy level, or ground state. Two neutrons can inhabit that level, but only if they have different quantum states. For that to be true, one neutron must have spin +1/2, and the other must have spin -1/2. A third neutron, however, must take a higher-energy position farther away from the center of the atom. The same exclusion rules apply, independently, to protons.
Lithium-6 has three protons and three neutrons; one proton and one neutron are in the higher-energy state, loosely bound to the core. Enter the L. Since a L particle is distinct from both protons and neutrons, it is exempt from the Pauli exclusion principle that governs those particles. As a result, it sinks directly into its ground state, joining the low-energy protons and neutrons at the center of the nucleus. The extra L binds the particles more tightly together but, unlike an added proton or neutron, takes up no additional space. The stabilized nucleus shrinks.
Tamura's team observed the shrinkage by precisely measuring gamma rays that emanate from lithium-6-L hypernuclei. The gamma rays reflect the shifting of particles' spins within hypernuclei--information that can help scientists determine not only a hypernucleus's size, but also how its components interact with one another.
"Nobody's been able to measure this with such high precision," says John Millener, a physicist at Brookhaven National Laboratory in Upton, New York. He hopes that understanding the interactions will shed light on so-far obscure aspects of nuclear physics. "We don't really have a theory for these interactions."
Hirokazu Tamura's home page