It may be possible to cram more neutrons into atomic nuclei than previously thought

The element that combines with chlorine to make salt has a whacking 28 neutrons and 11 protons in its atomic nucleus. The element is so extreme that few theoretical models predicted its existence. A nuclear physicist at the University of Tennessee, who was not involved in the work, said it was a surprise that the neutrons kept on hanging on.

The researchers at the Nishina Center created a small number of sodium-39 nuclei. Physicists have a hard time understanding nuclear structure. It suggests that tracing the process by which exploding stars create elements may be more difficult than thought.

Three years ago, an experiment with the RIKEN center produced a glimpse of a single nucleus. The experiment was repeated with a longer beam time.

The team shot a beam of nuclei through a target and then used a chain of magnets to shred them. Only a nucleus with a similar mass-to-charge ratio could slalom through, as a result of researchers tuning that chicane. A nucleus that was deposited in a detector revealed its charge. The charge and mass could be used to tally a nucleus. They spied nine sodium-39 nuclei when they fired 500 quadrillion calcium 48 nuclei through the target.

Predicting the nature of the nucleus can be difficult. A quantum mechanical effect favors nuclei with the same number of protons and neutrons. The balance is tilted towards fewer protons because the protons repel each other. Different theoretical approaches tend to work better in different mass ranges when compared to one another.

Brad Sherrill, a nuclear physicist at Michigan State University, is an author on the paper. Two years ago, Witold Nazarewicz, a nuclear theorist at Michigan State, and colleagues tried to predict all possible nuclei using average model predictions. There was a 50% chance of that happening. There is a question about theRIKEN result. The man says that. I don't think so. Does it matter? I agree.

He says it adds to the nuclear landscape. Physicists plot known and predicted nuclei on a chart with the number of protons climbing vertically and the number of neutrons increasing left to right The lower edge of the chart is called the neutron drips line. It marks the point at which it is impossible to cram more neutrons into a nucleus. Only element 10 is known about it.

It has served up surprises before. It goes from 16 neutrons for oxygen to 22 neutrons for fluorine. The jump had to be explained by forces not just among pairs of protons and neutrons, but also among trios. The drip line appears to leap by four neutrons from neon-34 to sodium-39.

Physicists have a goal. Half of the elements heavier than iron emerge from a supernova explosion, as the nucleus quickly absorbs neutrons from the explosion and undergoes radioactive decay. The Facility for Rare Isotope Beams at Michigan State will be able to identify the nucleus in the process. The nuclei may be harder to make if the line is further away.

In May, the first results from FRIB came in. The team reported in a paper in press at Physical Review Letters that they shredded a beam of calcium-48 to create the elements magnesium, aluminum, Silicon, and ­phosphorus. According to Heather Crawford, a nuclear physicist at Lawrence Berkeley National Laboratory, the half-life of magnesium-38 was short.

The beam was the same intensity as the one in the RIKEN study. Crawford says that in a few years, it will be possible to see the neutron drip line farther up the chart. One of the first things I expect to be done is pursue that.