The oxygen isotope is found to be unbound

Exploring the magic number of protons and neutrons in an experiment using radioactive isotopes to create a rare form of oxygen

Current theories state that atomic nuclei with certain numbers of protons and neutrons are inherently stable. This is because there are particles in the nucleus. It is hard to take away particles when a shell is filled with just the right number of protons. These are ‘magic’ numbers, and have been thought to include 2, 8, 20, 28, 50, 82 and 126 particles. In order for a nucleus to be stable it must have a magic number of both neutrons and protons.

The strongest force in the Universe is the nucleus of an atom. To unlock how elements are forged, the physics of neutron stars and more, scientists need to better understand this strong nuclear force, says Takashi Nakamura, a physicist at the Tokyo Institute of Technology. He and other researchers are testing theories about how atomic nuclei are held together by pushing them to extremes. Load lightweight nuclei with excess neutrons and see what happens.

The most abundant form of oxygen, O16, is doubly magic, because of its eight protons and eight neutrons. Oxygen-28, with 8 protons and 20 neutrons, has long been predicted to be doubly magic, too. Physicists have been able to detect it before.

Observing O28 required several experimental feats. Key to the whole operation were the intense streams of radioactive isotopes produced by the Riken RI Beam Factory in Wako, Japan. The scientists used a beam of calcium6-48 isotopes at a target to create a florine 29-isotope. The nucleus of this isotope has one more proton than does O28 The same number of neutrons is used. The scientists destroyed F29 by smashing it into a thick barrier of liquid hydrogen, which caused a hole in the nucleus and created O28.

This rare form of oxygen was too short-lived to be observed directly. Instead, the team detected its decay products: oxygen-24 plus four neutrons, a measurement that seemed impossible only a few years ago.

Scientists have found four neutrons at the same time for the first time. “They are like ghosts,” he says of neutrons. With no electrical charge, neutrons can’t be wrangled in the same way that protons can (O24, with its eight positively charged protons, could be ushered into a detector with magnets). The team used the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany as a home for a powerful detector and Riken instruments to observe individual neutrons. In this specialized detector, incoming neutrons are revealed when they knock protons around. The lead author of the study, a physicist from the Tokyo Institute of Technology, uses simulations to help verify tricky measurements.

Robert Janssens, physicist at the University of North Carolina at Chapel Hill, says that they have done their homework. They did all the checks you could do. It is a tour de force.

The life of the O28 nucleus and the limits of how many proton and neutrons can be combined in a nuclear system?

Although the team was unable to get an exact measurement of the lifetime of O28, it is thought that it fell apart as soon as it came into existence.

“It’s so simple and so complicated,” Janssens says. He adds, “We don’t know how many protons and neutrons you can put together in a nucleus.” What is the limit?