Researchers in the US and UK have confirmed that a short-lived isotope of tin is the latest member in an exclusive club of “doubly magic” nuclei, a nuclear equivalent to the noble gases. This is only the seventh of these rigidly spherical nuclei to have its magical qualities measured. And the experiment could provide clues to how heavy elements are created in the supernova explosions of massive stars.
Physicists have long known that protons and neutrons in nuclei occupy discrete orbital shells – in much the same way as electrons do in atoms. Indeed, when this idea was developed into the “nuclear shell model” it won Maria Goeppert-Mayer and J Hans D Jensen a share of the 1963 Nobel Prize for Physics.
Magic nuclei are those having the precise number of protons or neutrons required to fill each orbital shell to full capacity. Nuclei with magic neutron or proton numbers tend to be characterized by a stronger binding, greater stability, and are, therefore, more abundant in nature. In doubly magic nuclei, both proton and neutron shells are filled, which can make the binding even stronger.
Double the magic
To date, researchers have discovered just six doubly magic nuclei by confirming the spherical nature of their outer shells. Five of these – including helium-4 and oxygen-16 – are among the most common isotopes found in nature on account of their stability. But in 1998 researchers discovered the latest doubly magic nucleus, nickel-56, which is much less stable. It has a half life of 5.9 days on account of it having two neutrons fewer than nickel-58, which is far more abundant in nature.
People thought this wouldn’t be possible until the next generation of facilities come along Kate Jones, the University of Tennessee
Now, a group of researchers at the Oak Ridge National Laboratory in Tennessee have confirmed the existence of a seventh doubly magic nucleus, which is far more unstable still. Tin-132, with its 50 protons and 82 neutrons, was predicted to be doubly magic by the nuclear shell model but it has been very difficult to prove this because it decays with a half life of just 40 seconds.
To get around this, Kate Jones of the University of Tennessee has led a novel experiment, which turned standard procedures on their head. One favoured approach to identifying doubly magic nuclei is to strip neutrons from the target isotope and infer the structure of the doubly magic nucleus from properties of these dislodged neutrons. This can be done by creating a film of the target material and hitting it with a beam of deuterium, which is an isotope of hydrogen comprising one proton and one neutron.
Aggressor becomes the target
However, given that tin-132 is too short-lived to produce a substantial film, Jones’ team decided to switch the target with the firing beam. Using Oak Ridge’s Holifield Radiation Isotope Beam Facility (HRIBF), they accelerate a beam of tin-132 to a velocity of about 10% the speed of light and direct this at a target of polyethalene that has been deuterated: its hydrogen-1 atoms replaced with hydrogen-2. As the tin-132 isotopes collide with the target some of them could split the deuterium molecules by stripping a neutrons to form the heavier nuclei of tin-133, while the proton fell back towards the target.
The clever part is that the researchers can study the motion of these recoiling protons to reveal information about the structure of the tin nuclei. By analysing properties such as energy and angular distribution of these particles, the researchers could confirm that the stripped neutron rests in a separate orbital above the closed inner structure of the tin-132. The fact that the neutron falls into this “single particle state” is confirmation that tin-132 must have the robustly spherical nature reminiscent of a doubly magic nucleus.
Jones says that it is the “purity” of the single particle states that makes these results so convincing. “People thought this wouldn’t be possible until the next generation of facilities come along, but we are really excited to have managed it using our cyclotron accelerator whose design dates back to the 1950s.”
Short-lived nuclear landscape
Rituparna Kanungo, an experimental nuclear physicist at St Mary’s University in Halifax, Canada, is impressed by the inverse kinematics technique employed by the Holifield team. “This is a powerful but quite challenging experimental tool to study single-particle states,” she says. “These observations are important for developing the nuclear shell model in regions of the nuclear landscape where isotopes start to become short-lived.”
One fascinating area of physics that may benefit from this field of research is the study of how heavier elements are formed in highly explosive conditions. More than half of the elements heavier than iron are believed to originate from something known as the “R process”, which involves a succession of rapid neutron captures on seed nuclei, typically nickel-56. “The main contender for the site of production is supernova or neutron star mergers, but given the explosive nature of these sites it is very difficult to reproduce this process in the laboratory,” says Jones.
This research is published in Nature.
