Heavier nuclei are less stable—that's something we all learned in school. Adding more nucleons (protons and neutrons) makes atoms more likely to break apart. It's one reason why elements heavier than plutonium haven't been found in nature. However, new nuclear physics calculations predict an "island of stability," where heavier nuclei are long-lived, but at much higher numbers of nucleons than exist naturally. The details of this island depend on the shell structure of the nucleus, something that is difficult to calculate directly.

Through experiments on nobelium (No) and lawrencium (Lr) nuclei described in a new Science paper, researchers have measured more details of high-mass nuclear structure, thus helping to pin down this shell structure. E. Minaya Ramirez and colleagues performed the first precision mass measurements, which allowed them to find the energy binding the nucleus together. While neither nobelium nor lawrencium are completely stable, they are relatively long-lived for superheavy elements (yes, they belong to the island of stability). Knowing how they are put together reveals much about the very heavy elements, showing that the center of the island of stability exists for nuclei with 184 neutrons.

The number of protons (labeled Z) determines the chemical element, and the number of neutrons (written as N) determines the isotope. Thus, nobelium has Z=102 and lawrencium has Z=103, but each may have a wide range of N values. And both Z and N contribute to the stability of a nucleus. The shell model depicts this as a series of concentric layers or shells (in a simplified version at least). If a shell is incomplete, the nucleus is more likely to give up or accept a nucleon—or may be unstable, spontaneously decaying into another type of nucleus. Certain magic numbers of nucleons are particularly stable, such as the isotope of lead containing 126 neutrons (208Pb); these correspond to closed shells in the nucleus.

Isotopes are labeled with the symbol for the element from the periodic table (which reveals the number of protons) and the total number of nucleons A = Z + N as a prefix. So, the most common isotope of carbon on Earth is carbon-12, consisting of 6 protons and 6 neutrons, and is labeled 12C. The radioactive isotope used in carbon dating has two more neutrons, so it is labeled 14C.

Beyond lead (elements with Z greater than 82), however, no known element is completely stable, nor is any expected to be from theoretical models. One way to think of this is like an inverted pyramid built of child's blocks; each complete layer (which keeps the pyramid balanced) represents a closed shell, but the higher the structure stands, the more likely it is to collapse, even if every layer is complete.

In this sense, stability in the superheavy elements (SHE, Z > 100) is relative: if an element is long-lived compared to isotopes of similar composition, it's stable enough. For example, one isotope of nobelium (with N=157) has a half-life of 58 minutes, while other isotopes may not last even 5 seconds. The island of stability is a region of the periodic table where elements' half-lives increase after a large group of elements with very short decay times.

However, due to the difficulty in producing SHEs and their short lives (even in the island of stability), it's hard to measure their masses. The mass of a nucleus is not simply the sum of the masses of the nucleons; instead, some of the mass is converted to binding energy (via Einstein's formula E = mc2). The amount of binding energy is related to the shell structure, so even if the shell is open—it happens to be closed with lawrencium or nobelium—it reveals what the magic number should be to close the shell.

In the current experiment, the researchers confined the nuclei temporarily in a Penning trap, which stores moving charged particles using magnetic and electric fields. The motion within the trap reveals the mass (in a similar way to mass spectroscopy used in ordinary chemical applications). The authors measured the masses of 255No, 256Lr (both with N = 153) and 255Lr (N = 152) for the first time, revealing the details of the shell structure for those and similar isotopes. Particularly, they found a gap in the shell structure at N = 152—meaning there is a jump in the amount of binding energy if another neutron is added.

With precision mass measurements and knowledge of the shell structure, the researchers have made significant progress toward mapping the SHE region of the periodic table, including the island of stability. Their method can be extended to other similarly heavy isotopes, even ones that are produced in tiny quantities (like lawrencium). In turn, these results can be used to improve theoretical models, and lend strong support to the nuclear shell model.

Science, 2012. DOI: 10.1126/science.1225636 (About DOIs).