There are 118 elements. Each element is unique by virtue of the different numbers of electrons, protons, and neutrons its atoms contain.
Isotopes are atoms of a given element that vary only in the number of neutrons. This difference sets apart one isotope from another. Many isotopes are also unstable, especially those whose atoms have too few neutrons for the number of protons.
Unstable isotopes are short-lived, and often decay by releasing some energy to achieve a more stable configuration. Isotopes in nuclear physics constitute an ongoing area of research, with many isotopes not known to physics.
A study recently published in Physical Review Letters set out to find if one particularly unusual isotope, nitrogen-9, actually exists.
Why is nitrogen-9 special?
Atoms of the nitrogen-9 isotope are characterised by seven protons and two neutrons – which is an unusually high proton-to-neutron ratio. This disparity has a critical effect on the isotope’s stability, influencing its decay processes as well as overall behaviour.
For one, the high proton content places nitrogen-9 atoms beyond the conventional stability thresholds. That is, if all the atoms lived in a town where their location depended on how stable they were, nitrogen-9 would live outside the town, in a place where no other atom lives.
The question is whether it really exists in this state and, if so, how.
What are drip lines?
Physicists make sense of such stability limits using drip lines. The proton and neutron drip lines represent a boundary: add more particles beyond this boundary and the nucleus becomes unstable. They’re still working out the exact values at which the instabilities arise, however.
This said, they have also extensively examined the neutron drip line for the first 10 elements. For example, that oxygen’s heaviest particle-bound isotope is oxygen-24, with 16 bound neutrons. Beyond that 16, the nucleus becomes far too unstable.
But we haven’t yet been able to make full sense of the drip line for protons, especially for elements heavier than germanium on the periodic table. Germanium-58 itself hosts 32 bound protons.
Nuclei with a lopsided ratio of protons to neutrons – especially those located beyond the proton and neutron drip lines – throw up a significant challenge for physicists who are trying to classify them. So understanding them has become fundamental to nuclear physics.
What are nuclides?
The periodic table is a cornerstone in chemistry, showing all the known elements. However, it does not cater to isotopes or their characteristics.
So scientists have developed an analogous table for atomic nuclei characterised by a unique combination of protons and neutrons, a.k.a. nuclides.
A table or chart of nuclides is a two-dimensional graph of isotopes of the elements. One axis shows the number of neutrons (N) and the other shows the number of protons (Z) in each atomic nucleus. Each point on the graph denotes a nuclide.
This way, scientists can understand the relationships between isotopes quickly. They also spot isotones (nuclides with the same number of neutrons), isobars (nuclides with the same total number of protons and neutrons but different individual numbers), and isodiaphers (nuclides with the same differences between their numbers of neutrons and protons).
What did the new study do?
For the new study, a team of researchers from China and the U.S. used a sophisticated experimental setup to check for the existence of the exotic nitrogen-9 isotope.
They aimed an energised beam of oxygen-13 atoms at a target composed of beryllium-9 atoms. As the oxygen smashed into the beryllium, charged particles were produced. The team studied these particles using a high-resolution array comprising 14 detectors arranged to cover specific scattering angles.
These devices played a pivotal role in capturing and analysing particles resulting from the nuclear reactions.
The collision between oxygen-13 and beryllium-9 produced nitrogen-9 atoms of different energy configurations, or resonant states. This was the result of fragmentation reactions: the original nucleus undergoes fission, yielding smaller fragments. In this case, the oxygen projectiles lost three neutrons and one proton apiece.
In the course of these nuclear reactions, the detectors recorded the release of particles, including an alpha (α) particle – a clump of two protons and two neutrons – and five protons.
To reveal the characteristics of the nitrogen-9 resonant states, the researchers used the invariant-mass technique – i.e. they examined the energy distribution of events that released five protons and an alpha particle.
Does the nitrogen-9 nuclide exist?
Their analysis revealed two distinct peaks among the nitrogen-9 resonant states that weren’t due to statistical fluctuations (or noise) in the data.
These two resonant states refer to specific energy values at which a nucleus is particularly stable. In these states, specific nuclear reactions also become more likely than at other energies.
The observation of the two distinct peaks indicated to the team that the five protons were being emitted from both the ground state – the lowest energy state – and low-lying resonances, i.e. excited states relatively close in energy to the ground state.
The ½+ and ½- resonant states also played a crucial role. ½+ and ½- are designations that signify the angular momentum (or spin) and symmetry (or parity) of the states. Spin and parity are fundamental quantum properties of particles. (Parity refers to whether a particle is left- or right-handed.)
The researchers corroborated their findings using a theoretical framework called the Gamow shell model. This model is used to study systems that are maintained by weak bonds – such as an atomic nucleus whose proton and neutron numbers are close to the drip lines.
The features that the Gamow shell model predicted for these resonant states aligned well with the actual structural features of a nitrogen-9 nucleus that the team had deduced from the invariant-mass spectrum.
As a result, the team reported in its paper that it had found “strong evidence for the exotic nuclide nitrogen-9 produced in the fragmentation of an oxygen-13 beam”.
What are nuclei’s limits of existence?
This result of the new study has profound implications – including offering a new perspective on subatomic structures that physicists interpret as being nuclear states. On the nuclide chart, for example, nuclides have been found to exist only at certain locations, i.e. points denoted by specific numbers of protons and neutrons. The nitrogen-9 nuclide now adds a new point to this chart.
What was previously a ‘dead zone’ is now alive with possibility.
The identification of such fleeting creatures outside convention challenges previous interpretations, and paves the way for potentially more isotopes at the limits of existence defined by the drip lines.
Further, the theoretical insights provided by the Gamow shell model can help us comprehend the nature of resonant states like those of nitrogen-9. This enhances our understanding of the specific isotope under investigation as well as establishes a solid foundation for future studies in this field.
Certain exotic isotopes are also involved in astrophysical processes – like those by which stars synthesise various elements. So understanding their properties will also help scientists model these processes, and the origins of elements, better.
Tejasri Gururaj is a freelance science writer and journalist.
