Double Trouble: First Sighting of 2p Decay Between Isobaric Analog States

A few months ago, Bob Charity, Lee Sobotka and a team of researchers from Washington University working with the HiRA group at NSCL saw something at NSCL that nobody had ever seen before. The nuclear physicists from Washington University in St. Louis detected a new twist on a very rare form of nuclear decay – the emission of two protons in a single step between excited states of a very peculiar sort: isobaric-analog states or IAS.

Don’t worry; it isn’t as complicated as it sounds.


Bob Charity (professor at WU) and Bec Shane (graduate student at WU) standing with the device that made this work possible - the High Resolution Si Array, or HiRA for short.

One of the many fundamental processes that researchers attempt to understand by conducting experiments at NSCL is nuclear decay. When a nucleus is unstable, it will decay by shedding energy in the form of expelling particles like protons and neutrons or bits of energy like gamma rays or photons. Some isotopes like carbon-14 can take thousands of years to decay while others last tiny fractions of a second.

Because these latter types exist for but a fleeting instant, they can only be found in nature in the places where they are made like stars, supernova, neutron stars and other astronomical phenomena. This is how the heavy elements are formed. Exploding stars pump so much energy into the particles that they form exotic nuclei that quickly decay into the stable elements we see every day.

However, there are other places besides the cosmos that create these kinds of isotopes – nuclear accelerators like the fast fragmentation beams at NSCL.

IAS graphic

Quantum numbers are used to describe the energy and angular momentum of every particle in an atom. An isobaric-analog state is an excited state of a nucleus that has the exact same structure - or set of quantum numbers - as the ground state of another except, for example, one proton has become a neutron. In the figure above, the ground state of carbon-8 and the isobaric analog state in boron-8 have the exact same arrows in the exact same places pointing the exact same directions, but one red arrow (proton) in carbon-8 (top left) has become a blue arrow (neutron) in boron-8 IAS (top right). The "IAS" notation is necessary because this structure is an excited state in boron-8. In the ground state (bottom right) - or the state of lowest energy - one of the protons has an opposite spin, denoted by the red arrow pointing in the opposite direction.

When isotopes traveling half of the speed of light crash into other particles, they split apart, often into these very short-lived rare isotopes. Once made, researchers have little more than the blink of an eye to determine an isotope’s properties and how it decays into a more stable form. Understanding this not only gives scientists a better idea of how the universe works, it also informs them about where to look for new, never-before-seen isotopes.

Some of the quantum states of these unseen isotopes are called “isobaric –analogs” of other known isotopes. This means that the quantum state in one nuclide has exactly the same structure as another, but with a different mix of neutrons and protons (see sidebar).

Sobotka and his group first studied how carbon-8 decays. Carbon-8 is a very strange nucleus because it has so many more protons than neutrons; three times as many, in fact. But it turns out that how it decays is even stranger.

The proton-rich nucleus gets rid of its protons by one of the strangest decay modes ever seen - four proton emission via two separate steps of emitting two protons. Usually nuclei decay by emitting a single particle at a time but this is apparently not so for carbon-8.

Though interesting in itself, the researchers found something even more peculiar when they studied the decay of the isobaric-analog of the ground state of carbon-8, an excited state of boron-8.

There are two conceivable ways in which the excited state in boron-8 could decay by emitting one proton, making a brief pit stop at beryllium-7. However, one of these ways is energy forbidden and the other does not conserve isospin. Read the caption of the chart below for more details.

decay chart

The figure above illustrates the two decay processes described in the research. The blue graph describes two subsequent two-proton decays of carbon-8 into beryllium-6 and then to helium-4. The bottom red graph illustrates the newly discovered decay mode where an isotope in an isobaric-analog state (boron-8) decays directly into another isotope also in an isobaric-analog state (this one in lithium-6) directly through two-proton decay. Notice the red cross-hatched box along the arrow’s route. It indicates the energy region where an intermediate step is permissible by isospin conservation. However, it is too high and narrow (in energy) for the decay to take place. It is forbidden by energy conservation. The black cross-hatched potential intermediate steps would require an isospin breaking transition.

While conserving isospin is not a hard and fast rule, if there is any other way for the nucleus to decay, it will jump at that alternative. In this case the alternative, one that is both energy and isospin allowed, is to decay by emitting two protons in one step to an excited state in lithium-6, which is itself an isobaric-analog of the ground state of helium-6. This is the first time that decay by emitting two protons at the same time has been observed between isobaric analog states.

What’s more, the Washington University team believes that this twist on two-proton decay from excited IAS to excited IAS likely can be found in two more massive cases (see chart and caption below). One of these is in nitrogen-12 - analog of oxygen-12 - and the other is in fluorine-16 - the analog of neon-16 . However, there is one difference between these two cases and the one described above; the initial isobaric analog states have never been seen.

But thanks to the measurements made for the boron-8 (ISA) decay, Sobotka feels confident that his team can work backwards to figure out the excitation energy of these levels from the energies of the decay products. In fact, this is exactly what the Washington University team has proposed to do at NSCL in future experiments.

“We know the approximate energy where these states should be and we know how to find them – from the energy of the two protons and the residue they leave behind,” says Sobotka. “The only complication is that we need to detect a gamma ray as well as the three particles. This is because this residue is produced in an excited state that decays by emitting a gamma ray. However, by combining forces with the appropriate groups at NSCL, all the necessary technology exists to get the job done.”

If these experiments succeed, the Washington University team will have found several cases of a new subclass of nuclear decays, two-proton decays between IAS, a subclass not known to exist before their last experiment. Furthermore, unlike most other cases of two-proton emission, the IAS variety can be studied with large data sets of many thousands of events each.

“We’re adding to the portfolio of two-proton decay cases a new subclass, a subclass that even though particle-gamma coincidence experiments are required, can be studied with excellent statistics at NSCL.”

IAS decay charts

These energy level diagrams illustrate the two decay processes that Charity and Sobotka want to study next at NSCL. Notice that the red intermediate single-proton decay states on the left are sufficiently low in energy that an intermediate single-proton step might be possible. However, for the case of the IAS in fluorine-16 on the right, the potential intermediates are too high in energy to be allowed. Thus, the isospin allowed intermediate steps are energy forbidden. Without any viable energy and isospin allowed intermediates, this case is almost a sure bet to be another of the new class: an excited isobaric-analog state decaying to another excited isobaric-analog state via two-proton decay.