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.
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.
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.
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.”