Physicists headed up by a team at the University of Tennessee, Knoxville, and the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan say they have measured the alpha decay of tellurium-104 for the first time. The feat could help us better understand how alpha particles form inside atomic nuclei, which is one of the least understood problems in nuclear science.

Alpha radioactivity was discovered over 125 years ago and it is the process whereby an atomic nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons that are strongly bound. The alpha particle exits the nucleus by quantum-mechanical tunnelling through the energy barrier surrounding the nucleus. While this model broadly explains the lifetimes of radioactive nuclei, a big question remains, however, says study lead Robert Grzywacz at the University of Tennessee, Knoxville. This is: how do alpha particles form in the nucleus and where can they exist as “pre-formed” structures inside it before they leave?

Tellurium-104 is particularly suited to studying alpha radioactivity, he says, because it is predicted to have the highest chance of pre-forming alpha particles of all heavy nuclei. “Such a strong enhancement of preformation shouldn’t be possible in theory because the matter in heavy nuclei is uniformly distributed,” Grzywacz explains. “There must, therefore, be an extra mechanism that causes alpha particles to locally ‘clump’ or ‘cluster’.”

In their experiments, he and his colleagues set about measuring the alpha particles produced by tellurium-104. This was no easy task because this isotope of tellurium can only be observed during the decay of xenon-108, which itself is extremely difficult to make in the laboratory.

Pulses of alpha particles

The researchers did their work at Japan’s RIKEN accelerator complex, which consists of four coupled cyclotrons that accelerate a beam of  xenon-124 onto a beryllium production target. The collisions between the two produces xenon-108 and then tellurium-104. The tellurium-104 finally decays into tin-100.

Grzywacz and co-workers say they succeeded measuring pulses of alpha particles produced in short succession by the tellurium-104. They measured the half-life of the radioisotope as being 7.2 ns, which is the shortest known alpha decay half-life for alpha particle emission from a heavy nucleus. More importantly, when corrected for the tunnelling effect – using a parameter known as the reduced width, which separates quantum tunnelling from inherent alpha particle emission from the nucleus – they were able to confirm that the probability of an alpha particle pre-forming in the nucleus was much higher than expected from theory calculations.

The possibility that tellurium-104 could show such “superallowed” alpha decay was first put forward more than 60 years ago, but it has been impossible to observe experimentally, says Grzywacz. “We started our search for this decay more than 20 years ago at Oak Ridge National Laboratory and later at JAEA in Tokai (Japan). We proposed the present experiment in 2018 at RIKEN but because of the covid-19 pandemic, we had to repeat the proposal in 2022. The experiment was deemed to be of ‘very high priority’ and we performed our experiments almost exactly two years ago, in June 2024.”

The work, which is detailed in Nature, will be important for understanding how alpha particle form in nuclei and will push the theory to explain where and how nuclear cluster can form, says Grzywacz. “More than 300 nuclei – and importantly, almost all superheavy nuclei – decay via alpha particle emission. Lighter nuclei may also naturally form ‘alpha condensates’, but in heavier nuclei, it is not obvious how this happens.”

Grzywacz told Physics World that he and his colleagues will now need to measure alpha particle energies with better precision in order to constrain the preformation they have observed.

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Physicists have discovered that the quantum arrangement of protons and neutrons inside atomic nuclei plays a much bigger role in nuclear pairing than previously thought. That is the conclusion of an international team of physicists that has scattered high-energy electrons from calcium and iron nuclei. Their study improves our understanding of the strong nuclear force, which binds atomic nuclei together.

The team focused on the brief partnerships that form when a proton and a neutron come unusually close together inside a nucleus. Known as short-range correlated (SRC) pairs, these fleeting configurations involve only about 20% of all nucleons but account for almost all of the fastest-moving particles in nuclei. Because the two particles approach each other so closely, such pairs offer a rare opportunity to probe nuclear matter under extreme conditions.

The new measurements suggest that these pairs form according to quantum-mechanical rules linked to the shell structure of the nucleus rather than simply depending on how many protons and neutrons the nucleus contains.

Distance matters

Atomic nuclei contain protons and neutrons, collectively known as nucleons. In the standard shell model of the nucleus, these particles occupy different quantum states, or shells, much as electrons do in atoms.

However, the shell model does not tell the whole story. Nucleons occasionally come very close together, forming temporary pairs with exceptionally large velocities. Almost all of these pairs consist of one proton and one neutron.

According to team member Lawrence Weinstein of Old Dominion University in the US, such pairs can reveal what happens when nucleons approach each other so closely that their internal structures may begin to overlap.

“Nucleons are like people,” he says. “When they are far apart they do not interact, at moderate distances they can attract each other, but if they get too close they can repel each other violently.”

Quarks and gluons

The pairs therefore provide a way to investigate how the strong nuclear force behaves at very short distances and whether these close encounters affect the quarks and gluons inside nucleons, where quarks are the fundamental constituents of protons and neutrons and gluons are the particles responsible for binding them together.

Previous experiments had suggested that neutron-rich nuclei contain more short-range pairs than nuclei with similar numbers of protons and neutrons. However, those studies compared nuclei that also differed substantially in mass, making it difficult to determine the true cause of the effect.

To separate these possibilities, the researchers examined three carefully chosen nuclei: two calcium isotopes and an iron isotope.

“We studied calcium-40, calcium-48, and iron-54,” says Or Hen of the Massachusetts Institute of Technology, one of the authors of the study. “These let us see how the number of SRC pairs increased as we added eight neutrons from calcium-40 to calcium-48 and then added six protons from calcium-48 to iron-54.”

The measurements were carried out at the Thomas Jefferson National Accelerator Facility in Virginia. The researchers fired a beam of electrons at the nuclei and measured both the scattered electrons and protons knocked out of the target.

Reconstructed motion

By reconstructing the motion of the proton before the collision, they could determine whether it had belonged to a short-range correlated pair.

The team expected that adding large numbers of neutrons would significantly increase the number of proton-neutron pairs. Instead, the effect was surprisingly small.

“We found that adding 40% more neutrons only increased the probability of finding a proton in an SRC pair by 10%,” says Hen.

The additional neutrons occupied an outer quantum shell, while most of the protons remained in inner shells. The result suggests that the newly added neutrons rarely formed close-range pairs with protons in different shells.

The researchers then examined iron-54, which contains six additional protons occupying the same outer shell as the extra neutrons in calcium-48.

Dramatic effect

“Conversely, the added six protons in the outer orbital of iron-54 formed 50% more SRC pairs (relative to calcium-48), presumably with the outer-orbital neutrons in calcium-48,” Hen says.

The result points to an unexpected conclusion. Nucleons appear to prefer forming close-range pairs with partners occupying the same quantum shell rather than with particles located in different shells.

The finding also posed a challenge for existing theoretical models. Although some calculations reproduced part of the observed behavior, none predicted the strong increase seen in iron-54.

The work could have consequences beyond the structure of individual nuclei. Researchers have proposed that short-range pairs influence the properties of extremely dense matter, including the matter found inside neutron stars. The pairs may affect both the cooling of neutron stars and the relationship between pressure and density within these exotic objects.

Hen says that the team will now study wider range of nuclei. “We are extending this work to other stable nuclei from beryllium-9 to gold-197 to further study the effects of shell structure and mass on pair formation.”

Future experiments will also investigate unstable neutron-rich nuclei that cannot be studied using conventional targets. Those measurements should help determine whether the newly observed shell effects represent a general rule governing how short-range proton–neutron pairs form throughout nuclear matter.

The research is described in Nature.

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