The double magical core of the Lead-208 goes against expectations with its amazing shape properties

In a surprising event, an international team of scientists discovered that the heaviest known “double magic” nucleus, Lead-208 (208pb), exhibits unexpected geometric properties that current nuclear models cannot be accurately predicted.

Double magic refers to a nucleus with a full shell of both protons (82) and neutrons (126).

The studies published in the Physical Review Letter focus on the deformation and collective behavior of lead 208, particularly the moments of spectral quadruple leaves in two excited states.

The spectroscopic quadrupole moment measures how far the nucleus deviates from the perfect sphere and quantifies the distribution of charge within the nucleus.

Phys.org spoke to Dr. Jack Henderson of Surrey University, the first author of the study. “I fired germanium, tellurium, neodymium and erbium beams with lead foil,” explains Dr. Henderson.

“When these particles scatter from the foil, they can energize the lead nucleus, leaving a potentially excited state. The probability of this excitation occurring depends on the shape of the nucleus, so careful measurements allowed us to infer the deformation of lead 208.”

Magical numbers and nucleus shapes

Just as the way noble gases represent stable electronic compositions, magic and double magic nuclei provide stable nuclei. These “magic numbers” are 2, 8, 20, 28, 50, 82, and 126.

In the case of a magical nucleus, the number of neutrons or protons is equal to the number of magical nucleus, but in the case of a double magical nucleus, both are magical numbers.

This means that the nuclear shell is filled, and in the case of double magical nuclei, the more stable nuclei are double. The protons and neutrons are arranged symmetrically, resulting in a more spherical nucleus.

With 82 protons and 126 neutrons, the lead 208 sits at the intersection of two magical numbers, and is theoretically predisposed to double magic to maintain the sphere.

Explaining the importance of magical numbers, Dr. Henderson said, “Magical numbers are extremely important for understanding nuclear physics and related properties, so they are very important, as they give nearby nuclei more binding energy.”

“This distorts the nuclear landscape and is responsible for peak abundances of intense elemental nucleo-synthesis, for example, in the so-called rapid neutron capture process (R process).”

Coulomb excitation

Although the study of double magical nuclei is not new, researchers have used state-of-the-art experimental equipment to provide more definitive measurements, particularly in terms of signs and magnitude of deformation.

The researchers used Coulomb excitation. This is the way in which the nucleus is excited by electromagnetic interactions without the powerful force of the nucleus.

“Coulomb excitation is very sensitive to determining deformation, as it relies solely on electromagnetic forces that we have understood since the late 1800s,” Henderson explained.

The experimental setup consisted of four different beams (germanium, tellurium, neodymium, and erbium) directed at lead 208 foil.

Researchers measured the gamma rays emitted as the excited nucleus returns to its ground state, while tracking scattered particles with a CHICO2 detector array, using a Gretina array of high purity germanium detectors to measure the gamma rays emitted.

In particular, the researchers focused on two excited states and studied deformation, first quadrupole excitation, and first octopole excitation.

Data were analyzed using the Gosia semiclassical Coulomb excitation code.

Challenging models, impacts, future work

The researchers demonstrated that both the studied excited states show a large, negative spectral quadrupole moment.

When compared to predictions from three different theoretical approaches (nuclear shell model, sensory theory of density, and Hartley Fock calculations), we were unable to replicate the signs and magnitude of the observed deformation.

Dr. Henderson said, “The model does not seem to replicate our observability, and the reason for this is not entirely clear. One possibility is that the interactions of the nuclei used require a freedom of refinement or freedom that we didn’t recognize as important.”

As mentioned earlier, Lead-208 plays an important role in understanding how gravity forms in space events such as Neutron Star Mergers, or R-Process. Therefore, the effects range from nuclear studies to stellar evolution.

Researchers plan to tackle these challenges, with Dr. Henderson hinting at future work, saying, “One possibility is that adjacent nuclei, such as lighter and heavier isotopes of lead, may help us to better understand missing components of theoretical interpretation.”

“Another potential route is to improve your understanding of how nuclear shapes vibrate by looking for important signature states that have previously avoided critical identification.”

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