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Direct measurement of Enhanced octupole collectivity in 148Dy

Using the GRIFFIN spectrometer at TRIUMF, researchers measured an enhanced electric octupole transition strength of 46(3) W.u. in $^{148}$ Dy, providing direct evidence of significant octupole collectivity beyond $Z=64$ and extending the known boundaries of this phenomenon far from traditional octupole magic numbers.

Original authors: P. Spagnoletti, V. Vedia, E. Yuksel, Y. X. Yu, G. J. Fu, R. Umashankar, G. Andreetta, C. Andreoiu, A. A. Avaa, G. C. Ball, V. Bildstein, S. Buck, E. Cantacuzene, G. Colombi, I. Dillmann, L. P. Gaffney

Published 2026-09-21

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Original authors: P. Spagnoletti, V. Vedia, E. Yuksel, Y. X. Yu, G. J. Fu, R. Umashankar, G. Andreetta, C. Andreoiu, A. A. Avaa, G. C. Ball, V. Bildstein, S. Buck, E. Cantacuzene, G. Colombi, I. Dillmann, L. P. Gaffney, A. B. Garnsworthy, P. E. Garrett, G. F. Grinyer, G. Hackman, J. Liu, A. Ludlam, L. L. Luperi, Madhu, T. La Marca, D. Movilla-Quintero, S. Murillo Morales, R. Nicolas del Alamo, M. M. Rajabali, P. H. Regan, M. Rocchini, M. Scheck, J. Seger, S. Sekal, D. Stramaccioni, C. E. Svensson, A. Tsantiri, J. Williams, F. Wu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Inside the heart of every atom lies a nucleus, a dense cluster of protons and neutrons that usually holds a perfectly round or slightly flattened shape. For decades, physicists have known that under specific conditions, these nuclei can stretch into a pear shape, where one end is slightly heavier than the other. This unusual form arises from a subtle tug-of-war between the particles inside, creating a distortion that breaks the symmetry of the sphere. Scientists are deeply interested in these pear-shaped nuclei because they might hold the key to understanding why the universe is made of matter rather than antimatter. To find them, researchers look for specific combinations of protons and neutrons where this pear shape is most likely to appear, often near certain "magic" numbers that act like stable rungs on a ladder.

A team of researchers has now taken a significant step forward in mapping where these pear shapes exist. They focused on an isotope of the element dysprosium, specifically an atom with 66 protons and 82 neutrons. By studying how this atom decays, they were able to measure a property called the electric octupole transition strength. In simple terms, this measurement tells us how easily the nucleus can shift from a pear shape back to a round shape, or how strongly it vibrates in that pear-like mode. The stronger this signal, the more pronounced the pear shape is within the nucleus. This specific measurement is notoriously difficult to perform because the signals are faint and easily drowned out by other, more common types of nuclear decay.

To capture this elusive signal, the team created a beam of dysprosium atoms at a major research facility in Canada. They bombarded a target with high-energy protons to create a rare, unstable version of the element, which they then filtered and slowed down until it was just right for observation. These atoms were implanted into a tape system surrounded by a sophisticated array of detectors. As the atoms decayed, they emitted gamma rays, which are high-energy flashes of light. The researchers used a specialized timing technique to measure exactly how long the excited nucleus waited before releasing its energy. By combining this precise timing with a count of how often the nucleus chose to release energy directly to its ground state versus taking a detour through an intermediate step, they could calculate the strength of the octupole vibration.

The results were striking. The team determined that the transition strength for this dysprosium isotope is 46 units, a value that stands as the largest ever measured in this specific chain of elements. This finding confirms that the tendency to form pear shapes does not stop at the previously known boundary of 64 protons but extends further, reaching at least 66 protons. The data suggests that the region of the atomic chart where these shapes are most prominent is larger than previously thought, pushing the limits of where we expect to find them. This measurement is not just a number; it is direct evidence of enhanced octupole collectivity in this specific atom, indicating a strong pear-like vibration, though whether this represents a permanently stable deformation remains an open question in this region.

The researchers compared their experimental results with complex computer simulations designed to predict nuclear behavior. While some of the theoretical models came close, none of them fully predicted the magnitude of the strength they observed in the dysprosium atom. The simulations suggested the effect would be weaker, or they failed to capture the sharp increase seen in the data. This gap between the theory and the measurement highlights that our current understanding of how protons and neutrons interact to create these shapes is still incomplete. The models need to be refined to account for the specific ways these particles move across energy gaps that were previously thought to be less significant.

This work does more than just add a data point to a chart; it redefines the boundaries of where we should look for these exotic shapes. The discovery implies that the "pear-shaped" region extends further into the heavier elements than scientists had anticipated. It also suggests that other nearby isotopes, which have not yet been studied with this level of precision, might possess even stronger pear-like characteristics. The findings provide a clear direction for future experiments, urging researchers to look at even heavier elements to see if this trend continues. By pinning down exactly where these shapes are strongest, the study brings us closer to understanding the fundamental forces that shape the matter around us, without relying on guesswork or indirect hints.

Technical Summary: Direct Measurement of Enhanced Octupole Collectivity in $^{148}\text{Dy}$

Problem Statement
The existence of "pear"-shaped nuclei resulting from reflection symmetry breaking is driven by long-range octupole-octupole correlations, which are strongest when pairs of orbitals differing by $\Delta j = \Delta \ell = 3\hbar$ lie near the Fermi surface. While significant theoretical and experimental focus has been placed on the actinide ( $Z \sim 88, N \sim 134$ ) and lanthanide ( $Z \sim 56, N \sim 88$ ) regions, the precise boundaries of enhanced octupole collectivity remain unclear. Specifically, while $B(E3; 3^-_1 \to 0^+_1)$ transition strengths were observed to increase from $Z=50$ to $Z=64$ within the $N=82$ isotonic chain, it was unknown if this trend continued beyond $Z=64$ (Gadolinium) or if the Gd isotopes represented the peak of collectivity in this region. Furthermore, experimental access to electric-octupole (E3) transition strengths is challenging because E3 transitions often compete poorly with allowed E1 or E2 transitions.

Methodology
The authors performed a direct measurement of the mean lifetime of the $3^-_1$ level in $^{148}\text{Dy}$ and the branching fraction of its decay to the ground state.

Production: Excited states in $^{148}\text{Dy}$ were populated via the $\beta^+$ /EC decay of the $5^-$ isomer of $^{148m}\text{Ho}$ ( $T_{1/2} = 9.60(13)$ s). The isomer was produced by bombarding a Tantalum target with 480-MeV protons at the TRIUMF ISAC-I facility.
Beam Purification: Ho atoms were ionized using a laser ion source to suppress isobaric contaminants, followed by mass separation to produce a pure $^{148}\text{Ho}$ beam, which was implanted into a Mylar tape at the center of the GRIFFIN spectrometer.
Detection: The GRIFFIN spectrometer, equipped with 15 HPGe clover detectors and 8 LaBr $_3$ (Ce) fast-scintillator detectors (with active anti-Compton shielding), was used to collect decay data.
Lifetime Measurement: The mean lifetime of the $3^-_1$ level was determined using the Generalized Centroid Difference (GCD) method. This involved measuring the time difference between the feeding transition ( $5^-_1 \to 3^-_1$ at 660.8 keV) and the decay transitions ( $3^-_1 \to 0^+_1$ at 1687.5 keV and $3^-_1 \to 2^+_1$ at 1677.5 keV) using LaBr $_3$ (Ce) detectors.
Branching Fraction: The absolute $\gamma$ -ray branching fraction for the $3^-_1 \to 0^+_1$ transition was derived by measuring the intensity ratio of the 1687-keV (E3) and 1677-keV (E2) transitions in coincidence with transitions feeding the $3^-_1$ level. The contribution of the unobserved 10-keV E1 transition was accounted for using internal conversion coefficients.
Analysis: Data analysis included corrections for Compton background time correlations and the energy-dependent Prompt Response Difference (PRD) of the detector system.

Key Results

Mean Lifetime: The mean lifetime of the $3^-_1$ level in $^{148}\text{Dy}$ was determined to be $\tau = 640(38)$ ps. This value was consistent with results obtained via the convolution method ( $\tau = 640(25)$ ps).
Branching Fraction: The ground-state branching fraction for the $3^-_1 \to 0^+_1$ transition was measured as $I_{\gamma, \text{rel.}} = 0.862(11)$ .
Transition Strength: Combining the lifetime and branching fraction, the reduced electric-octupole transition strength was calculated as $B(E3; 3^-_1 \to 0^+_1) = 46(3)$ W.u.
Comparison: This value represents the largest measured $B(E3)$ strength across the entire $N=82$ isotonic chain and is one of the largest in the lanthanide region.

Theoretical Context and Comparison
The experimental results were compared with Quasiparticle Random Phase Approximation (QRPA) calculations using SkI3 and SkM* Skyrme energy-density functionals, as well as Large-Scale Shell-Model (SM) calculations.

QRPA: The SkM* interaction reproduced data for lighter isotones ( $^{142}\text{Nd}$ , $^{144}\text{Sm}$ , $^{146}\text{Gd}$ ) but significantly underestimated the enhancement observed in $^{148}\text{Dy}$ . The SkI3 interaction reproduced the $^{148}\text{Dy}$ result but overpredicted the strength in $^{146}\text{Gd}$ . Both models suggest the enhancement is driven by the increasing occupation of the $\pi 1d_{5/2}$ orbital and the availability of the $\pi 0h_{11/2}$ orbital, but neither fully captures the magnitude of the increase from $^{146}\text{Gd}$ to $^{148}\text{Dy}$ .
Shell Model: Standard SM calculations underestimated $B(E3)$ values by a factor of two. A shifted SM approach, accounting for missing cross-shell neutron excitations by referencing the $^{132}\text{Sn}$ core, provided a better baseline but still failed to reproduce the large increase observed between $^{146}\text{Gd}$ and $^{148}\text{Dy}$ .

Significance and Claims
The authors claim that this work provides direct evidence of enhanced octupole collectivity beyond $Z=64$ , extending the known boundaries of this phenomenon far from the traditional "octupole magic numbers" ( $Z=56, N=88$ ).

The measurement confirms that the trend of increasing $B(E3)$ strength continues past the quasi-doubly-magic $^{146}\text{Gd}$ , reaching a maximum in $^{148}\text{Dy}$ within the $N=82$ chain.
The result challenges existing theoretical models (QRPA and SM), indicating a need for more accurate descriptions of cross-shell octupole correlations.
The authors suggest that based on systematics and QRPA predictions, heavier Dysprosium isotopes ( $^{150,152}\text{Dy}$ ) and isotopes of Erbium and Ytterbium may possess even larger $B(E3)$ strengths, motivating future experimental studies in these regions to fully map the limits of octupole collectivity.
The study demonstrates the viability of fast-timing techniques combined with branching fraction measurements as a complementary approach to Coulomb excitation for determining $B(E3)$ values in nuclei where E3 decays are experimentally accessible.

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