⚛️ nuclear experiments

First observation of the fine structure of the Pygmy Dipole Resonance in a nucleus away from stability via the $\beta^-$ decay of $^{92}$ Rb to $^{92}$ Sr

Using the high-efficiency GRIFFIN spectrometer at TRIUMF, researchers performed a comprehensive $\gamma$ -ray spectroscopy study of the $\beta^-$ decay of $^{92}$ Rb to $^{92}$ Sr, revealing the fine structure of the Pygmy Dipole Resonance through the identification of 190 excited levels and 864 transitions while confirming the suppression of the Pandemonium effect and validating theoretical models.

Original authors: P. Spagnoletti, M. Ramalho, S. Tahtela, E. Kauppinen, J. Suhonen, C. Andreoiu, M. Scheck, V. Vedia, Z. Ahmed, D. Annen, G. C. Ball, G. Benzoni, S. S. Bhattacharjee, H. Bidaman, V. Bildstein, S. Buck

Published 2026-09-21

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CC BY 4.0

Original authors: P. Spagnoletti, M. Ramalho, S. Tahtela, E. Kauppinen, J. Suhonen, C. Andreoiu, M. Scheck, V. Vedia, Z. Ahmed, D. Annen, G. C. Ball, G. Benzoni, S. S. Bhattacharjee, H. Bidaman, V. Bildstein, S. Buck, R. Caballero-Folch, R. J. Coleman, S. Devinyak, I. Dillmann, I. Djianto, F. H. Garcia, A. B. Garnsworthy, P. E. Garrett, B. Greaves, C. J. Griffin, G. F. Grinyer, E. G. Fuakye, G. Hackman, D. Hymers, D. Kalaydjieva, R. Kanungo, K. Kapoor, E. Kasanda, W. Korten, N. Marchini, K. Mashtakov, A. Nannini, K. Ortner, B. Olaizola, C. Natzke, C. M. Petrache, M. Polettini, A. Radich, M. Rocchini, N. Saei, M. Satrazani, M. Siciliano, M. Singh, C. E. Svensson, D. A. Torres, R. Umashankar, E. Wadge, T. Zidar, M. Zielinska

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 a nuclear reactor, atoms split apart, creating a chaotic spray of new, unstable particles. As these fragments settle, they shed excess energy by emitting tiny, ghostly particles called antineutrinos. For decades, physicists have tried to predict exactly how many of these particles emerge and at what energies, but their calculations have consistently missed the mark, showing a deficit of about six percent compared to what detectors actually see. This discrepancy, known as the Reactor Antineutrino Anomaly, suggests that our understanding of the decay process is incomplete. The missing piece likely lies in the complex behavior of these unstable fragments, specifically how they transition from one state to another. To solve this, scientists must map the energy levels of these fragments with extreme precision, looking for subtle patterns that standard models might overlook.

A team of researchers has now peeled back a layer of this mystery by studying a specific unstable atom, rubidium-92, which is a major contributor to the high-energy antineutrino stream. Using a massive, highly sensitive instrument called GRIFFIN at the TRIUMF research facility in Canada, they observed the decay of rubidium-92 into strontium-92. When the rubidium atom decays, it transforms into strontium, but not just into a single, calm state. Instead, it populates a vast number of excited energy levels within the new strontium nucleus. The researchers were able to detect 864 distinct gamma-ray transitions, which are flashes of light emitted as the strontium nucleus relaxes from these high-energy states. This allowed them to construct a detailed map of 190 excited levels, most of which had never been seen before.

The primary goal was to measure how often the decay feeds into these various levels, a process that had previously been obscured by a systematic error known as the Pandemonium effect. This error occurs when detectors miss the faint, numerous flashes of light from complex decays, leading scientists to incorrectly assume the energy went somewhere else. By using a detector array with exceptional resolving power, the team captured these weak signals, revealing a much more accurate picture of the decay. They found that the energy distribution matched recent, independent measurements that used a different method, giving them high confidence that they had successfully corrected the historical errors. This clarity is essential because the way these atoms decay directly shapes the antineutrino spectrum that reactors produce.

What makes this discovery particularly striking is what the team found in the higher energy regions of the strontium nucleus. They observed that the decay frequently feeds into a specific cluster of states known as the Pygmy Dipole Resonance. In simple terms, this is a collective vibration where the protons and neutrons in the nucleus slosh against each other in a specific way. While this phenomenon is known to exist, seeing it clearly populated through beta decay in an unstable nucleus like strontium-92 is a first. The researchers discovered that the decay process is surprisingly efficient at reaching these high-energy states, even though physics suggests it should be much harder to reach them than the lower-energy states.

To understand why this happens, the team compared their experimental data with complex theoretical calculations. These models showed that the wavefunctions—the mathematical descriptions of how the particles are arranged inside the nucleus—contain specific features that allow the decay to bypass the usual energy barriers. Essentially, the internal structure of the nucleus creates a pathway that makes these high-energy vibrations accessible. This finding mirrors similar observations in a neighboring nucleus, zirconium-96, but the team found that the strontium-92 decay populates far more levels and shows a much more fragmented pattern of energy release.

The implications of this work extend beyond just understanding one specific atom. By confirming that these Pygmy Dipole Resonance states are significantly populated during the decay of fission fragments, the study provides a crucial piece of the puzzle for the Reactor Antineutrino Anomaly. It suggests that the structure of these unstable nuclei plays a larger role in shaping the antineutrino spectrum than previously thought. The data also offers a vital reference for future experiments, such as those using Coulomb excitation, which aim to measure the full electric dipole strength of these nuclei. Ultimately, this research demonstrates that with the right tools, scientists can resolve the fine details of nuclear decay, correcting long-standing errors and revealing the intricate dance of particles that governs the behavior of matter at the subatomic scale.

Technical Summary: First Observation of the Fine Structure of the Pygmy Dipole Resonance in a Nucleus Away from Stability via the $\beta^-$ Decay of $^{92}$ Rb to $^{92}$ Sr

Problem and Motivation
The measurement of $\beta$ -feeding intensities in nuclei with large $Q_\beta$ values is historically plagued by the "Pandemonium effect," a systematic error arising when high-resolution but low-efficiency $\gamma$ -ray detectors fail to resolve the complex, fragmented $\gamma$ -ray cascades of highly excited states. This leads to the misplacement or non-observation of transitions, resulting in inaccurate $\beta$ -feeding distributions. This issue is critical for the "Summation Method" used to predict reactor antineutrino spectra, particularly for the Reactor Antineutrino Anomaly (RAA). The decay of $^{92}$ Rb ( $T_{1/2} = 4.48$ s, $J^\pi = 0^-$ , $Q_\beta = 8095$ keV) is a major contributor to the high-energy region of the reactor antineutrino spectrum. While recent Total Absorption Spectroscopy (TAS) studies [11, 12] corrected previous underestimations of feeding to high-lying levels (above 4.5 MeV), TAS lacks the resolution to probe the detailed structure of these levels. Specifically, it remains unclear which features of the wavefunctions of high-lying $1^-$ levels allow them to compete with energetically favored decays to lower-lying states. Furthermore, the population of the Pygmy Dipole Resonance (PDR) via $\beta$ -decay in unstable nuclei remains an underexplored phenomenon.

Methodology
The authors performed a comprehensive high-resolution $\gamma$ -ray spectroscopy study of excited states in $^{92}$ Sr populated via the $\beta^-$ decay of $^{92}$ Rb. The experiment was conducted at the TRIUMF-ISAC facility using the GRIFFIN spectrometer, equipped with 15 Compton-suppressed HPGe clover detectors.

Beam Production: $^{92}$ Rb and $^{92}$ Sr ions were produced by bombarding a uranium carbide target with 480-MeV protons, followed by mass separation.
Data Acquisition: Ions were implanted into a moving tape at the center of the array. Data were collected with an implantation time of 45 s (approx. 10 half-lives) and a 13 s counting window, at intensities up to $3 \times 10^6$ pps.
Analysis: A total of 864 unique $\gamma$ -ray transitions were identified and placed in a level scheme involving 190 excited states. Spin and parity assignments were derived from $\gamma$ - $\gamma$ angular correlations, $\log ft$ values, and selection rules.
Theoretical Framework: Experimental results were compared against Large-Scale Shell Model (LSSM) calculations using the KSHELL code and the $jj45pnb$ Hamiltonian. Additionally, Multiple-Commutator Model (MCM) calculations were employed to analyze the microscopic structure of the transitions, utilizing the Quasiparticle Random-Phase Approximation (QRPA) to compute wavefunctions for the $0^-$ ground state of $^{92}$ Rb and excited states in $^{92}$ Sr.

Key Results

Level Scheme Extension: The study populated 190 excited states and established 864 $\gamma$ -ray transitions in $^{92}$ Sr, with the majority identified for the first time.
Pandemonium Suppression: The $\beta$ -feeding intensities measured with GRIFFIN show excellent agreement with recent TAS results [12] below 4.5 MeV and map well to the TAS data structure above this energy. The high statistics and resolving power of GRIFFIN allowed for the identification of weak branches (e.g., 40 decays from the 6030 keV level, where only 3 were previously known), effectively suppressing the Pandemonium effect even in decays with large $Q_\beta$ windows.
Spin Assignments: Spins were assigned to 55 levels via angular correlations and constrained for 30 others using $\log ft$ values. A systematic trend was observed: transitions to lower-lying levels are predominantly first-forbidden, while transitions to high-lying $J=1^-$ levels in the PDR region are predominantly allowed Gamow-Teller transitions.
Shell Model Agreement: LSSM calculations successfully reproduced the experimental low-spin states up to 5 MeV, despite the complexity of the level scheme.
PDR Observation: The experimental data reveals significant $\beta$ -feeding to a large number of $1^-$ levels in the energy region of the Pygmy Dipole Resonance (PDR) in $^{92}$ Sr. MCM calculations indicate that these levels are populated via 1p-1h configurations that become fragmented over many states through coupling to higher-order quasiparticle states.
Dipole Strength: Spherical QRPA calculations show weak electric dipole strength near the neutron separation energy, carried by the $1^-$ states populated in the decay. The isovector strength comprises multiple proton and neutron excitations, while the lowest isoscalar strength is dominated by a specific single-particle transition.

Significance and Claims
The paper claims to provide the first observation of the fine structure of the Pygmy Dipole Resonance in a nucleus far from stability ( $^{92}$ Sr) via $\beta$ -decay. The study demonstrates that modern HPGe detector arrays (GRIFFIN) can effectively suppress the Pandemonium effect, providing highly complementary data to TAS studies for reactor physics applications.

The results offer an unprecedented detailed picture of the $1^-$ levels in the PDR region, revealing that the wavefunction features enabling the population of these high-lying levels (with low effective $Q$ -values) are well-reproduced by Multiple-Commutator Model calculations. This work highlights the role of nuclear structure in fission fragments in shaping the reactor antineutrino spectrum. The authors conclude that the detailed decay study of $^{92}$ Rb provides vital spectroscopic data for future Coulomb Excitation studies of $^{92}$ Sr to extract the full electric dipole strength and suggests that similar phenomena may exist in other key contributors to the reactor antineutrino spectrum, such as $^{142}$ Cs.

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