Exploring β decay and β-delayed neutron emission in exotic ^46,47Cl isotopes

In this paper, which Professor Tripathi and her collaborators published in Physical Review C, β^- and β-delayed neutron decays of ^46,47Cl are reported from an experiment carried out at the National Superconducting Cyclotron Laboratory using the Beta Counting System. The half-lives of both ⁴⁶Cl and ⁴⁷Cl were extracted. Based on the delayed γ-ray transitions observed, the level structure of N = 28 ⁴⁶Ar was determined. Completely different sets of excited states above the first 2⁺ state in ⁴⁶Ar were populated in the ⁴⁶Cl β⁢0⁢n and ⁴⁷Cl β⁢1⁢n decay channels. Two new γ-ray transitions in ⁴⁷Ar were identified from the very weak ⁴⁷Cl β⁢0⁢n decay. Furthermore, ⁴⁶Cl β⁢1⁢n and ⁴⁷Cl β2⁢n were also observed to yield different population patterns for levels in ⁴⁵Ar, including states of different parities. The experimental results allow Professor Tripathi and her collaborators to address some of the open questions related to the delayed neutron emission process. For isotopes with large neutron excess and high Q_β values, delayed neutron emission remains an important decay mode and can be utilized as a powerful spectroscopic tool. Experimental results were compared with shell-model calculations using the FSU and V_MU effective interactions.

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Study of alpha resonances in ¹¹B via the ⁷Li(⁷Li,t) reaction

The near-threshold proton resonance in ¹¹B, recently identified and characterized through independent experiments, plays a crucial role in understanding the unexpectedly high decay branch observed in the β-delayed proton emission (β−⁢p) from the neutron halo nucleus ¹¹Be. Critical to interpreting this process is the determination of whether or not this state has any significant α-decay width (Γ_α). A coincidence measurement was conducted to study the alpha-transfer reaction ⁷Li(⁷Li,t)¹¹B* → ⁷Li+α employing the Super-Enge Split Pole Spectrograph coupled with a silicon detector at Florida State University. The experiment aimed to investigate the structure of α resonances in ¹¹B in the E_x = 9.0 MeV to 14 MeV energy region. In particular, to explore and constrain the α-decay branch of the near-threshold proton resonance at E_x = 11.44 MeV. No significant α-decay strength was observed from this state. Additionally, the experimental results provided insights into the state at E_x = 12.55 MeV, previously proposed as the T = 3/2 isobaric analog state of the ¹¹Be ground state. The observed decay pattern, dominated by α+⁷Li, suggests a strong T = 1/2 component, indicating the need for further experimental measurements. Spearheaded by Professor Almaraz-Calderon's former student, Eilens Lopez Saavedra, the result were published in Physical Review C.

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Determination of proton and neutron contributions to the 0⁺(g.s.)→2⁺(1) excitations in ⁴²Si and ⁴⁴S

A collaboration, led by Ursinus College Professor Lew Riley and including Professors Cottle, Spieker, and Volya, has measured the 0⁺(g.s.)→2⁺(1) transition in the neutron-rich N = 28 isotope ⁴²Si using the probes of intermediate-energy Coulomb excitation and inelastic proton scattering in inverse kinematics at the Facility for Rare Isotope Beams (FRIB) with beam particle rates of about 5 particles/s. The results of these two measurements allowed them to determine M_n/M_p, the ratio of the neutron and proton transition matrix elements for the 0⁺(g.s.)→2⁺(1) transition. In addition, the collaboration measured the 0⁺(g.s.)→2⁺(1) transition in the isotone ⁴⁴S using inverse kinematics inelastic proton scattering. By comparing the ⁴⁴S proton-scattering result with a recent intermediate-energy Coulomb excitation result on the same transition, the group was able to determine M_n/M_p for the 0⁺(g.s.)→2⁺(1) transition in this nucleus as well. This work strengthens the evidence that ⁴²Si has a stable quadrupole deformation in its ground state and that ⁴⁴S does not. Both conclusions are further supported by shell-model calculations carried out with the FSU interaction. The results were published in Physical Review C.

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Role of the isovector spin-orbit potential in mitigating the CREX-PREX dilemma

Pioneering electroweak measurements of the neutron skin thickness in lead-208 and calcium-48 are challenging our understanding of nuclear dynamics. Many theoretical models suggest that the slope of the symmetry energy controls the development of a neutron skin in neutron-rich nuclei. This led to the expectation that if lead-208 exhibits a large neutron skin, calcium-48 should as well. Given that the PREX Collaboration reported a relatively thick neutron skin in lead, we anticipated that calcium would also have a significant neutron skin. Instead, the CREX Collaboration reported a thin neutron skin in calcium. Although many suggestions have been proposed, the “CREX-PREX dilemma” remains unsolved. Recently, an intriguing scenario has emerged, suggesting that an enhanced isovector spin-orbit interaction could simultaneously account for both results. Following this approach, Professor Piekariewicz and his graduate students, Athul Kunjipurayil and Marc Salinas, performed relativistic mean-field calculations with an increased isovector spin-orbit potential. Their findings indicate that, while this modification significantly affects the structure of calcium-48, it has only a marginal impact on lead-208, thereby bringing the results into better agreement with experiment. However, the strong enhancement required to mitigate the CREX-PREX dilemma destroys the agreement with a successful spin-orbit phenomenology, primarily by modifying the well-known ordering of spin-orbit partners. The results were published in Physical Review C as an Editors' Suggestion.

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Particle-γ coincidence experiments at Fox Lab

Since its foundation in the 1960s, the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University (FSU) pursued research at the forefront of nuclear science. In this invited contribution, Dr. Almaraz-Calderon and Dr. Spieker present recent highlights from nuclear structure and reaction studies conducted at the John D. Fox Superconducting Linear Accelerator Laboratory, also featuring the general experimental capabilities at the laboratory for particle-γ coincidence experiments. Specifically, they focus on light-ion induced reactions measured with the Super-Enge Split-Pole Spectrograph (SE-SPS) and the CATRiNA neutron detectors, respectively. Some results obtained with the CeBrA demonstrator for particle-γ coincidence experiments at the SE-SPS are presented. A highlight from the first experimental campaigns with the combined CLARION2-TRINITY setup, showing that weak reaction channels can be selected, is discussed as well. The results were published as part of the topical issue "Modern Advances in Direct Reactions for Nuclear Structure" in Frontiers in Physics, Section Nuclear Physics.

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CeBrA demonstrator commissioned at FSU SE-SPS

A highly selective experimental setup for particle-γ coincidence experiments at the Super-Enge Split-Pole Spectrograph (SE-SPS) of the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University (FSU) using fast CeBr₃ scintillators for γ-ray detection has been commissioned. A new publication reports on the results of characterization tests for the first five CeBr₃ scintillation detectors of the CeBr₃ Array (CeBrA) with respect to energy resolution and timing characteristics. Results from the first particle-γ coincidence experiments successfully performed with the CeBrA demonstrator and the FSU SE-SPS are also presented. The new setup enables very selective measurements of γ-decay branching ratios and particle-γ angular correlations using narrow excitation energy gates, which are possible thanks to the excellent particle energy resolution of the SE-SPS. In addition, nuclear level lifetimes in the nanoseconds regime can be determined by measuring the time difference between particle detection with the SE-SPS focal-plane scintillator and γ-ray detection with the fast CeBrA detectors. Selective excitation energy gates with the SE-SPS exclude any feeding contributions to these lifetimes. This research was supported by the National Science Foundation (NSF), United States under Grant No. PHY-2012522 (WoU-MMA: Studies of Nuclear Structure and Nuclear Astrophysics) and by Florida State University, United States.

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CLARION2+TRINITY setup commissioned

A new Compton-suppressed HPGe and charged-particle array, CLARION2-TRINITY, has been commissioned at the FSU John D. Fox Laboratory in collaboration with Oak Ridge National Laboratory. The TRINITY charged-particle array is comprised of 64 Cerium-doped Gadolinium Aluminum Gallium Garnet (GAGG:Ce) crystals configured into five rings spanning 7–54 degrees, and two annular silicon detectors that can shadow or extend the angular coverage to backward angles with minimal γ-ray attenuation. GAGG:Ce is a non-hygroscopic, bright, and relatively fast scintillator with a light distribution well matched to SiPMs. Count rates up to 40 kHz per crystal are sustainable. Fundamental characteristics of GAGG:Ce were measured and presented in this article, including light- and heavy-ion particle identification (PID) capability, pulse-height defects, radiation hardness, and emission spectra. The CLARION2 array consists of up to 16 Compton-suppressed HPGe Clover detectors configured into four rings (eight HPGe crystal rings) using a non-Archimedean geometry that suppresses back-to-back coincident 511-keV gamma rays. The entire array is instrumented with 100- and 500-MHz (14 bit) waveform digitizers which enable triggerless operation, pulse-shape discrimination, fast timing, and pileup correction. Two examples of experimental data taken during the commissioning of the CLARION2-TRINITY system are also presented: a PID spectrum from ¹⁶O+¹⁸O fusion-evaporation, and PID and Doppler-corrected γ-ray spectra from ⁴⁸Ti+¹²C Coulomb excitation.

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