arXiv subjects

S. Zhu

Publications and source records attributed to S. Zhu.

At least 19 recordsLinked to original sources

arXiv2026

SN 2007it on the RISE -- a radio detection of an interacting supernova 18 years post-explosion

We report the first detection of radio emission from the Type II supernova SN 2007it, located at a distance of 12.2 Mpc in NGC 5530. The observations were obtained with the Australian Telescope Compact Array (ATCA) more than 18 yr after the explosion as part of the Rebrightening in Interacting Supernova Emission (RISE) program, which monitors nearby core-collapse supernovae for late-time interaction with dense circumstellar material. SN 2007it was detected on 2026 April 8 (08:00-12:00 UTC) at 5.5 GHz with a flux density of $3.30 \pm 0.13$ mJy and at 9.0 GHz with $3.54 \pm 0.24$ mJy. Its non-detection in publicly available 0.88 GHz ASKAP data from 2026 January 11 suggests either rapidly rising emission or significant internal absorption at lower frequencies. We assess the prospects for detection at other wavelengths and encourage coordinated follow-up observations across the radio, optical, X-ray, and $\gamma$-ray band

astro-ph.HE↗

arXiv2025

High-spin spectroscopy and the onset of quasicollective structures in $^{69}$Ga

The intermediate- and high-spin level structure of the odd-$A$ $^{69}$Ga nucleus was investigated via the $^{26}$Mg($^{48}$Ca, $p4n\gamma$) fusion evaporation reaction at a beam energy of 195 MeV. The experiment was performed using the Gammasphere multidetector array in conjunction with the Fragment Mass Analyzer (FMA), with mass and charge identification achieved via an ionization chamber placed at the focal plane of the spectrometer. Coincidence relationships between the $^{69}$Ga reaction products and emitted $\gamma$ rays were analyzed to establish the level sequences, while angular distribution and angular correlation measurements were used to propose spin and parity assignments. As a result, the level scheme of the nucleus has been considerably extended. Near the ground state, the structure of $^{69}$Ga is well described by single-particle excitations, with shell-model calculations using the JUN45 and jj44b effective interactions providing a satisfactory interpretation of the observed levels. At spins in excess of 21/2 $\hbar$, three sequences of $E2$ transitions have been delineated, suggesting the onset of collectivity. An interpretation within the framework of the tilted-axis-cranking covariant density functional theory is proposed which reveals the role of $g_{9/2}$ protons and neutrons in this angular momentum regime.

nucl-ex↗

arXiv2025

Fifteen years of Gamma-Ray Burst observations at Very High Energies with H.E.S.S

We present results from the High Energy Stereoscopic System (H.E.S.S.) follow-up observations of Gamma-ray Bursts (GRBs) between 2004 and 2019. We are focusing on non-detections and providing the most extensive set of very-high-energy (VHE, >100 GeV) upper limits to date. We use this catalogue to constrain the properties of VHE-detected GRBs and compare them to those detected at VHE. Our study finds that VHE-detected GRBs are not a distinct population but are instead associated with bright X-ray afterglows and low redshifts. In addition, we model the multi-wavelength emission of a few of the observed GRBs and discuss the results in the context of their obtained microphysical parameters. The results from this work help put current VHE observations into perspective and highlight the capabilities of next-generation instruments, in detecting fainter and more distant GRBs at VHE.

astro-ph.HE↗

arXiv2025

Test-time Controllable Image Generation by Explicit Spatial Constraint Enforcement

Recent text-to-image generation favors various forms of spatial conditions, e.g., masks, bounding boxes, and key points. However, the majority of the prior art requires form-specific annotations to fine-tune the original model, leading to poor test-time generalizability. Meanwhile, existing training-free methods work well only with simplified prompts and spatial conditions. In this work, we propose a novel yet generic test-time controllable generation method that aims at natural text prompts and complex conditions. Specifically, we decouple spatial conditions into semantic and geometric conditions and then enforce their consistency during the image-generation process individually. As for the former, we target bridging the gap between the semantic condition and text prompts, as well as the gap between such condition and the attention map from diffusion models. To achieve this, we propose to first complete the prompt w.r.t. semantic condition, and then remove the negative impact of distracting prompt words by measuring their statistics in attention maps as well as distances in word space w.r.t. this condition. To further cope with the complex geometric conditions, we introduce a geometric transform module, in which Region-of-Interests will be identified in attention maps and further used to translate category-wise latents w.r.t. geometric condition. More importantly, we propose a diffusion-based latents-refill method to explicitly remove the impact of latents at the RoI, reducing the artifacts on generated images. Experiments on Coco-stuff dataset showcase 30$\%$ relative boost compared to SOTA training-free methods on layout consistency evaluation metrics.

cs.CV↗

arXiv2024

Evolution of chirality from transverse wobbling in $^{135}$Pr

Chirality is a distinct signature that characterizes triaxial shapes in nuclei. We report the first observation of chirality in the nucleus $^{135}$Pr using a high-statistics Gammasphere experiment with the $^{123}$Sb($^{16}$O,4n)$^{135}$Pr reaction. Two chiral-partner bands with the configuration $\pi(1h_{11/2})^1\otimes\nu(1h_{11/2})^{-2}$ have been identified in this nucleus. Angular distribution analyses of the $\Delta I = 1$ transitions connecting the two bands reveal a dominant dipole character, and quasiparticle triaxial rotor model calculations show good agreement with the data. Since the simultaneous observation of chirality and transverse wobbling in $^{135}$Pr relies critically on these angular distribution results, we also address and refute the experimental and theoretical criticisms raised in a recent work by Lv et al., presenting additional evidence that further strengthens our interpretation. This marks the first observation of both hallmarks of triaxiality-chirality and wobbling-in the same nucleus.

nucl-ex↗

arXiv2024

Determination of the spins and parities for the 0$_{4}^{+}$ and 0$_{5}^{+}$ states in $^{100}$Zr

Two 0$^{+}$ states at 1294.5 and 1774.0 keV, together with three 2$^{+}$ and one 4$^{+}$ levels, were identified or unambiguously spin-parity assigned for the first time in $^{100}$Zr utilizing $\gamma$-ray spectroscopy and $\gamma$-$\gamma$ angular correlation techniques with the Gammasphere spectrometer, following the $\beta^{-}$ decay of neutron-rich, mass separated $^{100,100m}$Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around $\approx$1500 keV. According to the measured relative B(E2)$_{relative}$ transition probabilities, the 0$_{5}^{+}$ state exhibits decay properties which more closely align with those predicted for a spherical shape, while the 0$_{4}^{+}$ level is suggested to be associated with a weakly-deformed shape similar to one related to the 0$_{2}^{+}$ state.

nucl-ex↗

arXiv2023

Hardware implementation of quantum stabilizers in superconducting circuits

Stabilizer operations are at the heart of quantum error correction and are typically implemented in software-controlled entangling gates and measurements of groups of qubits. Alternatively, qubits can be designed so that the Hamiltonian corresponds directly to a stabilizer for protecting quantum information. We demonstrate such a hardware implementation of stabilizers in a superconducting circuit composed of chains of $π$-periodic Josephson elements. With local on-chip flux- and charge-biasing, we observe a softening of the energy band dispersion with respect to flux that is exponential in the number of frustrated plaquette elements, in close agreement with our numerical modeling.

quant-ph↗

arXiv2023

Experimental Study of the $^{\textbf{38}}$S Excited Level Scheme

Information on the $^{38}$S level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam $γ$-ray spectroscopy. Prompt $γ$-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling $^{38}$S residues were selected by the Fragment Mass Analayzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of $^{38}$S residues and identify any associated $γ$-ray transitions. The new level information, including the extension of the even-spin yrast sequence through $J^π = 8^{(+)}$, was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-$Z$ $N=22$ isotones from $Z=14$ to $20$, and for $^{36}$Si-$^{38}$S in particular, was also presented with an emphasis on the role and influence of the neutron $1p_{3/2}$ orbital on the structure in the region.

nucl-ex↗

arXiv2022

E2 Rotational Invariants of $0^+_1$ and $2^+_1$ states for $^{106}$Cd: the Emergence of Collective Rotation

The collective structure of $^{106}$Cd is elucidated by multi-step Coulomb excitation of a 3.849 MeV/$A$ beam of $^{106}$Cd on a 1.1 mg/cm$^2$ $^{208}$Pb target using GRETINA-CHICO2 at ATLAS. Fourteen $E2$ matrix elements were obtained. The nucleus $^{106}$Cd is a prime example of emergent collectivity that possesses a simple structure: it is free of complexity caused by shape coexistence and has a small, but collectively active number of valence nucleons. This work follows in a long and currently active quest to answer the fundamental question of the origin of nuclear collectivity and deformation, notably in the cadmium isotopes. The results are discussed in terms of phenomenological models, the shell model, and Kumar-Cline sums of $E2$ matrix elements. The ${\langle 0_2^+ ||E2||2_1^+ \rangle}$ matrix element is determined for the first time, providing a total, converged measure of the electric quadrupole strength, $\langle Q^2 \rangle$, of the first-excited $2_1^+$ level relative to the $0_1^+$ ground state, which does not show an increase as expected of harmonic and anharmonic vibrations. Strong evidence for triaxial shapes in weakly collective nuclei is indicated; collective vibrations are excluded. This is contrary to the only other cadmium result of this kind in $^{114}$Cd by C. Fahlander et al., Nucl. Phys. A485, 327 (1988), which is complicated by low-lying shape coexistence near midshell.

nucl-ex↗

arXiv2022

High quality superconducting Nb co-planar resonators on sapphire substrate

We present measurements and simulations of superconducting Nb co-planar waveguide resonators on sapphire substrate down to millikelvin temperature range with different readout powers. In the high temperature regime, we demonstrate that the Nb film residual surface resistance is comparable to that observed in the ultra-high quality, bulk Nb 3D superconducting radio frequency cavities while the resonator quality is dominated by the BCS thermally excited quasiparticles. At low temperature both the resonator quality factor and frequency can be well explained using the two-level system models. Through the energy participation ratio simulations, we find that the two-level system loss tangent is $\sim 10^{-2}$, which agrees quite well with similar studies performed on the Nb 3D cavities.

physics.app-ph↗

arXiv2022

Emergence of an island of extreme nuclear isomerism at high excitation near $^{208}$Pb

Metastable states with T$_{1/2}$ = 8(2) ms in $^{205}$Bi and T$_{1/2}$ = 0.22(2) ms in $^{204}$Pb, with $\approx $ 8 MeV excitation energy and angular momentum $\ge $ 22 $\hbar $, have been established. These represent, by up to two orders of magnitude, the longest-lived nuclear states above an excitation energy of 7 MeV, ever identified in the nuclear chart. Additionally, the half-life of the 10.17 MeV state in $^{206}$Bi has been determined to be 0.027(2) ms, the next highest value in this highly excited regime. These observations indicate the emergence of an island of extreme nuclear isomerism arising from core-excited configurations at high excitation in the vicinity of the doubly closed-shell nucleus $^{208}$Pb. These results are expected to provide discriminating tests of the effective interactions used in current large-scale shell-model calculations.

nucl-ex↗

arXiv2021

Shapes, Softness and Non-Yrast Collectivity in 186W

Non-yrast, excited states in neutron-rich $^{186}$W were populated via inelastic-scattering reactions using beams of $^{136}$Xe nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particle-$ γ$ coincidence techniques using the Gammasphere array of High-Purity Germanium detectors and the compact heavy-ion counter, CHICO2. The $ K^π = 2 ^{+} $ ($γ$), $ K^π = 0^{+}$ and $ K^π = 2^{-} $ (octupole) rotational side bands were extended to spins $ 14\hbar $, $ 12\hbar $, and $ 13\hbar $, respectively. A staggering pattern observed in the energies of levels in the $ K^π = 2^{+} $ band was found to be consistent with a potential that gets softer to vibration in the $ γ$ degree of freedom with increasing spin. The odd-even staggering of states in the $ K^π = 2^{-}$ band was found to exhibit a phase opposite to that seen in the $ γ$ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in $^{186}$W.

nucl-ex↗

arXiv2021

Isomers in 203Tl and core excitations built on a five-nucleon-hole structure

Isomers with three- and five-nucleon-hole configurations have been established in $^{203}$Tl. These include newly identified levels with a three-nucleon structure: {\it I}$^{π}$ = (15/2$^{-}$) with {\it T}$_{1/2}$ = 7.9(5) ns, and {\it I}$^{π}$ = (35/2$^{-}$) with {\it T}$_{1/2}$ = 4.0(5) ns. In addition, five-quasiparticle states: {\it I}$^{π}$ = (39/2$^{-}$) with {\it T}$_{1/2}$ = 1.9(2) ns, and {\it I}$^{π}$ = (49/2$^{+}$) with {\it T}$_{1/2}$ = 3.4(4) ns have also been established. The previously determined long-lived decay [{\it T}$_{1/2}$ = 6.6(3) $μ$s from this work] is associated with isomerism of the {\it I}$^{π}$ = (29/2$^{+}$) state. Levels above this long-lived isomer have been identified through a delayed-prompt coincidence measurement. Five-nucleon-hole states with excitation energies {\it E}$_{x}$ $\approx $ 7 MeV have been established as well as possible octupole excitations of the $^{208}$Pb core built on these levels. The level scheme of $^{203}$Tl is extended up to {\it E}$_{x}$ $\approx $ 11 MeV with the inclusion of 25 new transitions. Empirical and shell-model calculations have been performed to aid in the description of the observed states which are found to be predominantly of intrinsic character.

nucl-ex↗

arXiv2021

Ground-state and decay properties of neutron-rich 106Nb

The ground-state properties of neutron-rich 106Nb and its beta decay into 106Mo have been studied using the CARIBU radioactive-ion-beam facility at Argonne National Laboratory. Niobium-106 ions were extracted from a 252Cf fission source and mass separated before being delivered as low-energy beams to the Canadian Penning Trap, as well as the X-Array and SATURN beta-decay-spectroscopy station. The measured 106Nb ground-state mass excess of -66202.0(13) keV is consistent with a recent measurement but has three times better precision; this work also rules out the existence of a second long-lived, beta-decaying state in 106Nb above 5 keV in excitation energy. The decay half-life of 106Nb was measured to be 1.097(21) s, which is 8% longer than the adopted value. The level scheme of the decay progeny, 106Mo, has been expanded up to approximately 4 MeV. The distribution of decay strength and considerable population of excited states in 106Mo of J >= 3 emphasises the need to revise the adopted Jpi = 1- ground-state spin-parity assignment of 106Nb; it is more likely to be J => 3.

nucl-ex↗

arXiv2020

Shape Coexistence at Zero Spin in 64Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (3463 keV), with a collective 2+ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

nucl-ex↗

arXiv2020

Multi-nucleon transfer in the interaction of 977 MeV and 1143 MeV $^{204}$Hg with $^{208}$Pb

A previous study of symmetric collisions of massive nuclei has shown that current models of multi-nucleon transfer (MNT) reactions do not adequately describe the transfer product yields. To gain further insight into this problem, we have measured the yields of MNT products in the interaction of 977 (E/A = 4.79 MeV) and 1143 MeV (E/A = 5.60 MeV) $^{204}$Hg with $^{208}$Pb. We find that the yield of multi-nucleon transfer products are similar in these two reactions and are substantially lower than those observed in the reaction of 1257 MeV (E/A = 6.16 MeV) $^{204}$Hg + $^{198}$Pt. We compare our measurements with the predictions of the GRAZING-F, di-nuclear systems (DNS) and improved quantum molecular dynamics (ImQMD) models. For the observed isotopes of the elements Au, Hg, Tl, Pb and Bi, the measured values of the MNT cross sections are orders of magnitude larger than the predicted values. Furthermore, the various models predict the formation of nuclides near the N=126 shell, which are not observed.

nucl-ex↗

arXiv2020

First candidates for γ vibrational bands built on the [505]11/2- neutron orbital in odd-A Dy isotopes

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the 155Gd(α,2n)157Dy and 148Nd(12C, 5n)155Dy reactions at 25 and 65 MeV, respectively. We report high-K bands, which are conjectured to be the first candidates of a Kπ= 2+ γ vibrational band, built on the [505]11/2- neutron orbital, in both odd-A 155, 157Dy isotopes. The coupling of the first excited K=0+ states or the so-called \b{eta} vibrational bands at 661 and 676 keV in 154Dy and 156Dy to the [505]11/2- orbital, to produce a Kπ=11/2- band, was not observed in both 155Dy and 157Dy, respectively. The implication of these findings on the interpretation of the first excited 0+ states in the core nuclei 154Dy and 156Dy are also discussed.

nucl-ex↗

arXiv2020

Observation of $γ$-vibrations and alignments built on non-ground-state configurations in 156Dy

The exact nature of the lowest $K^π=2_γ^+$ rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the $γ$ degree of freedom perpendicular to the nuclear symmetry axis. Very few such $γ$-bands have been traced past the usual back-bending rotational alignments of high-j nucleons. We have investigated the structure of positive-parity bands in the N=90 nucleus 156Dy, using the 148Nd(12C,4n)156Dy reaction at 65 MeV, observing the resulting $γ$-ray transitions with the Gammasphere array. The even- and odd-spin members of the $K^π=2_γ^+$ $γ$-band are observed to 32+ and 31+ respectively. This rotational band faithfully tracks the ground-state configuration to the highest spins. The members of a possible $γ$-vibration built on the aligned yrast S-band are observed to spins 28+ and 27+. An even-spin positive-parity band, observed to spin 24+, is a candidate for an aligned S-band built on the seniority-zero configuration of the $0_2^+$ state at 676 keV. The crossing of this band with the $0_2^+$ band is at $\hbarω$= 0.28(1) MeV and is consistent with the configuration of the $0_2^+$ band not producing any blocking of the monopole pairing.

nucl-ex↗