Excited Nuclear States Research Articles

A Constraint-Based Orbital-Optimized Excited State Method (COOX).

In this work, we present a novel method to directly calculate targeted electronic excited states within a self-consistent field calculation based on constrained density functional theory (cDFT). The constraint is constructed from the static occupied-occupied and virtual-virtual parts of the excited state difference density from (simplified) linear-response time-dependent density functional theory calculations (LR-TDDFT). Our new method shows a stable convergence behavior, provides an accurate excited state density adhering to the Aufbau principle, and can be solved within a restricted SCF for singlet excitations to avoid spin contamination. This also allows the straightforward application of post-SCF electron-correlation methods like MP2 or direct RPA methods. We present the details of our constraint-based orbital-optimized excited state method (COOX) and compare it to similar schemes. The accuracy of excitation energies will be analyzed for a benchmark of systems, while the quality of the resulting excited state densities is investigated by evaluating excited state nuclear forces and excited state structure optimizations. We also investigate the performance of the proposed COOX method for long-range charge transfer excitations and conical intersections with the ground-state.

Constraining the axial-vector X17 interpretation with 12C data

Recent findings of an unexpected, narrow resonance in the e+e− decay spectra of excited states of 8Be, 4He and 12C by the ATOMKI collaboration have received considerable experimental and theoretical attention, whereby a new, 17-MeV vector-like or axial-vector-like boson termed X17 was conjectured as an explanation of the anomaly. Further analysis of all existing constraints disfavors a vector X17 scenario. For a similar analysis of the axial-vector scenario, a calculation of the reduced matrix element of a spin-dipole operator between the excited nuclear state 12C(17.23) and the carbon ground state is required. In the present work, we compute the aforementioned reduced matrix element under the assumption that the state 12C(17.23) is well represented by the 2s1/21p3/2−1 particle-hole shell-model excitation of the ground state, as supported by experimental data. Within such a framework, our results indicate that, like the vector scenario, the axial-vector interpretation of X17 shows strong tensions with the other existing constraints on the nucleon coupling of a conjectured X17.

Spin-flip-induced quadrupole resonance in odd- A exotic atoms

We examine the possible existence of quadrupole resonances in exotic atoms containing odd-A nuclei. We find that the spin flip of the deexciting exotic particle can induce a resonance, altering the orbital angular momentum of the nucleus by one quantum. This process results in an excited nucleus with a suppression in x-ray photon emissions during the exotic atom cascade. We provide specific cases of antiprotonic atoms of stable elements where this resonance effect is expected to occur. The study of this phenomena may provide insight into the strong interaction of deeply bound antiprotonic states in the proximity of the unpaired nucleon, and serve as a tool for probing short-lived excited nuclear states. Published by the American Physical Society 2024

Lifetime measurements in the yrast band of 187Hg

Lifetimes of excited nuclear states in 187Hg have been measured using the 159Tb (32S, p3n)187Hg reaction at a beam energy of 154 MeV at the Inter University Accelerator Center, Delhi. In the experiment, the lifetimes of the five lowest yrast states have been determined using the recoil distance Doppler-shift (RDM) method. The extracted transition quadrupole moments show a sharp increase with increasing level spin, indicating a major shape transition happening in this nucleus. For a better insight into the predicted shape coexistence picture in 187Hg nucleus, the experimental results are compared with the results of the total Routhian surface calculations. The results indicate the shifting of deformed structures from weakly prolate at low-spins to strongly prolate at high spins in 187Hg nucleus.

Loading and identifying various charged thorium ions in a linear ion trap with a time-of-flight mass spectrometer

Various charged thorium ions such as singly charged, doubly charged, and triply charged thorium ions trapped in the ion trap can be used to excite the Th-229 first nuclear excited state via the electronic bridge process. We present an integration of a linear ion trap with a time-of-flight mass spectrometer to investigate trapped Th-232 ions. Various charged thorium ions are produced by laser ablation and dynamically loaded into the ion trap. After sufficient collisional cooling, thorium ions are extracted along one of the radial directions for time-of-flight mass spectrometry by rapidly quenching the trapping potential and applying high-voltage extracting pulses. The charge states of thorium ions are identified and the maximum mass resolutions of thorium ions reach ∼100 with initial 300 K collisional cooling. The velocity distributions of ablated various charged thorium ions are measured, and the results agree well with Monte Carlo simulation. Lifetimes of thorium ions are determined to be a few tens of seconds in the ion trap, which are helpful for further spectroscopic studies of Th-229 nuclear transition.

Optical transmission enhancement of ionic crystals via superionic fluoride transfer: Growing VUV-transparent radioactive crystals

The 8−eV first nuclear excited state in Th229 is a candidate for implementing a nuclear clock. Doping Th229 into ionic crystals such as CaF2 is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of Th229 prohibits the growth of high-quality single crystals with high Th229 concentration; radiolysis causes fluoride loss, increasing absorption at 8eV. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at 8eV by annealing Th229-doped CaF2 at 1250∘C in CF4. This technique allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants, which can be used in deep-UV optics, laser crystals, scintillators, and nuclear clocks. Published by the American Physical Society 2024

The measurement of the E2 nuclear resonance effects in kaonic atoms at DAΦNE: The KAMEO proposal

KAMEO (Kaonic Atoms Measuring Nuclear Resonance Effects Ob-servables) is a proposal for an experiment aiming to perform the first consistent measurement of the E2 nuclear resonance effects in kaonic molybdenum A=94,96,98,100 isotopes. The E2 nuclear resonance mixes atomic states, due to the electrical quadrupole excitation of nuclear rotational states. It occurs in atoms having the energy of a nuclear excitation state closely matching an atomic de-excitation state energy, and affects the rates of X-ray atomic transitions matching the energy of the resonance. The measurement E2 nuclear resonance effect in KMO isotopes allows the study of the strong kaon-nucleus interaction in a rotational excited nuclear state. Moreover, the effect enables the K- to access an inner atomic level not easily reachable by the kaon normal cascade, due to the nuclear absorption. The KAMEO proposed apparatus consists of 4 enriched Mo A=94,96,98,100 isotope strips, exposed to the kaons produced by the DAφNE collider, for kaonic atoms formation, with a high-purity germanium detector, cooled with liquid nitrogen, used to measure the X-ray atomic transitions. The DAφNE collider is located at the National Laboratories of Frascati (LNF-INFN), in Italy. It is already suited for kaonic atoms measurement by the SIDDHARTA-2 collaboration.

Fabrication and characterization of thin 94Zr target with gold backing for nuclear lifetime measurements using DSAM

For lifetime measurements of excited nuclear states in the sub-picosecond range, a thin target with a thick backing of high-Z material is required. To meet these specifications, a thin isotopic94Zr target of thickness 1.14 ± 0.05 mg/cm2, with backing of 4.7 ± 0.2 mg/cm2 thick Au layer, has been fabricated using the cost-effective cold rolling technique at the target laboratory of Inter-University Accelerator Centre (IUAC), New Delhi. The thickness verification and purity check of the fabricated target have been carried out by non-destructive Energy Dispersive X-ray Fluorescence (EDXRF) and Rutherford Backscattering Spectrometry (RBS) techniques. The results of these measurements are in good agreement with each other. The elemental composition, purity and surface morphology have been explored by Energy Dispersive X-ray Spectrometry (EDS) and Scanning Electron Microscopy (SEM) techniques. The fabricated target has been successfully used with a16O ion beam in a recent γ-ray spectroscopy experiment for lifetime measurements using the Doppler Shift Attenuation Method (DSAM) at IUAC, New Delhi.

The Contributions to Registration Efficiency of The Fast Neutron Reactions on The Nuclei of The Heavy Oxide Scintillators

The results of the study of the contributions of the interaction reactions of fast neutron sources of 239Pu-Be and 252Cf to the counting efficiency of registration by oxide scintillators CdWO4, ZnWO4, Bi4Ge3O12 and Gd2SiO5, presented. The amount of gamma quanta per input neutron emitted from final nuclei excited in the reactions of inelastic scattering (n, nʹγ)in, resonant scattering (n, n)res and capture (n, γ)res and radiation capture (n, γ)cap was measured. PMT R1307 operating in single-electron mode was used as a photodetector, the background rate was ~ 5*103 s-1. The measured efficiency ε for scintillators ø40x40 mm was 752 for ZWO, 532 for CWO, 37 for GSO, and 23 for BGO in "counts/neutron" units, measurement error rate ~ 3-5%. The formation of the detector response is influenced by the parameters of the scintillator nuclei, such as the values of the interaction cross sections in the resonance region, the density of nuclear levels of the final nuclei, the lifetime of excited nuclear states, the upper limit of the resonance region of the cross section, as well as the scintillation time and geometric parameters of the scintillators. A phenomenological model of the response of an oxide scintillator to fast neutrons is proposed.

The study of the X17 anomaly with the PADME experiment

In dark matter studies, the absence of experimental evidences other than the astrophysical observations, has triggered new approaches. Nowadays, many particle physics experiments at accelerators are trying to contribute by looking for signals of hidden particles postulated by different theoretical extensions of the Standard Model. This results in a vaste hunting for new particles with a wide range of properties with the intention to also give reason to other unexplained particle physics phenomena. Within this scenario is inserted the Positron Annihilation into Dark Matter Experiment (PADME) ongoing at the Laboratori Nazionali di Frascati of INFN that is looking for signals of hidden particles by studying the annihilations of a positron beam with the electrons of a fixed target. PADME had, up to now, two data taking periods devoted to the search of a dark photon signal, but its setup turned out to be also suited to explore the existence of a protophobic new boson of mass 17 MeV/c 2 postulated to explain an anomalous effect observed by a Hungarian group while studying nuclear excited states. In this paper it is reported an overview of the PADME experiment and of the modifications implemented to allow a dedicated data taking at 282 MeV beam energy, meant to produce the new particle at resonance.

New fast-timing [formula omitted]-ray detection system in Korea: KHALA

KHALA, the acronym for Korea high-resolution array of LaBr3(Ce), has been developed to measure the lifetimes of the nuclear excited states. This system consists of 36 cylindrical LaBr3(Ce) scintillator crystals with a diameter and length of 1.5 inches each. It is suitable for measuring γ-ray emission time from a few tens of picoseconds to a few tens of nanoseconds. The data acquisition electronics of KHALA are fully digitized for both energy and timing information, resulting in the quick data processing. Detailed performance specifications, such as the energy resolution, timing response, and photopeak detection efficiency, of KHALA are presented.

Nuclear Spin Relaxation in Cold Atom-Molecule Collisions.

We explore the quantum dynamics of nuclear spin relaxation in cold collisions of 1Σ+ molecules with structureless atoms in an external magnetic field. To this end, we develop a rigorous coupled-channel methodology, which accounts for rotational and nuclear spin degrees of freedom of 1Σ+ molecules and their interaction with an external magnetic field as well as anisotropic atom-molecule interactions. We apply the methodology to study the collisional relaxation of the nuclear spin sublevels of 13CO molecules immersed in a cold buffer gas of 4He atoms. We find that nuclear spin relaxation in the ground rotational manifold (N = 0) of 13CO occurs extremely slowly due to the absence of direct couplings between the nuclear spin sublevels. The rates of collisional transitions between the rotationally excited (N = 1) nuclear spin states of 13CO are generally much higher due to the direct nuclear spin-rotation coupling between the states. These transitions obey selection rules, which depend on the values of space-fixed projections of rotational and nuclear spin angular momenta (MN and MI) for the initial and final molecular states. For some initial states, we also observe a strong magnetic field dependence, which can be understood by using the first Born approximation. We use our calculated nuclear spin relaxation rates to investigate the thermalization of a single nuclear spin state of 13CO(N = 0) immersed in a cold buffer gas of 4He. The calculated nuclear spin relaxation times (T1 ≃ 1 s at T = 1 K at a He density of 10-14 cm-3) display a steep temperature dependence decreasing rapidly at elevated temperatures due to the increased population of rotationally excited states, which undergo nuclear spin relaxation at a much faster rate. Thus, long relaxation times of N = 0 nuclear spin states in cold collisions with buffer gas atoms can be maintained only at sufficiently low temperatures (kBT ≪ 2Be), where Be is the rotational constant.

Growth and characterization of thorium-doped calcium fluoride single crystals

We have grown ^{232}Th:CaF_2 and ^{229}Th:CaF_2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of ^{229}Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of ^{229}Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the vertical gradient freeze method on 3.2 mm diameter seed single crystals with a 2 mm drilled pocket, filled with a co-precipitated CaF_2:ThF_4:PbF_2 powder in order to grow single crystals. Concentrations of 4cdot 10^{19} cm^{-3} have been realized with ^{232}Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of ^{229}Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the ^{229}Th concentration to <5times 10^{17} cm^{-3}.

Scheme for the excitation of thorium-229 nuclei based on electronic bridge excitation

Thorium-229 possesses the lowest first nuclear excited state, with an energy of approximately 8 eV. The extremely narrow linewidth of the first nuclear excited state, with an uncertainty of 53 THz, prevents direct laser excitation and realization of the nuclear clock. We present a proposal using the Coulomb crystal of a linear chain formed by ^{229}Th^{3+} ions, where the nuclei of ^{229}Th^{3+} ions in the ion trap are excited by the electronic bridge (EB) process. The 7P_{1/2} state of the thorium-229 nuclear ground state is chosen for EB excitation. Using the two-level optical Bloch equation under experimental conditions, we calculate that 2 out of 36 prepared thorium ions in the Coulomb crystal can be excited to the first nuclear excited state, and it takes approximately 2 h to scan over an uncertainty of 0.22 eV. Taking advantage of the transition enhancement of EB and the long stability of the Coulomb crystal, the energy uncertainty of the first excited state can be limited to the order of 1 GHz.

Nuclear Equation of State from Nuclear Collective Excited State Properties

Nuclear Equation of State from Nuclear Collective Excited State Properties

Status of the X17 search in Montreal

At the Montreal Tandem accelerator, an experiment is being set up to measure internal pair creation from the decay of nuclear excited states using a multiwire proportional chamber and scintillator bars surrounding it from the Daphne experiment. The acceptance covers a solid angle of nearly 4π. Preamplifiers and the data acquisition hardware have been designed and tested. The 7LiF target, mounted on an Al foil and water-cooled is in a thin carbon fiber section of the beamline. The experiment will focus at first on a measurement of the internal pair creation from the 18.15 MeV state of 8Be with the aim of observing independently the X17 particle discovered by the ATOMKI experiment. Assuming the ATOMKI evaluation of the electron-pair production rate from X17, Geant4 simulation predicts observation of a clear signal after about 2 weeks of data taking with 2 μA proton beam. The IPC measurement could eventually be extended to the giant dipole resonance of 8Be, as well as to other nuclei, in particular to 10B.

Precise Q -value measurements of Ag112,113 and Cd115 with the Canadian Penning trap for evaluation of potential ultralow Q -value β decays

An ultra-low Q value $\beta$-decay can occur from a parent nuclide to an excited state in the daughter with $Q_{UL}$ <1 keV. These decays are of interest for nuclear $\beta$-decay theory and as potential candidates in neutrino mass determination experiments. To date, only one ultra-low Q value $\beta$-decay has been observed -- that of $^{115}$In with $Q_\beta$ = 147(10) eV. A number of other potential candidates exist, but improved mass measurements are necessary to determine if the decays are energetically allowed and, in fact, ultra-low. We performed precise $\beta$-decay Q value measurements of $^{112,113}$Ag and $^{115}$Cd and combined them with nuclear energy level data for the daughter isotopes to determine if the potential UL Q value $\beta$-decay branches of $^{112,113}$Ag and $^{115}$Cd are energetically allowed and <1 keV. The Canadian Penning Trap at ANL was used to measure the cyclotron frequency ratios of singly-charged $^{112,113}$Ag and $^{115}$Cd ions with respect to their daughters. From these measurements, the ground-state $\beta$-decay Q values were obtained. The $^{112}$Ag, $^{113}$Ag, and $^{115}$Cd $\beta$-decay Q values were measured to be 3990.16(22) keV, 2085.7(4.6) keV, and 1451.36(34) keV, respectively. These results were compared to energies of excited states in $^{112}$Cd at 3997.75(14) keV, $^{113}$Cd at 2015.6(2.5) and 2080(10) keV, and $^{115}$In at 1448.787(9) keV, resulting in $Q_{\textrm{UL}}$ values of --7.59(26) keV, 6(11) keV, and 2.57(34) keV, respectively. The potential UL Q value decays of $^{112}$Ag and $^{115}$Cd have been ruled out. $^{113}$Ag is still a possible candidate until a more precise measurement of the 2080(10) keV, 1/2$^{+}$ state of $^{113}$Cd is available. In the course of this work we have found the ground state mass of $^{113}$Ag reported in the 2020AME to be lower than our measurement by 69(17) keV (a 4$\sigma$ discrepancy).

Lasers for the observation of multiple order nuclear reactions

Nuclear reaction rates become nonlinear with respect to flux (cm−2s−1) in extreme environments such as those found during stellar nucleosynthesis and terrestrial nuclear detonations. To observe these effects directly in the laboratory, extremely high particle fluences (cm−2) are necessary but not sufficient. Reactor-based neutron sources, such as the Institut Laue-Langevin’s high-flux neutron reactor, were previously the closest to meeting this challenge, albeit over ∼hour time scales. In ultra-high flux environments, where multiple reactions occur on picosecond time scales, nuclei are unable to return to their ground states between reactions; consequently, reactions take place on excited nuclei. To accurately model high-flux environments, data on the cross-sections of excited nuclear states are required, which differ significantly from those of ground states due to spin/parity effects. In order to replicate these effects in the laboratory, short high-fluence pulses on the order of the lifetime of a typical nuclear excited state (generally ≲1 ns) are required. Particle beams generated by high-intensity lasers are uniquely positioned to meet this need with the potential to produce fluences of 1017 protons/cm2 and 1022 neutrons/cm2 over a few pico-seconds or less. In addition to providing a quantitative analysis of the rates of multiple rapid reactions in general, the present work examines a number of laser-based experiments that could be conducted in the near future to observe multiple rapid reactions for laboratory-based astrophysics and the measurement of exotic cross-sections.

Timing characteristics of the R13089-100 photomultiplier tube coupled with 1.5" × 1.5" CeBr3 scintillators and its application in lifetime measurements

The fast-timing performance of the Hamamatsu Photomultiplier Tube (PMT) model R13089-100 is investigated using 1.5" × 1.5" CeBr3 scintillators. The time walk was measured using the Mirror-Symmetric Centroid Difference (MSCD) method for various PMT Bias voltages and CFD delays, to optimize the parameters for measurement of the lifetime of nuclear excited states via the γ-γ fast-timing technique. The 152Eu and 133Ba sources were used for investigation on timing characteristics. Lifetimes of 161 and 383 keV states of 133Cs from the decay of 133Ba radioactive source have been determined for different bias voltages of PMT and CFD delays, with two 1.5" × 1.5" CeBr3 detector setup. The lifetime of 1522 keV state of 209Po from the decay of 209At, produced from α induced fusion evaporation reaction, has also been determined. The results reported in the present paper show that the time resolution and time-walk characteristics of the Hamamatsu PMT R13089-100 can be improved in low-energy region using higher PMT bias voltage and shorter CFD delay.

Prospects for Neutron Reactions on Excited States in High-Density Plasmas

With the reactions of high flux neutrons, such as in a DT plasma, there is a prospect of seeing new kinds of neutron-nucleus reactions for the first time. If neutrons excite a heavy nucleus, for example, there is possibility of a second neutron reacting on excited states of the residual nucleus before that nucleus has de-excited to its ground state. The possibility of such reactions on excited states has rarely been considered. The cross section for neutron induced fission on the isomeric state of 235U has been measured (D’Eer et al., Phys. Rev. C, 1988, 38: 1270–1276) and calculated (Younes et al., 2003, Maslov, 2007), and reactions on rotationally excited nuclear states has been calculated (Kawano et al., Phys. Rev. C, 2009, 80: 024611). In high flux plasmas, however, a much wider range of reactions is possible. We therefore need to consider excited states at much higher-energies than previously modeled, and then estimate whether second neutrons are likely to rescatter on those excited states. To determine the likelihood of such rescattering events, we first need to know the probable time series of nuclear decays of those excited states. The lifetimes of many low-lying states have been measured experimentally, but now we need to know the lifetimes of the many higher excited states that could be produced from incident 14 MeV neutrons. These are too numerous to be measured and also too numerous to be calculated individually, so statistical Hauser-Feshbach decay models are used. I show some lifetime calculations for 89Y, 169Tm, and 197Au targets, and predictions for the number of rescattering events in plausible plasma scenarios.