Isomeric State Research Articles

Manipulation of nuclear isomers with lasers: mechanisms and prospects

Over one hundred years have passed since the nuclear isomer was first introduced, in analogy with chemical isomers to describe long-lived excited nuclear states. In 1921, Otto Hahn discovered the first nuclear isomer $^{234m}$Pa. After that, step by step, it was realized that different types of nuclear isomers exist, including spin isomer, K isomer, seniority isomers, and ``shape and fission'' isomer. The spin isomer occurs when the spin change $\Delta I$ of a transition is very large. The larger $\Delta I$, the lower the electromagnetic transition rates, the longer the half-lives. The K-isomer exists due to the significant change in K, where K is the projection of the total angular momentum on the symmetry axis. The seniority isomers arise due to a very small transition probability in seniority conserving transitions around semi-magic nuclei, where the seniority, which corresponds to the number of unpaired nucleons, is a reasonably pure quantum number. For a so-called shape isomer, the inhibition of the decay transition comes from the associated shape changes. It is caused by that a nucleus is trapped in a deformed shape which is its secondary minimum and is hard to decay back to its ground state.

Experimental study of the isomeric state in N16 using the Ng,m16(d,He3) reaction

The isomeric state of $^{16}\mathrm{N}$ was studied using the $^{16}\mathrm{N}{}^{g,m}(d,^{3}\mathrm{He})$ proton-removal reactions at 11.8 MeV/u in inverse kinematics. The $^{16}\mathrm{N}$ beam, of which 24% was in the isomeric state, was produced using the Argonne Tandem-Linac Accelerator System (ATLAS) in-flight system and delivered to the Helical Orbit Spectrometer (HELIOS), which was used to analyze the $^{3}\mathrm{He}$ ions from the $(d,^{3}\mathrm{He})$ reactions. The simultaneous measurement of reactions on both the ground state and the isomeric states, reduced the systematic uncertainties from the experiment and in the analysis. A direct and reliable extraction of the relative spectroscopic factors was made based on a distorted-wave Born approximation approach. The experimental results suggest that the isomeric state of $^{16}\mathrm{N}$ is an excited neutron-halo state. The results can be understood through calculations using a Woods-Saxon potential model, which captures the effects of weak binding.

Probing ^{93m}Mo Isomer Depletion with an Isomer Beam.

The isomer depletion of ^{93m}Mo was recently reported [Chiara etal., Nature (London) 554, 216 (2018)NATUAS0028-083610.1038/nature25483] as the first direct observation of nuclear excitation by electron capture (NEEC). However, the measured excitation probability of 1.0(3)% is far beyond the theoretical expectation. In order to understand the inconsistency between theory and experiment, we produce the ^{93m}Mo nuclei using the ^{12}C(^{86}Kr,5n) reaction at a beam energy of 559MeV and transport the reaction residues to a detection station far away from the target area employing a secondary beam line. The isomer depletion is expected to occur during the slowdown process of the ions in the stopping material. In such a low γ-ray background environment, the signature of isomer depletion is not observed, and an upper limit of 2×10^{-5} is estimated for the excitation probability. This is consistent with the theoretical expectation. Our findings shed doubt on the previously reported NEEC phenomenon and highlight the necessity and feasibility of further experimental investigations for reexamining the isomer depletion under low γ-ray background.

Photoswitchable Fluorescent Proteins: Mechanisms on Ultrafast Timescales.

The advancement of super-resolution imaging (SRI) relies on fluorescent proteins with novel photochromic properties. Using light, the reversibly switchable fluorescent proteins (RSFPs) can be converted between bright and dark states for many photocycles and their emergence has inspired the invention of advanced SRI techniques. The general photoswitching mechanism involves the chromophore cis-trans isomerization and proton transfer for negative and positive RSFPs and hydration–dehydration for decoupled RSFPs. However, a detailed understanding of these processes on ultrafast timescales (femtosecond to millisecond) is lacking, which fundamentally hinders the further development of RSFPs. In this review, we summarize the current progress of utilizing various ultrafast electronic and vibrational spectroscopies, and time-resolved crystallography in investigating the on/off photoswitching pathways of RSFPs. We show that significant insights have been gained for some well-studied proteins, but the real-time “action” details regarding the bidirectional cis-trans isomerization, proton transfer, and intermediate states remain unclear for most systems, and many other relevant proteins have not been studied yet. We expect this review to lay the foundation and inspire more ultrafast studies on existing and future engineered RSFPs. The gained mechanistic insights will accelerate the rational development of RSFPs with enhanced two-way switching rate and efficiency, better photostability, higher brightness, and redder emission colors.

Predissociation spectroscopy of cold CN−H2 and CN−D2

The vibrational predissociation spectra of the CN–H2 and CN–D2 complexes are measured in regions between 450 and 3100 cm in an ion trap at different temperatures at the free electron lasers for infrared experiments (FELIX) Laboratory. Strong differences between the vibrational spectra of the two para and ortho nuclear spin isomers (CN–(p-H2) or CN–(o-H2) and CN–(p-D2) or CN–(o-D2)) are detected. In the case of CN–H2 we could assign the observed bands with the help of an accurate quantum calculation. The spectrum is dominated by the first overtone of the hindered H2 rotation in combination with the hindered CN rotation of the CN–(o-H2) nuclear spin isomer. The energetically higher-lying CN–(p-H2) isomer was not observed due to efficient ortho-para ligand exchange. The C–N stretching mode was recorded at 2040(1) cm. For the CN–D2 system several vibrational modes of both spin isomers overlap in their low-lying intermonomer streching and bending region, but the main observed bands are attributed to the more stable CN–(p-D2) spin isomer. The D–D stretching vibration was found to be at 2898(1) cm.

Applications of the dynamical generator coordinate method to quadrupole excitations

We apply the dynamical generator coordinate method (DGCM) with a conjugate momentum to a nuclear collective excitation. To this end, we first discuss how to construct a numerically workable scheme of the DGCM for a general one-body operator. We then apply the DGCM to the quadrupole vibration of $^{16}$O using the Gogny D1S interaction. We show that both the ground state energy and the excitation energies are lowered as compared to the conventional GCM with the same number of basis functions. We also compute the sum rule values for the quadrupole and monopole operators, and show that the DGCM yields more consistent results than the conventional GCM to the values from the double commutator. These results imply that the conjugate momentum is an important and relevant degree of freedom in collective motions.

Alternative Fuel: Hydrogen and its Thermodynamic Behaviour

Hydrogen is a contender for alternative energy. Hydrogen fuel cell vehicles and hydrogen-based low-carbon fuels will contribute to the decarburization of the mobility sector, shipping and aviation. Hydrogen is used as a rocket fuel. In addition, petroleum refining, semiconductor manufacturing, aerospace industry, fertilizer production, metal treatment, pharmaceutical, power plant generator, methanol production, commercial fixation of nitrogen from air reduction of metallic ores. Also, hydrogen is used to turn unsaturated fats into saturated fats and oils. In the enhancement of NMR and MRI signals, parahydrogen is used. Parahydrogen and orthohydrogen are nuclear spin isomers of hydrogen. At room temperature, the normal hydrogen at thermal equilibrium consists of 75% orthohydrogen and 25% parahydrogen. The development of hydrogen technology requires knowledge of the thermophysical properties of hydrogen. The second virial coefficient characterizes the primary interaction between the molecules. Therefore, knowledge of the second virial coefficient enables one to determine the pairwise molecular interaction and, in turn, the thermodynamic behaviour of hydrogen. The present study is based on three parameter modified Berthelot Equation of state aims to determine the second virial coefficient of hydrogen and its isomers, i.e., orthohydrogen and parahydrogen, over a wide range of temperatures, from the boiling point to the Boyle point. The obtained results are compared with those of the van der Waals Equation of state, Berthelot Equation of state, Tsonopoulos correlation, McGlashan & Potter correlation, Yuan Duan correlation, Van Ness & Abbott correlation, and McGlashancorrelation. The results of this work agree well with those of other correlations in the high temperature region. Doi: 10.28991/HEF-2022-03-02-05 Full Text: PDF

Nuclear Excitation by Electron Capture in Excited Ions.

A nuclear excitation following the capture of an electron in an empty orbital has been recently observed for the first time. So far, the evaluation of the cross section of the process has been carried out widely using the assumption that the ion is in its electronic ground state prior to the capture. We show that by lifting this restriction new capture channels emerge resulting in a boost of more than three orders of magnitude to the electron capture resonance strength.

Role of Dielectric Screening in Calculating Excited States of Solvated Azobenzene: A Benchmark Study Comparing Quantum Embedding and Polarizable Continuum Model for Representing the Solvent.

The low energy excited states of the conformational isomers of solvated azobenzene are calculated with several DFT methods accounting for the solute-solvent interaction implicitly with the polarizable continuum model or explicitly with subsystem DFT. For the latter, embedding potentials are calculated for 21 sampled snapshots of the solvent molecules. First, we find that accounting for the solvent implicitly or explicitly has little effect on the predicted cis-trans S1 excitation energy gap. Second, we find that azobenzene's S1 cis and trans energies are accurate as long as a screened range-separated hybrid exchange-correlation functional is employed. Finally, we also tested a simplified workflow whereby a single, averaged, embedding potential is used. Unfortunately, we find larger deviations against the experiment for the simplified workflow. This highlights a basic flaw in the approach, where the time scale of solvent averaging is much longer than that of the solute's electronic polarization.

The CD8α hinge is intrinsically disordered with a dynamic exchange that includes proline cis-trans isomerization

T cells engineered to express artificial chimeric antigen receptors (CARs) that selectively target tumor-specific antigens or deleterious cell types offer transformative therapeutic possibilities. CARs contain an N-terminal extracellular antigen recognition domain, C-terminal intracellular signal transduction domains, and connecting hinge and transmembrane regions, each of which have been varied to optimize targeting and minimize toxicity. We find that a CD22-targeting CAR harboring a CD8α hinge (H) exhibits greater cytotoxicity against a low antigen density CD22+ leukemia as compared to an equivalent CAR with a CD28 H. We therefore studied the biophysical and dynamic properties of the CD8α H by nuclear magnetic resonance (NMR) spectroscopy. We find that a large region of the CD8α H undergoes dynamic chemical exchange between distinct and observable states. This exchanging region contains proline residues dispersed throughout the sequence that undergo cis-trans isomerization. Up to four signals of differing intensity are observed, with the most abundantly populated being intrinsically disordered and with all prolines in the trans isomerization state. The lesser populated states all contain cis prolines and evidence of local structural motifs. Altogether, our data suggest that the CD8α H lacks long-range structural order but has local structural motifs that transiently exchange with a dominant disordered state. We propose that structural plasticity and local structural motifs promoted by cis proline states within the CD8α H are important for relaying and amplifying antigen-binding effects to the transmembrane and signal transduction domains.

Effect of nuclear deformation on the observation of a low-energy super-Gamow-Teller state

A well-known structure with concentrated Gamow-Teller (GT) transitions is the Gamow-Teller resonance, which has been observed at higher-energy regions (usually $>6$ MeV) of nuclear excitation. It has been found that the GT strength can also concentrate in the lowest ${J}^{\ensuremath{\pi}}={1}^{+}$ GT state named the ``low-energy super-GT (LeSGT) state'' when the initial even-even nucleus has the structure of ``LS-closed-shell core nucleus $+$ 2 neutrons (or 2 protons)'' and the final nucleus ``LS-closed-shell core nucleus $+$ 1 proton and 1 neutron.'' Such concentrations are realized with the core nuclei $^{4}\mathrm{He}$, $^{16}\mathrm{O}$, and $^{40}\mathrm{Ca}$, corresponding to the shell closures of $s$, $p$, and $sd$ shells, respectively. It is natural to speculate that the LeSGT state may also be observed in the $A=82$ systems if the $N=Z=40$ shell gap is significant and $^{80}\mathrm{Zr}$ represents a good core nucleus corresponding to the $pf$ shell closure. Possible conditions that allow the formation of the LeSGT state in the $^{82}\mathrm{Zr}\ensuremath{\rightarrow}^{82}\mathrm{Nb}$ charge-exchange reaction (or $^{82}\mathrm{Mo}\ensuremath{\rightarrow}^{82}\mathrm{Nb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay) are discussed by evaluating the results of projected shell model calculations, which are based on deformed model space. Our calculations show that with increasing deformation, the LeSGT feature found in the spherical limit (zero deformation) evolves gradually into a broad distribution in the higher-energy region. This lets us conclude that no LeSGT state is expected in $^{82}\mathrm{Nb}$ because the shape of the $^{80}\mathrm{Zr}$ core nucleus is ellipsoidal.

Study of nuclear low-lying excitation spectra with the Bayesian neural network approach

Nuclear low-lying excitation spectra are studied with the Bayesian neural network (BNN) approach by taking 02+, 21+, and 41+ states as examples. The BNN approach can well describe the low-lying excitation energies in a large energy scale from about 0.1 MeV to about several MeV, by including an input related to nuclear collectivity besides proton and neutron numbers and employing the logarithm of excitation energy as the output. Comparing with the sophisticated microscopic collective Hamiltonian model, the BNN approach significantly improves the description of nuclear low-lying excitation energies, which can generally reproduce the experimental data within about 1.12 times including those of transitional nuclei and magic nuclei. Taking Mg, Ca, Kr, Sm, and Pb isotopes as examples, it is found that the BNN approach well describes the isotopic trend of low-lying excitation energies, including those abrupt increases at magic numbers due to the shell effect, very low excitation energies of 02+ states due to the shape coexistence, and complex nuclear shape evolution due to the shape phase transition.

Combined Coulombic and magnetic contributions to dispersion in e + C12 collisions

Dispersion corrections to elastic electron scattering from a $^{12}\mathrm{C}$ nucleus are calculated within the second-order Born approximation. Three strong transient nuclear excitations are considered, at 4.439 MeV $({2}^{+})$, 17.7 MeV $({1}^{\ensuremath{-}}),$ and 23.5 MeV $({1}^{\ensuremath{-}})$. The Friar-Rosen-type Coulomb term is supplemented with the transverse part of the photon propagator. It is found that the dispersive cross-section changes in the first diffraction minimum reach nearly 10%, rising more slowly with collision energy beyond 400 MeV. Magnetic scattering can be strong for the individual excited states and lowers the dispersion effect considerably at energies below 400 MeV. However, it contributes at most 10% at the higher energies investigated.

Experimental study of the [formula omitted]Cl beam production at intermediate energies

The isomeric content of a 34Cl beam produced in the intermediate-energy projectile fragmentation of a 150 MeV/u 36Ar beam on a 3 mm-thick Be target was studied. β-delayed γ-ray spectroscopy was used to measure the population of 34Cl fragments in the ground vs. isomeric states at zero degrees relative to the incoming primary beam for four different momentum settings of the fragment separator near the predicted central velocity of these fragments, as well as, at two non-zero-degree settings for one momentum setting. Of the settings explored, which excluded rigidities within 0.5% of the value predicted to maximize total 34Cl yield due to unreacted primary beam, the maximum rate for the production of 34mCl was found at a rigidity setting 0.75% below the predicted peak 34Cl yield. The maximum population of the isomeric state relative to the ground state was observed at a rigidity 1.25% below the predicted maximum 34Cl yield. Studies such as this are important in generating the understanding needed for producing isomer-enriched rare-isotope beams.

P-process chaser detector in [formula omitted] - [formula omitted] coincidences

We propose two types of neutron-γ1−γ2 triple coincidence detectors (not constructed) to chase gamma transitions to produce p-nuclei following the neutron emission in the (γ,n) reaction. Neutrons are detected with 24 3He counters embedded in a polyethylene moderator in Type I detector and with 6 liquid scintillation detectors in Type II detector, respectively. γ rays are detected with two high-purity germanium detectors and four LaBr3(Ce) detectors. The detector which is referred to as p-process chaser detector is used to search for mediating states in 180Ta through which the isomeric and ground states in 180Ta are thermalized in the p-process. A search is made for both resonant states and unresolved states in high nuclear-level-density domain.

First coupling of the FRS particle identification and the FRS-Ion Catcher data acquisition systems: The case of 109In

For the first time, the FRagment Separator (FRS) and the Multiple-Reflection Time-Of-Flight Mass-Spectrometer (MR-TOF-MS) particle identification (PID) systems at GSI have been coupled. This new approach adds to the standard FRS PID an additional unambiguous identification of the fragments and the possibility to identify and count long-lived isomeric states (>ms). For this purpose, single-event timestamp information given by a common clock was used to correlate both systems. Two methods were implemented to improve the signal-to-background ratio by more than a factor 2 in the high resolution mass spectrum obtained with the MR-TOF-MS for the 109In isotope. Moreover, the coupling of the systems allows an improvement in the on-line monitoring of the FRS-Ion Catcher (IC) efficiency and extraction time. In addition, range calculations were implemented in the on-line monitoring; a powerful tool for real-time optimization of stopped beam experiments.

Isospin dependence of nuclear level density at A≈ 120 mass region

Nuclear level density and the level density parameter of 115,119,127Te isotopes are experimentally determined by analyzing the backward angle neutron energy spectra from 4He+112,116,124Sn reactions. Measurements are done in the compound nuclear excitation energy range of 25-42 MeV. Statistical model analysis is performed to test different phenomenological prescriptions of the level density parameter. Our data efficiently scan the explicit dependences of level density on three key factors: nuclear deformation, neutron-proton asymmetry, and the separation from the most stable isobar. It is observed that the experimental data are best described when deviation in atomic number from the corresponding β-stable isobar is explicitly taken into account. Further, nuclear level densities determined from the measured spectra show a reduction for 115,127Te in comparison to that of 119Te, which lies closer to the β-stability line.

Dynamical Control of Nuclear Isomer Depletion via Electron Vortex Beams.

Some nuclear isomers are known to store a large amount of energy over long periods of time, with a very high energy-to-mass ratio. Here, we describe a protocol to achieve the external control of the isomeric nuclear decay by using electron vortex beams whose wave function has been especially designed and reshaped on demand. Recombination of these electrons into the isomer's atomic shell can lead to the controlled release of the stored nuclear energy. On the example of ^{93m}Mo, we show theoretically that the use of tailored electron vortex beams increases the depletion by 4 orders of magnitude compared to the spontaneous nuclear decay of the isomer. Furthermore, specific orbitals can sustain an enhancement of the recombination cross section for vortex electron beams by as much as 6 orders of magnitude, providing a handle for manipulating the capture mechanism. These findings open new prospects for controlling the interplay between atomic and nuclear degrees of freedom, with potential energy-related and high-energy radiation source applications.

Mapping the N=40 island of inversion: Precision mass measurements of neutron-rich Fe isotopes

Nuclear properties across the chart of nuclides are key to improving and validating our understanding of the strong interaction in nuclear physics. We present high-precision mass measurements of neutron-rich Fe isotopes performed at the TITAN facility. The multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS), achieving a resolving power greater than 600000 for the first time, enabled the measurement of Fe63–70, including first-time high-precision direct measurements (δm/m≈10−7) of Fe68–70, as well as the discovery of a long-lived isomeric state in Fe69. These measurements are accompanied by both mean-field and ab initio calculations using the most recent realizations which enable theoretical assignment of the spin-parities of the Fe69 ground and isomeric states. Together with mean-field calculations of quadrupole deformation parameters for the Fe isotope chain, these results benchmark a maximum of deformation in the N=40 island of inversion in Fe and shed light on trends in level densities indicated in the newly refined mass surface.Received 19 July 2021Revised 24 December 2021Accepted 16 March 2022DOI:https://doi.org/10.1103/PhysRevC.105.L041301©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBinding energy & massesNuclear structure & decaysRare & new isotopesProperties59 ≤ A ≤ 89TechniquesAb initio calculationsMean field theoryTime-of-flight mass spectrometryNuclear Physics

Excitation functions of proton-induced nuclear reactions on ^{86}Sr, with particular emphasis on the formation of isomeric states in ^{86}Y and ^{85}Y

Cross sections of proton-induced nuclear reactions on enriched ^{mathrm {86}}Sr target were measured by the activation technique up to proton energies of 44 MeV. The isomeric cross-section ratios for ^{mathrm {86m,g}}Y and ^{mathrm {85m,g}}Y as a function of projectile energy were deduced from their measured data. The present experimental data for the nuclear reaction products, namely ^{mathrm {86m}}Y, ^{mathrm {86g+xm}}Y, ^{mathrm {85m}}Y, ^{mathrm {85g}}Y, ^{mathrm {84}}Rb and ^{mathrm {83}}Rb were compared with the results of nuclear model calculations using the code TALYS, which combines the statistical, precompound, and direct interactions. In general, the experimental cross-section data as well as the isomeric cross-section ratios are reproduced well by the model calculations, provided the input model parameters are properly chosen and the level structure of the product nucleus is thoughtfully considered. The quality of the agreement between experimental data and model calculations was numerically quantified. For products formed via emission of a light complex particle as well as multi-nucleons (e.g., alpha and 2p2n), the contribution of the latter process starts increasing when its energy threshold is crossed.