Neutron Separation Threshold Research Articles

Understanding the relationship between electric dipole polarizability and photonuclear (-2) moment in odd-A actinide nuclei

The nuclear electric dipole (E1) polarizability (α E1) is mainly dominated by the dynamics of the giant dipole resonance (GDR). α E1 is proportional to the (-2) moment of the total photo nuclear cross-section (σ −2). This research investigates the relationship between α E1 and σ −2, along with the effects of the Pygmy Dipole Resonance (PDR) and GDR in odd-mass actinide nuclei. For the first time, α E1 and σ −2 values have been calculated using the Translational and Galilean Invariant Quasiparticle Phonon Nuclear Model (TGI-QPNM) approach for odd-A actinide nuclei. According to TGI-QPNM results, E1 dipole transitions in the GDR region significantly contribute to σ −2 due to the energy weighting factor. Below the neutron separation threshold, the PDR in neutron-rich nuclei shows a contribution of about 5% to σ −2 values. In this region, E1 polarizability can reach values of 20%–25%. The α E1 values indicate the presence of PDR in these nuclei. Additionally, the Adaptive Neuro-Fuzzy Inference System (ANFIS), a new machine learning method, has been performed to analyze the relationship between α E1 and σ −2. The ANFIS results have been compared with those from the TGI-QPNM and experimental data. The TGI-QPNM model achieves an R2 of 0.85–0.95, while the ANFIS model achieves an R2 of 0.99. Moreover, the study suggests that the ANFIS model, consistent with TGI-QPNM results, could be an effective tool for estimating σ −2 in odd-A actinide nuclei.

The electric dipole characteristics of well-deformed 171,173Yb

The electric dipole response of well-deformed 171,173Yb in the giant dipole resonance (GDR) and pygmy dipole resonance (PDR) range has been theoretically analyzed using the translation and Galileo invariant quasiparticle phonon nuclear model (TGI-QPNM). The TGI-QPNM consists of an axially symmetric Woods-Saxon Potential, monopole pairing, dipole-dipole residual interaction, and the restoration terms for broken translation and Galilean symmetries. Numerical calculations show the existence of considerable E1 excitations around the neutron separation threshold (S n ) in both isotopes. The TGI-QPNM results of the photoabsorption cross-section give a double-humped shape in both nuclei, consistent with the available experimental data. The integrated moments (σ 0, σ −1) and the centroid energies in the GDR region are also reproduced well.

Low-lying structure of 15C: Information on the N=8 shell gap

The low-lying structure of 15C has been investigated via the neutron-removal d(16C, t) reaction. The experiment was performed at GANIL using a secondary 16C beam produced by fragmentation in the LISE spectrometer at 17.2 MeV/nucleon with an intensity of 5 × 104 pps and 100% purity. The angle and energy of the light ejectile were detected by three MUST2 telescopes. The missing mass technique was used to reconstruct the excitation energy of 15C. In this spectrum, two bound states were observed (gs and the first excited state) and two unbound resonant states above the neutron separation threshold (S n = 1.218 MeV). From the differential cross sections, information on the angular momentum of the transferred nucleon and spectroscopic factors were deduced. The excitation energies and the deduced spectroscopic factors of the negative parity states placed above the neutron separation energy are an important measurement of the 2p-1h configurations in 15C. Our results show good agreement with shell-model calculations with the YSOX interaction and show a sensitivity to the N=8 shell gap.

Spin inhibition in γ -decay probabilities for states above Sn in Sm and Dy nuclei

When a compound nucleus is formed at an excitation energy above the neutron-separation threshold, it is assumed that its de-excitation will proceed via neutron emission. However, if the excited nucleus is in a high-spin state, but does not have enough excitation energy to conserve angular momentum by either photon emission after neutron emission or relative angular momentum carried off by the neutron, the nucleus will de-excite via $\ensuremath{\gamma}$-emission instead. This effect, the spin inhibition, provides an insight into the structure of the compound nuclei and can aid the understanding of the distribution of the populated spin states in a compound nuclear reaction. In this work, the effects of spin inhibition on the $\ensuremath{\gamma}$-decay probabilities from states in $^{146,147}\mathrm{Sm}$ and $^{160}\mathrm{Dy}$ are presented. For high-spin states above the ${S}_{n}$, spin inhibition is able to suppress neutron emission, and de-excitation via $\ensuremath{\gamma}$-ray emission is observed for states up to 3 MeV above the ${S}_{n}$.

Microscopic Structure of the Low-Energy Electric Dipole Response of ^{120}Sn.

The microscopic structure of the low-energy electric dipole response, commonly denoted as pygmy dipole resonance (PDR), was studied for ^{120}Sn in a ^{119}Sn(d,pγ)^{120}Sn experiment. Unprecedented access to the single-particle structure of excited 1^{-} states below and around the neutron-separation threshold was obtained by comparing experimental data to predictions from a novel theoretical approach. The novel approach combines detailed structure input from energy-density functional plus quasiparticle-phonon model theory with reaction theory to obtain a consistent description of both the structure and reaction aspects of the process. The presented results show that the understanding of one-particle-one-hole structures of the 1^{-} states in the PDR region is crucial to reliably predict properties of the PDR and its contribution to nucleosynthesis processes.

Photo response of Dy164

Background: Little data is available for the pygmy dipole resonance (PDR) in axially deformed nuclei. Photon-scattering experiments are complicated by high level densities in the PDR region and the small energy difference of transitions to the ground state and to excited states.Purpose: We report on an experimental study of the low-energy dipole strength distribution of the well-deformed nucleus $^{164}\mathrm{Dy}$ between 4.0--7.7 MeV.Methods: The low-lying photoresponse of $^{164}\mathrm{Dy}$ has been investigated using the method of nuclear resonance fluorescence using a quasimonochromatic linearly polarized $\ensuremath{\gamma}$-ray beam in the energy range of 4.0--7.7 MeV in steps of 0.2 MeV.Results: For excitation energies between 4 MeV and 5 MeV, sufficiently low level densities allow for the identification of individual states, including level energies, reduced transition widths and branching ratios. Energy-averaged mean decay branching ratios, mean population ratios and partial absorption cross sections were determined above 5 MeV up to the neutron-separation threshold at 7.7 MeV. A Lorentzian-shaped enhancement of the partial photo absorption cross section followed by decays back to the ground-state band is found at 6.10(5) MeV with a width of 0.77(23) MeV. A comparison with results from complementary measurements is performed using the framework of the statistical model.Conclusions: The experimental results for the mean population ratios deviate systematically from the statistical model simulation by 30(6)%. However, they are in agreement within one standard deviation of the simulation.

High-sensitivity investigation of low-lying dipole strengths in Sn120

Background: The term Pygmy Dipole Resonance (PDR) denotes an accumulation of electric dipole excitations below and around the neutron separation threshold. It may be important, e.g., for the nucleosynthesis of heavy nuclei or the symmetry energy in the Equation of State (EoS). For a deeper understanding of the PDR, systematic studies are essential.Purpose: The tin isotopic chain is a well-suited candidate to investigate the systematics of the PDR, and the ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}$) reactions on $^{112,116,120,124}\mathrm{Sn}$ have already been measured in experiments using bremsstrahlung. It was claimed that the extracted electric dipole transition strengths of these isotopes increase with increasing neutron-to-proton ratio with the exception of $^{120}\mathrm{Sn}$. Furthermore, previous results from elastic photon scattering experiments on $^{120}\mathrm{Sn}$ are in disagreement with corresponding $(p,{p}^{\ensuremath{'}})$ Coulomb excitation data. To examine this discrepancy an additional high-sensitivity bremsstrahlung experiment on $^{120}\mathrm{Sn}$ was performed.Method: The Nuclear Resonance Fluorescence (NRF) method is used, which is based on the scattering of real photons. The bremsstrahlung experiment presented in this work was performed with a maximum energy of ${E}_{\ensuremath{\gamma},\mathrm{max}}=9.5\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ at the $\ensuremath{\gamma}\mathrm{ELBE}$ facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Besides a state-to-state analysis, the quasicontinuum was investigated as well.Results: Above ${E}_{x}=4$ MeV 228 dipole transitions were clearly identified; 163 were observed for the first time. Assuming that all identified dipole transitions have electric dipole character the summed electric dipole strength equals $\ensuremath{\sum}B(E1)\ensuremath{\uparrow}=369(49)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{4pt}{0ex}}{e}^{2}{\text{fm}}^{2}$ [which amounts to 0.58(8)% of the Thomas-Reiche-Kuhn sum rule] for transitions from 4 MeV to ${S}_{n}=9.1$ MeV. This is an enhancement of a factor 2.3 compared to the previously published $^{120}\mathrm{Sn}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ results. This increase can be explained by the contribution of many weak, previously not included transitions in the state-to-state analysis. The photoabsorption cross sections deduced from the quasicontinuum analysis exceed those of the $(p,{p}^{\ensuremath{'}})$ experiment in average by about $50%$ between 5.9 and 8.7 MeV.Conclusion: The newly extracted summed $B(E1)$ value of the state-to-state analysis is larger than those of $^{112,116}\mathrm{Sn}$ and smaller than that of $^{124}\mathrm{Sn}$. The difference between the electric dipole transition strengths deduced from isolated peaks of the present ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}$) data and those from the inelastic proton scattering experiment above 6.3 MeV is still striking. The analysis of the photoabsorption cross section including the quasicontinuum of levels overcomes this problem and the results are in the order of magnitude of the $(p,{p}^{\ensuremath{'}})$ data and continue the $(\ensuremath{\gamma},n)$ cross sections at the neutron separation threshold.

Experimental M1 response of Ar40 as a benchmark for neutrino-nucleus scattering calculations

The excitation of atomic nuclei via magnetic dipole transitions is closely related to the inelastic neutral-current neutrino-nucleus (NC-$\ensuremath{\nu}\mathrm{A}$) scattering process due to the similarity of the transition operators. NC-$\ensuremath{\nu}\mathrm{A}$-scattering serves for the detection of supernova neutrinos and poses a significant source of background in modern liquid-argon based high-energy neutrino detection experiments. To enable tests of the reliability of predictions for neutrino-nucleus scattering, the magnetic dipole response of $^{40}\mathrm{Ar}$ below 7.7 MeV was characterized in a nuclear resonance fluorescence experiment using quasimonoenergetic $\ensuremath{\gamma}$-ray beams. The linear polarization of the beams allowed for assignments of electric or magnetic character to previously known dipole excitations. A total magnetic dipole strength of $0.{36}_{\ensuremath{-}0.05}^{+0.04}\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{N}^{2}$ was identified in the energy range of the present experiment. Combined with data from previous measurements, the full magnetic dipole strength of $^{40}\mathrm{Ar}$ below the neutron separation threshold was investigated. Due to the low background in the energy range within the bandwidth of the $\ensuremath{\gamma}$-ray beams, the previous sensitivity limit was improved. A large-scale nuclear shell model calculation in the $sd\text{\ensuremath{-}}fp$ space satisfactorily agrees with the data in terms of excitation energies and strengths of the observed ${1}^{+}$ states.

The concept of nuclear photon strength functions: A model-independent approach via ([formula omitted]) reactions

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.

Combining [formula omitted]-ray and particle spectroscopy with SONIC@HORUS

The particle spectrometer SONIC for particle-γ coincidence measurements was commissioned at the Institute for Nuclear Physics in Cologne, Germany. SONIC consists of up to 12 silicon ΔE-E telescopes with a total solid angle coverage of 9%, and will complement HORUS, a γ-ray spectrometer with 14 HPGe detectors. The combined setup SONIC@HORUS is used to investigate the γ-decay behaviour of low-spin states up to the neutron separation threshold excited by light-ion inelastic scattering and transfer reactions using beams provided by a 10 MV FN Tandem accelerator. The particle-γ coincidence method will be presented using data from a 92Mo(p,p’γ) experiment. In a 119Sn(d,X) experiment, excellent particle identification has been achieved because of the good energy resolution of the silicon detectors of approximately 20 keV. Due to the non-negligible momentum transfer in the reaction, a Doppler correction of the detected γ-ray energy has to be performed, using the additional information from measuring the ejectile energy and direction. The high sensitivity of the setup is demonstrated by the results from a 94Mo(p,p’γ) experiment, where small γ-decay branching ratios have been deduced.

Unexpected high-energy γ emission from decaying exotic nuclei

The N=52Ga83β decay was studied at ALTO. The radioactive 83Ga beam was produced through the ISOL photofission technique and collected on a movable tape for the measurement of γ-ray emission following β decay. While β-delayed neutron emission has been measured to be 56–85% of the decay path, in this experiment an unexpected high-energy 5–9 MeV γ-ray yield of 16(4)% was observed, coming from states several MeVs above the neutron separation threshold. This result is compared with cutting-edge QRPA calculations, which show that when neutrons deeply bound in the core of the nucleus decay into protons via a Gamow–Teller transition, they give rise to a dipolar oscillation of nuclear matter in the nucleus. This leads to large electromagnetic transition probabilities which can compete with neutron emission, thus affecting the β-decay path. This process is enhanced by an excess of neutrons on the nuclear surface and may thus be a common feature for very neutron-rich isotopes, challenging the present understanding of decay properties of exotic nuclei.

Continuum discretised BCS approach for weakly bound nuclei

The Bardeen–Cooper–Schrieffer (BCS) formalism is extended by including the single-particle continuum in order to analyse the evolution of pairing in an isotopic chain from stability up to the drip-line. We propose a continuum discretised generalised BCS based on single-particle pseudostates (PS). These PS are generated from the diagonalisation of the single-particle Hamiltonian within a transformed harmonic oscillator basis. The consistency of the results versus the size of the basis is studied. The method is applied to neutron rich oxygen and carbon isotopes and compared with similar previous works and available experimental data. We make use of the flexibility of the proposed model in order to study the evolution of the occupation of the low-energy continuum when the system becomes weakly bound. We find an increasing influence of the non-resonant continuum as long as the Fermi level approaches the neutron separation threshold.

Nuclear Resonance Fluorescence off 54Cr: The Onset of the Pygmy Dipole Resonance

Low-lying electric and magnetic dipole excitations (E1 and M1) below the neutron separation threshold, particularly the Pygmy Dipole Resonance (PDR), have drawn considerable attention in the last years. So far, mostly moderately heavy nuclei in the mass regions around A = 90 and A = 140 were examined with respect to the PDR. In the present work, the systematics of the PDR have been extended by measuring excitation strengths and parity quantum numbers of J = 1 states in lighter nuclei near A = 50 in order to gather information on the onset of the PDR. The nuclei 50,52,54Cr and 48,50Ti were examined via bremsstrahlung produced at the DArmstadt Superconducting electron Linear Accelerator (S-DALINAC) with photon energies up to 9.7 MeV with the method of nuclear resonance fluorescence. Numerous excited states were observed, many of which for the first time. The parity quantum numbers of these states have been determined at the High Intensity Gamma-ray Source (HIγS) of the Triangle Universities Nuclear Laboratory in Durham, NC, USA. Informations to the methods and the experimental setups will be provided and the results on 54Cr achieved will be discussed with respect to the onset of the PDR.

First evidence of low energy enhancement in Ge isotopes

The γ-strength functions and level densities of 73,74 Ge have been extracted from particle-γ coincidence data using the Oslo method. In addition the γ-strength function of 74 Ge above the neutron separation threshold, Sn = 10.196 MeV has been extracted from photoneutron measurements. When combined, these two experiments give a γ-strength function covering the energy range of ∼1-13 MeV for 74 Ge. This thorough investigation of 74 Ge is a part of an international campaign to study the previously reported low energy enhancement in this mass region in the γ-strength function from ∼3 MeV towards lower γ energies. The obtained data show that both 73,74 Ge display an increase in strength at low γ energies.

Photoneutron cross section measurements on Sm isotopes

The Extreme Light Infrastructure - Nuclear Physics, one of the three pillars of the Extreme Light Infrastructure Pan-European initiative, is a new large scale facility dedicated to nuclear physics with extreme electromagnetic fields. ELI-NP will host two 10 PW lasers and a very brilliant Gamma beam system with unprecedented intensity and energy resolution parameters. We propose to perform photon induced nuclear reactions using the very brilliant γ -ray beams provided by the Gamma beam system to examine in detail the photon absorption process and its decay modes. Here the experimental program related to nuclear research on reactions above the neutron separation threshold, which is under preparation at ELI-NP, is presented.

Dipole strength distributions from HIGS Experiments

A series of photon scattering experiments has been performed on the double-beta decay partners 76 Ge and 76 Se, in order to investigate their dipole response up to the neutron separation threshold. Gamma-ray beams from bremsstrahlung at the S-DALINAC and from Compton-backscattering at HIGS have been used to measure absolute cross sections and parities of dipole excited states, respectively. The HIGS data allows for indirect measurement of averaged branching ratios, which leads to significant corrections in the observed excitation cross sections. Results are compared to statistical calculations, to test photon strength functions and the Axel-Brink hypothesis.

Study of the pygmy dipole resonance in 94Mo using the coincidence technique

Abstract The ( α , α ′ γ ) reaction at E α = 136 MeV was used to study the electric dipole response in the open-shell vibrational nucleus 94Mo below the neutron-separation threshold. The coincidence experiment has been performed at the Kernfysisch Versneller Instituut in Groningen, The Netherlands, exploiting the Big-Bite Spectrometer and an array of large volume High-Purity Germanium (HPGe) detectors. Due to the excellent energy resolution and high selectivity to transitions stemming from the pygmy dipole resonance, singles α-scattering cross sections could be determined for individual electric dipole excitations between 4 and 8 MeV. For three of the excited low-lying J π = 1 − states in 94Mo a γ-decay branch into the J π = 2 1 + state could be observed. The experiment extends the systematic studies of the pygmy dipole resonance by real-photon scattering ( γ , γ ′ ) experiments and ( α , α ′ γ ) experiments. Recently, a ( γ , γ ′ ) experiment on 94Mo was performed at the Darmstadt High-Intensity Photon Setup at the S-DALINAC in Darmstadt, Germany, permitting the comparison of B ( E 1 ) ↑ strength distribution and α-scattering cross sections.

Dipole response of neutron-rich Sn isotopes

The neutron-rich isotopes 129–133Sn were studied in a Coulomb excitation experiment at about 500 AMeV using the FRS-LAND setup at GSI. From the exclusive measurement of all projectile-like particles following the excitation and decay of the projectile in a high-Z target, the energy differential cross section can be extracted. At these beam energies dipole transitions are dominating, and within the semi-classical approach the Coulomb excitation cross sections can be transformed into photoabsorption cross sections. In contrast to stable Sn nuclei, a substantial fraction of dipole strength is observed at energies below the giant dipole resonance (GDR). For 130Sn and 132Sn this strength is located in a peak-like structure around 10 MeV excitation energy and exhibits a few percent of the Thomas-Reiche Kuhn (TRK) sum-rule strength. Several calculations predict the appearance of dipole strength at low excitation energies in neutron-rich nuclei. This low-lying strength is often referred to as pygmy dipole resonance (PDR) and, in a macroscopic picture, is discussed in terms of a collective oscillation of excess neutrons versus the core nucleons. Moreover, a sharp rise is observed at the neutron separation threshold around 5 MeV for the odd isotopes. A possible contribution of 'threshold strength', which can be described within the direct-breakup model is discussed. The results for the neutron-rich Sn isotopes are confronted with results on stable nuclei investigated in experiments using real photons.

Spin flip transitions inC13andF19probed with the (π−, γ) reaction

Photon spectra from radiative capture of stopped negative pions captured by $^{13}\mathrm{C}$ and $^{19}\mathrm{F}$ nuclei have been measured with a high resolution pair spectrometer. In the bound region transitions have been observed to $^{13}\mathrm{B}$(g.s.), $^{13}\mathrm{B}$(3.5 MeV), $^{19}\mathrm{O}$(g.s.), $^{19}\mathrm{O}$(4.9 MeV), and $^{19}\mathrm{O}$(6.3 MeV). In addition, strong transitions to narrow states above the neutron separation threshold were observed in both cases. The results for $^{13}\mathrm{C}$ are in agreement with detailed shell model calculations of this process. Major strength is observed for states formed by coupling extra nucleons or holes to the spin-quadrupole ${d}_{\frac{5}{2}}$ and ${d}_{\frac{3}{2}}$, ${J}^{\ensuremath{\pi}}={2}^{\ensuremath{-}}$ states in $^{16}\mathrm{O}$. The $^{13}\mathrm{C}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})^{13}\mathrm{B}_{\mathrm{g}.\mathrm{s}.}$ branching ratio is shown to be consistent with beta decay and electroexcitation data, through a combined phenomenological analysis. The observed transition strengths are accounted for by matrix elements of the spin-density operators ${[\ensuremath{\sigma}\ifmmode\times\else\texttimes\fi{}{Y}_{0}]}^{1}{t}^{+}$ and ${[\ensuremath{\sigma}\ifmmode\times\else\texttimes\fi{}{Y}_{1}]}^{2}{t}^{+}$.NUCLEAR REACTIONS $^{13}\mathrm{C}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})$, $^{19}\mathrm{F}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})$; measured normalized $I({E}_{\ensuremath{\gamma}})$; deduced upper limit on $\ensuremath{\delta}(\frac{E2}{M1})$ for $^{13}\mathrm{C}$\ensuremath{\rightarrow}$^{13}\mathrm{B}$(g.s.) at $q=125$ MeV/c, systematics of SIGDR in $12\ensuremath{\le}A\ensuremath{\le}20$. Stopped ${\ensuremath{\pi}}^{\ensuremath{-}}$; pair spectrometer; 850 keV at 129.4 MeV.