Giant Electric Dipole Resonance Research Articles

Constraints on the dipole photon strength for the odd uranium isotopes

Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies. Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest. Methods: The γ-ray spectra from neutron-capture reactions on the U234, U236, and U238 nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV. Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths. Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section.3 MoreReceived 24 August 2021Accepted 31 January 2022DOI:https://doi.org/10.1103/PhysRevC.105.024618Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsModels & methods for nuclear reactionsNeutron physicsNuclear reactionsRadiative captureResonance reactionsPropertiesA ≥ 220TechniquesNuclear data analysis & compilationNuclear Physics

Charge-exchange dipole excitations in deformed nuclei

Background: The electric giant-dipole resonance (GDR) is the most established collective vibrational mode of excitation. A charge-exchange analog, however, has been poorly studied in comparison with the spin (magnetic) dipole resonance (SDR).Purpose: I investigate the role of deformation on the charge-exchange dipole excitations and explore the generic features as an isovector mode of excitation.Methods: The nuclear energy-density functional method is employed for calculating the response functions based on the Skyrme-Kohn-Sham-Bogoliubov method and the proton-neutron quasiparticle-random-phase approximation.Results: The deformation splitting into $K=0$ and $K=\ifmmode\pm\else\textpm\fi{}1$ components occurs in the charge-changing channels and is proportional to the magnitude of deformation as is well known for the GDR. For the SDR, however, a simple assertion based on geometry of a nucleus cannot be applied for explaining the vibrational frequencies of each $K$ component. A qualitative argument on the strength distributions for each component is given based on the non-energy-weighted sum rules taking nuclear deformation into account. The concentration of the electric dipole strengths in low energy and below the giant resonance is found in neutron-rich unstable nuclei.Conclusions: The deformation splitting occurs generically for the charge-exchange dipole excitations as in the neutral channel. The analog pygmy dipole resonance can emerge in deformed neutron-rich nuclei as well as in spherical systems.

Giant dipole resonance parameters from photoabsorption cross-sections * * Supported by the IAEA Coordinated Research Project F41032 (20466), the National Natural Science Foundation of China (U1432247, 11775013, 11305270, 11465005, U1630143), and the Science Challenge Project (TZ2018001) and the Key Laboratory fund key projects (6142A080201)

The structural effect is believed to have no influence on the decay properties of medium and heavy-mass nuclei at excitation energies above the pairing gap. These properties can be described by statistical properties using so-called photon strength functions for different multipolarities, and directly related to the photoabsorption cross-section ( ). is dominated by the electric giant dipole resonance at energy MeV. In this study, we construct two kinds of systematic giant dipole resonance parameters by fitting the experimental photoabsorption cross-sections. One is based on the microscopic relativistic quasiparticle random phase approximation approach, whereas the other is estimated by the phenomenological models within the Lorentzian representation. Both of them are demonstrated ot efficiently describe the experimental photoabsorption cross-sections available for medium to heavy-mass nuclei, and they can obtain more reliable predictions for the unknown nuclear system.

Resonances in odd-odd182Ta

Enhanced γ-decay on the tail of the giant electric dipole resonance, such as the scissors or pygmy resonances, can have significant impact on (n,γ) reaction rates. These rates are important input for modeling processes that take place in astrophysical environments and nuclear reactors. Recent results from the University of Oslo indicate the existence of a significant enhancement in the photon strength function for nuclei in the actinide region due to the scissors resonance. Further, the M1 strength distribution of the scissors resonances in rare earth nuclei has been studied extensively over the years. To investigate the evolution and persistence of the scissor resonance in other mass regions, an experiment was performed utilizing the NaI(Tl) γ-ray detector array (CACTUS) and silicon particle telescopes (SiRi) at the University of Oslo Cyclotron laboratory. Particle-γ coincidences from the 181 Ta(d,p) 182 Ta and 181 Ta(d,d') 181 Ta reactions were used to measure the nuclear level density and photon strength function of the well-deformed 181 Ta and 182 Ta systems, to investigate the existence of resonances below the neutron separation energy.

The Pygmy Dipole Resonance – status and new developments

At energies well below the isovector electric Giant Dipole Resonance, a concentration of electric dipole strength is observed in many stable and radioactive nuclei. This low-lying excitation mode is usually denoted as Pygmy Dipole Resonance (PDR). Different theoretical approaches can reproduce the gross features, like the energetic position and partly the excitation strength of the PDR, but they differ considerably in its underlying structural description. More sophisticated experiments looking for the isospin character, the detailed decay pattern, and the single-particle structure of the low-lying E1 excitations are therefore mandatory to gain a deeper understanding. This manuscript will give an overview about the most recent experimental results and present new experimental tools for the future.

Self-Consistent Calculations of the Electric Giant Dipole Resonances in Light and Heavy Nuclei

While bulk properties of stable nuclei are successfully reproduced by mean-field theories employing effective interactions, the dependence of the centroid energy of the electric giant dipole resonance on the nucleon number A is not. This problem is cured by considering many-particle correlations beyond mean-field theory, which we do within the quasiparticle time blocking approximation. The electric giant dipole resonances in 16O, 40Ca, and 208Pb are calculated using two new Skyrme interactions.

Mechanisms for the appearance of prescission γ-rays and their related T-odd asymmetries

Conditions for the appearance and observation of prescission γ-rays emitted by a fissioning nucleus before its separation into fission fragments were investigated within the quantum theory of fission. It was demonstrated that these conditions can be fulfilled in the γ-decay of giant electric isovector dipole resonances in a fissioning nucleus that become excited due to the nonadiabaticity of the collective deformation motion of the nucleus at the final stages of its prefission evolution. Angular and energy distributions of prescission γ-rays emitted by unpolarized fissioning nuclei were analyzed. Characteristics of T-odd asymmetries in angular distributions of prescission γ-rays were investigated for fission of unpolarized target nuclei induced by polarized cold neutrons, and these correlations were shown to be similar in nature to the T-odd ROT correlations earlier found for α-particles emitted in ternary nuclear fission.

Scission gamma rays

Gamma rays probably emitted by the fissioning nucleus 236U* at the instant of the break of the neck or within the time of about 10−21 s after or before this were discovered in the experiment devoted to searches for the effect of rotation of the fissioning nucleus in the process 235U(\( \vec n \), γf) and performed in a polarized beam of cold neutrons from the MEPHISTO Guideline at the FRM II Munich reactor. Detailed investigations revealed that the angular distribution of these gamma rays is compatible with the assumption of the dipole character of the radiation and that their energy spectrum differs substantially from the spectrum of prompt fission gamma rays. In the measured interval 250–600 keV, this spectrum can be described by an exponential function at the exponent value of α = −5 × 10−3 keV−1. The mechanism of radiation of such gamma rays is not known at the present time. Theoretical models based on the phenomenon of the electric giant dipole resonance in a strongly deformed fissioning nucleus or in a fission fragment predict harder radiation whose spectrum differs substantially from the spectrum measured in the present study.

Pygmy dipole resonance in nuclei

A brief survey of the state of the modern microscopic theory of the so-called pygmy dipole resonance in nuclei is given—in particular, some unresolved problems are listed. It is emphasized that, in order to explain the pygmy dipole resonance, it is necessary but not sufficient to take into account the coupling of single-particle degrees of freedom to photon degrees of freedom. The results of the calculations performed for the first time for the isovector pygmy dipole resonance and the isovector electric giant dipole resonance in 124Sn within a self-consistent approach involving, in addition to the standard quasiparticle random-phase approximation, a single-particle continuum and quasiparticle-phonon coupling of single-particle degrees of freedom to phonon degrees of freedom are presented. The results are found to be in satisfactory agreement with experimental data. The calculation of the isoscalar strength function in the energy region of the pygmy dipole resonance revealed that the nuclear-structure mechanism does not provide the isoscalar-strength suppression observed at energies in excess of 7 MeV in (α, α′γ) reactions; therefore, this suppression may stem from the reaction mechanism.

Structure inE1strength distribution built on the 15.1 MeVT=1state inC12

The measurement of the giant electric dipole resonance built on the isobaric analog state in the self-conjugate even-even nucleus $^{12}\mathrm{C}$ is reported covering a wide excitation energy range. This is done using the $^{11}\mathrm{B}$($p,\ensuremath{\gamma}$) reaction leading to the 15.1 MeV $T=1$ state in $^{12}\mathrm{C}$ over a proton energy range of 7 to 24.5 MeV. Two resonances of 1.3 and 2 MeV widths are observed at excitation energies of 24.4 and 28.8 MeV superimposed on a smoothly varying cross section. The \ensuremath{\gamma} transitions from both the resonances are assigned $E1$ multipolarity. The structured $T=0$ component of the $E1$ strength distribution is thus very different from that built on the first $T=1$ state in the neighboring nucleus $^{14}\mathrm{N}$ and those built on the ground states of $^{12}\mathrm{C}$ and $^{14}\mathrm{C}$.

What do we really know about photon strength functions?

It is believed that decay properties of medium-weight and heavy nuclei at excitation energies above the pairing gap are not governed by structural effects but more likely by statistical properties that can be described via so-called photon strength functions (PSFs) for different multipolarities. These quantities are directly related to photoabsorption cross section (σγ). It is well known that PSFs/σγ are dominated by giant electric dipole resonance (GEDR) with the maximum at about 15 MeV and width of about 5 MeV. On the other hand, shapes of PSFs at the low-energy tail of GEDR are rather poorly known. Available information on PSFs for γ-ray energies below about 10 MeV obtained from different experiments will be combined and discussed within existing models of PSFs. A special emphasis will also be devoted to possible resonance structures at low energies.

Evaluation of Neutron Capture Gamma-ray Spectra for 89Y, 93Nb, 127I, 133Cs, 141Pr, 197Au, natTl, and 209Bi

The neutron capture gamma-ray spectra for 8 nuclides, 89Y, 93Nb, 127I, 133Cs, 141Pr, 197Au, natTl, and 209Bi, were calculated by using the Hauser-Feshbach statistical model, and their results were compared with the available experimental data. Two dominant ingredients to perform the statistical calculation were the level densities described by the Gilbert-Cameron approach with an improved systematics, and the gamma-ray transmission coefficients described by gamma-ray strength functions. Although various gamma-ray strength functions with a Lorentzian formula have been developed by using the photonuclear data or a microscopic analysis, they have failed to reproduce the occasional anomalous bumps observed near or below a neutron binding. In this work, we could reproduce the bumps well by adding a Lorentzian with an energy-temperature dependent width into a giant electric dipole resonance with an enhanced generalized Lorentzian. In addition, we introduced a correction function so as to compensate for the shortcomings due to missing levels or level-cuts.

Effect of nuclear deformation on the electric-dipole strength in the particle-emission threshold region

The role of nuclear deformation in the photoabsorption cross section in the tail region of the electric giant dipole resonance (GDR) is studied in terms of a deformed oscillator model and a Nilsson-plus-random-phase-approximation model. It is found within the framework of these approaches that extra electric dipole strength is generated at energies below the GDR maximum if the nuclear system is spatially deformed. This is important in the prediction of stellar photodisintegration rates, knowing that an extra strength can affect these rates even below the particle separation energies through the so-called \ensuremath{\gamma} process. Because the nuclear deformation is governed by shell effects, this extra strength does not directly correlate with the neutron excess.

Compilation of giant electric dipole resonances built on excited states

Giant Electric Dipole Resonance (GDR) parameters for γ decay to excited states with finite spin and temperature are compiled. Over 100 original works have been reviewed and from some 70 of them, about 350 sets of hot GDR parameters for different isotopes, excitation energies, and spin regions have been extracted. All parameter sets have been brought onto a common footing by calculating the equivalent Lorentzian parameters. The current compilation is complementary to an earlier compilation by Samuel S. Dietrich and Barry L. Berman (At. Data Nucl. Data Tables 38 (1988) 199–338) on ground-state photo-neutron and photo-absorption cross sections and their Lorentzian parameters. A comparison of the two may help shed light on the evolution of GDR parameters with temperature and spin. The present compilation is current as of July 2006.

Systematics of hot giant electric dipole resonance widths

Giant Electric Dipole Resonance (GDR) parameters for γ decay to excited states with finite spin and temperature have been compiled by two of the authors ( nucl-ex/0605004 ). Over 100 original works have been reviewed and from some 70 of them, more than 300 sets of hot GDR parameters for different isotopes, excitation energies, and spin regions have been extracted. All parameter sets have been brought onto a common footing by calculating the equivalent Lorentzian parameters. Together with a complementary compilation by Samuel S. Dietrich and Barry L. Berman [At. Data Nucl. Data Tables 38 , 199-338, (1988)] on ground-state photo-neutron and photo-absorption cross sections and their Lorentzian parameters, it is now possible by means of a comparison of the two data sets to shed light on the evolution of GDR parameters with temperature and spin.

Microcanonical entropies and radiative strength functions ofV50,51

The level densities and radiative strength functions (RSFs) of $^{50,51}$V have been extracted using the ($^3$He,$\alpha \gamma$) and ($^3$He,$^3$He$^{\prime} \gamma$) reactions, respectively. From the level densities, microcanonical entropies are deduced. The high $\gamma$-energy part of the RSF is described by the giant electric dipole resonance. A significant enhancement over the predicted strength in the region of $E_{\gamma} \lesssim 3$ MeV is seen, which at present has no theoretical explanation.

Radiative strength functions inMo93−98

Radiative strength functions (RSFs) in $^{93\ensuremath{-}98}\mathrm{Mo}$ have been extracted using the $(^{3}\mathrm{He},\ensuremath{\alpha}\ensuremath{\gamma})$ and $(^{3}\mathrm{He},{^{3}\mathrm{He}}^{'}\ensuremath{\gamma})$ reactions. The RSFs are U shaped as function of \ensuremath{\gamma} energy with a minimum at around ${E}_{\ensuremath{\gamma}}=3 \mathrm{MeV}$. The minimum values increase with neutron number because of the increase in the low-energy tail of the giant electric dipole resonance with nuclear deformation. The unexpected strong increase in strength below ${E}_{\ensuremath{\gamma}}=3 \mathrm{MeV}$, here called soft pole, is established for all $^{93\ensuremath{-}98}\mathrm{Mo}$ isotopes. The soft pole is present at all initial excitation energies in the $5\ensuremath{-}8\text{\ensuremath{-}}\mathrm{MeV}$ region.

Thermal properties and radiative strengths inDy160,161,162

The level densities and radiative strength functions (RSFs) in 160,161Dy have been extracted using the (3He,alpha gamma) and (3He,3He' gamma) reactions, respectively. The data are compared to previous measurements on 161,162Dy. The energy distribution in the canonical ensemble is discussed with respect to the nucleon Cooper pair breaking process. The gross properties of the RSF are described by the giant electric dipole resonance. The RSF at low gamma-ray energies is discussed with respect to temperature dependency. Resonance parameters of a soft dipole resonance at Egamma = 3 MeV are deduced.

Evaluation of Photonuclear Reaction Data on Tantalum-181 up to 140MeV

Photonuclear cross sections of 181Ta are evaluated up to an incident photon energy of 140MeV which is the threshold energy for pion production. Re-analyses were performed on the (γ, n) and (γ, 2n) data measured at Saclay and Livermore, and reference data were reconstructed. The absorption cross sections were evaluated with the giant electric dipole resonance (GDR) model below 40MeV. The calculation by the statistical model with preequilibrium correction reproduced the reference data of all the photoneutron cross sections consistently. From 40 to 140 MeV, the quasideuteron model (QDM) was adopted to evaluate photoabsorption cross sections and the results are compared with the measurements. The decaying processes including n, p, d, t, 3He, and α particle emission up to 140MeV were theoretically evaluated by the ALICE-F code.

Radiative neutron captures by neutron-rich nuclei and the r-process nucleosynthesis

The radiative neutron capture by neutron-rich nuclei is estimated with an improved description of the electric giant dipole resonance. In addition, 3 major effects affecting the capture rates by exotic neutron-rich nuclei are studied. These concern the existence of a low-energy E1 pygmy resonance, the overestimate of the statistical predictions for resonance-deficient nuclei and the direct capture mechanism. The total ( n, γ) reaction rates including these 3 effects are evaluated for 3100 neutron-rich nuclei and used in parametric r-process calculations to analyze their impact on the r-abundance distribution.