Giant Resonance Region Research Articles

Electric dipole polarizability of low-lying excited states in atomic nuclei

New equations for the electric dipole polarizability α E1 of low-lying excited states in atomic nuclei—and the related (−2) moment of the total photo-absorption cross section, σ −2—are inferred in terms of electric dipole and quadrupole matrix elements. These equations are valid for arbitrary angular momenta of the initial/ground and final/excited states and have been exploited in fully converged 1ℏ ω shell-model (SM) calculations of selected p- and sd-shell nuclei that consider configuration mixing; advancing previous knowledge from 17O to 36Ar, where thousands of electric dipole matrix elements are computed from isovector excitations which include the giant dipole resonance (GDR) region. Our results are in reasonable agreement with previous SM calculations and follow—except for 6,7Li and 17,18O—Migdal’s global trend provided by the combination of the hydrodynamic model and second-order non-degenerate perturbation theory. Discrepancies in 6,7Li and 17O arise as a result of the presence of α-cluster configurations in odd-mass nuclei, whereas the disagreement in 18O comes from the mixing of intruder states, which is lacking in the SM interactions. More advanced ab initio calculations of the dipole polarizability for low-lying excited states covering all the isovector states within the GDR region are missing and could be very valuable to benchmark the results presented here and shed further light on how atomic nuclei polarize away from the ground state

Research on natAg(γ,xn)103-106Ag reaction cross sections with the bremsstrahlung end-point energy of 70 and 75 MeV

The flux-weighted average cross sections of natAg(γ,xn)103-106Ag reactions were measured with the bremsstrahlung end-point energy of 70 and 75 MeV using an activation and off-line γ-ray spectrometric technique. The GEANT4 code was used for the simulation of the bremsstrahlung spectra. The cross sections of natAg(γ,xn) 103-106Ag reactions as a function of mono-energetic photons were theoretically calculated by using the TALYS-1.96 code. The data of the present work and literature at different bremsstrahlung end-point energy are found to be in agreement with the theoretical values within the uncertainty limit. It can be seen that the flux-weighted average cross sections of the natAg(γ,xn)105.106Ag reactions increase with bremsstrahlung energy up to the Giant Dipole Resonance region. Thereafter, a small decrease trend with the bremsstrahlung energy was seen due to the opening of another reaction channel.

Theoretical consideration of isomeric ratios in some photonuclear reactions induced by bremsstrahlung with endpoint energy in giant dipole resonance region using Talys 1.95 code

Isomeric ratios of isomeric pairs produced by photonuclear reactions (γ, n) on Se, Ce, Eu and Hg targets induced by bremsstrahlung with endpoint energies in the giant dipole resonance region have been theoretically calculated using TALYS 1.95 code in combination with Geant4 simulation. The computed isomeric ratios as a function of the bremsstrahlung endpoint energies in the range of 10 to 25 MeV resulted from convolution between calculated differential cross-sections using six level density models available in TALYS 1.95 and the bremsstrahlung spectra simulated by the GEANT4 toolkit. Moreover, for each level density model, eight gamma strength functions have been employed. The calculated results are compared to the experimental data in the existing literature for Talys 1.95 model evaluation.

Investigation of level density and Gama strength function for photoneutron reaction in medical linacs in Beamline

Photoneutron reaction cross-sections of 197Au, 187Re, 186W, 181Ta, 94,95,96,97,98,100Mo isotopes were calculated through the TALYS 1.95 nuclear reaction code. The energy range of the incident photon chosen as 7 MeV–30 MeV corresponded to the range of the giant dipole resonance region, which is also an applicable energy range in radiotherapy for many commercial medical linear accelerators. Calculations were performed using three phenomenological level density models available in code, namely the Constant Temperature Fermi Gas Model, the Back-shifted Fermi Gas Model, and the Generalized Superfluid Model. The most convenient level density model for each reaction was chosen using relative variance calculations. The cross-section calculations were repeated using gamma strength function models, Kopecky-Uhl generalized Lorentzian model, Brink-Axel Lorentzian model, and Goriely's hybrid model and the best level density model was kept constant. The calculated data and the experimental data from the international Experimental Nuclear Reaction Data Library were analysed and compared graphically.

Twenty years operating of Flerov nuclear reaction laboratory microtron MT-17 in Vietnam

Mictrorons are accelerators of electrons and simultaneous sources of bremsstrahlung photon flux and fission neutrons. In 1982, a microtron of seventeen trajectories Microtron MT - 17 of FLNR put into operation at the National Institute of Physics of Vietnam. Thouf very modest, raicrotrons are very useful for developing countries such i Vietnam in both fundamental and applied physics research During the recei twenty years by using the above mentioned Microtron MT - 17 at microtrons from other institutes we have carried out different investigation In this report we present some results obtained in the studies of photonucle. reactions and photon activation analysis in the giant dipole resonance region

Energy transitions and half-life determinations of 47Sc isotopes from photonuclear reaction on Ti target

For the activation of the target nucleus, bremsstrahlung photons generated in a medical linear accelerator can be used. Due to particle separation energy limitations for low photon energies in the giant resonance region ([Formula: see text]18[Formula: see text]MeV), the reaction exit channels are restricted to the ejection of one or two particles only. Thus, the gamma spectrum is relatively clear, and the use of ancillary detectors coupled to HPGe is not highly needed. In this study, the bremsstrahlung photons have been used for the activation of the titanium target. [Formula: see text]Sc isotopes are observed from the natural titanium target via photo-proton reactions. Decay radiations from the isotopes and half-life of [Formula: see text]Sc which is from the most abundant natural titanium have been determined consistent with the literature values.

Implementation of a Geant4-based code using low-energy gamma-nuclear final state model for photonuclear studies

In this work, we developed a Geant4-based code, IS-2020, for photonuclear studies below 200 MeV covering the giant dipole resonance region (GDR). It uses G4LowEGammaNuclearModel, which consists of preequilibrium and equilibrium reaction mechanisms, for sampling the final states of photonuclear reactions. Simulations for photonuclear reactions on 96Mo, 90Zr, and the strongly-deformed nucleus 153Eu targets have been performed for its validation. Moreover, a good agreement was also observed between experimental relative yields and the simulation results of 59Co(γ,2n)57Co and 50Cr(γ,2n)48Cr reactions induced by bremsstrahlung. The implemented code will be a useful and reliable tool for photonuclear studies, including designing the experimental setup, estimating the yield of outgoing products, and interpreting the measured data.

Updated neutron-multiplicity sorting method for producing photoneutron average energies and resolving multiple firing events

Photoneutron measurements in the Giant Dipole Resonance region have been recently performed using quasimonochromatic Laser Compton Scattering γ-ray beams at NewSUBARU (Japan) for 59Co, 89Y, 103Rh, 139La, 159Tb, 165Ho, 169Tm, 181Ta, 197Au and 209Bi. Experimental (γ,xn) with x = 1–4 photoneutron cross sections have been obtained with a direct neutron-multiplicity sorting method associated to a flat-efficiency neutron detector, which operates under single firing conditions obtained by low reaction rates. In this paper, we discuss recent developments to the neutron-multiplicity sorting procedure dedicated to (i) resolving multiple firing events and to (ii) extracting the average energies of (γ,xn) photoneutrons. The method is tested on 159Tb experimental data.

Experimental studies on photoabsorption by endohedral fullerene ions with a focus on Xe@C60 + confinement resonances

A brief overview on the status of experimental investigations into photoionization and fragmentation of fullerenes and endohedral fullerenes by absorption of a single short-wavelength photon is presented. The focus of this paper is on the endohedral Xe@C60 + molecular ion in which a xenon atom is centrally encapsulated inside a fullerene cage. Confinement resonances that result from the interference of Xe photoelectron matter waves emerging from the cavity were studied by exposing Xe@C60 + ions to synchrotron radiation of 60 to 150 eV energy which is the region of the well-known giant atomic Xe excitation resonance. Photoions Xe@C n q+ (with final charge states of the product ions and numbers of carbon atoms left in the cage) were recorded as a function of photon energy and cross sections for the individual reaction channels were determined. In addition to previous work, new final channels ( and ) were observed, and thus, about 70% of the oscillator strength expected for the encapsulated Xe atom in the investigated energy range could be recovered. The present results establish the first and, so far, only conclusive experimental observation of confinement resonances in an endohedral fullerene. The data are in remarkable agreement with relativistic R-matrix calculations that accompanied the previous experimental work.

Theoretical investigation of giant dipole resonance in 146−152Nd isotopes with QRPA

We investigate the electric dipole ([Formula: see text]) structure properties of the deformed [Formula: see text]Nd nuclei in the giant dipole resonance (GDR) region within the framework of the quasiparticle random-phase approximation (QRPA). Translational and Galilean invariance (TGI) QRPA with separable isovector dipole–dipole residual interaction have been employed for the calculations. We have computed the photoabsorption cross-section and then we have compared with the experimental data. Our calculations revealed that while the photoabsorption cross-section shows a Lorentzian line in the neighborhood of spherical geometry, it starts to shift to an asymmetric shape by increasing deformation in [Formula: see text]Nd isotopes by increasing neutron number. In addition to this, we have also observed that the splitting of the [Formula: see text] strength distribution and the separation between [Formula: see text] and [Formula: see text] branches are increasing. We have calculated the contribution of the electric and magnetic parts of total dipole strength up to 20[Formula: see text]MeV for the nuclei of interest. This calculation shows that the electric part dominates the total dipole strength and [Formula: see text] excitation dominates the electric part.

Theoretical calculation by Talys code in combination with Geant4 simulation for consideration of [formula omitted] reactions of Eu isotopes in the giant dipole resonance region

In this work, the differential cross-sections of 151Eu(γ,n)150Eu and 153Eu(γ,n)152Eu photonuclear reactions in the giant dipole resonance region (GDR) were considered by theoretical models available in Talys 1.8 code. Isomeric ratios of these two reactions induced by bremsstrahlung with end-point energies in (GDR) region were then simulated using the calculated cross-section in combination with Monte-Carlo simulation using Geant4 toolkit based code which was implemented to simulate bremsstrahlung and photonuclear reaction. Moreover, a simple model was added into Geant4 for populating multi-isomeric states. The theoretical results were compared with the experimental data from the existing literature to examine the validity of the theoretical modes.

Photonuclear reactions by relativistic electron channeling radiation

The research for newly developing branch of nuclear physics, the nuclear photonics, has been accompanied since the beginning with the studies on intense MeV photon sources. One of the possible solution is in the use of channeling radiation. The channeling radiation spectrum for sub-GeV–several GeV electrons is characterized by a sharp maximum at photon energies up to several MeV, which is enough to excite separate nuclear levels as well as (γ,n) reaction for light Be and D nuclei. This maximum may even reach the region of giant dipole resonance for heavier nuclei. At equal radiator thickness the channeling radiation flux may exceed in more than one order that of bremsstrahlung. Thus, channeling radiation can be efficiently utilized in studying photonuclear reactions as well as generating pulsed neutron beams at sub-GeV electron accelerators. The latter is illustrated by detailed calculations of the neutrons yield from the light D and Be targets irradiated by channeling radiation. The non-trivial dependence of neutrons yield on the energy of incident electron beam and on the electron beam alignment with respect to the crystal channeling planes is revealed.

Isoscalar E0, E1 , and E2 strength in Fe54 and Zn64,68

The giant resonance region from $10\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}l{E}_{x}l62\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ in $^{54}\mathrm{Fe}, ^{64}\mathrm{Zn}$, and $^{68}\mathrm{Zn}$ has been studied with inelastic scattering of 240-MeV $\ensuremath{\alpha}$ particles at small angles, including ${0}^{\ensuremath{\circ}}$. Between 70 and $105%$ of the expected isoscalar $E0$ strength has been identified below ${E}_{x}=40\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ for each of the nuclei. Between 71 and $108%$ of the $E1$ strength has been identified while $60%$ of $E2$ strength in $^{54}\mathrm{Fe}$ and $^{68}\mathrm{Zn}$ and $120%$ of the strength in $^{64}\mathrm{Zn}$ have been identified. The strength distributions for $^{54}\mathrm{Fe}$ and $^{64,68}\mathrm{Zn}$ are compared with the predictions from HF-RPA calculations with the KDE0v1 Skyrme interaction.

Time-Reversed Particle-Vibration Loops and Nuclear Gamow-Teller Response.

The nuclear response theory for charge-exchange modes in the relativistic particle-vibration coupling approach is extended to include for the first time particle-vibration coupling effects in the ground state of the parent nucleus. In a framework based on the effective meson-nucleon Lagrangian, we investigate the role of such complex ground-state correlations in the description of Gamow-Teller transitions in ^{90}Zr in both (p, n) and (n, p) channels. The particle-vibration coupling effects are calculated without introducing new parameters. We find that this new correlation mechanism is fully responsible for the appearance of the strength in the (n, p) branch. Comparison of our results to the available experimental data shows a very good agreement up to excitation energies beyond the giant resonance region when taking into account a phenomenological admixture of the isovector spin monopole transitions. The parent-daughter binding-energy differences are also greatly improved by the inclusion of the new correlations.

Reference database for photon strength functions

Photon strength functions describing the average response of the nucleus to an electromagnetic probe are key input information in the theoretical modelling of nuclear reactions. Consequently they are important for a wide range of fields such as nuclear structure, nuclear astrophysics, medical isotope production, fission and fusion reactor technologies. They are also sources of information for widely used reaction libraries such as the IAEA Reference Input Parameter Library and evaluated data files such as EGAF. In the past two decades, the amount of reaction gamma-ray data measured to determine photon strength functions has grown rapidly. Different experimental techniques have led to discrepant results and users are faced with the dilemma which (if any) of the divergent data to adopt. We report on a coordinated effort to compile and assess the existing experimental data on photon strength functions from the giant dipole resonance region to energies below the neutron separation energy. The assessment of the discrepant data at energies around or below the neutron separation energy has been possible only in a few cases where adequate information on the model-dependent analysis and estimation of uncertainties was available. In the giant dipole resonance region, we adopt the recommendations of the new IAEA photonuclear data library. We also present global empirical and semi-microscopic models that describe the photon strength functions in the entire energy region and reproduce reasonably well most of the experimental data. The compiled experimental photon strengths and recommended model calculations are available from the PSF database hosted at the IAEA (URL:www-nds.iaea.org/PSFdatabase).

Isomeric yield ratios of 195m,gHg and 197m,gHg in the 196Hg(γ, n) and 198Hg(γ, n) reactions induced by bremsstrahlung energy within giant dipole resonance region

We have measured the isomeric ratios of isomeric pairs 195m,gHg and 197m,gHg produced in 196Hg(γ, n)195m,gHg and 198Hg(γ, n)197m,gHg reactions within the giant dipole resonance (GDR) region by the activation method. In order to improve the accuracy of the experimental results, the necessary corrections were made in the gamma activity measurements and data analysis. The results were analyzed, discussed and compared with the similar data from literature to examine the role of excitation energy, nucleon configuration and the reaction channel effects in the isomeric ratios. The isomeric ratios from 18 to 24 MeV for 197m,gHg and from 14 to 24 MeV for 195m,gHg are the first time measurements.

Electric and magnetic dipole modes in high-resolution inelastic proton scattering at 0°

Inelastic proton scattering under extreme forward angles including $0^\circ$ and at energies of a few hundred MeV has been established as a new spectroscopic tool for the study of complete dipole strength distributions in nuclei. Such data allow an extraction of the electric dipole polarizability which provides important constraints parameters of the symmetry energy, which determine the neutron skin thickness and the equation of state (EOS) of neutron-rich matter. Also new insight into the much-debated nature of the pygmy dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1 resonance can be studied in heavy nuclei, where only limited experimental information exists so far. Together with much improved results on the isoscalar spin-M1 strength distributions in $N = Z$ nuclei, these data shed new light on the phenomenon of quenching of the nuclear spin response. Using dispersion matching techniques, high energy resolution ($\Delta E/E \leq 10^{-4} \,\,$ full width at half maximum, FWHM) can be achieved in the experiments. In spherical-vibrational nuclei considerable fine structure is observed in the energy region of the isovector giant dipole resonance (IVGDR). A quantitative analysis of the fine structure with wavelet methods provides information on the role of different damping mechanisms contributing to the width of the IVGDR. Furthermore, level densities can be extracted from a fluctuation analysis at excitation energies well above neutron threshold, a region hardly accessible by other means. The combination of the gamma strength function (GSF) extracted from the E1 and M1 strength distributions with the independently derived level density permits novel tests of the Brink-Axel hypothesis underlying all calculations of statistical model reaction cross sections in astrophysical applications in the energy region of the PDR.

Interplay between low-lying isoscalar and isovector dipole modes: A comparative analysis between semiclassical and quantum approaches

We perform Time Dependent Hartree-Fock (TDHF) calculations to investigate the small amplitude dipole response of selected neutron-rich nuclei and Sn isotopes. A detailed comparison with the dipole strength predicted by Random-Phase Approximation (RPA) calculations is presented for the first time. TDHF results are also confronted to Vlasov calculations, to explore up to which extent a semi-classical picture can explain the properties of the nuclear response. The focus is on the low-energy response, below the Giant Dipole Resonance region, where different modes of non negligible strength are identified. We show that the relative weight of these excitations evolves with nuclear global features, such as density profile and neutron skin, which in turn reflect impor tant properties of the nuclear effective interaction. A thorough analysis of the associated transition densities turns out to be quite useful to better characterize the mixed isoscalar(IS)/isovector(IV) nature of the different modes and their surface/volume components. In particular, we show that the dipole response in the so-called Pygmy Dipole Resonance region corresponds to isoscalar-like surface oscillations, of larger strength in nuclei with a more diffuse surface. The ratio between the IV and IS Energy Weighted Sum Rule fractions exhausted in this region is shown to almost linearly increase with the neutron skin thickness in Sn isotopes.

Investigation of gamma strength functions and level density models effects on photon induced reaction cross–section calculations for the fusion structural materials 46,50Ti, 51V, 58Ni and 63Cu

Scientists have been focused on fusion reactor studies to overcome the increasing energy demand. The materials, which have the potential to be used in fusion reactors must be resistant to the harmful effects of radiation in the manner of material itself. Selection of the appropriate materials to be used in nuclear reactors has a crucial importance to achieve the maximum efficiency and security. Ti, V, Ni and Cu are known as some of the constructional fusion materials. Existence of many knowledge about those materials provides countless advantages to the researchers and one of them is the cross–section, which basically means the probability of a nuclear reaction's occurrence. In addition to the cross–section, there exist some other parameters, which could be pointed as gamma strength function and level density models that affect the theoretical calculations. In this study, photon induced reaction cross–sections of 46,50Ti, 51V, 58Ni and 63Cu target isotopes have been calculated by using TALYS 1.8 code with different gamma strength functions in the giant dipole resonance region. For gamma strength functions Kopecky-Uhl generalised Lorentzian Model, Brink-Axel Lorentzian Model, Hartree-Fock BCS tables, Hartree-Fock-Bogolyubov tables and Goriely's Hybrid Model have been employed. To appoint the best gamma strength function model, the relative variance calculations have been performed. Also, reaction cross–sections have been recalculated by using the best gamma strength function models through the different level density options. Constant Temperature Fermi Gas Model, Back Shifted Fermi Gas Model and Generalised Super Fluid Model have been employed for level density calculations. Experimental data for the investigated reactions have been taken from EXFOR library and used for comparisons of the obtained calculation results.

Isomeric yield ratio of 152m1Eu(8−)–152m2Eu(0−) produced in (γ, n) photonuclear reaction in the giant dipole resonance region

In this work, the isomeric ratio of 152m1Eu(8−)–152m2Eu(0−) produced from the 153Eu(γ, n)152Eu reaction have been measured in the whole giant dipole resonance region by the activation method. In order to improve the accuracy of the experimental results, the necessary corrections were made in the gamma activity measurements and data analysis. The results were discussed and compared with the similar data from literature to examine the excitation energy, the spin difference and the nuclear reaction channel effects and can be used for the interpretation on the structure of the nucleus and the mechanisms of nuclear reactions. The data for bremsstrahlung end-point energies of 19–23 MeV are first-time measurements.