Neutron Separation Energy Research Articles

Nuclear Structure Studies with Gamma-Ray Beams

In stable and weakly bound neutron-rich nuclei, a resonance-like concentration of dipole states has been observed for excitation energies below the neutron-separation energy. This clustering of strong dipole states has been named the Pygmy Dipole Resonance (PDR) in contrast to the Giant Dipole Resonance (GDR) that dominates the E1 response. Understanding the PDR is presently of great interest in nuclear structure and nuclear astrophysics. High-sensitivity studies of E1 and M1 transitions in closed-shell nuclei using monoenergetic and 100% linearly-polarized photon beams are presented.

Even-odd Effects in the Prompt Emission in Fission of Even-even Actinides

The even-odd effects in prompt emission received less attention compared to the extensively studied Z even-odd effects in fragment distributions. In this context, the present work is focusing on the even-odd effects (in both Z and N) appearing in the prompt emission of even-even nuclei fissioning spontaneously (252Cf, 236,238,240,242,244Pu) and by thermal neutron induced fission (233,235U, 239Pu). The study shows that these effects are mainly due to the Z even-odd effects in fragment distributions and ΔZ and the N even-odd effects in average neutron separation energies. The most pronounced even-odd effect in prompt emission is observed for the prompt neutron multiplicity, its global size decreasing with increasing fissility. The N even-odd effect in <Sn>, influences the global even-odd effect in prompt neutron multiplicity often in an opposite way compared to the Z even-odd effect. The global N even-odd effect in <Sn> is considerably higher in the case of 236-244Pu(SF) compared to other neutron induced fissioning nuclei with comparable mass. <ν>(TKE) shows an increase of the Z even-odd effect with increasing TKE. Compared to 233,235U(nth,f), 239Pu(nth,f) and 252Cf(SF) for which these effects show a decrease for heavier fissioning systems, in the case of 236-244Pu(SF) this behaviour is not visible. The comparison of even-odd effects of 240Pu(SF) and 239Pu(nth,f) showed i) a higher even-odd effect in Y(Z) and ΔZ for SF confirming the Z even-odd effect decrease with increasing excitation energy and ii) the higher N even-odd effect in <Sn> for SF impacts the prompt emission leading to a lower Z even-odd effect in prompt neutron multiplicity of 240Pu(SF) compared to 239Pu(nth,f).

The statistical properties of111,112,113Sn studied with the Oslo method

The γ -ray strength function and level density of 111, 112, 113 Sn are being studied at the Oslo Cyclotron Laboratory (OCL) up to the neutron binding energy by applying the Oslo method to particle-γ coincidence data. The preliminary results for the γ -ray strength function are discussed in the context of the results for the more neutron-rich Sn-isotopes previously studied at OCL.

Investigation of dipole strength up to the neutron separation energy atγELBE

The bremsstrahlung facility at the ELBE accelerator offers the possibility to investigate dipole strength distributions up to the neutron-separation energies with photon up to 16 MeV in energy. The facil- ity and various results for nuclides measured during recent years are presented. One example is the study of the N = 80 nuclide 136 Ba. The other presented example is the study of the chain of xenon isotopes from N = 70 to N = 80 which aimed to investigate the influence of nuclear deformation an neutron excess on the dipole strength in the pygmy region. An overview of the analysis is given. GEANT4 simulations were performed to determine the non-nuclear background that has to be removed from the measured spectra. This opens up the possibility to take into account also the strength of unresolved transitions. Simulations of gamma-ray cascades were carried out that consider the transitions from states in the quasi-continuum and allow us to estimate their branching ratios. As a result, the photoabsorption cross sections obtained from corrected intensities of ground- state transitions are compared with theoretical predictions and results within the chain of isotopes. With the help of the measured dipole distribution it is possible to describe gamma-ray spectra following neutron capture more precisely.

Level densities and thermodynamical properties of Pt and Au isotopes

The nuclear level densities of $^{194-196}$Pt and $^{197,198}$Au below the neutron separation energy have been measured using transfer and scattering reactions. All the level density distributions follow the constant-temperature description. Each group of isotopes is characterized by the same temperature above the energy threshold corresponding to the breaking of the first Cooper pair. A constant entropy excess $\Delta S=1.9$ and $1.1$ $k_B$ is observed in $^{195}$Pt and $^{198}$Au with respect to $^{196}$Pt and $^{197}$Au, respectively, giving information on the available single-particle level space for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is revealed by sequential peaks in the microcanonical caloric curve.

Investigation of nuclear structure of 30–44S isotopes using spherical and deformed Skyrme–Hartree–Fock method

Nuclear many-body system is usually described by a mean-field built upon a nucleon–nucleon effective interaction. In this work, we investigate ground state properties of the sulfur isotopes covering a wide range from the line of stability up to the dripline region (30–44S). For this purpose the Hartree–Fock mean field theory in coordinate space with a Skyrme parameterization SkM* has been utilized. In particular, we calculate the nuclear charge, neutrons, protons, mass densities, the associated radii, neutron skin thickness and binding energy. The charge form factors have been also investigated using SkM*, SkO, SkE, SLy4 and Skxs15 Skyrme parameterizations and the results obtained using the theoretical approach are compared with the available experimental data. To investigate the potential energy surface as a function of the quadrupole deformation for isotopic sulfur chains, Skyrme–Hartree–Fock–Bogoliubov theory has been adopted with SLy4 parameterization.

Even–odd effects in prompt emission of spontaneously fissioning even–even Pu isotopes

Abstract The available experimental Y ( A , TKE ) data for 236,238,240,242,244 Pu(SF) together with the Zp model prescription with appropriate parameters allows the investigation of even–odd effects in fragment distributions. The size of the global even–odd effect in Y ( Z ) is decreasing from 244 Pu(SF) to 236 Pu(SF) confirming the general observation of a decrease of the even–odd effect with the fissility parameter. Charge polarizations (Δ Z ) and root-mean squares (rms) as a function of A of 236–244 Pu(SF) were obtained for the first time. In the asymmetric fission region both Δ Z ( A ) and rms ( A ) exhibit oscillations with a periodicity of about 5 mass units due to the even–odd effects. The total average charge deviations 〈 Δ Z 〉 (obtained by averaging Δ Z ( A ) over the experimental Y ( A ) distribution) are of about |0.5| for all studied Pu(SF) systems. The comparison of the calculated Δ Z ( A ) and rms ( A ) of 240 Pu(SF) with those of 239 Pu ( n th , f ) reported by Wahl shows an in-phase oscillation with a higher amplitude in the case of 240 Pu(SF), confirming the higher even–odd effect in the case of SF. As in the previously studied cases ( 233,235 U ( n th , f ) , 239 Pu ( n th , f ) , 252 Cf(SF)) the even–odd effects in the prompt emission of 236–244 Pu(SF) are mainly due to the Z even–odd effects in fragment distributions and charge polarizations and the N even–odd effects in the average neutron separation energies from fragments 〈 Sn 〉 . The size of the global N even–odd effect in 〈 Sn 〉 is decreasing with the fissility parameter, being higher for the Pu(SF) systems compared to the previously studied systems. The prompt neutron multiplicities as a function of Z , ν ( Z ) , exhibit sawtooth shapes with a visible staggering for asymmetric fragmentations. The size of the global Z even–odd effect in ν ( Z ) exhibits a decreasing trend with increasing fissility. The average prompt neutron multiplicities as a function of TKE show an increase of the even–odd effect with increasing TKE, with global effect sizes close to each other (a decrease of the effect for heavier fissioning nuclei is not observed here). The amounts of the global even–odd effect in Y ( Z ) and of the N even–odd effect in 〈 Sn 〉 of 240 Pu(SF) are larger compared to 239 Pu ( n th , f ) . This fact affects the prompt emission leading to a lower Z even–odd effect in the prompt neutron multiplicity of 240 Pu(SF) compared to 239 Pu ( n th , f ) .

Dipole strength ofTa181for the evaluation of theTa180stellar neutron capture rate

The photoabsorption cross section of $^{181}\mathrm{Ta}$ up to the neutron-separation energy is deduced using bremsstrahlung produced with an electron beam of 9.6 MeV energy. The analysis of the measured $\ensuremath{\gamma}$-ray spectra includes the quasicontinuum of levels at high energy. Simulations of $\ensuremath{\gamma}$-ray cascades are performed to estimate intensities of inelastic transitions and branching ratios of the ground-state transitions. The resulting photoabsorption cross section shows enhanced dipole strength in the energy range from 5 to 8 MeV, which may be related to a pygmy dipole resonance. The results of the present experiment are compared with predictions of a quasiparticle-random-phase approximation in a deformed basis. A combination of the present experimental data and $(\ensuremath{\gamma},n)$ data is used as an input to the statistical code talys applied to calculate cross sections and reaction rates of photonuclear reactions that are important for the nucleosynthesis of $^{180}\mathrm{Ta}$.

Formation of Medium-Heavy Elements in Rapid Neutron Capture Process

We predict the neutron drip-line and simulate the path for Cu — Sn, based on the calculation of binding energy in the frame-work of relativistic and non-relativistic mean field formalisms. We also compare the quadrupole deformation parameterβ 2 , and one neutron separation energy S n of these isotopic series with the results of finite range droplet model (FRDM) prediction. the results produced by RMF and ShF are comparable to each other and also agreeable with the FRDM model.

Low-energy dipole strength inSn112,120

The $^{112,120}\mathrm{Sn}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for $^{112}\mathrm{Sn}$ up to 9.5 MeV and for $^{120}\mathrm{Sn}$ up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in the background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation, allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficiently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.

Examining the enhancement of sub-barrier fusion cross sections by neutron transfer with positiveQ values

Background: Significant enhancement of sub-barrier fusion cross sections owing to neutron rearrangement with positive $Q$ values were found for many combinations of colliding nuclei. However, several experimental results on fusion reactions were reported recently in which such enhancement has not been observed in spite of a possibility for neutron rearrangement with positive $Q$ values.Purpose: We aim to clarify much better the mechanism of neutron rearrangement in sub-barrier fusion reactions to find the other requirements (beside positive $Q$ values) which favor (or prevent) sub-barrier fusion enhancement.Methods: A channel coupling approach along with the semiclassical model for neutron transfer has been used for analysis of available experimental data on sub-barrier fusion of heavy ions.Results: The role and interplay of different factors determining the enhancement of sub-barrier fusion (such as $Q$ values for neutron rearrangement, properties of collective excitations, and neutron binding energies) have been studied and clarified.Conclusions: (1) Only $1n$ and $2n$ transfers with positive $Q$ values have a noticeable impact on sub-barrier fusion. A positive $Q$ value for neutron rearrangement is a necessary but not sufficient requirement for additional sub-barrier fusion enhancement to take place. (2) The ``rigidity'' of colliding nuclei with respect to collective excitations is important for sub-barrier fusion enhancement due to neutron rearrangement with positive $Q$ values to be clearly visible. (3) The neutron binding energy of the ``donor'' nucleus has a strong impact only in the case of fusion of light weakly bound nuclei.

Mass predictions of the relativistic mean-field model with the radial basis function approach

The radial basis function (RBF) is a powerful tool to improve mass predictions of nuclear models. By combining the RBF approach with the relativistic mean-field (RMF) model, the systematic deviations between mass predictions of the RMF model and the experimental data are eliminated to a large extent and the resulting rms deviation is reduced from $2.217$ to $0.488$ MeV. Furthermore, it is found that the RBF approach has a relatively reliable extrapolative power along the distance from the $\ensuremath{\beta}$-stability line except for a large uncertainty around the region at magic number. From the deduced neutron separation energies, we found that the description of the nuclear shell structure and shape transition is also significantly improved by the RBF approach, thus improving agreement with the solar $r$-process abundances before $A=130$ and speeding up the $r$-matter flow. Therefore, a shorter irradiation time is enough to reproduce the solar $r$-process abundance distribution for the improved RMF mass model, which is closer to the irradiation time for those sophisticated mass models.

Even–odd effects in prompt emission in fission

Abstract Compared to the extensively studied proton even–odd effects in fission fragment distributions, the even–odd effects in prompt emission received less attention. In this context the present paper is focusing on the even–odd effects (in both Z and N) appearing in prompt neutron and gamma ray emission. The study of even–odd effects in the prompt emission of four fissioning nuclei with even Z and even N, 233,235U( n th , f ), 239Pu( n th , f ) and 252Cf(SF), shows that these effects are mainly due to the Z even–odd effects in fragment distributions and charge deviations and the N even–odd effect in average neutron separation energies. The most pronounced even–odd effect in prompt emission is for the prompt neutron multiplicity. The average multiplicity as a function of Z, ν ( Z ) , has a sawtooth shape and exhibits a visible staggering for asymmetric fragmentations. The size of the global even–odd effect in prompt neutron multiplicity shows a decrease with increasing fissility. The N even–odd effect in average neutron separation energy influences the global even–odd effect in prompt neutron multiplicity often in an opposite way compared to the Z even–odd effect. The average prompt neutron multiplicity as a function of TKE shows an increase of the Z even–odd effect with increasing TKE and this effect is decreasing for heavier fissioning systems. The Z even–odd effects in the average prompt γ-ray energy and the average prompt neutron energy have comparable sizes and are lower than the even–odd effects in prompt neutron multiplicity. Different average prompt emission quantities (as a function of A, of Z, of TKE and total average ones)—being dependent on fragment distributions—are affected by the even–odd effects in Y ( A , Z , TKE ) . The N even–odd effect in neutron separation energy and the Z even–odd effect in the charge deviation enter the primary multi-parametric matrices of prompt emission quantities (e.g. ν ( A , Z , TKE ) ) which are not depending on fragment distributions.

A New Approach to Estimating the Probability for β-delayed Neutron Emission

The probability for neutron emission following β decay, Pn, is a crucial property for a wide range of physics and applications including nuclear structure, r-process nucleosynthesis, the control of nuclear reactors, and the post-processing of nuclear fuel. Despite much experimental effort, knowledge of Pn values is still lacking in very neutron-rich nuclei, requiring predictions from either systematics or theoretical models. Traditionally, systematic predictions were made by investigating the Pn value as a function of the decay Q value and the neutron separation energy in the daughter nucleus. A new approach to Pn systematics is presented which incorporates the half-life of the decay and the Q value for β-delayed neutron emission. This prescription correlates the known data better, and thus improves the estimation of Pn values for neutron-rich nuclei. Such an approach can be applied to generate input values for r-process network calculations or in the modeling of advanced fuel cycles.

A theoretical study of energy spectra and two-neutron separation energies of 106–122Cd isotopes in the transitional region of IBM

In this paper, the properties of Cd(106-122)isotopes are considered in the U(5)-SO(6)transitional region of IBM1. With employing a transitional Hamiltonian which is based on affine SU(1,1)Lie Algebraic technique, the energy levels and B(E2)transition rates are calculated. The results are compared with the most recent experimental values where an acceptable degree of agreement is achieved. Also, the energy ratios, control parameters and empirical two neutron separation energies suggest spherical to gamma soft shape transitions in these nuclei where the Cd (114,122)nuclei are considered as the best candidates for the E(5)critical symmetry.

Extracting the electric dipole breakup cross section of one-neutron halo nuclei from inclusive breakup observables

How to extract an electric dipole (E1) breakup cross section \sigma(E1) from one- neutron removal cross sections measured by using 12C and 208Pb targets, \sigma_(-1n)^C and \sigma_(-1n)^Pb, respectively, is discussed. It is shown that within about 5% error, \sigma(E1) can be obtained by subtracting \Gamma \sigma_(-1n)^C from \sigma_(- 1n)^Pb, as assumed in preceding studies. However, for the reaction of weakly-bound projectiles, the scaling factor \Gamma is found to be two times as large as that usually adopted. As a result, we obtain 13-20% smaller \sigma(E1) of 31Ne at 250 MeV/nucleon than extracted in a previous analysis of experimental data. By compiling the values of \Gamma obtained for several projectiles, \Gamma=(2.30 +/- 0.41)\exp(- S_n)+(2.43 +/- 0.21) is obtained, where S_n is the neutron separation energy. The target mass number dependence of the nuclear parts of the one-neutron removal cross section and the elastic breakup cross section is also investigated.

Surface diffuseness correction in global mass formula

By taking into account the surface diffuseness correction for unstable nuclei, the accuracy of the macroscopic–microscopic mass formula is further improved. The rms deviation with respect to essentially all the available mass data falls to 298 keV, crossing the 0.3 MeV accuracy threshold for the first time within the mean-field framework. Considering the surface effect of the symmetry potential which plays an important role in the evolution of the “neutron skin” toward the “neutron halo” of nuclei approaching the neutron drip line, we obtain an optimal value of the symmetry energy coefficient J=30.16 MeV. With an accuracy of 258 keV for all the available neutron separation energies and of 237 keV for the α-decay Q-values of super-heavy nuclei, the proposed mass formula is particularly important not only for the reliable description of the r process of nucleosynthesis but also for the study of the synthesis of super-heavy nuclei.

Electromagnetic Dipole Strength in 124,128,134Xe

The electromagnetic dipole strength in several even nuclei in the chain of Xenon isotopes has been investigated at the bremsstrahlung facility of the ELBE accelerator in Dresden, Germany and at the HIγS facility in Durham, USA. The goal of these measurements is to extend the knowledge about the general behavior of the dipole strength in the energy region below the neutron separation energy under the aspect of neutron excess and nuclear deformation.

Deformation-driven p-wave halos at the drip line: 31Ne.

The halo structure of 31Ne is studied using 1n-removal reactions on C and Pb targets at 230 MeV/nucleon. A combined analysis of the cross sections of these nuclear and Coulomb dominated reactions that feed directly the 30Ne ground-state reveals 31Ne to have a small neutron separation energy, 0.15(-0.10)(+0.16) MeV, and spin-parity 3/2-. Consistency of the data with reaction and large-scale shell-model calculations identifies 31Ne as deformed and having a significant p-wave halo component, suggesting that halos are more frequent occurrences at the neutron drip line.

Nuclear Level Density within Extended Superfluid Model with Collective State Enhancement

For nuclear level densities, an enhanced generalized superfluid model with different vibrational enhancement factors is studied. The ready-to-use tables for the asymptotic value of a-parameter of level density as well as for additional shift to excitation energy for 291 atomic nuclei are prepared using the chi-square fit of the theoretical values of neutron resonance spacing and cumulative number of low-energy levels to the experimental values. The systematics of these parameters as a function of mass number and neutron excess are obtained. The vibrational enhancement descriptions are additionally tested for a wide range of excitation energies. Simple methods of the vibrational enhancements lead to increasing the level density due to vibrational states by factor ∼ 2 to 3 for the isotopes A ∼ 100 with the excitation energies near the neutron binding energies.