Level density parameters for the back-shifted fermi gas model in the mass range 40 < A < 250

The parameters a and Δ for the Fermi gas model with fictive ground state are determined for about 220 nuclei from experimental level densities at low excitation energy and at the neutron binding energy. In. agreement with previous results it is found that for most nuclei the fictive ground state is back-shifted relative to the conventionally shifted ground state as determined by the pairing energy. Shell effects are evident at the mass numbers 90, 140 and 208 for both the level density parameter a and the back-shift. A comparison is given with previous results and different experimental data on level densities.     

Experimental indication of the onset of nuclear deformation in neutron-rich Sr isotopes at mass 98

A partial decay scheme for 0.1 sec ⁹⁸Rb has been deduced from γ-singles, γ-multispectra ana γγ coincidence measurements taken at the OSTIS mass-separator system at the Institut Laue-Langevin. The low-lying levels of ⁹⁸Sr follow the energy level systematics of the even-AN = 60 isotones. The onset of nuclear deformation in even-A neutron-rich Sr isotopes appears to occur at mass 98, as the first 2⁺ level energy drops from the nearly constant value of about 800 keV for masses 90–96 to 144 keV at mass 98. Energy level systematics indicate that a transition in the nuclear structures of the more neutron-rich nuclei near mass 100 occurs rather sharply at neutron number N = 60.     

Diffuseness parameter of even-even nuclei in the range of 58 ≤ A ≤ 250 and the coupled channel optical model

An interaction potential with a radial dependence in the Woods-Saxon form is used to describe low energy neutron data in the context of the coupled channel optical model. A single value of the diffuseness parameter was used for even-even nuclides over a wide range of A: a = 0.65 fm. Traditional and nontraditional magic and nonspherical nuclei were exceptions. The deviations can be used to find or verify the existence of nontraditional magic numbers.     

Oscillator parameters in nuclei

The dependence on nucleon numbers of the harmonic oscillator length parameter b or energy spacing ℏω in nuclei is determined, using an extensive tabulation of nuclear charge radii and an empirical expression for the difference between proton and neutron radii. Various possible constraints on the parameters or radii are discussed. Alternative parametrizations are compared and a preferred “universal” parametrization is proposed. It is argued that the established Blomqvist–Molinari formula is perfectly adequate for determining the oscillator parameter for specific applications.     

Quasielastic knockout of light fragments from 12C and 16O nuclei by intermediate-energy pions

Quasielastic deuteron and triton knockout from ¹²C and ¹⁶O nuclei has been studied infull kinematics using a 0.72-GeV/c pion beam. The momentum distributions of the intranuclear quasideuteron motion, excitation-energy spectra of the residual nuclei, and the effective numbers N _d ^eff of quasideuterons are determined. The parameters of the quasideuteron intranuclear motion are in reasonable agreement with the results obtained in other beams. The N _d ^eff in the nuclei from ⁶Li to ¹⁶O measured in full kinematics are virtually independent of the atomic number in contrast to the analogous values in the inclusive deuteron-knockout reaction induced by protons. The phenomenon of triton knockout from these nuclei is observed, which makes possible estimation of the cross section of backward pion-triton elastic scattering in yet unexplored regions of energy and momentum transfer.     

Collective properties of neutron-rich Ru, Pd, and Cd isotopes

Collective properties of neutron-rich Ru, Pd, and Cd isotopes are reviewed, combining the original results from the IGISOL β-decay experiments with recent experimental and theoretical progress. The transitional nature of Ru and Pd nuclei is discussed via the low-lying level systematics, including the low-lying 0^?+? states. Although the role of an anharmonic quadrupole vibrator in Cd nuclei was recently questioned, level systematics for the three-phonon quintuplet in ^116,118,120Cd are presented, and an outlook of the spectroscopic methods for the level lifetime or B(E2) values is given.     

Time features of delayed neutrons and partial emissive-fission cross sections for the neutron-induced fission of 232Th nuclei in the energy range 3.2–17.9 MeV

The energy dependence of the relative abundances of delayed neutrons and the energy dependence of the half-lives of their precursors in the neutron-induced fission of ²³²Th nuclei in the energy range 3.2–17.9 MeV were measured for the first time. A systematics of the time features of delayed neutrons is developed. This systematics makes it possible to estimate the half-life of delayed-neutron precursors as a function of the nucleonic composition of fissile nuclei by using a single parameter set for all nuclides. The energy dependence of the partial cross sections for emissive fission in the reaction ²³²Th(n, f) was analyzed on the basis of data obtained for the relative abundances of delayed neutrons and the aforementioned half-lives and on the basis of the created systematics of the time features of delayed neutrons. It was shown experimentally for the first time that the decrease in the cross section after the reaction threshold in the fission of ²³²Th nuclei (it has a pronounced first-chance plateau) is not an exclusion among the already studied uranium, plutonium, and curium isotopes and complies with theoretical predictions obtained for the respective nuclei with allowance for shell, superfluid, and collective effects in the nuclear-level density and with allowance for preequilibrium neutron emission     

Intermediate valence and semiconductor-metal transition of TmS1−xSex and TmSe1−yTey

A complete series of TmS_1?xSe_x and TmSe_1?yTe_y single crystals has been prepared in order to enable a systematic investigation of the valence change of the Tm-ion between nearly +3 for metallic TmS (x = 0) and +2 for semiconducting TmTe (y = 1). A discontinuous semiconductor-metal transition (SMT) is observed at y = 0.33, associated with a lattice parameter decrease of Δa₀/a₀ = ?0.037. Using the lattice parameter as a measure for the Tm-valence results in a valence change of 0.65 at the SMT. All other physical parameters are likewise discontinuous at the SMT. For instance, the effective Bohr magneton number evaluated from the high-temperature Curie-Weiss susceptibility jumps by 16% at the SMT which gives a valence change of 0.26, just 40% of the value deduced from a₀. In the metallic state (y ? 0.33, all x) the valence number of the Tm-ion evaluated from μ_eff is systematically lower than that resulting from a₀. For TmSe a valence of +2.83 is obtained from a₀ + 2.62 from μ_eff. All metallic samples undergo a metamagnetic phase transition. The type of order changes gradually to antiferromagnetic on approaching TmS. Application of Falicov's model for the phase transition into the metallic state with intermediate valence implies that the system be close to the critical point for Δa₀/a₀ = ?0.037, in contradiction to the experimental evidence.     

Three dynamical boson-fermion symmetries for odd-odd nuclei

For odd-odd nuclei we introduce three dynamical boson-fermion symnetries associated with j_π=3/2, j_υ=3/2 and U^B(5) symmetry of the boson core. The corresponding energy formulas and quantum numbers for low-lying states are explicitly given.     

Effective boson number calculations in Mo and Cd isotopes

The effects of the neutron-proton interaction on the low-lying levels of Mo and Cd isotopes have been considered in the frame of the IBA-1 model by taking into account an effective boson number (N _eff). Both an empirical procedure based on previous IBA-2 mixing calculations and theN _p N _n scheme provide comparableN _eff values. Level spectra and electromagnetic transitions are investigated. The results support the idea that IBA-1 calculations with a suitableN _eff can largely simulate IBA-2 mixing calculations, taking advantage of simplicity and a smaller number of parameters.     

Measurements of effective delayed neutron fraction in a fast neutron reactor using the perturbation method

A perturbation method is proposed to obtain the effective delayed neutron fraction β_(eff) of a cylindrical highly enriched uranium reactor.Based on reactivity measurements with and without a sample at a specified position using the positive period technique,the reactor reactivity perturbation △ρ of the sample in β_(eff) units is measured.Simulations of the perturbation experiments are performed using the MCNP program.The PERT card is used to provide the difference dκ of effective neutron multiplication factors with and without the sample inside the reactor.Based on the relationship between the effective multiplication factor and the reactivity,the equation β~(eff)=dκ/△ρ is derived.In this paper,the reactivity perturbations of 13 metal samples at the designable position of the reactor are measured and calculated.The average β_(eff) value of the reactor is given as 0.00645,and the standard uncertainty is 3.0%.Additionally,the perturbation experiments for β_(eff) can be used to evaluate the reliabilities of the delayed neutron parameters.This work shows that the delayed neutron data of ~(235)U and ~(238)U from G.R.Keepin's publication are more reliable than those from ENDF-B6.0.ENDF-B7.0,JENDL3.3 and CENDL2.2.     

Nuclear level densities in <Superscript>208</Superscript>Bi and <Superscript>209</Superscript>Po from the neutron spectra in the (<Emphasis Type="Italic">p,n</Emphasis>) reactions on <Superscript>208</Superscript>Pb and <Superscript>209</Superscript>Bi nuclei

The spectra of neutrons from the (p, n) reactions on the ²⁰⁸Pb and ²⁰⁹Bi nuclei were measured in the proton-energy range 8–11 MeV. These measurements were performed by using a time-of-flight spectrometer of fast neutrons on the basis of the pulsed tandem accelerator EGP-15 of the Institute of Physics and Power Engineering (Obninsk, Russian Federation). A high resolution and stability of the time-of-flight spectrometermade it possible to identify reliably low-lying discrete levels alongwith the continuum section of the neutron spectra. The measured data were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed by using the precise formalism of Hauser-Feshbach statistical theory together with the generalizedmodel of a superfluid nucleus and the back-shifted Fermi gas model for the nuclear-level density. The nuclear-level densities in ²⁰⁸Bi and ²⁰⁹Po were determined along with their energy dependences and model parameters. Our results are discussed together with available experimental data and recommendations of model systematics.     

In-beam conversion electrons from 141Pr, 143Pm, 144Pm and 145Pm excited by (α, xn) and (p,n) reactions

Electromagnetic transitions between low-lying states in the odd proton nuclei ₅₉¹⁴¹Pr₈₂, ₆₁¹⁴³Pm₈₂, ₆₁¹⁴⁴Pm₈₃ and ₆₁¹⁴⁵Pm₈₄ with neutron numbers N ≈ 82 were studied by means of in-beam electron-gamma spectroscopy. The triple-focusing electron spectrum selector (TESS) was used to measure in-beam conversion electrons. The TESS, giving very good electron line spectra with small background, was found to be very powerful for in-beam spectroscopy. Accurate values for the internal conversion coefficients (ICC) were obtained by measuring simultaneously both conversion electrons and γ-rays. M2, E3 and some other multipolarities were uniquely assigned from the ICC for the γ-transitions between low-lying states. Properties of these transitions and energy levels are discussed. Analyses of the M2 and E3 transition rates gave, respectively, an isospin-spin (magnetic) core polarization effect and an octupole core polarization effect. The l-forbidden M1 transitions were analyzed in terms of the tensor terms.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

A SYNTHESIS LEVEL DENSITY FORMULA

Based on the recommended level density formula of the Fermi gas model and considering the modifications at low excitation energy,a synthesis level density formula is proposed.From the mean neutron resonance spacing D_exp for 248 nuclei and the cumulative numbers of levels for 529 nuclei,a set of parameters is determined.     

Effect of Wigner energy on the symmetry energy coefficient in nuclei

The nuclear symmetry energy coefficient(including the coefficient a_(sym)~((4)) of the I~4 term) of finite nuclei is extracted by using the differences of available experimental binding energies of isobaric nuclei.It is found that the extracted symmetry energy coefficient a_(sym)~*(A,I) decreases with increasing isospin asymmetry I,which is mainly caused by Wigner correction,since e_(sym)~* is the summation of the traditional symmetry energy e_(sym) and the Wigner energy ew.We obtain the optimal values J = 30.25±0.10 MeV,a_(ss)=56.18±1.25 MeV,a_(sym)~((4)) = 8.33±1.21 MeV and the Wigner parameter x= 2.38 ±0.12 through a polynomial fit to 2240 measured binding energies for nuclei with20 ≤ A ≤ 261 with an rms deviation of 23.42 keV.We also find that the volume symmetry coefficient J■ 30 MeV is insensitive to the value x,whereas the surface symmetry coefficient a_(ss) and the coefficient a_(sym)~((4)) are very sensitive to the value of x in the range 1≤x≤4.The contribution of the a_(sym)~((4)) term increases rapidly with increasing isospin asymmetry I.For very neutron-rich nuclei,the contribution of the a_(sym)~((4)) term will play an important role.     

Microscopic study of positive-parity yrast bands of 224−234Th isotopes

The positive-parity bands in ^224???234Th are studied using the projected shell model (PSM) approach. The energy levels, deformation systematics, B(E2) transition probabilities and nuclear g-factors are calculated and compared with the experimental data. The calculation reproduces the observed positive-parity yrast bands and B(E2) transition probabilities. Measurement of B(E2) transition probabilities for higher spins and g-factors would be a stringent test for our predictions. The results of theoretical calculations indicate that the deformation systematics in ^224???234Th isotopes depend on the occupation of low k components of high j orbits in the valence space and the deformation producing tendency of the neutron–proton interaction operating between spin orbit partner (SOP) orbits, the [(2g_9/2) _π –(2g_7/2) _ν ] and [(1i_13/2) _π –(1i_11/2) _ν ] SOP orbits in the present context. In addition, the deformation systematics also depend on the polarization of (1h_11/2) _π orbit. The low-lying states of yrast spectra are found to arise from 0-quasiparticle (qp) intrinsic states whereas the high-spin states turn out to possess composite structure.