Proton-neutron Configurations Research Articles

Ternary fission analysis of Fm242,258 nuclei using equatorial and collinear cluster tripartition configurations

The quantum mechanical fragmentation theory--based three cluster model is employed to investigate the ground-state ternary fission of two Fm isotopes nuclei having atomic mass ${A}_{P}=242$ and 258. The mass asymmetry coordinate and the relative separation among the decaying fragments play a crucial role for the estimation of the fragmentation structure and related barrier penetration process. First, the choice of third fragment (${A}_{3}$) is fixed by minimizing the probable ${A}_{3}$ fragments having different proton neutron configurations. Further, the fission fragment combinations (${A}_{1}+{A}_{2}+{A}_{3}$) are identified for the fixed third fragments by selecting the channel of lower ternary fragmentation potential and higher relative fission yield. Two type of tripartition of radioactive nuclei are considered such as equatorial cluster tripartition (ECT) and collinear cluster tripartition (CCT). A comparative analysis of ternary fragmentation potential and relative fission yield within ECT and CCT geometrical arrangement is carried out for different choices of third fragment, i.e., ${A}_{3}=1$ to ${A}_{P}$/3. The choice of most probable fragments suggest that the proton and neutron magic shell closures play essential role in the ternary mass division. Finally, a relative analysis of binary and ternary fragmentation is worked out for better insight of dynamics involved.

Nuclear structure study for the neutron-rich nuclei beyond 132Sn: In-beam gamma-ray spectroscopy of 136Sn and 132Cd

The neutron-rich nuclei 136Sn and 132Cd have been studied in the purpose of nuclear structure for the nuclei beyond the doubly-magic nucleus 132Sn. The 2+1 → 0+ gs transitions were identified for these two nuclei using in-beam γ-ray spectroscopy in coincidence with one- and two-proton removal reactions, respectively, at the RIKEN Radioactive Isotope Beam Factory. The 2+ 1 state in 136Sn is found to be similar to that for 134Sn indicating the seniority scheme may also hold for the heavy tin isotopes beyond N = 82. For 132Cd, the 2+ 1 state provides the first spectroscopic information in the even-even nuclei locating in the region “southeast” of 132Sn and the result is discussed in terms of proton-neutron configuration mixing. In both these two nuclei, it was found that the valence neutrons play an essential role in their low-lying excitations.

Electromagnetic transitions in multiple chiral doublet bands**Supported by National Natural Science Foundation of China (11675094, 11622540, 11545011, 11405096, 11461141001, U1432119), Shandong Natural Science Foundation (ZR2014AQ012), and Young Scholars Program of Shandong University, Weihai (2015WHWLJH01)

Multiple chiral doublet bands (MχD) in the 80, 130 and 190 mass regions are studied by the model of γ = 90° triaxial rotor coupled with identical symmetric proton-neutron configurations. By selecting a suitable basis, the calculated wave functions are explicitly exhibited to be symmetric under the operator Â, which is defined as rotation by 90° about the 3-axis with the exchange of valance proton and neutron. We found that both M1 and E2 transitions are allowed between levels with different values of A, while they are forbidden between levels with same values of A. Such a selection rule holds true for MχD in different mass regions.

First spectroscopic information from even-even nuclei in the region “southeast” of Sn132 : Neutron-excitation dominance of the 21+ state in Cd132

The neutron-rich nucleus Cd132 has been studied at the RIKEN Radioactive Isotope Beam Factory using in-beam γ-ray spectroscopy with two-proton removal reactions from Sn134. A γ-ray transition was observed at 618(8) keV and was assigned to the 21+→0g.s.+ decay. The 21+ state provides the first spectroscopic information from the even-even nuclei located in the region southeast of the doubly magic nucleus Sn132. By comparing with the 21+ excitation energies in the semimagic nuclei Sn134 and Cd130, it is found that neutron excitations dominate the 21+ state in Cd132, in a similar manner to Te136. The results are discussed in terms of proton-neutron configuration mixing. © 2016 American Physical Society.

Isomeric levels in92Rb and the structure of neutron-rich92,94Rb isotopes

The medium-spin structure of the ${}^{92}$Rb nucleus, populated in spontaneous fission of ${}^{248}$Cm and ${}^{252}$Cf, has been studied using the EUROGAM2 and the GAMMASPHERE Ge arrays, respectively. Two isomers have been found in ${}^{92}$Rb at 284.2 and 1958.2 keV with half-lives of ${T}_{1/2}=54(3)$ ns and ${T}_{1/2}=7(2)$ ns, respectively. A measurement of neutron-induced fission of ${}^{235}$U, at the PF1B cold-neutron beam facility of the Institut Laue Langevin, Grenoble, has been performed to confirm the assignment of the ${T}_{1/2}=54(3)$ ns, 284.2-keV isomeric level to the ${}^{92}$Rb nucleus. Excited levels in ${}^{92}$Rb and ${}^{94}$Rb nuclei have been calculated in a large-scale shell-model framework. Isomers observed in these two nuclei have been interpreted as proton-neutron configurations involving the high-$j$ $\ensuremath{\nu}1{h}_{11/2}$ and $\ensuremath{\pi}1{g}_{9/2}$ orbitals.

Odd-odd nuclei as the core-particle-hole systems and chirality

Odd-odd nuclei treated as core-particle-hole systems with various collective cores and various particle-hole configurations are investigated within the Core-Particle-Hole Coupling (CPHC) model. A new symmetry, called the S-symmetry, is identified as a combination of the α-parity of the collective core and the proton-neutron symmetry of the valence proton and neutron in particle-hole configurations involving single-particle states with the same quantum numbers. It is found that the S-symmetric odd-odd nuclei show signatures which are usually considered as fingerprints of nuclear chirality, namely doublet band structure with a particular pattern of electromagnetic transitions. Reported results imply that the rigid rotor with a symmetric valence proton-neutron configuration is only a special case of the system with the novel S-symmetry. Therefore, it is an open question whether the chiral fingerprints discussed so far identify uniquely the orthogonal coupling of angular momentum in the intrinsic system.

Near-yrast structure ofCs142andCs144

Excited states in $^{142}\mathrm{Cs}$ and $^{144}\mathrm{Cs}$, populated in the spontaneous fission of $^{248}\mathrm{Cm}$ and $^{252}\mathrm{Cf}$ and in thermal neutron-induced fission of $^{235}\mathrm{U}$ and $^{242}\mathrm{Am}$ were studied by means of $\ensuremath{\gamma}$ spectroscopy using the EUROGAM2 and Gammasphere multidetector Ge arrays and the LOHENGRIN fission-fragment separator, respectively. In $^{142}\mathrm{Cs}$, a band and an isomer with a half-life of ${T}_{1/2}=11(3)$ ns have been identified. Spins and parities have been proposed for excited levels in this nucleus. In $^{144}\mathrm{Cs}$ excited levels have been observed. A ${T}_{1/2}=1.1(1)$ $\ensuremath{\mu}$s isomer was found with a $\ensuremath{\gamma}$ cascade, which probably feeds this isomer. There is also an indication of a nanosecond isomer in $^{144}\mathrm{Cs}$. Quasiparticle-rotor model calculations done in this work allowed proton-neutron configurations to be proposed for levels in $^{142}\mathrm{Cs}$ and $^{144}\mathrm{Cs}$.

Short-lived isomers inRb94

The medium-spin structure of the neutron-rich, odd-odd nucleus {sup 94}Rb was studied by means of {gamma}-ray spectroscopy. Excited levels were populated in the neutron-induced fission of {sup 235}U and in the spontaneous fission of {sup 252}Cf and {sup 248}Cm. Two isomeric states were found at 1485.2 and 2074.8 keV with half-lives of 18 and 107 ns, respectively. The probable structures of the two isomers involve the fully aligned, proton-neutron configurations [{pi}(g{sub 9/2}) x {nu}(g{sub 7/2})]{sub 8{sup +}} and [{pi}(g{sub 9/2}) x {nu}(h{sub 11/2})]{sub 10{sup -}}, respectively. These new data give information on the single-particle energies in the region.

Proton-neutron configurations in 236g, mAm and its EC-decay daughter 236Pu

The EC decay of 236Am has been studied using a gas-jet coupled on-line isotope separator. A half-life analysis revealed that there are two EC-decaying states in 236Am: the 5- state with T1/2 = 3.6(2) min and the (1-) state with T1/2 = 2.9(2) min. The 1185.5 keV level in 236Pu was found to be the K isomer with Kπ = 5- and t1/2 = 1.2(3) μs. EC transitions from 236g, mAm to the 1185.5, 1311.5, and 1340.8 keV levels in 236Pu show small log ft values of 4.9, 5.3, and 4.8, respectively, indicating that the π5/2+[642] → ν5/2+[633] transition largely contributes to these transitions, and thus, the populated levels should be the π5/2-[523]π5/2+[642] two-quasiparticle states. The Kπ = 0- octupole band established in 236Pu is located at higher energy than those in 238,240Pu, which implies that the octupole correlations become weak at 236Pu.

Measurements of EC and Weak α Decays of Neutron-deficient Transuranium Isotopes

Measurements of EC and weak α decays of neutron-deficient americium and curium isotopes have been performed using a gas-jet coupled on-line isotope separator. Decay schemes of 235, 236Am have been established for the first time, and proton-neutron configurations in 235, 236Am and their EC-decay daughters 235, 236Pu have been evaluated. Qα values were derived from measured α-particle energies for 235, 236 Am and 238Cm as well as for the new isotopes 233 Am and 237Cm.

Coulomb excitation of radioactive 132,134,136Te beams and the low B(E2) of 136Te.

The B(E2;0(+)-->2+) values for the first 2+ excited states of neutron-rich 132,134,136Te have been measured using Coulomb excitation of radioactive ion beams. The B(E2) values obtained for 132,134Te are in excellent agreement with expectations based on the systematics of heavy stable Te isotopes, while that for 136Te is unexpectedly small. These results are discussed in terms of proton-neutron configuration mixing and shell-model calculations using realistic effective interactions.

Decay Studies of Neutron-deficient Am, Cm, and Bk Nuclei Using an On-line Isotope Separator

Spectroscopic studies of heavy and superheavy nuclei give usinformation on various nuclear properties in heavy element re-gion. Measured α-decay energies lead to the precise determina-tion of atomic masses which are one of the most essential quan-tities to define nuclear decay properties, fission and fusion prob-abilities, and shell stability in heavy nuclei. Gamma-ray spec-troscopy following α, β, and orbital-electron capture (EC) de-cays and fine structure of α-particle spectra reveal excited statesin daughter nuclei, single-particle energies, proton-neutron con-figurations, and nuclear deformation.Neutron-deficient Am, Cm, and Bk nuclei predominantly de-cay by EC, and their α-decay probabilities are very small. Thisproperty makes experimentalstudies for these nuclei difficult. Inorder to measure γ rays following the EC decay and very weakαtransitions, it is quite important to isolate the nuclei of interestfrom a large amount of other reaction products. The traditionalmethod to separate heavy nuclei is chemical separation owingto its high separation efficiency. Another efficient method is in-flight separation using recoil separators. However, these meth-ods are not so suitable to apply to the neutron-deficient Am,Cm, and Bk nuclei; their half-lives are in the order of a few min-utes, and their α-decay intensities are very small. In this work,we have studied EC and α decays of the neutron-deficient Amand Cm nuclei using the gas-jet coupled JAERI on-line isotopeseparator (ISOL)

Interacting boson model plus broken-pair description of high-spin dipole bands

The interacting boson model with broken pairs has been extended to include mixed proton-neutron configurations in the fermion model space. The extended version of the model has been used to describe high-spin bands in the transitional nucleus ${}^{136}$Nd. Model calculations reproduce ten bands of positive and negative parity states, including the two dipole high-spin structures based on the $(\ensuremath{\pi}{h}_{11/2}{)}^{2}$ $(\ensuremath{\nu}{h}_{11/2}{)}^{2}$ configuration.

Stretched proton-neutron configurations in fp-shell nuclei: (II). Systematics

The systematics of the binding energies of stretched proton-neutron configurations ( f 7 2 , g 9 2 ) 8 − , ( p 3 2 , g 9 2 ) 6 − , ( g 9 2 , p 3 2 ) 6− and ( g 9 2 ) 2 9 + are studied over a wide range of f p-shell nuclei. The effective proton-neutron interaction energies deduced from the data are nearly constant for ( p 3 2 , g 9 2 ) 6 − and ( g 9 2 ) 2 9 + states while the ( f 7 2 , g 9 2 ) 8 − configuration reveals an additional repulsive term proportional to the partial filling of the f 7 2 orbit in the target ground state. Two-body matrix elements are extracted. A crude shell model, which predicts that the excitation energy of a stretched state is equal to the sum of the single-particle energies, works well for the 6 − and 9 + states, but fails for the 8 − levels due to neglect of the additional interactions described above. The physics underlying the empirically introduced basic assumptions of the crude shell model is discussed. The binding energies are found to be linearly dependent on the mass number A and the isospin T z component and are well described by the weak-coupling model of Bansal and French. The derived parameters agree with averaged values of a similar analysis for the single-particle states in the corresponding odd-even neighbours. The data indicate a significant change of the particle-hole energies with closure of the proton f 7 2 shell.

Stretched proton-neutron configurations in fp-shell nuclei: (I). Experimental results of the (α, d) reaction

The (α, d) reaction has been investigated at bombarding energies around 55 MeV on 40,42,44,48Ca, 50,52,54Cr, 54,56,58Fe, 58,60,62,64Ni, 64,66Zn and 70,72Ge targets. The angular distributions of prominent transitions were analyzed with the distorted-wave Born approximation and shown to correspond to unnatural-parity stretched proton-neutron configurations. Strong transitions of ( g 9 2 ) 2 9 + and ( p 3 2 , g 9 2 ) 6− character were identified in most final nuclei. Additionally, dominant transitions with ( f 7 2 , g 9 2 ) 8− structure were observed for target nuclei with an open proton ( f 7 2 shell. These stretched high-spin states could be consistently described by an average normalization constant N = 225 ± 85 indicating their relatively pure two-nucleon structure. In the lower-mass Sc and Mn isotopes, configurations built on g 9 2 states are increasingly damped because of the strong fragmentation of the underlying single-particle strength.

IBFFM yrast states in odd-odd nuclei associated withO(6) andSU(3) limits

In the Interacting Boson-Fermion-Fermion Model (IBFFM) for odd-odd nuclei the yrast states are calculated for\(\tilde j_\pi = \tilde h_{1 1/2} ,\tilde j_v = \tilde i_{1 3/2} \), proton-neutron configurations coupled to theO(6) andSU(3) boson core. The results are discussed in terms of the strongly coupled/ decoupled band type criterion and in comparison to the peaceful and conflicting case of asymmetric rotor model.

Level structure in double odd 104Ag

The level structure of the odd-odd 47 104Ag 57 nucleus has been studied using the 103Rh(α,3nγ) 104Ag reaction. The assignment of γ-rays to 104Ag and the construction of the level scheme are based on relative excitation function and γ- γ- Δt coincidence experiments. Spins and parities of levels are proposed on the basis of the angular distribution of γ-rays and conversion electron measurements. Excited states with spin value up to J π = 13 + have been observed. The 212.1 keV; J π = 7 + level was found to decay with T 1 2 = 1.4±0.2 ns while for the 112.6 and 387.0 keV levels upper limits of T 1 2 < 1 ns and T 1 2 ≦ 0.4 ns , respectively, were obtained. A positive-parity band with spin sequence ΔI = 1 based on the J π = 5 + ground state has been identified. This band is interpreted as resulting from the coupling of the π g 9 2 ν g 7 2 proton-neutron configuration to the even-even moderately deformed core. Performed calculations on the basis of the Nilsson model (at deformation ε = 0.15) with Coriolis interaction reproduce well the observed band and the electromagnetic transition probabilities when taking into account attenuation of the Coriolis interaction α = 0.55.

Nuclear data sheets for A = 198

Nuclear structure information available to April 1977 has been reviewed. The level properties obtained from various reaction and decay experiments are shown in the drawings. Experimental methods, references, and comments are given in the text. Proton-neutron configurations are associated with a number of levels in odd—odd nuclei.

Proton-neutron correlations in nuclei with N = 30

Structure of the nuclei with N = 30 and Z = 20–28 is investigated by the nuclear shell model within the proton-neutron configurations (1f 7 2 ) z−20 p × (2p 3 2 , 2p case1 2 , 1f case5 2 ) 2 n. Effective proton-neutron interactions determined by a least-squares fit to the observed spectra of N = 29 nuclei are adopted. Agreement of the calculated spectra with experimental spectra is satisfactory. Strong correlations between protons and neutrons break down the pairing scheme and lower the first J = 2 levels in doubly even nuclei, which is shown from the resultant wave functions. A relation between the shell model and collective rotational model is discussed concerning the calculated rotation-like spectrum of 56Fe. Electromagnetic properties and spectroscopic factors of single-nucleon transfer reactions are calculated. They are in good agreement with experiments.

Realistic Partial State Densities for Proton-Neutron Configurations in Nuclei near Closed Shells

Using Nilsson single-particle level schemes as input, we have calculated partial state densities for excited proton-neutron configurations with specified particle-hole numbers. The recursion relation method used is general and free of any mathematical approximations. Results are given for nuclides in the Sn region and for several Pb isotopes. The strong shell effects in these nuclei are clearly reflected in their state densities. The results are compared to densities obtained from the equidistant-spacing model. This model not only fails completely at low excitation energies as expected, but it also disagrees in several cases with the average behavior of the realistic densities at higher energies.