Pairing-isopairing competition in odd-odd nuclei

We discuss the systematics of 0⁺, v = 0, T = |T_Z| + 1 levels in odd-odd nuclei and conclude about the ground state isospin and seniority of these nuclei. For N = Z nuclei starting from the $\text{1f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ shell v_g.s = 0 |T_g.s = |T_z + 1. Otherwise v_g.s = 2, T_g.s = |T_z|.     

Gamow-Teller transitions from 58Ni to discrete states of 58Cu

Under the assumption that isospin is a good quantum number, symmetry is expected for the transitions from the ground states of T = 1, T _z = ±1 nuclei to the common excited states of the T _z = 0 nucleus situated between the two nuclei. The symmetry can be studied by comparing the strengths of Gamow-Teller (GT) transitions obtained from a (p, n)-type charge-exchange reaction on a target nucleus with T _z = 1 with those from the β-decay of the T _z = - 1 nucleus. The A = 58 system is the heaviest for which such a comparison is possible. As a part of the symmetry study, we measured the GT transitions from ⁵⁸Ni (T _z = 1) to ⁵⁸Cu (T _z = 0) by using the zero-degree (³ He, t) reaction at 150 MeV/nucleon. With the achieved resolution of 50 keV, many hitherto unresolved GT states have been identified. The GT transition strengths were obtained for states up to 8 MeV excitation, i.e., near to the Q window limitation ( Q _EC = 9.37 MeV) of the β-decay from ⁵⁸Zn (T _z = - 1) to ⁵⁸Cu. The strength distribution is compared with that from shell-model calculations. Received: 24 November 2001 / Accepted: 30 January 2002     

The T = Tz + 2 GDR and the dipole states of the A = 6 and 14 nuclei

The isospin properties of the giant dipole resonance (GDR) built on states with isospin T = T_z + 1 are discussed. Results of a shell-model calculation with realistic interactions for dipole transitions to various low-lying states in ⁶Li, ⁶He, ¹⁴N and ¹⁴C are presented. The T = T_z + 2 component of the GDR in ¹⁴N decaying to the 0⁺T = 1 state is predicted at the surprisingly low energy E_x = 26.0 MeV.     

The deformation of the ground and excited states in the Ag and Sn nuclei

The microscopic calculation of the potential energies in the ground and excited states of Ag and Sn nuclei has been performed. The single particle Nilsson potential and the shell correction Strutinski method have been used. The weak sensitivness to nonaxial deformation has been found for even neighbours of these nuclei. The small tendency towards prolate deformation of the ground and excited one-quasiparticle states originating from theg _9/2 proton subshell in^101–105Ag odd isotopes has been noticed. The behaviour with quadrupole e and hexadecapole ε₄ deformation of the ground and two-quasiparticle excited 0⁺ states originating from thed _5/2,g _9/2 andg _7/2 proton subshells andh _11/2 neutron subshell in^112–118Sn has been investigated. The small quadrupole deformation of the excited 0⁺ states has been found what is in agreement with the experimental data concerning the rotational bands build on the first excited 0⁺ states in Sn isotopes.     

The g-factor of the yrast 4+ state in 20Ne from transient-field precession measurements

Transient field precessions of the first excited 2⁺ and 4⁺ states in ²⁰Ne nuclei traversing polarized gadolinium have been measured. The deduced g-factor of the 4⁺ state, g = + 0.08(20), agrees with our earlier reported value of g = ? 0.10(19) measured in iron by the same technique. The significant reduction in the value of the $\text{g-}\text{factor}\text{,}\text{g}\text{?}\text{= ? 0.01(14)}$, relative to that of the 2 ⁺ state, g = + 0.54(4), is incompatible with the pure T = 0 character expected for low-lying states in this self-conjugate nucleus. In addition, the lifetime of the 4⁺ state has been measured to be τ = 95(13) fs, in agreement with a previous measurement.     

A note on the induced pseudo-scalar coupling constant in μ− + 12C(g.s)→ 12B(1+,g.s+ ν μ

The sum of the induce pseudo-scalar g_P and the second-class induced tensor g_T couplings in muon capture is found to be (g_P + g_T)g_A= (13.3 + 1.8) by comparing with recent data on the recoil polarization of ¹²B(1⁺) in the μ^? + ¹²C→¹²B+ ν_μ process, even aaking into account the corrections due to the gamma decay of the excited states of ¹²B to ¹²B(1⁺;g.s.). With the canonical value for g_P and in the absence of meson exchange effects, our results indicate a large positive value for g_T which is in strong contradiction with the conclusion of the Louvain-Saclay—ETH experiment.     

Lifetime measurements of levels in 34Ar: Isoscalar and isovector matrix elements for E2 analogue transitions

Mean lifetimes have been measured for the low-lying levels in ³⁴Ar(T_z = ? 1) excited by the bombardment of ³He-implanted Au targets with a beam of 80 MeV ³²S. The lifetimes determined by fitting Doppler-broadened γ-ray lineshapes are as follows (E_x in keV, τ in fs): 2091, 460 ± 60; 3288, 280 ± 50; 3871, > 270 and 4128, 1301⁺¹⁷⁰_?130. The resulting transition matrix elements for the 2⁺₁ → 0⁺₁ and 2⁺₂ → 0⁺₁ transitions are compared with those for the analogue transitions in ³⁴S (T_z = + 1) and ³⁴Cl(T_z = 0) to determine the isoscalar and isovector components. The component values are compared with theoretical calculations and with values of the ratio of neutron to proton matrix elements determined by inelastic scattering with strongly interacting probes.     

Optical absorption spectrum of Cr3+ ion doped in zinc cesium sulphate hexahydrate

Optical absorption spectrum of Cr³⁺ ion doped in zinc cesium sulphate hexahydrate single crystal has been studied both at room and liquid nitrogen temperatures. From the nature and position of the bands a successful interpretation of all the bands could be made assuming octahedral symmetry for the Cr³⁺ ion in the crystal. The observed bands are assigned to the transitions from the ground ⁴A_2g(F) state to the excited ²E_g(G), ²T_1g(G), ⁴T_2g(F) and ⁴T_1g(F) states.The crystal field parameters Dq = 1735 cm^?1, B = 635 cm^?1 and C = 4.75 B are found to give a good fit to the observed band positions.     

169Tm NMR study of the high-Tc superconductor TmBa2Cu3O6.92

The ground state of Tm³⁺ ion (4t²,³H₆) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100÷130 cm^?1. The pulsed NMR of¹⁶⁹Tm nuclei (spinI=1/2) in a magnetically-oriented TmBa₂Cu₃O_6.92 power is studied at temperatures 1.5÷4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (≥80% of total amount, having relaxation timesT ₁ ⁽¹⁾ ≈35 ms,T ₂ ⁽¹⁾ =80÷250 μs) and slowly relaxing Tm2 nuclei (≦20%,T ₁ ⁽²⁾ =300÷1000 μs). The¹⁶⁹Tm NMR spectra are described by the spin-Hamiltonian ?₁ = ??Σγ _i H _i I _i (i = x,y,z), where |γ _x ⁽¹⁾/2π| = 5.3(1), |γ _y ⁽¹⁾/2π| = 6.6(1), |γ _x ⁽²⁾/2π| = |γ _y ⁽²⁾/2π| = 5.1(1), |γ _z ⁽¹⁾/2π| = |γ _z ⁽²⁾/2π| = 2.3(1) kHz/Oe, and axesx, y, z coincide with the crystal axesa, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic (CEF of D_2h symmetry) and tetragonal (D_4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5÷4.2 K. The smallestT ₂ ⁽¹⁾ -values (≈80 μs) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystalc-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H‖c as well as the origin of unusually largeT ₂ ⁽²⁾ -values remain unclear.     

Negative parity states in 16O from coupled-cluster equations

The extension of the coupled-cluster theory to excited states of closed-shell nuclei is presented. The method is applied to the negative parity particle-hole spectrum of ¹⁶O. Some approximations found necessary in the application of the theory at the three-body level are discussed. Numerical calculation bears out characteristic discrepancies to experimental energy levels for some T = 0 states whereas agreement with experiment is found for T = 1 states.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.     

The structure ofT=1 levels inA=14 nuclei

States in¹⁴C, populated via the¹¹B(α, p) reaction at 14 MeV bombarding energy, were investigated with the Doppler Shift Attenuation Method (DSAM). The analysis ofpγ coincidence spectra taken in a Ge(Li) detector at 0? and 120? with the particle detector near 0? with respect to the beam direction yielded the following lifetimes and lifetime limits for the states at 6.09, 6.59, 7.01 and 7.34 MeV, respectively, <20 fs, <1,200 fs, <7 fs and 160±60 fs. Shell model calculations using the MSDI and an empirical interaction fitted to nuclear states in 1p and 2s 1d shell nuclei, are shown to account very well for the experimental levels andγ-transition rates of 5 different multipolarities. The structure of the (J ^*,T)=(2⁺, 1) levels is discussed in the light of the experimentally observed T_z-dependence of the 2⁺, 1→0⁺, 1E2 matrix elements.     

Present status and future aspects of nuclear structure close to 100Sn

The present status of experimental approach to ¹⁰⁰Sn in the spectroscopy of excited states is landmarked by the T_z=3/2 nuclei between ⁹⁵Pd and ¹⁰¹In and the T_z=1 nuclei ⁹⁴Pd and ⁹⁸Cd. The detection limits with Pre-EUROBALL γ-arrays and ancillary detectors are below the 10^?5 level of the total fusion- evaporation residue cross section. A large scale shell model analysis of the existing data reveals the shell structure at ¹⁰⁰Sn, which shows a remarkable similarity to ⁵⁶Ni. Evidence for an increasing proton-neutron interaction in approaching the N=Z line is deduced from high spin isomers and spherical yrast lines. The effective E2 operator for protons and neutrons and implications for a low lying particle-hole (ph) E2 excitation in ¹⁰⁰Sn are discussed.     

Two-particle transfer reactions leading to giant pairing resonances

The triplet of τ = 1, 0⁺ two-particle states in the lead region such that the state τ_z = 1 is ²¹⁰Pb(g.s.) are analysed. It is found that these three states have the same pairing-collective features. However, the states with τ_z ≠ 1 are built upon high-lying single-particle configurations. They can be considered pairing giant resonances. The pairing giant resonance states are found to be strongly excited in two-particle transfer reactions.     

The g-factor of the first excited 2+ state in 26Mg from transient field precession measurements

Transient field (TF) precessions of the first excited 2⁺ state in ^24,26Mg have yielded g = +0.50 (13) for ²⁶Mg. Contrary to an earlier measurement the present data is in good agreement with Hartree-Fock calculations. The large effective TF of 280 (30) T at Mg nuclei for ν_ion ≈ 2.0 ν₀ is attributed to K-shell polarization of the ions.     

Investigations of the Spin-Hamiltonian Parameters for the Rhombic Mo5+ Centers in Ca1−x Y x MoO4 Crystal

The spin-Hamiltonian parameters (g factors g _i and hyperfine structure constants A _i , where i = x, y, z) of the rhombic Mo⁵⁺ center in Ca_1?x Y _x MoO₄ crystal are calculated from the high-order perturbation formulas based on the two-mechanism model for the rhombic d¹ tetrahedral clusters with the ground state |d _z 2〉. In these formulas, besides the contributions due to the widely applied crystal-field (CF) mechanism concerning CF excited states, those due to the charge-transfer (CT) mechanism (which is omitted in CF theory) concerning CT excited states are considered. The calculated results are in reasonable agreement with the experimental values. The calculations show that because of the great relative importance of CT mechanism for the components of spin-Hamiltonian parameter along x and y axes, the accurate and complete calculations of spin-Hamiltonian parameters for Mo⁵⁺ and other high valence state dⁿ ions in crystals should take account of both the CF and CT mechanisms. The defect model of the rhombic Mo⁵⁺ center is also confirmed from the calculations.     

Study of the36Cl(n,p)36S reaction in the neutron energy range up to 10 keV

The³⁶Cl(n,p)³⁶S reaction cross-section was measured by the time-of-flight method in the IBR-30 pulsed reactor of the JINR. The measured cross-section shows three not previously observed neutron resonances with energies E_n = 1.3; 3.5 and 8.2 keV for which the parameters A_p=gΓ_nΓ_p/Γ: 0.07 ±0.01; 0.08 + 0.03 and 1.7 + 0.3 eV were determined, respectively. Comparison of these results with the excited states obtained by the³⁶S(p, γ)³⁷Cl reaction made possible a more exact determination of the scale energy of the³⁷Cl nucleus excited states.     

Increased statistics and observation of the gT,gT′, and gT′&#x27; 2++resonances in the Glueball enhanced channel π−p→φφn

In a new experiment we have obtained 3006 new events in the reaction π⁻p→φφn, approximately doubling the statistics which now total 6658. A refined partial wave analysis of the φφ system again reveals three resonances (g_T, g_T′, g_T′') with I^GJ^PC=0⁺2⁺⁺. The absence of the OZI suppression, and the observation of only three J^PC=2⁺⁺ states (g_T, g_T′, g_T′') which comprise virtually all the cross section are unusual characteristics of the data. The large φφn signal occurs over a mostly structureless and incoherent φK⁺K⁻n background. All these unusual characteristics are well explained if these states are produced by 1–3 glueballs (multigluon resonances).