M1 resonances in unstable magic nuclei

Within a microscopic approach which takes into account RPA configurations, the single-particle continuum and more complex 1p 1 h?phonon configurations isoscalar and isovector M1 excitations for the unstable nuclei^56,78Ni and^100,132Sn are calculated. For comparison, the experimentally known Ml excitations in⁴⁰Ca and²⁰⁸Pb have also been calculated. In the latter nuclei good agreement in the centroid energy, the total transition strength and the resonance width is obtained. With the same parameters we predict the magnetic excitations for the unstable nuclei. The strength is sufficiently concentrated to be measurable in radioactive beam experiments. New features are found for the very neutron rich nucleus⁷⁸Ni and the neutron deficient nucleus¹⁰⁰Sn.     

Extended Theory of Finite Fermi Systems for magic and nonmagic nuclei

Consideration is given to a short review of the main features, recent results, and prospects of the Extended Theory of Finite Fermi Systems (ETFFS), which has been applied in the past 15 years to collective excitations in the neutral channel for nuclei with and without pairing. The theory is an extension of the Migdal standard TFFS to include in a consistent way the single-particle continuum and more complex 1p1h ⊗ phonon or 2qp ⊗ phonon configurations beyond the RPA or QRPA ones; i.e., the theory takes into account all three known mechanisms of giant-resonance width. To the most extent, the theory was developed and applied to nuclei without pairing. A quantitative explanation of the giant-resonance widths was obtained, with the complex configurations contributing about half of the width. In addition, a large part of the observed giant-resonance gross and fine structures can be directly traced back to the specific complex configurations, and the recent results of the (α, α′) experiments in ⁴⁰Ca and ⁵⁸Ni could be explained. Consistent use of the Green’s function method makes it possible to include and calculate some effects which were practically unstudied earlier. These are ground-state correlations induced by complex configurations and “refined” basis effects, in particular, the second (or quasiparticle-phonon) mechanism of pairing. Both of them can be studied in current experiments. In the past five years, the ETFFS has been developed and applied actively to even—even and odd-mass nuclei with pairing. Calculations of the E1 pygmy resonance in Ca and Sn isotopes have shown that this phenomenon, which is important for (n, γ) and (γ, n) reactions, cannot be explained without allowing for complex configurations. Consideration of the single-particle continuum and the practical universality of the interaction parameters allow the ETFFS to be used for calculations of unstable nuclei. The prospects and status of the necessary development of a self-consistent ETFFS for nuclei with pairing are discussed. The text was submitted by the author in English.     

High spin states in121Te

High spin states of¹²¹Te, populated in the¹¹⁴Cd(¹¹B,p3n) reaction, have been studied throughγ-ray spectroscopy. The level scheme has been established up toJ ^π=51/2^?. Three-quasiparticle states, based on the πg _7/2 ²?νh _11/2 andπg _7/2 d _5/2?νh _11/2 configurations, have been identified. A favoured 39/2^? state is suggested to be the fully aligned [πg _7/2 ²]₆₊?[νh _11/2 ³ _27/2? yrast non-collective oblate configuration. This assignment is supported by Total Routhian Surface (TRS) calculations which also suggest a similar oblate assignment to the states atJ ^π=21/2^? and 23/2^?. A higher 47/2^? state is also found and is suggested to be the fully aligned [πg _7/2 ²]₆₊?[νh _11/2 ⁵]_35/2? configuration.     

Transition probabilities and static moments of excited states in even-even nuclei

A new approach is made to calculate the generalized single-particle density matrixp ^μv in even-even nuclei. It is shown that this quantity is included in the three particle response function, for which we derived a renormalized equation. Taking into account in a consequent way the effective particle-hole interaction we received a formula for the static moment of excited states and the transition probability between such states which is essentially different from the usual RPA theory.     

A modified RPA for open-shell nuclei

A modified RPA suitable for open-shell nuclei is constructed using the Green function method. Unlike the standard RPA, this modification makes allowance for the spread of bound single-particle and single-hole states over more complex configurations which is done due to a more realistic approximation of single-particle Green functions. The calculations of the nucleon-nucleus S-matrix and of the probability amplitudes for the single-particle transitions of the nucleus from the ground state into scattering states are examined. For the separable effective forces these quantities are expressed in closed form.     

Description of charge-exchange resonances in deformed nuclei

The (p, n) and (n, p) transition strength functions with excitation of the GT resonance, other 1⁺ states, spin-dipole with λ^π=0^- andE1 charge-exchange resonances in deformed nuclei in the regions 156≦A≦168 and 236≦A≦240 are calculated in the RPA. It is shown that the GT resonance has a maximum at 18–20 MeV, and in the region of 5–6 MeV around maximum (60–70)% of strength is concentrated. The spindipole resonance with λ^π=0^-, 1^- and 2^? strength is distributed within 14–33 MeV and theE1 strength within 25–29 MeV. The latter is splitted withΔE equal to 0.6–2 MeV into two peaks withI ^π I=1^-0 and 1^?1. In the region of 4–7 MeV around maximum 73–77% ofE1 strength is concentrated. The total (n,p) transition strength is 10–200 times as small as the total (p, n) transition strength.     

Odd nuclei in the “Wild west”

Much has been learned recently concerning the structure of odd-odd nuclei in thef-p-g shell. Regular rotational bands appear above a complex of low-lying levels. The yrast π=+ bands have the most consistent structure because of their uniqueπg _9/2?νg _9/2 structure. Their moments of inertia and signature splittings show a sharp change atI≈9h, indicating a change from single-particle to collective behavior. Large alternations arising from the single-particle-core coupling characterize theB(M ₁) strengths. There is more variation in the π=? bands due to the variety of possible structures.     

Shell model and octupole states in148Gd from in-beam experiments

Through (α, 4n) and (τ, 3n) reactions the high-spin states in the two-neutron nucleus¹⁴⁸Gd were populated up toI ^π=21? at 7.2 MeV, including numerous states above the yrast line. The¹⁴⁸Gd energy spectrum is interpreted in terms of the spherical shell model. Identification of the (νf _7/2 i _{1 3/2})_10? state gives the νi _13/2 single particle energy free of octupole admixtures as 2.1(1) MeV. Eight high-spin states between 1.2 and 3.7 MeV were identified as the couplings of the two valence-particles to the¹⁴⁶Gd octupole phonon, and three above-lying levels are assigned as double-octupole excitations including a 12⁺ state which decays by anE3-E3 stretched cascade. All these octupole levels can be quantitatively predicted from the one-particle x phonon spectrum of¹⁴⁷Gd. The high-spin states above 3 MeV are four-quasiparticle excitations ofπ ⁺¹ π ^?1 ν ² andπ ² ν ² type and their energies are in good accord with shell model estimates.     

Triaxiality, chirality and γ-softness

Current work explores the impact of γ-softness on partner bands built on the πh_11/2νh_11/2 particle-hole configurations in triaxial odd-odd nuclei. The results of calculations conducted using a core-particle-hole coupling are presented. The model Hamiltonian includes the collective core, the single-particle valence nucleons, and separable quadrupole-quadrupole interactions. The Kerman-Klein method was applied to find eigenstates, which provided a convenient way for exploring core effects. Calculations were made for triaxial cores with various γ-softness using the General Collective Model keeping the expectation value for the triaxiality parameter fixed at (γ)=30°. The degeneracy in the πh_11/2νh_11/2 bands results from the calculations for the γ-rigid core but is lifted for the γ-unstable core.     

Phonon–particle coupling effects in the single-particle energies of semi-magic nuclei

A method is presented to evaluate the particle–phonon coupling (PC) corrections to the single-particle energies in semi-magic nuclei. In such nuclei, always there is a collective low-lying 2⁺ phonon, and a strong mixture of single-particle and particle–phonon states often occurs. As in magic nuclei the so-called g _L ² approximation, where g _L is the vertex of the L-phonon creation, can be used for finding the PC correction δΣ^PC(ε) to the initial mass operator Σ₀. In addition to the usual pole diagram, the phonon “tadpole” diagram is also taken into account. In semi-magic nuclei, the perturbation theory in δΣ^PC(ε) with respect to Σ₀ is often invalid for finding the PC-corrected single-particle energies. Instead, the Dyson equation with the mass operator Σ(ε) = Σ₀ + δΣ^PC(ε) is solved directly, without any use of the perturbation theory. Results for a chain of semi-magic Pb isotopes are presented.     

TheN=83 nucleus149Dy from Gamow-Teller decay of its 11/2− and 1/2+ 149Ho parents

A high-sensitivityγ-spectroscopic study of the¹⁴⁹Hoπh _11/2 andπs _1/2 β-decays using mass separated sources has located dominant 0⁺ → 1⁺ GT decay strength associated with decay of pairedh _11/2 protons, leading to 3п-states in the¹⁴⁹Dy daughter nucleus. In theirγ-decay low-lying¹⁴⁹Dy levels characteristic of anN=83 nucleus are excited. They include theνf _7/2,νp _3/2,νh _9/2 andνp _1/2 single particle- and theνs ₁ ^2/?1 andνd ₃ ^2/?1 two-particle one-hole states, as well as the νf _7/2 × 3^? andνf _7/2 × 2⁺ particle-phonon multiplets. A synopsis is given of these excitations in theN=83 isotones from¹⁴⁹Nd to¹⁵³Yb. The¹⁴⁹Dy GT decay strength is discussed in terms of the¹⁴⁷Tb₈₂ and¹⁴⁸Dy₈₂ decays. The strength function results are also compared with independent¹⁴⁹Ho 11/2^? decay data from the literature based on totalγ-ray absorption measurements.     

Neutrino-12C reactions and the LSND and KARMEN experiments on neutrino oscillations

We present new theoretical results of the flux-averaged ¹²C(ν _e , e ^?)¹²N and ¹²C(ν _μ , μ ^?)¹²N cross sections with ν _μ (ν _e ) coming from the decay-in-flight (decay-at-rest) of π ⁺(μ ⁺). These cross sections are relevant for the interpretation of the recent experiments on neutrino oscillation performed by the LSND and KARMEN collaborations. The microscopic approaches used are charge-exchange random phase approximation (RPA), charge-exchange RPA among quasiparticles (QRPA), and the Shell Model. We show that the exclusive cross sections are in nice agreement with the experimental values for both reactions when a large-scale shell-model calculation is performed. Concerning the inclusive cross section for ν _μ coming from the decay-in-flight of π ⁺, the calculated value keeps overestimating the experimental one by 20–30%, while the inclusive cross section due to ν _e coming from the decay-at-rest of μ ⁺ is in agreement within experimental error bars with the measured values. The shell-model prediction for the decay-in-flight neutrino cross section is reduced compared to the RPA one because of the different kind of correlations in the calculation of the spin modes (in particular, the quenching of the 1⁺) and partially due to the shell-model configuration basis, which is not large enough, as we show using arguments based on sum rules.     

Rotational and vibrational states in 115Te

High-spin states of the ¹¹⁵Te were studied by in-beam spectroscopy with the ⁸⁹Y (²⁹Si, p2n) fusion-evaporation reaction at a beam energy of 108 MeV. γ?γ coincidence and γ?γ angular correlation analyses were employed for determining the level scheme of ¹¹⁵Te. We have identified two vibrational-like bands built on the νh_11/2 and νg_7/2 quasiparticle states and the noncollective oblate states from the full alignment of quasiparticle configurations. In addition, a regular ΔI = 2 sequence with positive-parity was found for the first time in odd-A Te nuclei. This sequence is interpreted as a deformed structure resulting from three-quasiparticle alignment having the [π(g_7/2, h _11/2) ? ν(h _11/2)] configuration. Calculations of total Routhian surfaces and cranked shell model were performed and were used for assigning quasiparticle configurations to the states in ¹¹⁵Te.     

g-factor measurement of the two lowest 10+ states in 140Sm

The g-factors of the two 10⁺ states in ¹⁴⁰Sm at 3172 and 3210 keV have been measured by means of the TDPAD method. The obtained values are g = −0.176(20) and g = 1.27(9), respectively. The reaction ¹¹⁶Cd (²⁸Si, 4n)¹⁴⁰Sm, at incident beam energy of 125 MeV, has been used. The measured values clearly show that these states can be described as rather pure (νh_11/2)⁻² and (πh_11/2)² configurations.     

Nuclear levels in 238Np

Low and high energy spectra from thermal neutron capture in ²³⁷Np have been studied over the energy ranges 25 to 650 keV and 2600 to 5500 keV. Primary transitions from neutron capture in four resonances have been observed between about 4800 and 5400 keV. Using 12 MeV deuterons, (d, p) spectra at three angles have been observed with a magnetic spectrograph. A nuclear level scheme for ²³⁸Np has been constructed by combining the results of the above measurements with previous data from a study of the ^242mAm α-decay. The Nilsson model has been used to interpret the level structure. Including results from the previous α-decay study, nine rotational bands can be assigned. The Nilsson configurations (K^π [Nn₃ΛΣ]) and band-head energies are: 2⁺π[642↑]?ν[631↓], 0.0 keV; 3⁺π[642↑]+ν[631↓], 86.6 keV; 3^?π[523↓]+ν[631↓], 136.0 keV; 2^?π[523↓]?ν[631↓], 182.8 keV; 5⁺π[642↑]+ν[622↑], 278.1 keV; 0⁺π[642↑]?ν[622↑], 332.5 keV; 5^?π[523↓]+ν[622↑], 342.6 keV; 0^?π[523↓]?ν[622↑], 286.0 keV; 6^?π[642↑]+ν[743↑], 301 keV. The measured (d, p) reaction cross sections are compared with theoretical calculations based on these assignments. The Gallagher-Moszkowski rule is found to be valid in the four cases where we have observed both parallel and antiparallel coupled bands with $\text{K}{}^{\mathrm{+}}\text{= Ω}{}_{\mathrm{<mtext>p</mtext>}}\text{+Ω}{}_{\mathrm{<mtext>n</mtext>}}$ and $\text{K}{}^{\mathrm{?}}\text{= |Ω}{}_{\mathrm{<mtext>p</mtext>}}\text{?Ω}{}_{\mathrm{<mtext>p</mtext>}}\text{|}$. The lowest levels of the two K = 0 bands have spin I = 1; Newby odd-even shifts can be determined in both cases.     

Emergence of new magic numbers,N = 16 and 32 by tensor interaction in Skyrme–Hartree–Fock theory

Melting of N = 20 shell and development of N = 16 and 32 shells for neutron-rich nuclei have been studied extensively by including tensor interaction in Skyrme–Hartree–Fock theory optimized to reproduce the splitting Δ1f shells of ^40,48Ca and ⁵⁶Ni nuclei. Evolution of gap generated by the energy difference of single-particle levels ν2s _1/2 and ν1d _3/2 has been found to be responsible for shell closure at N = 16. The splitting pattern of spin–orbit partners 2p shell model state in Ca, Ti, Cr, Fe and Ni isotopes indicates the formation of a new shell at N = 32 region.     

Study of the π+π+π-π- system centrally 0 produced by incident π+ andp beams at 85 GeV/c

The reactions π⁺ p→π⁺(π⁺π⁺π^-π^-)p andpp→p(π⁺π⁺π^-π^-)p, where the (π⁺ π⁺ π^- π^-) system is centrally produced have been studied at 85 GeV/c. The π⁺π⁺π^-π^- mass spectrum shows evidence for thef ₁ (1285) meson with a mass of 1278±2 MeV and width 41±12 MeV which decays mainly through ρ⁰(770)π⁺π^-. Thef ₁(1285) is also observed in the ηπ⁺π^- channel. There is no significant evidence for ηππ or 4π decay modes of thef ₁(1420). The ρ⁰ρ⁰ production is found to be small. A Dalitz plot analysis of the 3π system shows evidence fora ₂ (1320) production and for a large contribution of theJ ^PC=1⁺⁺ ρπ wave.     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

On the role of the Δ(1232) on the transverse nuclear response in the (e,e′) reaction

The transverse nuclear response to an electromagnetic probe which is limited to create (or destroy) a particle-hole (ph) or delta-hole (Δh) pair is analyzed. Correlations of the random phase approximation (RPA) type and self-energy insertions are considered. For RPA correlations we have developed a scheme which includes explicitly the Δ and the exchange terms. Self-energy insertions over ph and Δh bubbles are studied. Several residual interactions based on a contact plus a (π + ϱ)-meson exchange potential are used. All calculations are performed in non-relativistic nuclear matter. The main effect of the Δ is to reduce the intensity over the nuclear quasi-elastic peak. Exchange RPA terms are very important, while the terms with self-energy insertions depend strongly on the residual interaction employed. Our results are compared with data for ⁴⁰Ca at momentum transfer q = 410 and q = 550 MeV/c, where the longitudinal response is also briefly discussed.     

In-beamγ-ray spectroscopy of the neutron deficient100Ag

The neutron deficient nucleus¹⁰⁰Ag, three proton holes below and three neutron particles above the N=Z=50 shell closure at¹⁰⁰Sn, has been identified and studied by in-beam spectroscopy. The reactions⁴⁶Ti(⁵⁸Ni,3pn) at 231 MeV and⁶⁴Zn(⁴⁰Ca,3pn) at 167 MeV of the respective⁵⁸Ni and⁴⁰Ca beams were used, and states up to 8.7 MeV excitation energy and spin I?20 were found in two mainγ-ray cascades forming band-like structures of even and odd parity. Large scale shell model calculations suggest predominantπg ₉ ^2/?3 ν(d_5/2,g_7/2)³ andπg ₉ ^2/?3 ν(d_5/2,g_7/2)²h_11/2 structures with maximum spins I ^π = 19⁺ and I ^π =22^?, respectively, for the two level sequences. The influence ofπp_1/2 vs.νh_11/2 excitations is discussed for low lying odd-parity levels.