Electromagnetic transitions in some odd-proton deformed nuclei

In-beam measurements of nanosecond lifetimes applying the method of delayed γ-γ coincidences were performed in the (p, n) reaction. Analysing the time spectra with the centroid shift method, the following half-lives of excited nuclear states in the subnanosecond region could be found: $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(353.2}\text{keV in}{}^{161}\text{Ho}\text{) = 0.52±0.15}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(252.7}\text{keV in}{}^{161}\text{Ho}\text{) ≦ 0.2}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(579.4}\text{keV in}{}^{161}\text{Ho}\text{) ≦ 0.2}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(431.2}\text{keV in}{}^{163}\text{Ho}\text{) = 0.37±0.15}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(439.9}\text{keV in}{}^{163}\text{Ho}\text{) = 0.35±0.15}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(471.3}\text{keV in}{}^{163}\text{Ho}\text{) ? 0.2}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(612.8}\text{keV in}{}^{163}\text{Ho}\text{) ? 0.3}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(295.6}\text{keV in}{}^{171}\text{Lu}\text{) = 0.85±0.20}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(469.2}\text{keV in}{}^{171}\text{Lu}\text{) ≦ 0.2}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(357.0}\text{keV in}{}^{173}\text{Lu}\text{) = 0.40±0.08}\text{ns and}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(449.0}\text{keV in}{}^{173}\text{Lu}\text{) = 0.58±0.12}\text{ns}$. Following half-lives in ¹⁷³Lu have been remeasured: $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(425.3}\text{keV}\text{) = 0.84±0.20}\text{ns and}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(434.9}\text{keV}\text{) = 0.38±0.10}\text{ns}$. Absolute γ-ray transition probabilities are deduced and compared with Nilsson model predictions including pairing correlations. Coriolis mixing calculations are performed for K-allowed as well as for K-forbidden transitions.     

Electromagnetic transitions in some odd-neutron deformed nuclei

Using the reactions ^{155, 157}Gd(α,2n), ¹⁷⁸Hf(n,γ) and ^{177Hf(α, 2n}, the following half-lives of excited nuclear states have been measured: $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(188.1}\text{keV in}{}^{157}\text{Dy}\text{) = 1.00 ± 0.15}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(161.9}\text{keV in}{}^{157}\text{Dy}\text{) = 1.3 ± 0.2 μ}\text{S}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(177.6}\text{keV in}{}^{159}\text{Dy}\text{) = 9.0 ± 0.5}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(614.3}\text{keV in}{}^{179}\text{Hf}\text{) = 0.50 ± 0.15}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(720.7}\text{keV in}{}^{179}\text{Hf}\text{) ≦ 0.3}\text{ns}$, $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(516.4}\text{keV in}{}^{179}\text{Hf}\text{) < 0.2}\text{ns and}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(309.0}\text{keV in}{}^{179}\text{W}\text{) = 1.53 ± 0.10}\text{ns}$. A Ge(Li) timing system was employed. Electromagnetic transition probabilities are calculated in the Nilsson model including pairing and band mixing effects. Comparisons between theoretical and experimental results are performed.     

Electromagnetic transitions in some doubly odd deformed nuclei

Nanosecond lifetimes of excited states in doubly odd deformed nuclei have been determined by in-beam measurements applying the method of delayed γ-γ coincidences as well as by experiments in the radioactive decay with the method of delayed γ-ce coincidences, respectively. Analysing the time distributions and delayed γ-ray spectra, the following half-lives of isomeric states could be obtained for the first time: $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(63.7}\text{keV in}{}^{160}\text{Tb}\text{) = 60 ± 5}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(138.7}\text{keV in}{}^{160}\text{Tb}\text{) = 5.7 ± 0.5}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(82.0}\text{keV in}{}^{156}\text{Ho}\text{) = 1.25 ± 0.20}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(87.2}\text{keV in}{}^{156}\text{Ho}\text{) = 58.5 ± 3.5}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(139.2}\text{keV in}{}^{158}\text{Ho}\text{) = 1.85 ± 0.10}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(38.3}\text{keV in}{}^{162}\text{Ho}\text{) = 1.2 ± 0.2}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(179.9}\text{keV in}{}^{162}\text{Ho}\text{) = 8.7 ± 0.2}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(342.8}\text{keV in}{}^{164}\text{Ho}\text{) = 2.6 ± 0.2}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(295.1}\text{keV in}{}^{166}\text{Ho}\text{) = 1.10±0.15}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(44.6}\text{keV in}{}^{162}\text{Tm}\text{) = 1.40±0.15}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{,(163.4}\text{keV in}{}^{162}\text{Tm}\text{) = 1.1 ± 0.1}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(220.1}\text{keV in}{}^{178}\text{Ta}\text{) = 8.5 ± 1.0}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(289.5}\text{keV in}{}^{178}\text{Ta}\text{) = 2.0 ± 0.5}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(392.8}\text{keV in}{}^{178}\text{Ta) ≈ 1 ns, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(316.5}\text{keV in}{}^{186}\text{Re}\text{) = 0.20 ± 0.10}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(300.2}\text{keV in}{}^{188}\text{Re}\text{) = 1.5 ± 0.2}\text{ns and}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(482.2}\text{keV in}{}^{188}\text{Re}\text{) = 0.26 ± 0.10}\text{ns}$. Furthermore, upper limits for the half-lives of fifteen excited states in ¹⁶⁰Tb, ^{164, 166}Ho and ^{186, 188}Re have been estimated. For eight isomeric levels in ^{186, 188}Re, the lifetimes earlier determined have been remeasured. Unlike previous studies, the existence of isomeric states at 87.2 keV in ¹⁵⁶Ho and at 179.9 keV in ¹⁶²Ho is suggested. Absolute γ-ray transition probabilities are deduced and compared with single-particle estimates according to Weisskopf and Nilsson, the latter also including pairing correlations. The K-, Ω- and f-forbidden transitions can qualitatively be explained in terms of configuration mixings. Experimental El, ΔK= 1 transition matrix elements in odd-odd deformed nuclei are supposed to be appreciably influenced by higher-order vibrational admixtures coupled via RPC and p-n interaction mixings.     

Electromagnetic and weak interactions with nuclei: One-body densities in 6LI

Eleastic and inelastic magnetic electron scattering, and other electromagnetic data, are used to accurately determine one-body densities (and wave functions) for the valence nucleons in ⁶Li. Semi-leptonic weak processes are then analyzed in an essentially model-independent way.     

Partial transitions in radiative pion capture on light atomic nuclei

The partial transitions in radiative pion capture on light nuclei are studied within the shell model with intermediate coupling. The probabilities of capture from s- and p-states of a mesoatom and the total yield of γ-quanta have been calculated and compared with experimental data.     

Quenched magnetic transitions from high spin states in light nuclei

Recent investigations of stretchedM 6 transitions in thes-d shell have uncovered the unexpected systematic trend that isoscalarM 6 transitions are two to three times more retarded than the isovector ones. We present the result of a microscopic calculation analyzing the different mechanisms usually invoked in the formation of the quenching and suggest that an additional argument based on the difference in structure between isoscalar and isovector transition densities may be playing a predominant role in explaining the data.     

Allowed β-transitions,weak magnetism and nuclear structure in light nuclei

The general formalism for allowed β-transitions is derived in the framework of the elementary particle as well as in the impulse approximation treatment. Explicit expressions for the shape factor, the asymmetry and anisotropy of electrons emitted from oriented nuclei, the β-γ (CP) correlation and the β-γ angular correlation are given. Emphasis is imputed to the question how model independently these quantities can be evaluated. This problem has been studied in detail for the β-transitions in the nuclei ¹²B¹²N, ²⁰F²⁰Na, ²²Na and ²⁴Na²⁴Al. The computations of the nuclear matrix elements are carried out for the A = 12 system in intermediate coupling and for the sd-shell nuclei in the SU(6)-SU(3) coupling scheme using different residual interactions. Comparison to experiments is made. All results can be explained with ordinary CVC predictions without invoking second class currents.     

Electromagnetic processes in light nuclei: Results within a microscopic approach

Results obtained by investigating the electrodisintegration of A = 3 and 4 nuclei and the photodisintegration of A = 3, 4, 6, and 7 nuclei on the basis of precise solutions to dynamical problems involving NN and NNN forces are presented. The role of NNN forces, final-state relativistic effects, the contribution of meson-exchange currents, an anomaly in electrodisintegration at high momentum transfers and low energy transfers, sensitivity to nuclear dynamics in the exclusive electrodisintegration of ⁴He nuclei, and the soft-mode resonance in the photodisintegration of ⁶He nuclei are considered.     

Core polarization for l-forbidden M1 transitions in light nuclei

The experimental systematics of $\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?}\text{ld}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{1-}\text{forbidden}$ M1 transition rates in light nuclei with N, Z ? 20 are studied. Some surprising deviations from the expected behavior are qualitatively discussed in terms of an interplay between first- and second-order core polarization contributions. The B(M1) values in mirror pairs turn out to be close to each other within the experimental errors, the proton transition strengths being slightly larger. Isovector dominance is shown to account for this phenomenon.     

Electromagnetic response and breakup of light weakly bound nuclei in a dicluster model

The light weakly bound nucleus ⁷Li is studied within a dicluster α + t picture. Different observables obtained within our simple model are compared with previous calculations and experiments showing good agreement. In particular, we calculate dipole and quadrupole electromagnetic response to the continuum. The energy distribution of B(Eλ) values are consistent with the energy-weighted molecular sum rule and display a sizable contribution of non-resonant character arising from the weak binding property. The corresponding form factors for excitations to the continuum are used in a semiclassical coupled-channel scheme to get estimates for the breakup cross-section in a heavy-ion reaction. The nuclear contribution is found to play an important role in the process for bombarding energies around the Coulomb barrier. The masses and charges ratios of the two clusters are shown to lead to features of the cluster halo that may significantly differ from the one usually associated with one-nucleon haloes.     

Charge-exchange transitions in hot nuclei

A formalism based on the thermo field dynamics and allowing one to treat thermal effects on the strength distribution of charge-exchange transitions in hot nuclei is developed. The strength distributions of the allowed and first-forbidden p → n transitions are calculated for the neutron-rich nucleus ⁸⁰Ge at different temperatures. Then the electron capture rates on the same nucleus are calculated at temperatures and densities corresponding to an advanced stage of stellar evolution. Published in Russian in Yadernaya Fizika, 2009, Vol. 72, No. 8, pp. 1373–1384. The text was translated by the authors.     

Axial polarizability and weak currents in nuclei

The effect of the isobaric excitations on the weak axial coupling constants in nuclei is studied through P.C.A.C. We first establish the Klein-Gordon equation for the virtual pion field in the nucleus; it takes into account pion rescattering. The influence of isobar excitation is contained in the axial polarizability coefficient which is linked to the p-wave π-N scattering volume. The derivation of this equation stresses its analogies with electromagnetism. We give then a basic relation between the axial current and the pionic field. It incorporates the effects of the isobars in the axial polarizability, which leads naturally to an electromagnetic analog. We show that this relation leads in heavy nuclei to a quenching of the axial coupling constant by the Lorentz-Lorenz factor, which may originate from the short range or the Pauli correlations, depending on the range of the π-N forces. Hence this quenching may have a different origin than the existence of short-range correlations and may arise from a Pauli blocking effect. On the other hand, the pseudoscalar coupling constant is found to be strongly suppressed. In finite nuclei, these basic quenchings can be masked by surface effects, the general features of which are studied with the help of a solvable model. This model is further used to obtain the asymptotic pion field which is linked to the effective pion-nucleus coupling constant and can be determined experimentally through π-nucleus dispersion relations. We find that this quantity is quenched, in agreement with recent experimental data.     

Effective potential and weak noise transitions

We review the notion of effective potential for stochastic processes and discuss its possible applications. We calculate this function up to first order in a parameter measuring the intensity of the noise for a general nonlinear system. The result is applied exhibiting a transition induced by weak noise.     

Electric dipole transitions in neutron-rich nuclei

The structure of neutron-rich nuclei in several isotopes is investigated by shell model calculations. We study the electric dipole (E1) transitions in C isotopes focusing on the interplay between the low-energy Pigmy strength and the giant dipole resonance (GDR). Reasonable agreement is obtained with available experimental data for the photoreaction cross sections in ¹²C, ¹³C, and ¹⁴C with the inclusion of the quenching effects. A low-energy peak in the dipole strength in ¹⁵C is associated with a single-particle motion of the 1s_1/2 valence neutron relative to the ¹⁴C core. The calculated transition strength below the GDR in C isotopes heavier than ¹⁵C is found to exhaust about 50–80% of the cluster sum rule value and 12–16% of the classical Thomas-Reiche-Kuhn sum rule value. Next, we point out that the quadrupole and magnetic moments in the odd C isotopes strongly depend on configuration, which will be useful to determine the spin parities and the deformations of the ground states of these nuclei. The electric quadrupole (E2) transitions in even C isotopes are also studied. The isotopic dependence of the E2 transition strength is found to be reasonably well explained, although the calculated strength largely overestimates the unexpectedly small strength observed in ¹⁶C. The E1 strength in ¹⁸N and ¹⁹N as well as in Ne isotopes is also investigated.     

Rotation-induced shape transitions in Dy nuclei

Lifetimes of states with spins up to 30? have been measured in the nuclei ¹⁵⁶Dy, ¹⁵⁷Dy, and ^ll58Dy using the recoil-distance technique together with inverse reactions of the type $\text{Mg}\text{(}{}^{136}\text{Xe}\text{, x}\text{n}\text{)}$. The applied method, which benefited from the high velocities of the fusion residues as well as from improvements of the recoil-distance technique, allowed us to determine lifetimes and feeding times down to 0.1 ps. Below the first backbending the resultant B(E2) values in the ground-state band of ^{156, 158}Dy increase faster with increasing rotational frequency than expected for rigid rotors, reaching values similar to those observed for the well-deformed neutron-rich Dy isotopes. In contrast to this, the E2-transition probabilities between high-spin states are clearly retarded. The retardation gradually evolves from the rotation alignment of nucléons and indicates deformation changes most likely towards a triaxial shape. From the analysis of the side-feeding times of the high-spin yrast states it could be furthermore deduced that the E2 component of the preyrast γ-decay stems from transitions along highly collective bands.     

Low-lying Gamow-Teller transitions in spherical nuclei

The Pyatov Method has been used to study the low-lying Gamow-Teller transitions in the mass region of 98 ⩽ A ⩽ 130. The eigenvalues and eigenfunctions of the total Hamiltonian have been solved within the framework of proton-neutron quasiparticle random-phase approximation. The low-lying β decay log(ft) values have been calculated for the nuclei under consideration.     

Nuclear multifragmentation and phase transitions in hot nuclei

Nuclear multifragmentation is a new, multibody, decay mode of very hot nuclei. The key properties of this process that were measured are considered, such as the space-time and temperature characteristics. The experimental data for the critical temperature of the nuclear liquid-gas-phase transition are analyzed. Thermal multifragmentation is interpreted as a result of spinodal decomposition, which is actually the specific nuclear liquid-fog-phase transition of the first order. The text was submitted by the author in English.