J. M. Espino

High-spin spectroscopic study of 167Lu

Abstract Three decay sequences associated with the 7 2 + [404], 9 2 − [514] and 1 2 − [541] Nilsson states of 167 Lu have been established to high spin using the 123 Sb( 48 Ca,4n) 167 Lu reaction. The quasineutron and quasiproton band crossings, the energy signature splittings, and the B (M1)/ B (E2) values are discussed and compared with similar quantities for the lighter lutetium isotopes. Configuration-and isotopic-dependent deformations, reflected in such comparisons, indicate that 167 Lu has a larger and more stable prolate deformation than the lighter isotopes and that the 1 2 − [541] configuration has a larger quadrupole deformation than the 9 2 − [514] configuration. The shift of the AB neutron band crossing occurring in the 1 2 − [541] sequences to a larger rotational frequency cannot be understood in terms only of deformation differences. The configuration and neutron-number dependent shapes are also discussed in a “gauge”-space analysis.

Signature inversion in semi-decoupled bands: residual interaction between protons and neutrons ☆

Abstract Semi-decoupled bands based on the πh 9 2 ⊗ vi 13 2 configuration are observed in 162 Tm, 164 Tm and 174 Ta. Spins assigned to these bands imply an inversion of the expected signature splitting, which is interpreted as being the result of a residual proton-neutron interaction.

Proton-proton correlations observed in two-proton decay of 19Mg and 16Ne

Proton-proton correlations were observed for the two-proton decays of the ground states of 19 Mg and 16 Ne. The trajectories of the respective decay products, 17 Ne + p + p and 14 O + p + p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.

α-particle production in the scattering of 6He by 208Pb at energies around the Coulomb barrier

New experimental data from the scattering of 6He + 208Pb at energies around and below the Coulomb barrier are presented. The yield of breakup products coming from projectile fragmentation is dominated by a strong group of α particles. The energy and angular distribution of this group have been analyzed and compared with theoretical calculations. This analysis indicates that the α particles emitted at backward angles in this reaction are mainly due to two-neutron transfer to weakly bound states of the final nucleus.

Structure of excited bands in 162Tm and residual interactions

Abstract The nucleus 162Tm has been populated by the 130Te(37Cl,5n) reaction, and the γ-cascades detected with the NORDBALL array. Eight coupled rotational bands involving the proton orbitals, g 7 2 [404] 7 2 + , d 3 2 [411] 1 2 + , d 5 2 [402] 5 2 + , h 11 2 [523] 7 2 − and h 9 2 [541] 1 2 − and neutron orbitals i 13 2 [642] 5 2 + , h 9 2 [523] 5 2 − and f 7 2 [521] 3 2 − have been established in 162Tm, to spins varying from 18 to 47ℏ. Various phenomena which can be related to residual neutron-proton interactions are discussed. In particular, signature inversion in the π [541]1 2 − ⊗ ν [642]5 2 + bands based on the h 9 2 quasiproton and i 13 2 quasineutron, rewpectively, may be explained by a fairly large p-n interaction. In addition, interactions at the crossings of bands with constituent quasiparticles differing both in parity and signature have been extracted with strength of the order of a few (4–12) keV. Such interactions also require terms beyond the standard mean field description.

Interaction strength and shape difference for the h9/2 and h11/2 configurations in163Tm

The strongly shape driving πh9/2[541]l/2− configuration with α=+1/2 exhibits some anomalous, and so far unexplained, features concerning the crossing frequency, ħωc, the aligned angular momentum, ix, and interaction strength, at the alignment of the first pair of i13/2 quasineutrons in several odd-Z rare earth-nuclei. The h9/2[541]1/2− and h11/2[523]7/2− bands have been studied in the stably deformed rare-earth nucleus163Tm to investigate these features. A difference in band crossing frequency of ∼ 80 keV between the two bands is found. Rotational bands built on these two configurations have been found to cross in the spin range I=25/2–29/2 ħ. Theγ-decay pattern between the two bands is established in the crossing region and analysed in terms of a moderate shape difference between them. A theoretical estimate of the size of the interaction strength between the two bands is presented and compared to the experimental value. The observed band structure in163Tm is very similar to that of167Lu which has 2 protons and 2 neutrons in addition. This observation is discussed in relation to the similarity of the yrast bands of the two even-even “core” nuclei162Er and166Yb, for which theγ-transition energies are identical within ∼0.2 keV below the vi13/2 crossing.

NpNn correlations in rotating nucle

Abstract The empirical correlation observed for kinematic and dynamic moments of inertia (J(1) and f(2), respectively) of Z= 64–78 nuclei with NpNn for small angular momenta disappears at large angular momenta. The nearly constant value of J (1) ( = 60.6 ± 3.0 MeV −1 2 ) corresponding to the lowest 20+ and 22+ states of these nuclei, independent of NpNn or A, reflects: (i) a shift of the transition from spherical to deformed shapes nearer to the closed shells in rotating systems; (ii) the reduction of pair correlations above the band crossings in rotating systems; and (iii) the occupation of the most alignable configuration just above the N = 82 shell closure. These features emphasize the microscopic nature of the nuclear moment of inertia.

β-delayed three-proton decay ofAr31

The beta decay of 31Ar, produced by fragmentation of a 36Ar beam at 880 MeV/nucleon, was investigated. Identified ions of 31Ar were stopped in a gaseous time projection chamber with optical readout allowing to record decay events with emission of protons. In addition to \b{eta}-delayed emission of one and two protons we have clearly observed the beta-delayed three-proton branch. The branching ratio for this channel in 31Ar is found to be 0.07(2)%.

Electromagnetic properties of 161Tm at high spin

Abstract The decay of the yrast and near-yrast high-spin states in 161Tm has been observed following the reaction 128Te(37Cl,4n). Extension of the known level scheme reveals previously unobserved crossings in the bands built on the 7 2 − [523] , 7 2 + [404] and 1 2 − [541] Nilsson states. The use of both gold- and terbium-backed targets has enabled the measurement by the Doppler shift attenuation method of lifetimes of states in the h 11 2 yrast band in the spin range 41 2 ⩽ I ⩽ 67 2 . The results show little evidence for change in the nuclear deformation over this interval in spin (Qt ≈ 6 eb). From lifetime measurements and B( M1 ) B( E2 ) values in the h 11 2 band, absolute B(M1) values for five transitions (for states with 41 2 ⩽ I ⩽ 51 2 ) were determined. These results provide some evidence for a reduction in the proton spin projection on the deformation axis relative to its value at low rotational frequency. The signature splitting in the Routhians and B( M1 ) B( E2 ) values for the h 11 2 band is discussed in terms of the inclusion of non-axially-symmetric (γ-deformed) components in the nuclear wavefunction.

The distribution of nuclear quantum states in ‘‘cold’’ rotating nuclei

A statistical analysis of the distribution of level spacings for states with the same spin and parity is described in which the average spacing is calculated for the total ensemble. The resulting distribution of level spacings for states of deformed nuclei with A=155–185 and Z=62–77 is the closest to that of a Poisson distribution yet obtained for nuclear levels. However, when only the even‐spin, positive‐parity data for even‐even nuclei are considered, the level‐spacing distribution becomes double peaked. The anomalously‐large separations are shown to be the result of the low energy of the strongly‐correlated, completely‐paired yrast configuration of even‐even nuclei. Average values of the level spacings also are discussed as a function of spin, parity, and nuclear type (even‐even, even‐Z–odd‐N, etc.). Likewise, deviations from a Poisson distribution for several spacings (s) less than about 60 keV are compared with similar values for 116Sn on an absolute scale. Such discrepancies are attributed to intera...