N. Roy

Electromagnetic decay of 10+ states and yrast isomers in 208Pb

Abstract Gamma decay of the four previously known 10 + states between 4.8 and 5.9 MeV of excitation in 208 Pb, and isomerism of the lowest 10 + [T 1 2 =500(50) ns ] and 8 + [T 1 2 =3.2(5) ns ] states have been established with α - γ coincidence measurements in 209 Bi(t, αγ ) 208 Pb. Wavefunctions for the 10 + states were deduced in the model space involving 1p-1h excitations only. These show that the π h 9 2 h 11 2 −1 strength is nearly evenly distributed among the 10 + states, in disagreement with results from (e, e′) data.

Quadrupole moments of low-energy isomers in 210Po

Abstract The quadrupole interaction frequencies of the 8 + , 11 − , 13 − and 16 + isomers in 210 Po have been measured in a Bi single crystal by the DPAD method with the 209 Bi(t, 2n) 210 Po reaction. The results for the quadrupole moments, normalized to Q (8 + 1 ) = −55.2 (20)fm 2 , are | Q (11 − )| = 86 (11)fm 2 , | Q (13−)| = 90(7)fm 2 and | Q (16 + )| = 129.7 (20) fm 2 . The quadrupole moment of the 0 i 13 2 proton orbital was deduced, and found to be surprisingly small. The results are compared with shell model calculations using the surface delta interaction as residual interaction.

Oblate L=1 bands in 194,196–201Pb and 193Hg

Abstract Reports of recent experiments have included observations of regular and irregular bands in neutron deficient Pb isotopes with A =194, 196–201. The bands are populated strongly in (HI, xn ) reactions. The shared characteristics of the bands include: 1) bandhead energies of a few MeV; 2) high bandhead spin; 3) large alignments; 4) small dynamic moments of inertia, and 5) strong L =1 transitions and weaker L =2 crossover transitions, with B( M 1) B( E 2) ≈ 20 μ 2 e 2 b 2 . Lifetimes of band members in the 198 Pb regular band are B (M1) ≈ 1 W.u. and B (E2) ≈ 12 W.u. (with large errors). These observations are consistent with an interpretation of the regular structures as collective oblate bands with both proton and neutron excitations involved; the closed proton shell at Z =82 is broken, and coupled to v( i 13 2 ) −n excitations. The irregular structures may correspond to triaxial shapes, with similar orbits involved. A similar structure has been also found in 193 Hg.

Analysis of High‐Multiplicity Gamma‐Ray Events

A significant improvement in the measured peak‐to‐background ratio of highly‐correlated gamma‐ray cascades can be achieved through the use of high‐multiplicity gamma‐ray coincident events. We studied this effect using data from the 176Yb(22Ne,6n)192Hg reaction at a beam energy of 122 MeV using HERA at the LBL 88‐Inch cyclotron facility. An enriched set of all four‐ and higher‐fold coincidences was culled from these data.

Superdeformed bands in193Hg and194Hg

A superdeformed band, consisting of 12 transitions, is assigned to193Hg on the basis of excitation function and cross bombardment results. A second band is identified which may be the signature partner of this band. In addition, another rotational band is observed which is assigned as the third superdeformed band in194Hg. The similarities of these three bands to those already known in the mercury region are discussed briefly.

Observation of superdeformed bands in194Hg

Two rotational bands, with energy spacings characteristic of superdeformed shapes, have been observed following bombardment of150Nd with48Ca. The more intensively populated band consists of 18 transitions and is assigned to194Hg. The depopulation of this band occurs around spin 10. The second band, consisting of at least 16 transitions, was populated less strongly and is tentatively assigned to194Hg also. The lowest level in this band is assigned spin 8. The energy differences between transitions for both bands decrease from ∼40 keV at low rotational frequencies to ∼30 keV at the highest observed frequencies. The moments of inertia of the bands are similar to those of the two previously observed superdeformed bands in191,192Hg. The similarities and differences of the four known bands in the mercury region are discussed.

Decay of the 21/2− isomer in211Bi and structure of thevg 9 2/2 πh9/2 levels

Gamma rays and conversion electrons have been measured in211Bi populated by the209Bi(t,p) reaction, and the near yrast levels were observed up to 21/2−. The 21/2− state is isomeric withT1/2=70(5) ns. No alpha decaying isomer was found in211Bi. ObservedM1 transitions reveal mixing of the210Pb parent states in the levels of211Bi. The levels and transitions are well reproduced by the shell model with experimental matrix elements and pure configurations.

In-Beam Studies of 96Zr and 98Zr: Collective Excitations

Nearly two decades ago signatures of deformation in the ground state bands of 100Zr and 102Zr were identified, and the rapid change in the deformation of heavy zirconium nuclei noted.1 It is now well accepted that the short-range proton-neutron interaction between the 1g9/2 and 1g7/2 spin-orbit partners plays an important role in producing ground state deformation in this region.2 Nevertheless, recent studies of zirconium nuclei, including those in the transition region,3–10 continue to refine our understanding of the interplay between single-particle and collective degrees of freedom. In this report we discuss some aspects of the level structures of 96Zr and 98Zr with emphasis on collective excitations.

Conversion-Electron Expffiment To Characterize The Decay Of The 237 Np Shape Isomer

Conversion electrons from the decay of low-lying levels of 237Np have been measured to detect the population of these levels by 2i/ gamma-ray decay of the Np shape isomer. Analysis of the 208-keV transition L conversion-electron peak _gives an upper limi3J?f about 17 pb for the population of the 3/2 267-keV level in Np from the shape isomer decay. Model calculations are compared with the measured limit. Improvements are suggested for this experiment.