摘要:

Recent experimental data on the formation and decay of states at−and below−the level of neutron binding energy (compound and pre‐compound states) give insight into the structural nature of the highly excited states as well as into the processes of fragmentation of simple excitation modes over compound states. The experimental data from &agr;‐decay of compound states indicate essential fragmentation of few quasiparticle components as compared with the single particle component. There is an indication on the variation of &agr;‐particle strength function on the energetic interval 5–10 keV. Single‐particle excitation of compound and pre‐compound states play an essential role in &ggr;‐decay of neutron resonances of heavy nuclei in the vicinity of the 4S shell.

ISSN:0094-243X    

DOI:10.1063/1.41279

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The status of Nuclear Physics is briefly reviewed, and the blanks in our knowledge are highlighted. The nature of the (n,&ggr;) reaction and its contribution to our knowledge of nuclear properties is discussed and a recent study of the188Os (n,&ggr;)188Os reaction is cited as an example. A comparison is then made with the properties of the more fashionable, heavy ion induced, fusion‐evaporation reaction which provides information about states of high spin and neutron‐deficient nuclei far from stability. The recent observation of the N=Z nucleus84Mo is used as an example. Future developments in the use of the (n,&ggr;) and (HI,xn&ggr;) reactions and in low temperature Nuclear Physics generally are outlined.

ISSN:0094-243X    

DOI:10.1063/1.41301

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have studied118−126Te with Sn(&agr;,xn&ggr;) reactions and have extended the knowledge of these nuclei up to moderate angular momenta. In the122,124,126Te isotopes we have searched for intraband transitions in the previously proposed 4p‐2h intruder band. For119,121,123Te we have found additional negative‐parity states. The structure of these excitations built on an h11/2neutron is examined in terms of the interacting boson‐fermion model.

ISSN:0094-243X    

DOI:10.1063/1.41281

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The mesonic enhancement of the time‐like component of the weak axial current in nuclear matter is very large and is best observed via its effect on the decay rate of &Dgr;J=0(&pgr;i&pgr;f=−)&bgr; decay. Studies in the A=16,40, and 90 regions yield enhancements of 40–60% over the impulse approximation. The lead region is a rich source of information on these first‐forbidden decays. This study is the first to extract information on mesonic enhancement from these decays.206Hg(0+)→206Tl(0−)→206Pb(0+) is chosen to exemplify the approach which has been applied to 10 or so first‐forbidden decays in A=205–214 nuclei. The nuclear wave functions are evaluated via large‐basis shell‐model calculations. The results indicate a much larger enhancement than expected and thus the possibility of some non‐nucleonic effect in addition to the mesonic enhancement considered to date.

ISSN:0094-243X    

DOI:10.1063/1.41282

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Neutron single‐particle states above the N=152 subshell have been studied by high‐resolution (d,p) reaction on a250Cf target. All of the orbitals between N=152 and N=164 subshells have been identified. A tentative assignment has been made for the 1/2−[761] Nilsson state.

ISSN:0094-243X    

DOI:10.1063/1.41283

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have investigated nuclei close to the proton drip line by using heavy‐ion fusion reactions to produce extremely neutron‐deficient nuclides. Their nuclear decay properties were studied by using on‐line isotope separators at Oak Ridge (UNISOR) and Berkeley (OASIS), the Oak Ridge National Laboratory velocity filter, and a fast helium‐gas‐jet transport system at Lawrence Berkeley Laboratory 88‐inch Cyclotron. Many isotopes, isomers, and &bgr;‐delayed‐proton and &agr;‐particle emitters were discovered. This contribution summarizes three topics that are part of our overall program: (a) decay rates of even‐even &agr;‐particle emitters, (b) mass excesses of181Pb,182Pb, and183Pb, and (c) &bgr;‐delayed proton emitters near N=82.

ISSN:0094-243X    

DOI:10.1063/1.41284

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The yrast and near‐yrast level structures of some very neutron‐rich nuclei have been studied by observing prompt &ggr;‐rays emitted from252Cf and248Cm fission fragments. &ggr;−&ggr; coincidence experiments have enabled levels with spins up to ∼14ℏ to be observed in strongly produced final fragments, viz those with N/Z ratios of ∼1.55 and ∼1.62, and with mass numbers A∼104 and ∼144 respectively, near to the peaks in the mass distributions. Level schemes have been constructed from the coincidence data using, where possible, previously identified transitions between levels at low excitation energy. Several new nuclei have also been discovered, and their partial decay schemes determined, using an identification technique based on gating on transitions in complementary fragments. New nuclei103,104Zr,107,108Mo and141,142Xe, with N/Z ratios higher than given above, have been studied in this way. Spectroscopic studies in the higher of the two mass regions have revealed the extent and details of the new region near N=88 where strong octupole correlations may give rise to stable octupole deformations. Spectroscopic studies near A=100 have revealed details of how the large deformations in this region change with mass number and how they are strongly influenced byh11/2intruder orbitals.

ISSN:0094-243X    

DOI:10.1063/1.41285

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Level lifetimes in the range between about 10 ps and 1 ns have been determined for the heavy isotopes of Zr, Nb, and Mo at the fission‐product separator JOSEF. The results allow the identification of spherical configurations for the lighter isotopes and the determination of the deformation of the heavier ones. It could be shown that a rapid onset of deformation takes place atN=60in the Nb chain since a deformation of &bgr;q=0.40(5) and 0.37(8) is deduced for101Nb and103Nb, respectively, while spherical shell‐model and particle‐phonon coupled states exist in99Nb. Hence, the situation in the Nb chain is similar to that in neighbors with smallerZ. The closed shell character of96Zr is confirmed by the half‐life of 127(10) ps of the 81+level at 4390 keV. This half‐life is in accordance with the expectation for a &pgr;(g9/2)2configuration and an effective chargeeeff= 1.65(6)e.The energy of the state evinces a strong single particle gap atZ=40.

ISSN:0094-243X    

DOI:10.1063/1.41286

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A technique to measure the lifetimes of nuclear states with half lives <10 ps has been developed in conjunction with the TRISTAN mass separator at the High Flux Beam Reactor at BNL. The method uses fast plastic and BaF2scintillators and Ge detectors in a triple coincidence (&bgr;‐&ggr;‐&ggr;) fast‐slow counting system. The timing information is derived from the fast plastic‐BaF2coincidence, while the higher resolution of the Ge detector (in slow coincidence) serves to insure that the &bgr;‐&ggr; event lies in the cascade of interest. The calibrations and corrections necessary to achieve precise results and the methods of data reduction and results from recent measurements on theA=97 mass chain are presented.

ISSN:0094-243X    

DOI:10.1063/1.41287

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Most recent development and applications of the Doppler shift attenuation method in measurements of nuclear lifetimes are discussed. Emphasis is put on the development of the Doppler shift attenuation technique performed in our laboratory. Main factors behind inconsistencies of the literature data are shortly estimated. The applications of current interest, i.e., the systematic studies of transition strengths in thesd‐shell nuclei, the superdeformation of rare earth nuclei at high spin, and the studies offp‐shell nuclei with a new ultra‐high resolution neutron capture &ggr;‐ray spectroscopy, are considered. The first use of molecular dynamics techniques in the simulation of the slowing‐down process combined with the simulation of Doppler‐broadened &ggr;‐ray lineshapes by Monte Carlo method, is discussed in the frame work of ultra‐low velocity Ti recoils produced through a neutron capture reaction.

ISSN:0094-243X    

DOI:10.1063/1.41288

出版商:AIP    

年代:1991

数据来源: AIP