Two proton high-spin excitations and dipole bands in192Hg

The¹⁹²Hg nucleus was populated in the¹⁶⁰Gd(³⁶S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of¹⁹²Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ?. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i_13/2 * h_9/2)_K=11 andπ (h_9/2) _K=8 ² , coupled to rotation-aligned quasi-neutron ν(i_13/2)ⁿ and quasi-proton π(h_11/2)² excitations.     

First observation of high spin states in90Tc

High spin states in⁹⁰Tc have been studied via the⁵⁸Ni (³⁵Cl, 2pn) ⁹⁰Tc reaction at beam energy of 124 MeV. Twenty-six newγ-rays were observed, a new level scheme was established and spins were assigned. Two band-like structures were established up to about 20? with an excitation energy of 6–7 MeV. The band (B) shows collective characteristic at high spin. We suggest it to be of a quasi-particle configurationπg_9/2?vf _5/2.     

Shell model study of high spin states in theN=50 nucleus93Tc

High spin states in theN=50 nucleus⁹³Tc were reinvestigated by using the reaction⁶⁴Zn (³⁵Cl,4p 2n) at a beam energy of 140 MeV. This was done particularly with a view to observe anyγ rays upto 2.7 MeV which may have been missed in our earlier study where the experimental conditions were set to observeγ rays upto 2 MeV. We found four newγ rays of energy: 2484, 2164, 2130 and 69 keV. We have placed theseγ rays in the level scheme and it now gets extended to 49/2^?. Though there is no substantial change in the level scheme, placing theγ rays in the level scheme has resulted into two important conclusions: (1) We have performed shell model calculations for⁹³Tc nucleus within a model space which encompasses an enlarged proton configuration and allows for the excitation of the neutron across theN=50 core. The excitation of a single neutron across theN=50 core satisfactorily explains the new level scheme. (2) The energy of the 17/2^? isomeric state is now unambiguously placed at 2185 keV.     

Shape coexistence and other aspects of nuclear structure in40Ar

Theγ-decay of⁴⁰Ar has been studied by particle-γ-ray coincidence measurements in the³⁷Cl(α, pγ) reaction at 12 and 13 MeV bombarding energy. Particle-γ-ray angular correlations and linear polarizations ofγ-rays were measured at 12 MeV. A lifetime measurement using the Doppler-shift attenuation method was performed at 11 MeV. The coexistence of spherical and deformed states in⁴⁰Ar could be concluded from the observation of aK ^π=0⁺ rotational band which has itsI ^π=0⁺ through 6⁺ members at 2,121, 2,524, 3,515 and 4,959 KeV excitation energy. The intrinsic quadrupole moment derived fromB(E2) values is ∥Q ₀∥=1,320 _?120 ⁺⁶⁰ mb. Negative-parity states with high spin were observed at 4,858(5^?), 4,494(5^?), 4,226(4^?) and 4,991 KeV(4^?) excitation energy. A complete account of all levels below 5 MeV excitation energy is obtained by a model in which twod _3/2 proton holes couple weakly to the⁴²Ca levels below 4.75 MeV excitation energy.     

High-spin structure in154Er

High spin states in the nucleus¹⁵⁴Er have been reinvestigated using the¹²³Sb(³⁵Cl, 4n) reaction and a variety of spectroscopic techniques including excitation functions,γ-γ coincidences,γ angular distribution and linear polarization measurements. From the measured energies, relative intensities and transition multipolarities a new level scheme has been deduced up to an excitation energy of ～12 MeV and spin 36. An interpretation of the experimental results is given in terms of the deformed Woods-Saxon orbitals. Gigantic backbending (superdeformation) effect is studied theoretically within the cranking model.     

Levels of195Au from the decays of195g Hg and195m Hg

The levels of¹⁹⁵Au were analyzed by observingγ-singles andγγ-coincidences following the¹⁹⁵Hg decay. Nine new lines are found which require two new levels at 778 and 946 keV. An interpretation of the level scheme is given on the basis of the de Shalit core-excitation model. A qualitative understanding of all levels up to 1 MeV excitation energy is obtained.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Spectroscopy of 32P

Particle-gamma angular correlations have been measured with the ²⁹Si(α,pγ)³²P reaction at bombarding energies E_α = 10.65, 10.69 and 11.00 MeV. Together with DSA lifetime measurements these experiments lead to seven unambiguous spin assignments to levels below 3.5 MeV excitation energy in ³²P. In addition, the measured mixing ratios and branching ratios yield many dipole and quadrupole transition strengths. The previously known 3.44 MeV level is a doublet with components at 3 443.0±0.6 and 3 444.0±0.9 keV.     

The gamma-decay of34Ar

States in³⁴Ar up to 5.5 MeV excitation energy, populated by the³²S(³He,nγ) reaction at 8–12 MeV bombarding energy, have been investigated with the Doppler shift attenuation method,nγ angular correlation andnγγ triple coincidence measurements. For the states at 2.09, 3.29 and 4.52 MeV the lifetimes 200±60 fs, 130±60 fs and 260±80 fs have been found, respectively. Thenγ correlations yielded the spin assignmentsJ ^μ=(2⁺) for the level at 4.13 MeV andJ ^μ=3^? for the 4.52 MeV state, and the mixing ratiosδ(3.29→2.09)=?0.12±0.05 andδ(4.13→3.29)=?0.52±0.15. The results are compared with shell model calculations and predictions from the weak coupling unified model.     

Spectroscopy of 32P (II)

Spins, parities and transition probabilities of levels in ³²P at E_x > 3.5 MeV have been determined with the ²⁹Si(α, pγ)³²P reaction at bombarding energies of E_α = 12.80, 12.93 and 16.30 MeV. Proton-gamma angular correlation experiments and DSA lifetime measurements lead to six unambiguous spin assignments and to many spin limitations. The measured mixing and branching ratios yield many transition strengths for dipole and electric quadrupole transitions. Five, sofar unknown, energy levels are reported. A doublet at 4.03 MeV excitation energy and a high-spin state (E_x = 4.27 MeV; J^π = 5^?) were observed. Shell-model predictions have been compared to the present experimental results.     

High-spin states in154Er

High-spin states in¹⁵⁴Er have been populated by bombarding¹⁴⁷Sm and¹⁴⁸Sm with¹²C particles. Excitation functions, lifetimes, angular distributions andγ-γ coincidences were measured. AT _1/2=35ns isomeric state atE _x～3 MeV has been found and is interpreted as a two-quasi-particle state with aligned angular momenta. A cascade of intense individual lines from states with spin up to at least 26 (excitation energy up to 8.543 MeV) was found to feed the isomeric state. The level sequence above this yrast trap exhibits an irregular pattern which cannot be easily interpreted in terms of collective modes.     

The decay of the 560 ns Isomer at 8.6 MeV in 64 147 Gd83

In (α,xn) and (¹²C, 5n) in-beamγ-ray experiments with pulsed beams the decay of the 560 ns high-spin isomer in¹⁴⁷Gd has been investigated. Extensive coincidence measurements identified in the isomeric decay more than eightyγ-rays which almost all were placed in the level scheme. Of crucial importance for establishing the highly complex decay was the comparison of data from the different reactions and the identification of transitions which bypass the 27 ns isomer at 3.58 MeV. Our data determine the excitation of the 560 ns isomer as 8.588 MeV and suggest 47/2 as its most probable spin.     

Study of the structure of16O above 25 Mev by the reaction13C(τ, α)12C*(γ15.1)

The nucleus¹⁶O has been investigated within the excitation range of 25–32 MeV by means of the reaction¹³C(τ, α)¹²C^* _15.1(γ)¹²C. Excitation functions for the 15.1 MeVγ-rays, taken for bombarding energiesE _τ=2.6–12 MeV at 0? and 90?, exhibit some marked structures with widths of 0.5 to 1 MeV. As the (τ, α)-reaction leads to the 15.1 MeV,T=1 state, levels in¹⁶O underlying these structures should have largeT=1 components. Angular distributions of theα-particles coincident to the 15.1 MeVγ-rays show patterns typical for a direct process and are, therefore, compared with DWBA calculations.     

The width of the giant dipole resonance built on excited states of Cu compound nuclei

Continuumγ- ray spectra from the decay of⁵⁹Cu formed at an excitation energy of 100 MeV and angular momenta up to 43? by means of the reaction 190 MeV³²S +²⁷Al have been measured and analyzed. The parameters of the Giant Dipole Resonance (GDR) have been extracted using the statistical model. The derived GDR width confirms the sizeable broadening of this resonance in⁵⁹Cu already reported in our earlier investigation at 77 MeV excitation energy (J_crit=38?). Estimates of the GDR width have been performed in the adiabatic approximation. Predicted values account qualitatively for the experimental data of⁵⁹Cu as well as of the heavier isotope⁶³Cu, in which the broadening was not seen up to 77MeV excitation (J_crit=35?). The present analysis demonstrates the strong sensitivity of the GDR to spin effects in this mass region.     

The structure of25Mg

Gamma-decay modes and spin(-parity) assignments of levels in²⁵Mg have been systematically investigated up to 10 MeV excitation energy by particle-γ-ray angularcorrelation measurements with the²⁴Mg(d, pγ) reaction at 6.5 MeV bombarding energy and with the²²Ne(α,nγ) reaction at 11.8, 12.5, 14.4 and 15.5 MeV bombarding energy. A level scheme has been established which is comprehensive up to 8.3 MeV excitation energy forI≦9/2 and up to 10 MeV for 9/2O d _5/2 — 1s _1/2-O d _3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent. The first intruder states are located near 6.8 MeV excitation energy. The collective properties of²⁵Mg beyond the well established rotational bands are investigated using both the new experimental information and theB(E2)'s obtained from the shell model. The spectrum of²⁵Mg is completely rotational for the first five to six MeV above the yrast line. Shell modelB (M 1)'s reflect the Nilsson model structure of²⁵Mg in great detail. The prospectiveI ^π=9/2^?, 13/2^?, and 15/2^? members of the established negative-parity,K=1/2 band are found in levels atE _x=7801, 9410, and 8896 keV.     

Competition between few nucleon emission andγ-ray deexcitation for the reaction systems65Cu+87Rb and40Ar+110Pd

Bothγ-ray and neutron emission have been studied for the reaction systems⁶⁵Cu(237MeV) +⁸⁷Rb→¹⁵²Dy^* and⁴⁰Ar(158MeV)+¹¹⁰Pd→¹⁵⁰Gd^*. By using a sum spectrometer in coincidence with neutron counters, Ge(Li) or Nal detectors, we have measured the totalγ-ray energy and the average totalγ-multiplicity distributions as well as the neutron spectra for various exit channels. These measurements provide strong evidence for thermal equilibrium in reactions involving a small number of emitted neutrons (i.e.⁸⁷Rb(⁶⁵Cu,n or 2n)) at rather high excitation energy (～54MeV). This statistical emission of only a few neutrons is controlled by very strong y-ray competition: theγ-entry line is found not to be parallel to the yrast line. Instead the energy gap is about 8MeV for J～27? and rises to at least 13MeV for J～36?. There are some indications that the main part of the energy from this gap is removed by statisticalγ-ray cascades. The main features of the experimental data for both entrance channels are well reproduced by statistical model calculations with proper attention to the yrast line position and an adjustement of the dipoleγ-ray normalization coefficient. It is conceivable that the y-ray enhancement that we introduce may be related to a lack of knowledge of the absolute level densities at high energy and spin, or possibly to the presence of new or additional degrees of freedom that may enter into the competition between neutron andγ-ray emission.     

Etude du noyau 63Cu par la reaction 60Ni(α, pγ)63Cu

The level structure and the decay properties of low-lying levels in ⁶³Cu have been investigated via the ⁶⁰Ni(α, pγ)⁶³Cu reaction at E_α = 11.7MeV. Using a Ge(Li) detector, the correlations of twenty-five primary γ-rays in coincidence with protons, stopped in an annular detector at approximatively 180° with respect to the beam direction, were measured. From these measurements, branching ratios, γ-ray mixing ratios and spin assignments have been obtained for most of the levels up to 2.4 MeV excitation.     

Gamma-spectroscopy within the island of high-spin isomers nearN=82

A NaI sum-spectrometer combined with Ge-counters has been used to characterize the members of the island of high spin isomers nearN=82. On the basis of half lives, totalγ-decay energies and discreteγ-lines, assignments of 22 isomers are given or confirmed. The isomers are localized to the region 82≦N≦86 andZ≦68, and the excitation energies vary from 3 MeV to 12.2 MeV. An empirical relation between spin and excitation energy is presented and on this basis isomeric spin values up to (33±2)? are deduced. The isomers are thought to be due to strong alignment of 2 to 8 shell-model particles in a spherical or possibly weakly oblate potential.     

High-spin states in38Ar

Theγ-decay of 93³⁸Ar levels between 6.84 and 10 MeV excitation energy has been measured using the³⁵Cl(α,pγ) reaction atE _α=14 MeV. A previous measurement of the same reaction could be exploited in the light of the present results, yielding spin(-parity) assignmentsJ ^π(7,070)=5^-, J(7,507)=7,5,J ^π(8,077)=7⁺, J(8,129)=5,6 and J(8,488)=7,5. Theγ-decay modes of yrast levels, which have been previously observed in heavy ion induced reactions, are revised leading to major changes in their spin-parity assignments. New candidates for theJ ^π=8^- and 9⁺ levels were found in the levels at 9,644 and 9,928 keV excitation energy, respectively. A comparison of yrast levels with shell-model calculations is satisfying.