R. S. Henderson, 2,7 K. P. Jackson, 2 S. Long, C. A. Miller, 2 W. C. Olsen, 3 B. M. Spicer, A. Trudel, and M. C. Vetterli University of Western Ontario, London, Ontario, Canada N6A 3K7 TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3 University of Alberta, Edmonton, Alberta, Canada T6G 2N5 Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia California Institute of Technology, Pasadena, California University of Melbourne, Parkville, Victoria 3052, Australia ~Received 19 September 1996 !

Cross sections have been measured up to 35 MeV excitation for the {sup 76}Se(n,p){sup 76}As reaction at five laboratory angles between 0{degree} and 15{degree}. The incident neutron energy was 198 MeV. The distribution of Gamow-Teller (GT) strength at excitation energies less than 10 MeV was deduced by carrying out a multipole decomposition of the data. The GT strength below 6 MeV excitation was found to be less than 0.9 units, in qualitative agreement with a quasiparticle random-phase approximation calculation. The present results have been compared with data from the {sup 76}Ge(p,n){sup 76}As reaction to obtain an estimate of a lower limit for the lifetime of {sup 76}Ge for {beta}{beta} decay. The estimate is in qualitative agreement with the measured lifetime. {copyright} {ital 1997} {ital The American Physical Society}

Cross sections have been measured up to 35 MeV excitation for the ${}^{76}$Se($n,p$)${}^{76}$As reaction at five laboratory angles between $0\ifmmode^\circ\else\textdegree\fi{}$ and $15\ifmmode^\circ\else\textdegree\fi{}$. The incident neutron energy was 198 MeV. The distribution of Gamow-Teller (GT) strength at excitation energies less than 10 MeV was deduced by carrying out a multipole decomposition of the data. The GT strength below 6 MeV excitation was found to be less than 0.9 units, in qualitative agreement with a quasiparticle random-phase approximation calculation. The present results have been compared with data from the ${}^{76}$Ge($p,n$)${}^{76}$As reaction to obtain an estimate of a lower limit for the lifetime of ${}^{76}$Ge for $\ensuremath{\beta}\ensuremath{\beta}$ decay. The estimate is in qualitative agreement with the measured lifetime.

The Coulomb excitation measurements for the230Th nucleus with32S,84Kr and142Nd projectiles are presented. The use of different projectiles allowed us to get information in the ground-state band and side bands. The energy spectrum of the ground-state band and of the lowest negative-parity band have been investigated up to the spin valueI=24+ andI=19−(21), respectively. Five side bands (Kπ=0+, 2 1 + , 2 2 + , 1−, 2−) were observed also. The branching ratios for a large number of transitions in the spin regionI≦10 for π=+1 andI≦9 for π=− 1 are analysed. The full set of experimental data contains information on the mixing of the adiabatic states and on the nuclear response to the electromagnetic field ofγ-radiation. It is shown that the experimental data may be explained taking into account the coupling of the ground-β- and twoγ-bands and also of theKπ=0−, 1− and 2− negative-parity bands. An enhancement of the transitions from theγ-to theβ-band in respect to the transitions from theγ to the ground band and from theβ- to the ground band is reported. The mixing of the negative-parity bands is found to be typical for the alignment of the octupole-vibrational angular momentum. The strong spin dependence of the intrinsic matrix elements of the electric-dipole operator follows from the branching ratios of inter- and intra-band transitions from theKπ=0− states.

The Coulomb excitation measurements for the230Th nucleus with32S,84Kr and142Nd projectiles are presented. The use of different projectiles allowed us to get information in the ground-state band and side bands. The energy spectrum of the ground-state band and of the lowest negative-parity band have been investigated up to the spin valueI=24+ andI=19−(21), respectively. Five side bands (K π=0+, 2 1 + , 2 2 + , 1−, 2−) were observed also. The branching ratios for a large number of transitions in the spin regionI≦10 for π=+1 andI≦9 for π=− 1 are analysed. The full set of experimental data contains information on the mixing of the adiabatic states and on the nuclear response to the electromagnetic field ofγ-radiation. It is shown that the experimental data may be explained taking into account the coupling of the ground-β- and twoγ-bands and also of theK π=0−, 1− and 2− negative-parity bands. An enhancement of the transitions from theγ-to theβ-band in respect to the transitions from theγ to the ground band and from theβ- to the ground band is reported. The mixing of the negative-parity bands is found to be typical for the alignment of the octupole-vibrational angular momentum. The strong spin dependence of the intrinsic matrix elements of the electric-dipole operator follows from the branching ratios of inter- and intra-band transitions from theK π=0− states.

The ground state differential cross sections for the (n, p) reaction at 0° on 6Li, 12C and 13C have been measured for 198 MeV neutrons with a precision of about 4% relative to the corresponding value for the H(n, p) reaction. These data, together with the accurately known ft values for the β- decay of 6He, 12B and 13B provide three calibrations of the (n, p) reaction as a probe of Gamow-Teller strength for isospin raising transitions.

Yrast states in the nucleus 220Ra were studied by means of the 208Pb(14C, 2n) reaction at 61 and 64 MeV. A staggering sequence of levels of positive and negative parity has been observed up to spin and parity Iπ = 16+ (18 +) and from Iπ = 5− to Iπ = 17−, respectively. These states are connected by strong E1 transitions competing with the stretched E2 transitions, the B(E1)B(E2) ratio being ~ 10 −6 fm−2. The ratio of the excitation energy of the 4+ state to that of the 2+ state is close to the vibrational limit. The moment of inertia associated with the negative-parity yrast states is slightly increasing with the rotational frequency ω. It is considerably higher than that of the positive-parity states at lower spins, the difference decreasing monotonically with increasing ω. The data are discussed with reference to the octupole vibrational picture as well as to the results of recent models predicting reflection-asymmetric shapes in the Ra-Th region.

The level scheme of230Th is presented, as deduced from Coulomb excitation experiments using32S,84Kr and142Nd projectiles. The ground-state band is followed up to spin (24+) and the octupole band up to 19−. A comparison is made with the analogous bands known in232Th and232U.