R.M. Diamond

ROTATIONAL STATES PRODUCED IN HEAVY-ION NUCLEAR REACTIONS

Abstract The de-excitation of the ground state rotational band in nine deformed even nuclei has been observed following heavy-ion nuclear reactions. The transitions from states up to spin 16 (on the average) were observed and their energies were measured with an accuracy of ±0.3%. The rotational spacings thus identified have been compared with several calculations, and much the best agreement is obtained with a simplified calculation by Davydov and Chaban taking into account the effects of centrifugal stretching of the nucleus.

ANGULAR DISTRIBUTIONS OF GAMMA RAYS PRODUCED IN REACTIONS WITH HEAVY IONS.

Abstract Angular distributions of the stretched E2 gamma-ray transitions cascading from rotational or vibrational states, populated by heavy-ion, x n reactions, have been measured. The reactions were induced by 4 He, 11 B, 14 N and 19 F ions on targets in the mass range from 159 to 186. In one case the variation of anisotropy with bombarding energy was measured. Large anisotropies were observed indicating the strong alignments of the decaying states. The problems involved in a theoretical calculation of the alignments are discussed. The implications of the strong angular distributions for nuclear structure measurements with these reactions are considered.

Collective levels in light even Ce isotopes

Abstract The heavy-ion reactions ( 16 O, 4n) and ( 20 Ne, 4n) have been used to populate levels in the neutron-deficient even cerium isotopes 136 Ce- 128 Ce. Prominet γ-rays in the “in-beam” spectra have been assigned to transitions between the members of the ground-state quasi-rotational bands. The results show that as the number of neutrons decreases these isotopes tend towards rotational behaviour. The cross sections for the neutron evaporation reactions used are considerably reduced for the most neutron-deficient systems due to competition from charged-particle emission. Such competition prevents us from studying Ce isotopes lighter than 128 Ce by this method.

Isomeric levels in the light thallium isotopes

Abstract A study has been made of the metastable states produced in certain heavy-ion reactions by observing the gamma-ray and conversion-electron transitions occurring between the beam bursts of the HILAC at the Lawrence Radiation Laboratory. Targets and projectiles were chosen which produce thallium isotopes ranging from mass 200 to mass 191. New E3 isomers were found in Tl200, Tl199 and Tl193. The isomer in Tl200 is apparently a 7+ level, in close analogy with other known even-mass thallium isotopes, Considerable evidence was accumulated that in the odd-mass thallium nuclei, including the previously studied Tl197 and Tl195, the metastable state has spin and parity 9 2 − . Such a state is not expected to occur at low energies in these nuclei, and its nature, therefore, presents an interesting problem.

Collective excitations in neutron-deficient barium, xenon and cerium isotopes

Abstract The collective excitations of some neutron-deficient barium, xenon and cerium isotopes have been studied with germanium counters. Various (heavy ion, xn) reactions have been used to produce 126Ba, 124Ba, 120Xe, 120Xe and 134Ce. The decay of 134Pr to 134Ce, with a half-life of 17±2 min, was also studied. Partial level schemes for the ground state rotational bands up to spin 8+ or 10+ are proposed for the nucleides investigated. The mean lifetime of the first excited state in 126Ba has been determined to be 270±50 psec and, hence gives the deformation of 126Ba as β = 0.24±0.02. The implications of the rotational energies and spacings as related to deformation in this region are discussed.

Ground state (quasi-) rotational levels in light Os, Pt and Hg nuclei

Abstract Energy levels in some neutron-deficient doubly even nuclei in the platinum region have been studied following heavy-ion reactions. Information of the ground state rotational (or quasi-rotational) bands in 178, 180, 182Os, 182, 184, 186, 188Pt and 188, 190Hg is presented. The levels suggest that the light platinum nuclei are very soft axially-symmetric rotors, rather different from the well-known γ-unstable or tri-axial heavy osmium nuclei. The energy-level systematics of this region are compared with those of the samarium region, and it is concluded that the two regions are basically similar, with the well-known 88–90 neutron discontinuity reasonably well duplicated between neutron-deficient osmium and platinum nuclei (proton number 76–78).

AN 8- ISOMERIC STATE IN THE 106-NEUTRON NUCLEI: 180W, 182Os, AND 184Pt

Abstract The decay of a two-neutron, 8−, isomeric state has been observed in three nuclei having 106 neutrons. The half-lives of these isomers are in the region of 1 msec. In each of the isotones 180mW, 184mOs and 184mPt, there were observed five prominent transitions. Four of these are the E2 transitions of the 8 → 6 → 4 → 2 → 0 cascade in the ground-state band. The fifth transition is a very hindered E1 that de-excites an 8−, 8 isomeric level and populates the 8+, 0 level of the ground-state band. Despite the appreciable difference between the properties of these nuclei, as can be seen by comparing the well-developed rotational spectrum of 180W with that of the poor rotor 184Pt, the isomeric transitions seem to exhibit a striking similarity. In 184mPt, the isomeric decay also populates other levels. Among them were tentatively identified two members of the β-Vibrational band. Their position relative to the ground-state band in this transitional-region nucleus is of special interest.

COULOMB EXCITATION OF 235U

Abstract The Coulomb excitation of 235 U has been studied using 4 He, 16 O, and 40 Ar projectiles. Rotational bands at 921 keV and 638 keV (tentative) have been characterized as the K 0 + 2 and K 0 −2 γ-vibrational bands, respectively, and a somewhat tentative band at 1053 keV has been assigned as the β-vibrational band based on the ground state. Two bands at 633 and 822 keV are identified as the 5 2 − [752] and 9 2 − [734] Nilsson states, respectively. The large B (E2) value for exciting these bands indicates that they are strongly Coriolis coupled to the ground state band, 7 2 − [743] . A three-parameter Coriolis calculation, involving all eight components of the j 15 2 shell-model orbital, can account adequately for all the rotational energies and transition probabilities observed in these three bands.

Quadrupole moments of the first excited states in 20Ne and 22Ne

Abstract Static quadrupole moments of the first excited states of 20 Ne and 22 Ne have been measured using the reorientation effect in projectile excitation. The Ne nuclei, accelerated by the Berkeley Hilac, were Coulomb excited by thin targets of 120 Sn, 130 Te or 148 Sm and the gamma-ray yields in coincidence with particles scattered at angles of 90° and 160° were evaluated using the de Boer-Winther Coulomb excitation program. The results obtained are Q( 20 Ne , 2 + ) = −0.24 ±0.03 b , B (E2, 20 Ne , 0 + → 2 + ) = 0.048±0.007 e 2 · b 2 , Q( 22 Ne , 2 + ) = −0.21 ±0.04 b, and B( E 2, 22 Ne , 0 + → 2 + ) = 0.033±0.006 e 2 · b 2 .

The neutron deficient iridium isotopes Ir182, Ir183 and Ir184

The new neutron-deficient iridium isotopes Ir/sup 182/, Ir/sup 183/ >s, and Ir/sup 184/ were produced by irradiations with the Berkeley heavyion linear accelerator. By means ot timed chemical separations, Nal scintillation spectrometers, and proportional counters, half lives were determined as follows: Ir/sup 182/ , 15 plus or minus 1 min; Ir/sup 183/ , 55 plus or minus 7 min; Ir/ sup 184/, 3.2 plus or minus 0.2 hr. The g amma spectra of l/sup 182/ and lr/ sup 184/ are very complex. each extendi ng above 4 Mev. Positon branches are also seen in both isotopes. Studies of the gamma spectra indicate that the energies of the first and second excited states of Os/sup 182/ are slightly higher than the corresponding states in Os/sup >84/. These data suggest that there is a maximum in the moment of inertia at Os/sup 184/ . (auth)