J. L. Durell

Rotational bands in the mass 100 region

Abstract Rotational bands up to spin of ≈10 h have been studied in even-even and odd- A nuclei in the mass 100 region, by observing prompt γ-rays from the spontaneous fission of 248 Cm. Transitions in the neutro-rich nuclei 103 Zr, 104 Zr, 107 Mo and 108 Mo have been observed for the first time, and evidence for a band in 105 Nb is presented. Rotational bands are also reported in 98–100 Sr, 99,101 Y, 100–102 Zr, 101,103 Nb and 102–106 Mo. In most cases these bands have been extended to much higher spins than previously known. The data on the even-even nuclei provide information on deformation trends in this region. The level structures of the odd- A nuclei indicates which Nilsson orbitals lie close to the Fermi level. Bands based on the 5 2 + [422] and 5 2 − [303] configurations are identified in the odd- A nuclei with Z = 39 and 41. Evidence is presented that certain bands observed in N = 61 and 63 nuclei are built on the 5 2 − [532] configuration, and the role of the neutron h 11 2 orbital in stabilizing the deformation in this region is discussed.

The role of triaxiality in the ground states of even-even neutron-rich Ru isotopes

Abstract Partial decay schemes for the very neutron-rich nuclei 108,110,112 Ru and 114 Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248 Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.

The observation of 84Mo

Abstract The 28 Si( 58 Ni, 2n) 84 Mo reaction has been studied at 195 MeV bombarding energy. The prompt gamma rays were detected in an array of twenty, escape-suppressed Ge detectors in coincidence with the recoiling nuclei, which were identified by A and Z with a recoil separator. A 443.8±0.3 keV gamma ray was observed from 84 Mo which is interpreted as the 2 + −0 + transition in this nucleus. This would indicate a quadrupole deformation e 2 ∼0.30. This result confirms that 76 Sr lie at the centre of the region of deformed nuclei with A ∼80.

Medium-spin structure of 96,97Sr and 98,99Zr nuclei and the onset of deformation in the A∼100 region

Abstract The N=58 , 59 nuclei 96,97Sr and 98,99Zr produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Regular rotational bands have been found in 97Sr and 99Zr, firmly establishing shape coexistence in both nuclei. Contrary to earlier suggestions, no rotational bands were found on top of 0 + 3 states in the N=58 nuclei 96Sr and 98Zr. It is likely, however, that the N=58 nuclei undergo a shape change along the yrast line from spherical ground state to weakly deformed medium-spin states. Numerous single-particle excitations of negative parity, observed in 97Sr and 99[loc=pre]Zr, suggest the important role of the h 11/2 shell in creating deformation in these nuclei. Quadrupole moments were determined for rotational bands in the N=58 , 59, 60, 62 and 64 Sr and Zr nuclei. Deformation parameters, which increase gradually from β 2 ∼0.1 at N=56 , through β 2 ∼0.2 at N=58 to β 2 ∼0.4 at N=64 , suggest that in strontium and zirconium isotopes the shape change happens gradually between N=56 and N=62 , and is most likely due to an occupancy of three or more deformation-driving orbitals of h 11/2 parentage.

Octupole correlations in neutron-rich, even-even barium isotopes

Abstract A = 140–148 even-even barium nuclei produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Spins and parities of excited levels have been firmly established from triple-γ angular correlation and direction-polarization correlation measurements. Known band structures have been extended and new bands were found. Band-crossing phenomena have been studied, which suggest termination of an alternating-parity band by particle alignment in 146Ba. The new data for 148Ba confirm predictions of a local decrease of electric dipole moment in 146Ba.

Properties of N=84, even-even nuclei populated in the spontaneous fission of 248Cm

Abstract: Excited states in the neutron-rich, N=84 nuclei 134Sn, 136Te and 138Xe, populated in the spontaneous fission of 248Cm, were studied to medium spins using the EUROGAM2 array. OXBASH code calculations support the experimental identification of maximum aligned configurations in these isotopes. Empirical shell model calculations agree with the proposed excitation energy of the neutron h9/2 excitation in the 132Sn region. A discrepancy between the observed and calculated excitation energy of the Iπ= 12+ level in 136Te indicates possible admixtures of collective excitations in this nucleus. Clear signs of collective excitations are observed in 138Xe.

Parities and multipolarities of gamma rays in neutron-rich odd-mass Ba nuclei

Abstract The nuclei 143,145,147 Ba produced in the spontaneous fission of 248 Cm have been studied using the EUROGAM phase II array. The parities and multipolarities of γ rays in these nuclei have been assigned from triple angular correlation, directional linear-polarisation correlation and internal conversion coefficient measurements. These data confirm an alternating-parity sequence connected by strong electric dipole transitions in 143 Ba. They also reveal a more weakly populated, complementary alternating-parity sequence. The two sequences together exhibit some characteristics of parity-doublet bands. Interpreting the E1 strengths in terms of a rotating intrinsic dipole moment gives a moment similar in size to that observed in 142 Ba and 144 Ba. No well-developed similar structure is seen in 145 Ba, although E1 transitions are observed, from which a value of the dipole moment is obtained. This value is smaller than the moment observed in 143 Ba. No features appropriate to strong octupole correlations were observed in 147 Ba.

Sub-Coulomb neutron transfers from 17O onto 40Ca, 44Ca and 48Ca

Abstract The reactions 48 Ca( 17 O, 16 O) 49 Ca, 44 Ca( 17 0, 16 O) 45 Ca and 40 Ca( 17 O, 16 O) 41 Ca have been observed at bombarding energies of 27,27 and 28.5 MeV, respectively. The cross sections for transitions to levels in the final nuclei with predominantly 2p 3 2 configurations were measured over the angular range 100–170°. The results have been analysed using a DWBA theory with Coulomb wave functions describing the entrance and exit channels. For a chosen wave function for the 17 O ground state, the analysis yields the rms radii of the 2p 3 2 single-particle orbits in the odd Ca isotopes. With the use of a simple potential model the rms radii of the 1f 7 2 orbits are determined within reasonable limits and the sizes obtained compared with the rms radii of the neutron excesses in the Ca isotopes as calculated from Coulomb energy differences. The latter estimates are significantly lower than the values suggested by the present experiments.

Triple angular correlations of γ rays following spontaneous fission

Abstract Triple angular correlations between γ rays following spontaneous fission of 248 Cm are studied using the EUROGAM array. Results, for transitions in even-even fission-product nuclei, agree with predictions of angular correlation theory. Efficient detector systems combined with new analysis techniques offer for the first time a convenient tool to determine multipolarities of prompt γ transitions in the fission-product nuclei.

First identification of excited states in 84–86Nb

We report on the observation of excited states in the niobium isotope with masses 84, 85 and 86. These nuclei lie in the shape transition region known to occur in nuclei with 35 less-than-or-equal-to Z less-than-or-equal-to 40 and N = 45. Unambiguous identification was achieved by detecting gamma-rays in coincidence with mass-separated recoils produced in heavy-ion fusion reactions. Rotational bands were observed in 84Nb and 85Nb which show decoupling schemes similar to those in the neighbouring nuclei. A rotational model was used to interpret these bands and the quasiparticle configurations involved. Two sequences of transitions were observed in 86Nb, at least one of which appears to be a rotational band.