W.R. Phillips

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.

Octupole correlation effects near Z=56, N=88

Abstract Partial decay schemes for the very neutron-rich nuclei 146, 148, 150Ce have been determined by the study of γ-γ coincidences in 252Cf fission fragments. Similar behavior to that seen in 144, 146Ba, 148Nd and 150Sm, and interpreted in terms of strong octupole correlations, has been observed in yrast levels of 146Ce but not in 148Ce and 150Ce. This is in agreement with the predictions of cranked mean field calculations.

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.

Sub-Coulomb transfer reactions on 208Pb with carbon and oxygen projectiles

Abstract Single-neutron transfers induced by 12, 13C and 16, 17, 18O projectiles on 208Pb and the 12C(17O, 16O)13C reaction have been studied at energies close to the Coulomb barrier. These processes are well described by the distorted-wave Born approximation. Coupled-channels effects are found to be small. Normalization factors have been determined for all projectile and target transitions, and also for the triton-deuteron overlap by comparison with previous measurements of the 208Pb(d, t)207Pb reaction. The root-mean-square (rms) radii of single-particle neutron wave functions in 208Pb and 209Pb were calculated using known spectroscopic factors. The distribution of the point neutron excess density in the surface region of 208Pb has been derived and its rms radius determined to be 5.93 ± 0.13 fm with a local potential model. This is in good agreement with theoretical predictions, but is considerably larger than estimates based on Coulomb energy differences. The phenomena of core polarisation by the odd particle or hole outside 208Pb is discussed using the single-particle orbitals determined in this work.

New neutron-rich nuclei 103,104Zr and the A~100 region of deformation.

Partial decay schemes in the neutron-rich nuclei {sup 103}Zr and {sup 104}Zr have been measured for the first time and rotational bands in {sup 100--102}Zr have been extended to spins of up to 10{h bar} by observing prompt {gamma} rays from the spontaneous fisson of {sup 248}Cm. These nuclei are among the most deformed known at low spin and excitation energy. The level structures in the odd-{ital A} nuclei show that the {ital h}{sub 11/2} intruder orbital plays an important role in stabilizing the deformation in this region.

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.

The spin(6) scheme and electromagnetic transitions in 191Ir and 193Ir

The Coulomb excitation of natural Ir and Pt targets has been measured with protons and α-particles of energies between 5.0 and 6.0 MeV. Accurate γ-ray angular distributions were taken to determine the relative yields of observed γ-rays. Using 194Pt as standard the reduced electric quadrupole transition probabilities to the ground states from Coulomb excited levels in the Ir and Pt isotopes were determined. Other B(E2) and B(M1) values were determined from the decay schemes obtained and from mixing ratios deduced from γ-ray angular distributions or from previously measured internal conversion coefficients. The detailed sets are compared with calcula- tions based on the Spin(6) scheme, which predicts several symmetries in 191Ir and 193Ir. The possible existence of wider supersymmetry multiplets in this region of nuclei is examined on the basis of comparison of electromagnetic properties of Ir nuclei with those of Pt and Os isotopes.

Some properties of low levels in 43Sc

The reaction 40Ca(4He, p)43Sc has been used to study excited states of 43Sc below 1.7 MeV. The chief modes of decay of the levels, and in some cases their spins, were determined by measuring the angular correlations between decay gamma rays and the appropriate proton groups. These were observed at near 180° in an annular solid state detector. Experiments were made at 4He bombarding energies of 9.0 and 9.35 MeV. Nine excited states above the ground state were observed at energies of 0.151, 0.472, 0.849, 0.880, 1.158, 1.181, 1.341, 1.414 and 1.644 MeV. The p-γ correlations establish spins of 52 for the 0.880 MeV level and 72 for the 1.341 MeV level. The following scheme is suggested by all the data available: 0.151 MeV level, spin and parity 32+ 0.472, 32−(52±); 0.880, 52±); 0.880, 52+; 1.158, (32+) 1.181, unknown; 1.341, 72+; 1.414, (72±); 1.644, (52+,32−).