L P Ekstrom

Rotational alignment in 158Er and 159Er

High-spin states in 158Er have been populated with the reactions 145Nd(16O,3n) and 146Nd(16O,4n) and high-spin states in 159Er by the reaction 145Nd(18O,4n). Gamma-gamma coincidence and angular correlation data were obtained using an array of five Ge(Li)-NaI(Tl) escape-suppressed spectrometers. Linear polarisation data were obtained using a sectored Ge(Li) detector and internal conversion data were obtained using a mini-orange spectrometer. In 158Er the yrast band has been observed up to Jpi =26+ and three negative-parity side bands have been observed up to spins Jpi =23-, 22- and 15- respectively. The ground-state band has been observed to continue after the first backbend in the yrast sequence to Jpi =20+ and the odd- and even-spin members of the gamma-vibrational band were observed up to 7+ and 10+, respectively. In 159Er the yrast band has been observed up to Jpi =57/2+ and four side bands have been identified to spins Jpi =15/2-, 39/2+, 49/2-, and 51/2- respectively. The bands observed are interpreted as configurations of quasiparticles using the cranked shell model. Residual quasiparticle interactions are calculated as a function of rotational frequency.

An on-line computer controlled time digitizer for half-life measurements

Abstract A novel time digitizer has been commissioned to facilitate the measurement of isometric lifetimes or the measurement of the lifetimes of nuclear reaction products from μs upwards. The time digitizer has been used in conjunction with a beam chopper on the Liverpool University EN tandem Van de Graaff accelerator to measure and store the time intervals between the beam off markers and every subsequent γ-ray occuring in the beam off period. The device has been interfaced as a pseudo ADC into the Liverpool on-line computer system such that the beam on and off periods and time dispersion can be set from the on-line display and are then used for spectrum labelling. Exponential decay spectra of 1000 channels or more may be quickly built up on magnetic tape for every γ-ray contained in a Ge(Li) spectrum and the decay of selected γ-rays or sections of the spectrum may be monitored via the on-line display. The system has been tested in the range 20 μs to 20 s against accurately measured isometric half-lives in 38 K, 24 Na and 77 Se. Further measurements have been carried out for several other cases where there has been disagreement in the literature.

Gamma-ray studies in 64Zn: levels above 3.1 MeV

The 61Ni ( alpha ,n gamma ) 64Zn reaction has been studied 8.6 MeV and 9.35 MeV bombarding energies together with the 56Fe(11B, p2n gamma )64Zn reaction at 30 MeV bombarding energy. The gamma -ray decay scheme of 64Zn has been deduced up to 5.7 MeV excitation by means of gamma - gamma coincidence experiments and n- gamma coincidence experiments. Spins and parities have been deduced for most of the high-spin states between 3.1 and 5.7 MeV excitation by means of gamma -ray angular distribution measurements together with gamma -ray linear polarisation measurements. The lifetimes of levels have been measured by DSAM enabling electromagnetic transition rates to be determined for many transitions. The results have been compared with shell-model, cluster-vibration model and interacting boson approximation calculations.

Lifetime and linear polarisation measurements with the 80Kr(α,nγ)83Sr and 82Kr(α,nγ )85Sr reactions

Mean lifetimes of 83Sr, 84Sr and 85Sr levels have been measured by DSAM. The reactions used were (?,n?) and (?, 2n?) with solid krypton targets. Angular distribution and linear polarisation measurements yielded several multipole mixing ratios and J? assignments. The J? of the 4.9 h isomer in 85Y is shown to be 2/9+.

Spin-alignment attenuation factors in mass 61–67 nuclei populated by (α, n) and (α, p) reactions

Gamma-ray angular distributions for 39 E2 transitions in A = 61–67 nuclei, excited in ( α , n) or ( α , p) reactions, are collected from the literature and reduced to spin-alignment attenuation factors α 2 and α 4 . The validity of the prediction for α 2 from statistical compound nucleus reaction theory is investigated. Also the sensitivity of this prediction to the optical-model parameters is examined. The present data support the rule 0 ⩽ α 4 ⩽ α 2 , but contradict the assumption of a Gaussian shape for the population of magnetic substates.

Gamma-ray studies of 62Zn

The reaction 58Ni(6Li,pn gamma )62Zn was used to populate levels in 62Zn at bombarding energies between 15 and 24 MeV. Gamma-gamma and neutron-gamma coincidence measurements at beam energies of 20 and 24 MeV were used to establish a decay scheme up to 7.4 MeV in excitation. Spins and parities of the levels together with branching and mixing ratios of the gamma-ray transitions have been deduced from angular distribution and linear polarisation measurements. Mean lifetimes of many levels have been measured using the doppler-shift attenuation method and the recoil-distance method. Comparisons are made with the shell model and the interacting boson approximation model.

Gamma-ray spectroscopy on 87Sr and the energy-B(E2) rule

In-beam gamma -ray measurements following the reaction 84Kr( alpha ,n gamma )87Sr have been undertaken. The level scheme has been established with gamma - gamma coincidence measurements. Mean lifetimes for several levels were obtained with the DSA method. New spin-parity assignments have been made for a number of levels from angular distribution and linear polarisation measurements. The mod g92/-1)(X) mod 88Sr(21+)) quintuplet is discussed in terms of a new energy-B(E2) rule.

Heavy-ion gamma-ray spectroscopy of 60Ni

Twenty excited states below 6.8 MeV and with spins up to J=(9) have been observed in 60Ni in the reaction 56Fe(7Li, p2n gamma ). Lifetimes of levels have been determined from DSA and RD measurements. Gamma-ray angular distribution and linear polarisation measurements have been made and unambiguous Jpi assignments deduced. Several new high-spin states above 4 MeV in excitation are proposed and the results have been compared with the predictions for high-spin states of various theoretical models.

Gamma-ray spectroscopy of 61Zn and a planar germanium polarimeter system

The decay scheme and level structure of the neutron-deficient isotope 61Zn have been investigated up to an excitation energy of 2.4 MeV using gamma -ray spectroscopy techniques following the 58Ni( alpha ,n gamma )61Zn reaction. Using beam energies in the range 10.75-16.0 MeV, excitation functions gamma - gamma coincidences, n- gamma coincidences, angular correlations and linear polarisations have been measured resulting in the determination of a decay scheme, excited state spins and parities and gamma -ray branching and mixing ratios. For the linear polarisation measurements a new design of Compton polarimeter is used. The design and operating characteristics of this polarimeter system are described in detail. Spin and parity assignments of Jpi =1/2-, 5/2-, 3/2-, 5/2-, 7/2-, 9/2- and 9/2(+) have been made for the levels determined at Ex=88, 124, 418, 756, 996, 1265 and 2002 keV respectively. The results are compared with shell-model calculations.

A study of the neutron-deficient isotope 66Ge

The neutron-deficient isotope 66Ge has been studied using gamma-ray spectroscopy techniques. States in 66Ge were populated using the 58Ni(10B,pn gamma )66Ge reaction at bombarding energies of 28.5, 29 and 30 MeV. A decay scheme has been established from the results of gamma-gamma and neutron-gamma coincidence experiments performed at a beam energy of 28.5 MeV. Angular distributions were measured at beam energies of 28.5, 29 and 30 MeV, and linear polarisations were measured at 28.5 MeV. The analysis of these data yields spin-parity assignments of 1694 keV (2+), 2175 keV (4+), 3686 keV (4+,5-), 4208 keV (6+,7-) and a probable 6+ assignment to the 3657 keV level. Spin restrictions are also placed on several other levels. The lifetimes of the 958, 1694 and 4208 keV levels are deduced from a recoil-distance-method experiment, performed at a beam energy of 29 MeV, to be 5.3+or-1.0 ps, 6.5+or-2.5 ps and 295+or-40 ps respectively. A limit of less than 3 ps has also been placed on the lifetime of the 2175 keV level.