J.R.B. Oliveira

SACI—a 4π plastic phoswich array ancillary detector system of a γ-ray spectrometer

A large solid angle low-Z charged particle detector system was built in order to improve the quality of the identification and selection of evaporation channels for in-beam g-spectroscopy measurements. The array consists of 11 plastic phoswich telescopes closely packed in a 4p arrangement and to be used as an ancillary system for the Pelletron Laboratory g-ray spectrometer. The new system was tested in g-particle and g–g-particle coincidence measurements using the 27 Al þ 16 O and 100 Mo þ 11 B reactions, at 53 and 43 MeV incident beam energies, respectively. The values for the particle detection efficiency and improvement in the peak to background ratios are considered acceptable for the usefulness of the system. r 2002 Elsevier Science B.V. All rights reserved.

Study of the effect of disorder on the local magnetism in Heusler alloys

A local study of the influence of annealing temperature that can provoke structural defects and chemical disorder on the magnetism of Heusler alloys has been carried out by measuring hyperfine interactions in Pd2MnZ (Z=Ge, In, Sn, and Sb). Perturbed angular correlation method has been used to measure the hyperfine fields at Cd111 probe nuclei substituting the Mn and Z sites as a function of temperature. The fraction of In111-nuclei-occupying Mn sites was found to increase with the annealing of sample at higher temperatures followed by quenching that can leave the alloy in a disordered state. As a consequence, it was possible to observe the effect of temperature in the ordering of the alloys. Moreover, electric quadrupole interaction measurements clearly indicated chemical disorder after quenching in Pd2MnSn and an improvement in the ordering of the Pd2MnGe structure at high temperatures.

First allowed bandcrossing in neutron deficient nucleus 141Tb

The neutron deficient 141Tb nucleus has been studied with the 92Mo (54Fe, ap) reaction at 240-MeV incident energy and the multidetector array GASP. For the yrast ph11/2 decoupled band, excited states up to 6.7 MeV and spin up to 47/2- have been observed. This band presents an upbend at rotational frequency of w = 0.38 MeV due to the alignment of h11/2 protons. The results are discussed in terms of the Cranking model.

Stopping power of Au for silver ions at low velocities

Energy loss measurements for the slowing down of Ag ions in Au, in the velocity range 1:6v0 < v < 4:4v0, where v0 is the Bohr velocity, are presented. The measurements were performed using the Doppler shift technique and also with a new method, where a secondary beam of low velocity heavy ions is produced by elastic scattering of the accelerated beam. The results are compared to the SRIM2000 calculations (www.srim.org) and to recent measurements in this velocity region.

Installation of the IMPAC technique in the Pelletron laboratory

A special target chamber for the direct production & recoil implantation of radioactive probes for perturbed angular correlation has been constructed and tested. Feasibility studies were made by reproducing the known quadrupole frequencies at 77Br and 111In probes in metallic zinc. Preliminary results were obtained on the lattice site dependence of Cd hyperfine field in Pd2MnSn Heusler alloy with the PAC technique.

Experimental Determination of the 6,7Li Nucleon Densities

In this work we present optical model analysis of new near barrier‐quasi‐elastic experimental data, for the systems 6,7Li+120Sn. From the analysis it was possible to extract the ground state nucleon densities of the weakly bound 6,7Li isotopes. The apparent discrepancies between the experimental densities, as compared with those based on Dirac‐Hartree‐Fock‐Bouguliubov (DHB) calculations were removed, considering in the reaction mechanism, the projectile break‐up and a positive polarization from couplings of 6,7Li states of the continuum.

An imaginary potential for dissipative processes in heavy ion reactions

In this work, the performance of new coupled channel calculations in describing heavy‐ion experimental data for energies up to 100 MeV/A, with the real and imaginary potential based on the Sao Paulo Potential (SPP), is presented. The imaginary potential, with no adjustable parameter, which describes dissipative processes such as deep‐inelastic (DIC) or break‐up, is inspired in high energy nucleon multi‐scattering theory.

High-K band in 140Gd

High-spin states in the neutron-deficient140Gd nucleus have been studied with the 92Mo(54Fe,a2p) reaction at a beam energy of 240 MeV. The level scheme of 140Gd was considerably extended from what was previously known. In this work we concentrate on one of the 9 bands observed, which presents relatively strong M1 transitions and negligible signature splitting, and has an isomeric band-head, indicating a strongly coupled or high-K configuration. We compare this band to the Kp = 8- bands observed in N=74 isotones and propose a similar configuration assignment, but with a somewhat larger deformation.

Implantation of 111In in the heusler alloys Pd2MnZ (Z=Sn,Sb,Ge,In) following heavy ion nuclear reactions : Measurement of magnetic hyperfine field using PAC spectroscopy

Ion implantation of the recoil 111In nuclei following heavy ion nuclear reactions 108Pd( 7Li,4n)111In and 108Pd( 6Li,3n)111In has been used to implant 111In probes in the Heusler alloys Pd2MnZ(Z=Sn,Sb,Ge,In). Perturbed Angular Correlation method was used to study the hyperfine magnetic field in these alloys. Direct implantation of 111In probe nuclei was used to great advantage in the present case resulting in large implantation efficiency. Only a few hours of irradiation time with moderate beam current of the order of 400-500 nA resulted in sufficient implanted 111In activity on the sample for good quality measurements. The hyperfine field was measured at 111In probe nuclei substituting Mn and Z sites as a function of temperature. The fraction of 111In nuclei occupying Mn atom sites was found to increases with the annealing of sample at higher temperatures.

Rotational and vibrational bands in 108Pd

The 108 Pd nucleus has been studied with the 100 Mo( 11 B, p2nγ) reaction at 43 MeV incident energy. γ − γ − t, γ −γ− charged particle coincidences and directional correlation ratios were measured using the γ spectrometer formed by four Compton suppressed HPGe detectors and a4π charged-particle ancillary detector system. The struture of the bands was interpreted within the framework of the cranked shell model and total Routhian calculations.