B. Kastelein

Nicole: New on-line orientation facility at ISOLDE/CERN

This paper presents off-line and on-line orientation results obtained with the new on line nuclear orientation device NICOLE in CERN. Magnetic moments of187,185Ptg and186Irm, electric quadrupole moment ratios between Pt isotopes with mass number 185, 187 and 189, the spin of the 2 h isomer of186Ir and a new decay scheme of184Au involving a metastable state are established. The shape variation versus mass number of the Pt isotopes leads to sign change of the spectroscopic quadrupole moment between A=187 and 185.

Electric quadrupole nuclear orientation of182Ta as graphite intercalation compound TaCl5

Nuclear orientation of182Ta achieved by electric quadrupole interaction in TaCl5 molecules intercalated in oriented graphite crystals has been observed. Assuming the electric field gradient Vzz along the c-axis and proper intercalation yields Vzz=+0.32 (4)×1018V/cm2. Other possibilities and sources of reduction of the nuclear orientation effect are considered.

Hyperfine interactions of light Ir, Re isotopes in iron

The technique of On-line Nuclear Orientation has been applied to light Ir and Re isotopes. Using both192IrFe and54MnNi thermometers, orientation as a function of temperature was measured over the range 10–100 mK. Pulsed implantation methods were used to obtain spectra with gamma transitions well identified and clear of contaminant activities. Magnetic moment/oriented state spin combinations were obtained for180,182,183Ir and for180,179Re. The results are briefly discussed.

Time-resolved and time-integral on-line nuclear orientation measurements of neutron-deficient Hg−Au−Pt−Ir nuclei

The methods of time-resolved and time-integral on-line nuclear orientation have been applied to study short lived nuclei with the NICOLE facility (Nuclear Implantation into Cold On-Line Equipment) at ISOLDE-3 in CERN using beams of182–186Hg. The half-lives in these decay chains are of the order of seconds and therefore comparable to the spin-lattice relaxation times of the nuclei in iron. As the relaxation rate depends strongly on the g-factor, g-factors of nuclei in the decay chains can be deduced from the observation of the time evolution of γ-ray anisotropy. Using this technique the existence of an isomer in184Au has been found and the g-factors of184Au,184mAu and182Au have been determined. Accurate half-lives have been extracted from the data.Time-integral nuclear orientation has been observed for short lived as well as longer lived isotopes of the Hg decay chains. From these measurements, after proper correction for incomplete relaxation, the magnetic moments of183mPt,183Ir and182Ir have been derived.The applicability of the time-resolved nuclear orientation technique for nuclei far from stability and its possible limitations is discussed.

Nuclear electric quadrupole interaction of implanted ions in graphite

A number of isotopes have been implanted into thin foils of highly oriented pyrolytic graphite in order to investigate the possibility of using this material as a catcher for on-line LT-NO experiments to measure the quadrupole interaction of short-lived nuclei.Pure nuclear electric quadrupole interaction of nuclei of188,189Ir,203Hg,69mZn,182m,183Re and111In has been observed. Values for the electric field gradient along theC-axis are: +10.0(3) +8.2(9), +5.4(2.6) and +1.5(1)·1022 V/m2, respectively, for Ir, Hg, Re and In in graphite. A value ofQ=+0.79(6) b is deduced for the quadrupole moment of189Ir. Two theoretical models provide a better understanding of the origin of the electric field gradient in graphite. The first is based on induced electric polarization of graphite atoms, while in the second one hybridization of impurity and graphite electronic wave functions is calculated.

BRUTE-FORCE ORIENTATION OF HF-178M2 AND MAGNETIC-MOMENTS OF HF-178M1 AND HF-178M2

Gamma-ray anisotropy measurements have been carried out with brute-force oriented178m2Hf (16+) to derive its magnetic moment 7.26 (±0.16; +0.2/−0) nm and E2/M1 mixing ratios of some γ-transitions observed in the decay of this isomer. This magnetic moment is in agreement with an earlier proposed four-quasi-particle configuration. The magnetic moment of the 8− isomer at 1147.7 keV (4.0 s) is estimated to be between 1.4 and 5.3 nm, which confirms mixing of the {p[514]9/2 +p[404]7/2}8− and {n[514]7/2+n[624]9/2}8− configurations, which summed together give the 16+ isomer.

Nuclear alignment of implanted160Tb in highly oriented pyrolytic graphite layers

Low-temperature nuclear alignment of160Tb ions implanted in Highly Oriented Pyrolytic Graphite has been observed by the anisotropic intensity distribution of γ-rays. The data can be understood with an effective spin Hamiltonian containing combined magnetic and electric hyperfine interactions. Values deduced for the hyperfine parameters are A/k=(101±7) mK for the magnetic interaction, and P/k=(−3.7±0.9) mK for the nuclear electric quadrupole interaction, under the assumption that B=0 and Δ=0. Measurements in magnetic fields of 0.2 and 1.0 T directed along the graphite c-axis showed thermal saturation due to the strongly reduced heat conductivity of HOPG in the presence of a magnetic field.

Hyperfine interactions of implanted nuclei in graphite observed by low temperature nuclear orientation

Pure nuclear electric quadrupole interaction of ion-implanted 203 Hg and 188 Ir in Highly Oriented Pyrolytic Graphite layers has been observed in low temperature nuclear orientation experiments. The electric field gradients at the nuclear sites were found to be positive, with values of +0.79(17)×10 19 V/cm 2 (ν Q =0.66 (14) GHz) for 203 Hg and +0.788 (40)×10 19 V/cm 203 (ν Q =0.987 (40) GHz) for 188 Ir. At 4 mK, the anisotropies in the directional distribution of γ-rays from these isotopes were saturated, but only at 35.2 (2.4) % for 203 Hg, respectively 8.59(18)% for 188 Ir, of the expected maximum value.

STUDY OF ELECTRIC QUADRUPOLE INTERACTIONS OF IN-111 AND CD-111 FOLLOWING ION-IMPLANTATION OF IN-111 INTO GRAPHITE

The nuclear electric quadrupole interaction of111In ion-implanted in hightly oriented pyrolytic graphite has been observed by means of low-temperature nuclear orientation and by means of perturbed angular correlations. From the first kind of experiment, it is concluded that a relatively large number of indium nuclei experience a well-defined macroscopic orientation, which is partly lost after the radioactive decay to cadmium. Indeed, the second kind of experiment revealed a broad distribution of electrid field gradients interacting with the 245 keV Cd excited state, as well as a small faction experiencing a unique electric field gradient. The experimental results are compared with theoretical calculations of the electric field gradient at various lattice positions, in which carbon and indium electronic wavefunctions are allowed to hybridize. Lattice positions of the covalent indium atom between the graphite layers can explain the measured electric field gradient ofV22=+1.47(11)·1022 V/m2, directed parallel to the graphitec-axis.