Electric quadrupole orientation of 188,189Ir in graphite

Samples of 188Ir and 189Ir in foils of highly oriented pyrolytic graphite (HOPG) were obtained by electron capture decay of 188Hg and 189Hg nuclei, which were implanted at room temperature with the aid of the ISOLDE III facility. Nuclear electric quadrupole interaction of these Ir isotopes has been observed with the technique of low temperature nuclear orientation. With the known value for the quadrupole moment of 188Ir, the electric field gradient of Ir in HOPG was determined to be Vzz=+9.96(28)x1022 V/m2. This again could be used to determine the quadrupole moment Q(189Ir)=+0.79(6) b. The quadrupole interaction is so strong that at 10 mK the fraction f of nuclei at good sites aligns completely. There is no sign that the remaining part does align even at the lowest temperatures. The fraction f is 8.9(2)% for 188Ir and 15.5(5)% for 189Ir. These low values are supposedly caused by: i) Lattice damage due to implantation or already existing; ii) partial occupation of positions between the graphite layers,; and iii) perturbations due to preceding electron capture decay. Spin and mixing ratios concerning the 1575, 1716 and 2050 keV transitions of 188Ir and of the 276 and 233 keV transitions of 189Ir are reported. (orig.)