ELECTRIC QUADRUPOLE ORIENTATION OF IR-188,189 IN GRAPHITE

Samples of Ir-188 and Ir-189 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 Ir-188, the electric field gradient of Ir in HOPG was determined to be V(zz) = +9.96(28) x 10(22) V/M2. This again could be used to determine the quadrupole moment Q(Ir-189) = +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 Ir-188 and 15.5(5)% for Ir-189. 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 Ir-188 and of the 276 and 233 keV transitions of Ir-189 are reported.