Measurement of 63 Ni and 59 Ni by accelerator mass spectrometry using characteristic projectile X - rays

The long-lived isotopes of nickel (Ni, Ni) have current and potential use in a number of applications including cosmic radiation studies, biomedical tracing, characterization of low-level radioactive wastes, and neutron dosimetry. Methods are being developed at LLNL for the routine detection of these isotopes by AMS. One intended application is in Hiroshima dosimetry. The reaction Cu(n,p)Ni has been identified as one of a small number of reactions which might be used for the direct determination of the fast neutron fluence emitted by the Hiroshima bomb. AMS measurement of Ni (t1/2 = 100 y) requires the chemical removal of Cu, which is a stable isobar of Ni. Following the electrochemical separation of Ni from gram-sized copper samples, the Cu concentration is further lowered to < 2 × 10 -8 (Cu/Ni) using the reaction of Ni with carbon monoxide to form the gas Ni(CO)4. The Ni(CO)4 is thermally decomposed directly in sample holders for measurement by AMS. After analysis in the AMS spectrometer, the ions are identified using characteristic projectile x-rays, allowing further rejection of remaining Cu. In a demonstration experiment, Ni was measured in Cu wires ( 2-20 g ) which had been exposed to neutrons from a Cf source. We successfully measured Ni at levels necessary for the measurement of Cu samples exposed near the Hiroshima hypocenter. For the demonstration samples, the Cu content was chemically reduced by a factor of 10 with quantitative retention of Ni. Detection sensitivity (3σ ) was ~20 fg Ni in 1 mg Ni carrier ( Ni/Ni ≈ 2 × 10 -11 ). Significant improvements in sensitivity are expected with planned incremental changes in the methods. Preliminary results indicate that a similar sensitivity is achievable for Ni (t1/2 = 10 5 y). Initial work has been undertaken on the application of this isotope as a biomedical tracer in living systems.