MEASUREMENT OF NI-63 AND NI-59 BY ACCELERATOR MASS-SPECTROMETRY USINGCHARACTERISTIC PROJECTILE X-RAYS

The long-lived isotopes of nickel (Ni-59, Ni-63) have current and pote ntial use in a number of applications including cosmic radiation studi es, biomedical tracing, characterization of low-level radioactive wast es, 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-63(n,p)Ni-63 has been ident ified as one of a small number of reactions which might be used for th e direct determination of the fast neutron fluence emitted by the Hiro shima bomb. AMS measurement of Ni-63 (t(1/2) = 100 y) requires the che mical removal of Cu-63, which is a Stable isobar of Ni-63. Following t he electrochemical separation of Ni from gram-sized copper samples, th e Cu concentration is further lowered to < 2 x 10(-8) (Cu/Ni) using th e reaction of Ni with carbon monoxide to form the gas Ni(CO)(4). The N I(CO)(4) is thermally decomposed directly in sample holders for measur ement by AMS. After analysis in the AMS spectrometer, the ions are ide ntified using characteristic projectile X-rays, allowing further rejec tion of remaining Cu-63. In a demonstration experiment, Ni-63 was meas ured in Cu wires (2-20 g) which had been exposed to neutrons from a Cf -252 source. We successfully measured Ni-63 at levels necessary for th e measurement of Cu samples exposed near the Hiroshima hypocenter. For the demonstration samples, the Cu content was chemically reduced by a factor of 10(12) with quantitative retention of Ni-63. Detection sens itivity (3 sigma) was similar to 20 fg Ni-63 in 1 mg Ni carrier (Ni-63 /Ni approximate to 2 x 10(-11)). Significant improvements in sensitivi ty are expected with planned incremental changes in the methods. Preli minary results indicate that a similar sensitivity is achievable for N i-59 (t(1/2) = 10(5) y), Initial work has been undertaken on the appli cation of this isotope as a biomedical tracer in living systems.