Highly sensitive and precise 129I-AMS system was developed at MALT(Micro Analysis Laboratory, Tandem accelerator), The University of Tokyo. In this new system, we chose a gas counter for the final detector instead of TOF used previously. Accelerator terminal voltage 4.0MV and charge state 7+ were selected in order to gain rather higher energy and lower magnetic rigidity. Higher energy is advantageous for the resolution of the gas counter. Lower magnetic rigidity requires lower magnetic field of the analyzing magnet so that it is easier to cool the magnet. In 129I-AMS system, most serious interference for the detection of 129I is the isotope ion 127I. MALT has, as a high-energy mass analyzing system, the combination of an analyzing magnet (momentum analysis) and an electrostatic (cylindrical) analyzer (ECA: energy analysis) before the final detector. However some amount of 127I ions reach the final detector even when ions of mass 129 are injected to the accelerator. These 127I ions were found to have 1.4% higher energy (as an average) than 129I by the detailed examination of the ECA profiles. Finally 127I interference successfully reduced by setting ECA value appropriately. As a result we obtained isotopic ratio of 129I/127I of 4.0 x 10^-14 for a lot named "Israel" of iodine regent kindly offered by PRIME lab. In this system, several fragments of molecular ions also come to the final detector. Expected molecular fragment ions 37Cl2+ and 92Zr(or Mo)5+ were clearly identified on the spectrum obtained by the gas counter. Although accelerator transmission efficiency of the system (4.0MV and 7+) is not so high (1.75 %), 10 cps of 129I counts can be obtained for the case of 129I/127I ratio of 1x10-^11 samples when 1x10-^5A of 127I ions are extracted at the ion source. Using this 129I-AMS system, 129I concentrations in several Japanese soils were measured as well as various iodine regents. Some of results follow: 129I/127 ratios of regents (Aldrich, Ionplus, Kanto and Wako) are all around 1.5-2.0x10^-13 which coincides with that of Chiba brine. Soils taken near a nuclear reprocessing plant has seriously (an order or two) higher 129I concentration compared to other region. 129I tends to be adsorbed strongly near the surface of the soils. Generally 129I concentration in forest soil is higher than a paddy field. We plan to investigate more global and detailed distribution and movements of 129I based on the AMS measurements of many soils from now on.
See more of Nuclear Chemistry, Isotope Forensics and Nonproliferation
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)