A Method For ～(129)I-AMS Measurement Based On 12UD Accelerator

¹²⁹I is a long-lived (15.7 Ma) radionuclide. Although all the ¹²⁹I producedduring the primordial nucleosynthesis has now decayed into ¹²⁹Xe, ¹²⁹I iscontinuously produced by natural processes in the earth's atmosphere and crust.In the atmosphere, cosmogenic ¹²⁹I is produced by cosmic-ray-inducedspallation of xenon and, in minor quantities, by neutron bombardment oftellurium. In the earth's crust, the main source of ¹²⁹I is spontaneous fission of²³⁸U and thermal neutron induced fission of ²³⁵U. However, the greatest sourcesof ¹²⁹I are A-bomb tests and the using of nuclear energy, so, the main source ofenvironmental ¹²⁹I is from anthropogenic with a great quantity of nuclearweapons test and nuclear facilities application. If ¹²⁹I high sensitivitymeasurement can be realized, it can provide valuable information forinvestigation of environment monitor and physical geography. Recently releasesof anthropogenic ¹²⁹I have played an important role in environment and havealready been paid attention to, it is not only for environment monitor, but alsocan provide data for ¹²⁹I in other applications. On the other hand, ¹²⁹I producedby nuclear reactions between cosmic rays and matter can be applied to study thechange of cosmic rays range from 5 Ma to 80 Ma, the chronometers ofastrophysics and deep sea sedimentation and so on.Although ¹²⁹I can be measured by NAA or ICP-MS, the sensitivity of thesemethods is limited. The sensitivity of ¹²⁹I AMS have 3～4 orders of magnitudehigher than that of NAA or ICP-MS. It is very difficult to measure ¹²⁹Iconcentration of water and atmosphere environment samples by NAA or ICP-MS because of the low concentration of ¹²⁹I (10⁶-10⁸atoms). AMS methodshave ultra-sensitive due to the ability to eliminate isobars, isotopes andmolecules interference, the ratio of radionuclides to stable isotopes can reach to10^-15, so AMS become an ideal method to measure ¹²⁹I.This work is supported by the National Natural Science Foundation. Thiswork was finished by cooperating with University of Tsukuba .The experimentis operated at the Tandem Accelerator Complex, University of Tsukuba(UTTAC), in Japan. The goal is to establish a high precision method for¹²⁹I-AMS. The main content of this work is as follows: 1) An electrostaticdeflector which can improve the accuracy of ¹²⁹I-AMS has been developed. Theelectrostatic deflector just set at an image of the 120°magnet. 2) The procedurefor ¹²⁹I-AMS standard samples has been developed.The AMS detection of ¹²⁹I measurement is simple because the absence ofisobaric interference. ¹²⁹Xe is the only stable isobar, it does not form negativeions. The major interference is isotope ¹²⁷I which have the same magneticrigidity of ¹²⁹I. The key to realize ¹²⁹I high sensitivity measurement is toeliminate ¹²⁷I interference by physical methods. The chemical form of sample isAgI. AMS measurements of ¹²⁹I can be performed by extracting I^- negative ions,after the ions pass though 120°magnet and injection magnet, the most of ¹²⁷I^-negative ions can be eliminated, but the ¹²⁹I sensitivity and accuracy is not sohigh, so an improved ¹²⁹I-AMS measurement method has been developed on the12UD tandem accelerator at the University of Tsukuba. As mentioned above,these improvements include making an electrostatic deflector and installing itbehind the 120°magnet to measure ¹²⁷I current and ¹²⁹I count alternately; using⁹⁷MoO₂ molecular pilot beam to stabilize accelerator terminal voltage, by thisway, the terminal voltage is kept stably within 0.1% accuracy. Combining withsecond stripper, this method is successful. The level of sensitivity is 7.83×10^-13for AgI reagent sample. The uncertainty of measurement is less than±7% for aseries of samples which values of ¹²⁹1/¹²⁷I ratio are (4.92～0.28)×10^-10. Calculating the distance between ¹²⁹I and ¹²⁷I at the position of a Faradaycup (IS-FC2) installed just after a 120°magnet, designing and installing anelectrostatic deflector at the chamber, and analyzing the experimental data aremy main contribution.