The Role of 129I in the Environment and Its Measurement at the ANTARES AMS Center
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AU9716162 The Role of 129I in the Environment and its Measurement at the ANTARES AMS Center D FINK, MAC HOTCHKIS, EM LAWSON, GE JACOBSEN, AM SMITH and C TUNIZ Australian Nuclear Science and Technology Organization, PMB 1 Menai, NSW, 2234 SUMMARY Anthropogenic production of several radionuclides during the nuclear era has resulted in a dramatic enhancement in their ambient concentrations relative to cosmogenic values for geophysical systems which are in exchange with the atmosphere and oceans. These environmental pulses have been archived in sediments and polar ice caps where for example profiles of ^Sr and 135Cs have been measured to establish global transport rates and deposition budgets for bomb-test products. 129I, half-life 16 Ma, is another nuclear fission product that is and has been periodically released into the atmosphere, but unlike the shorter lived 90Sr and l35Cs, had not found widespread utilization because previous detection via neutron activation analysis (l29I(2n,y)mI) was cumbersome and lacked the required sensitivity. AMS has resolved this problem for 129I measurements by reducing the required sample size, measurement time and atom-counting sensitivity to as little as 106 atoms that enable I29I/127I isotopic ratios as low as 2xlO14 to be measured on milligram samples within an hour. As a result of this new detection capability, an ever-growing interest and awareness in the application of I29I as an environmental tracer, radiometric dating tool and monitor of operations of the nuclear industry has been generated. In response to these advances we have commenced 129I AMS measurements at ANTARES INTRODUCTION Prior to nuclear testing, the two main mechanisms for 129I production were radiogenic - the spontaneous fission of 238U (99.3%) and neutron induced fission of 235U (0.7%) in the subsurface, - and cosmogenic - cosmic ray spallation of Xe in the atmosphere. These two modes shared on an equal basis supply of I29I to the global inventory of the hydrosphere. With an iodine ocean residence time of -300 ka being far shorter than the I29I 16 Ma half-life and considerably longer than the turn-over time in the marine system, it can be safely assumed that iodine is well mixed. Analogously to the behaviour of I4C between the biosphere and atmopshere, reservoirs actively exchanging iodine arrive at an isotopic equilibrium - but once the 129I becomes detached from the main reservoir such as below the bioturbation depth in marine sediments or closed in evaporite deposits and oil field brines, exchange ceases and the decay clock begins. The 'initial' equilibrium ratio is difficult to determine but is estimated from recent (pre-bomb) ground-waters and sediments to be 120xl0"14. Given that the lowest AMS I29I/127I measurement (blank) in an ancient (600 Ma) deep salt mine sample is 4xlO"14, dating of marine sediments, Fehn et al (1), evaporite and ore deposits, and oil brines, Fehn et al (2), can potentially reach -80 Ma. In U-rich ore bodies, where radiogenic I29I concentrations may be quite considerable, its use as an analog 62 tracer to study migration and distribution of fission products has been carried out, eg the Stripa Granite formation, Sweden, Fabryka-Martin et al (3). However, with the advent of the nuclear era, an overwhelming proportion of the global inventory of I29I is anthropogenically produced via induced fission of 235U (yield of 0.74%) and 239Pu (1.5%) from nuclear power and reprocessing plant operations. I29I release from the Chernobyl incident in 1986, Paul et al (4), was recorded in rain in Germany and as far afield as Israel with concentrations of 30xl09 and 2xl09 atoms l29I/litre, respectively - a factor of -300 higher compared to rain collected in 1982. Ocean profiles in the Gulf of Mexico from surface to ocean bottom (1500 m) show values of 129I/I27I ranging from -lOOxlO"12 (anthropogenic I29I) to lxlO"12 respectively, Schink et al (5). The distribution of large quantities of effluent emissions from coastal nuclear fuel reprocessing plants at La Hague, France, and Sellafield, Great Britain, have been investigated using 129I tracing along the zone of the ocean plume, Yiou et al (6). 129I/127I ratios in those ocean surface water samples and seaweeds in close proximity (tens of kms) to the plants show a 104-fold increase. Rough estimates indicate that an astonishing 1.2 tons of I29I has been released into the environment from Sellafield and La Hague alone over the past 25 years. Although the anthropogenic signal presents a serious complication to the use of cosmogenic 129I as a geologic and marine chronometer, the above studies clearly indicate the new roles of 129I to remotely monitor nuclear activity with respect to arms control and safeguard agreements, and as an oceanographic tracer. THE 129I PROGRAM and AMS MEASUREMENTS at ANT ARES A number of I29I projects, in conjunction with AINSE, (eg dating the secondary ore zone at Broken Hill, tracing and dating ground waters), the IAEA (distribution of 129I concentrations in the environs of a nuclear power plant using water, sediment and bilogical samples) and dealing with an AMS inter-laboratory calibration have been scheduled and are in progress. To deal with the rather diverse selection of sample materials which involve I29I studies at Ansto, comprehensive and efficient chemical procedures for pretreatment and reduction of raw samples into the desired form of Agl have been tested and established. Procedures for extraction of I from water, sediment, and biota are available. Due to the absence of formation of negative ion beams of the isobar 129Xe, identification of I29I above residual background events is made relatively easier than that for some of the other AMS radioisotopes. In fact, this has allowed the use of even small tandem accelerators running at 3MV for I29I measurements, Kilius et al (7). The major complication arises from the ever-present background of 127I ions which through scattering and charge exchange manage to leak through the magnetic and electrostatic filtering elements prior to the detector. Our measurments of I29I are being carried out at ANT ARES using the 55° beam-line equipped with the high resolution 22°, 5m radius electrostatic deflector (ESA). A new time-of-flght (TOF) system has been installed comprising a multi-channel plate (MCP) start detector and a silicon barrier stop detector (300 mm2), which also provides a total energy signal. The MCP operates at -3800V with a 19mm radius, 20ng/cm2 carbon foil positioned at 45° to the beam. The TOF path at present is 600 mm. Modifications are in progress to increase this to 1200mm and construct retractable MCP units with electrostatic mirror assemblies to enable a perpendicular foil orientation so as to minimize flight path straggling and improve the TOF transmission. We have achieved a TOF timing resolution (FWHM) of 500 psec, which is sufficient to completely resolve the 129I peak from the low background rate of 127I , which at equal rigidities, has a TOF difference from I29I of 1 nsec. The tandem is operated at 6.3 MV for the 11+ charge state to give final I29II1+ energies of 75 MeV. Particle 63 transmission varies from 1-3% depending on foil quality. The use of a gas stripper will considerably improve this. A 2-dimensional TOF vs E(Si) spectrum is used to give unambiguous identification of 129I events. The beam line is tuned with an equal-rigidity 127I beam to a Farady cup assembly positioned after the ESA and in front of the MCP unit. The analyzing magnet off-axis cup was positioned to intercept the I27III+ beam at the terminal voltage used for measuring the 129lI1+ rate. The bouncer could not be used simultaneously with 129I measurment due to excessive rates in the E(Si) detector during isotope cycling and was effectively used as a beam chopper with AM cup insertions for each mass changeover as protection. A measurement sequence involved 300 sec for 129I (bouncer off) and 50 sec for 127I (bouncer on). A series of measurements on different blanks and low level samples and standards has been carried out. Commercial Agl and chemically prepared Agl at ANSTO both give I29I/I ratios of 20xl014. Blanks supplied by M. Paul at the AMS lab at the Hebrew Univ Jerusalem, Israel and from M. Roberts at the AMS lab at Lawrence Livermore, USA have also been measured at ANSTO. These blanks are reported to have 129I/I ratios of 4x1014. Our result of 20x10"14 for these samples, equivalent to that for the chemistry blank processed at ANSTO, indicates that the potential of on-site 129I contamination from HIFAR operations into our chemistry preparation system is negligible and the source of our 129I events is most likley the result of 127I misidentification, ion- source cross talk and/or random coincident events from other lower mass background ions. Further tests are planned to improve our sensitivity into the 10"14 range. S. Vogt at PRIME lab, Purdue University, USA, has kindly donated a complete set of 129I standards with strengths ranging from 120 to 62200 x 10'14. We have used a number of these standards to assess our reproducibility, accuracy and mechanism of ion-source cross-talk. Due to rapid stripper foil deterioration and unsteady I ion-source output which both lead to unacceptable variations in the measurement of the standard, our reproducibility is at present no better than 10%. To date, we have successfully analyzed a suite of -20 samples consisting of waters, sediments and biota collected for the IAEA project. Our results for the ocean waters range from 10 to 300 xlO9 atoms l29I/litre which is consistent with that measured by Yiou et al (6).