AMS 13 Abstract:
During nuclear weapons testing several caesium isotopes were released into the environment. 137Cs has been used in many studies. However this isotope has a relatively short half-life (30a) and it has already undergone ~50 years of decay. Caesium 135, another fissile isotope, has a much longer half-life (about 2Ma) and could be used to replace 137Cs and the ratio between the two isotopes of Cs could be used to identify the source of Cs and to calculate the age of the source material.
However Cs-135 can be very difficult to measure. It cannot be gamma counted, as it is a pure beta emitter and beta counting is an impractical approach due to low decay rate. This leaves mass spectrometry as a viable option. Analyses using TIMS and ICP-MS have been established but their detection limits and sample preparation requirements suggest the possible use of AMS.
The development of an AMS technique for 135Cs requires the development of (1) a beam of Cs anions, (2) a method to separate 135Cs from 135Ba and other ions with the same mass to charge ratio and (3) production of standards and yield tracers to measure the efficiency of the analytical process.
We have used the IsoTrace AMS facility and: (1) Tested a number of different Cs compounds to identify methods to produce Cs beams, (2) Successfully separated 135Cs from 135Ba using an Isobar Separator for Anions (ISA). This reaction chamber selectively reacts 135Ba with oxygen while allowing 135Cs to pass into the accelerator and (3) Used 134Cs as an internal standard and yield tracer.
Currently, the limitations in the analysis of 135Cs are the beam current and cross contamination during sputtering. An array of different Cs molecules are being tested and optimized for greater and more stable beam currents.
Abstract from M.Sc. thesis:
The first measurements of the radioactive 135Cs and 134Cs isotopes were made on an accelerator mass spectrometer. The natural Ba interference was suppressed using an isobar separator for anions (ISA) in order to measure the less abundant isobaric 134Cs and 135Cs isotopes. It was found that the Ba interference could be suppressed by a factor of 2 × 105 while 25% of Cs was transmitted. Furthermore, through comparing the known natural abundance of Ba isotopes to the measured concentration in a sample it was shown that the ISA does not introduce significant mass dependant fractionation at the level of 0.8%. A slow sequential injection analysis technique was developed to measure 135Cs using 134Cs as a reference isotope. This technique also permitted the monitoring of Ba interference. The ionization efficiency of Cs when analyzed in the molecular anion form, CsF−2 , was on the order of 10−7 while the total measurement efficiency was 1.7×10−9. The abundance sensitivity of this system was found to be 135Cs/ 133Cs = (1.3 ± 1.7) × 10−10 , corresponding to a 3σ detection limit of 132.5 pg of analyte per target. Lastly, using the developed AMS techniques, beta spectroscopy, gamma spectroscopy, and isotope production, a measurement of the half life of 135Cs was made. The two measurements of the half life of 135Cs were 0.72 ± 0.32 Ma and 0.99 ± 0.42 Ma.