Characterisation of long-lived radioisotopes with AMS

As shown in the introduction, the biggest part of the long time radiotoxicity is produced by long-lived isotopes (e.g. 242Pu, minor actinides, long lived fission products). These isotopes are hard to detect because of their low specific activity. Moreover most of them are pure β-emitters without γ-emission. Therefor it is necessary to distinguish between these isotopes via mass spectroscopy which is independent of half-lives or concentrations.

For detecting heavy isotopes accelerator mass spectroscopy (AMS) is used. With AMS, isobars can be separated and furthermore there is no background because of molecules.

AMS of long-lived isotopes will be done within the joint research project at the GAMS facility of TU Munich which uses the Munich MP tandem accelerator of the MLL. For isobaric separation a gas filled magnet is used in this facility. In a normal magnet the ions are deflected according to:

B \rho = \frac{M v}{q}

Figure: Isobar separation in a gas filled magnet (source http://www.ams.physik.tu-muenchen.de)

Where B is the magnetic field, ρ is the radius, M the mass of the ion, v the velocity of the ion and q the charge of the ion. Assuming that one has filtered the ion velocity via time of flight or a Wien filter, a normal magnet in vacuum is sensitive to \frac{M}{q}. That means that isobars with the same charge are not separable (e.g. 58Fe20+ and 58Ni20+).

For separating isobars a gas filled magnet is used. If an ion enters a gas filled area it will exchange electrons with gas atoms. An effective charge state is created which is proportional to the number of protons of the ion. q_{\text{eff}} \propto v Z^{0.4}

Hence it is possible to separate isobars (e.g. 58Fe Z=26 and 58Ni Z=28) because:

B \rho \propto \frac{M}{Z^{0.4}}

The gas filled magnet can be used to separate isotopes up to mass 130. In this region long-lived fission products can be investigated (e.g. half live in parenthesis  79Se (0,5 Myr), 93Zr (1,5Myr), 99Tc(0,2Myr), 126Sn (0,2Myr), 129CI(16Myr) and 135Cs(2Myr) ).

For heavy isotopes (such as minor actinides) a separation of isobars is not necessary because there exists only one long-lived isotope for each mass number. Therefor a determination of the isotope mass is sufficient.


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