Washington University home page back to Department home page

Thermion Mass Spectrometer (TIMS)

An Micromass Sector54 mass spectrometer is currently being installed in the Mass spectrometer Laboratory. The machine is equipped with 9 Faraday collectors, an ion-counting Daly detector system. It will be used primarily for isotopic analysis of uranium (U) and lead (Pb) in small mineral samples and isotopic analysis of thorium (Th).

For TIMS analysis, a sample is loaded onto a metal filament (usually Re or Ta, although other refractory metals may be used) that is heated by applying a current through the filament. The sample is gradually heated until ions are detected. Most of the sample will leave the filament due to evaporation, which means that the atoms are not ionized and therefore will not make it to the detector. A small fraction of the atoms (about 1 in 104 or 105) are ionized and accelerated towards the collector. The beam of ionized atoms is accelerated and focussed at the source by the collimator before it passes through the magnet. A charged particle travelling through a magnetic field is subject to a force:

F = q V x B

where q = the charge of the particle, V = the magnitude of the electric field, and B = the magnitude of the magnetic field.

The exerted force changes the ion trajectories from straight lines into curves. The radius of curvature for each ion is dependant on the momentum of the ions, in addition to their charge and the strength of the magnetic field. The kinetic energy of the all ions leaving the filament is approximately the same, 1/2mv2 (where m = mass and v = velocity). Therefore the difference in momentum (mv) between different isotopes differentiates ions according to mass. Heavier ions are deflected less and follow a path with a larger radius of curvature, while low-mass ions are deflected more and follow a more curved path. Isotopes are differentiated by charge to mass ratio according to the following relationship:

m/q = 2 V / (Br)2

where m = the mass of the particle, and r = the radius of curvature.

The strength of the magnetic field is generated by running a current through a wire curved many times around to form a torus. Varying the current changes the exact magnitude of the magnetic field. The different isotopic masses are going to be catched in a collector which can be either in Farady beakers and measured simultanious or they are going to be reinforced through a Daly Photomultiplier in the scale of 103 and is going to be measured step wise. The final ion beam intensities are converted directly into isotope ratios.