The Chalk River AMS time-of-flight detection system

At Chalk River, the final step in the identification of 129 I is a time-of-flight (TOF) measurement over a 2.5 m path. The start / stop signals are derived from microchannel plates that detect accelerated secondary electrons emitted from thin carbon films. This system provides a resolution of 400 ps (FWHM) for the 50 MeV iodine ions and the TOF peaks are closely Gaussian. Resolution is limited by the uniformity of the start-detector foil. Masses 128 and 130 are separeted from 129 I by about 2 ns. To improve transmission and to reduce energy straggling, the start detector has been constructed so that the beam is unobstructed by grids. In addition to TOF, the total energy ( E ) of the ions is measured with a passivated implanted planar silicon (PIPS) detector. Although 129 I standards and NaI blanks require only TOF for unambiguous identification of 129 I, many environmental samples are characterized by a plethora of peaks where both TOF and E are essential for unambiguous identification of 129 I.