The Chalk River AMS time-of-flight detection system

Abstract

At Chalk River, the final step in the identification of 129I 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 129I 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 129I standards and NaI blanks require only TOF for unambiguous identification of 129I, many environmental samples are characterized by a plethora of peaks where both TOF and E are essential for unambiguous identification of 129I.