A mass spectrometer for fission fragments based on time-of-flight and energy measurements

Abstract With the aim of determining masses of fission fragments by measuring velocities and kinetic energies, specially adapted time-of-flight detectors and ionization chambers were developed. The start and stop detectors for the time-of-flight sense the electrons emitted from a thin foil upon passage of fragments. The time resolution achieved is ⋍ 100 ps . The ionization chambers have the electric field arranged parallel to the particle trajectory, and with isobutane as the counting gas, the intrinsic energy resolution for fragments with m ⋍ 100 amu is typically 400 keV. The combination of both the time-of-flight and ionization chamber, to produce an energy-time-of-flight spectrometer, was tested with fragments from the 235 U(n, f) reaction on an external thermal neutron beam at the ILL/Grenoble. In the light group of fission fragments, all masses are resolved individually with a mass resolving power m / δm = 170 for m = 95.