Time-of-Flight Detector System of the IBEX-Lo Sensor with Low Background Performance for Heliospheric ENA Detection

IBEX-Lo on the Interstellar Boundary Explorer (IBEX) will map energetic neutral H atoms (ENA) from the termination shock at energies of 10 - 2000 eV and also image the interstellar O flow in spring and fall. The sensor combines a mechanical collimator to restrict the detectable arrival direc- tions, an atom to negative ion conversion surface, an electrostatic analyzer, post-acceleration up to 20 keV, and time-of-flight (TOF) analysis, providing species separation and effective background sup- pression. Because the flux of the heliospheric ENAs is very low a triple coincidence system has been fabricated, tested, and calibrated for IBEX-Lo This system uses secondary electrons produced at two consecutive carbon foils, followed by detection of the ions in a micro-channelplate. These signals are combined into three independent TOF measurements. The combination of several TOF measurements is very effective at suppressing background to unprecedented levels. This scheme allows identifica- tion of minor species, whose fluxes are several orders of magnitude below the main species. Results from the testing of both the engineering and the flight unit will be discussed in the light of the IBEX science objectives.