The invention relates to an ionization cell for a mass spectrometer (2), comprising: an ionization housing (10) comprising a first and a second electron input groove (11, 26) and one side (16) of which has an output groove (15) for passing ionized particles (14a, 14b, 14c) therethrough, a first working filament (13) placed opposite said first electron input groove (11) and intended to be supplied to produce an electron beam (12), and a second backup filament (22) placed opposite said second electron input groove (26) and intended to be supplied in the even the first working filament (13) fails so as to produce the electron beam, said input groove (26) being placed outside a front region (F) located opposite said first input groove (11). The invention also relates to a leak detector with a mass spectrometer, comprising such an above-described ionization cell.