Simulation investigation on the pulse/analog dual-mode electron multiplier with discrete arc-shaped dynodes

To satisfy the demand of mass spectrometers for high sensitivity and high resolution ion detection, a type of pulse/analog dual-mode, arc-shaped, discrete-dynode electron multiplier (DM-ADD-EM) with 20-stage dynode structure was proposed, and its gain and time characteristics were investigated by three-dimensional numerical simulation. Each of the 2nd-20th dynodes has an arc-shaped substrate consisting of a long arc segment and a short arc segment, attached with a pair of side baffles. The simulation results indicate that the two side baffles play a role in focusing the electron beam to the central regions between them, reducing the number of secondary electrons escaping from the dynode array and, therefore, raising the electron collection efficiency of dynodes. As the radius (R) of arc-shaped substrates increases, the device gain rises. In the case of the 3.6-mm R, there is an optimum long-arc-segment center angle (α = 79°) at which the DM-ADD-EM reaches relatively high analog gain and pulse gain together with preferable time response, and its dynodes in the pulse section can be better protected from electron impact in analog output mode. In addition, the long-arc-segment center angle of the 12th-17th dynodes was further optimized to 84° for suppressing ion feedback. A dynode-configuration-optimized DM-ADD-EM with SiO₂-doped MgO-Au secondary electron emission film achieves a pulse gain of 7.2 × 10⁸, an analog gain of 1.3 × 10⁴, a pulse rise time of 3.8 ns, and a pulse width of 9.2 ns under the analog-section/pulse-section voltages of −1800 V/1000 V, exhibiting significantly improved pulse gain and better time response. These results provide a basis for the design and fabrication of high-performance EMs.