Patent GB-1288493-A (GB)
IPC: H01J49/40
Filed: 1970-03-05
Abstract: 1288493 Particle spectrometers NATIONAL RESEARCH DEVELOPMENT CORP 19 April 1971 [5 March 1970] 10722/70 Heading HID In a particle detection system of the kind in which particles of a given type are arranged to control of a pulse signal and are detected by a detector 3 after having traversed the flight path, the detector 3 is gated by a pseudo-random binary signal which is similar to the pulse signal but is delayed by a variable amount with respect thereto, and the response of the detector is intergrated over an integral number of cycles of the signal for any given amount of the delay. The integral number of cycles may also be variable. Temporal variations in the response of the detector provide information concerning the relative abundances of particles having different values of a given parameter, e.g. the mass/charge ratio of ions in the case of a mass spectrometer or the enegy of the particles in the case of an electron spectrometer or molecular beam spectrometer. In the mass spectrometer shown, gas molecules entering an open-sided metal box 5 through a grid 8 are ionized by an electron beam from a filament 6 entering the base through a grid 7. The box 5 and filament 6 are respectively biased 200v. and 100v. positive with respect to a drift tube 4 provided at each end with a grid 10, 11. A grid 9 adjacent the open side of the box is biased 10v. positive thereto. Grid electrodes 12 and 13 are respectively biased 200v. negative with respect to the base 5 and 210v. positive with respect to the drift tube 4. A grid electrode 15 maintained at the potential of the drift tube 4 is provided adjacent the modulation grid 14 so that the modulation characteristic of the grid 14 is similar to that of the grid electrode 13. Ions are passed or blanked by the grid 13 by applying a pulse signal from a generator 18 controlled by a clock pulse generator 19 to the box 5 and electrodes 9 and 12 so as to switch them between potentials 5v. positive and 5v. negative with respect to their mean potentials. Similarly, ions are passed or blocked by the grid 14 by applying a delayed pulse signal from generator 21 controlled by a scanning shift resistor 22 to switch the grid 14 between potentials of 180 and 235v. positive with respect to the drift tube 4. The pulse signals are applied via a pair of gold-plated strip transmission lines 17 having a common conductor connected to the drift tube 4. The output of the detector 3 is fed via an integrating circuit 27 having a variable time constant to the vertical deflection system of an oscilloscope 26. The shift register 22 is operated by clock pulses from variable frequency generator 24 so that the delay is repeatedly scanned through a desired range, the delay being stepped during the scan by one digit interval for each clock pulse applied to the shift register 22. The output of the shift register 22 is also used to control a time base generator 25 supplying the horizontal deflection system of the oscilloscope 26. In the case of a molecular beam spectrometer in which the particles detected are molecules in an excited state induced by bombardment with the electron beam, control is effected by using the input pulse signal to switch the beam on and off, and the detector is gated by applying the delayed signal to one of its electrodes or to a gating circuit connected to its output.