Patent GB-908906-A (GB)
Title: A device for automatically controlling the exposure time of charge-carrier-beam apparatus
IPC: H01J37/22
Filed: 1960-09-05
Abstract: 908,906. Electron microscopes. ZEISSSTIFTUNG, CARL, [trading as ZEISS, C. [Firm of]]. Sept. 5, 1960 [Sept. 8, 1959], No. 30493/60. Class 39 (1). [Also in Group XX] In a device for automatically controlling the time of exposure of a film to the beam in a charged beam carrier apparatus, a collector electrode is positioned in the beam, means are provided for storing the charge received by the collector electrode and converting it into a time interval when a shutter positioned in the beam path is opened, the shutter being automatically closed again at the end of the said time interval. The arrangement is shown as applied to an electron microscope, the beam 4 of which finally impinges on a fluorescent screen 10, observable through a window 11. Secured to the screen, but insulated from it, is a collector electrode 12, also formed as a fluorescent screen. Behind the screen is a photographic plate 14, which can be utilized by swinging screen 10 out of the way (Fig. 2). Fig. 1 shows the circuit in the initial rest position. The collector electrode 12 is connected, via a D.C. voltage amplifier 15, and closed switches 18, 17, to a storage condenser 16, which thus stores a voltage proportional to the current density of the beam. A comparison condenser 19 is charged to a given initial negative voltage from a contact 24 via switch 25. A thyratron 21, in parallel with condenser 19, has a relay 26 in the anode line which actuates a mechanical shutter 27 within the electron microscope. If a photographic picture is to be taken, the screen 10 is swung away about pin 13, the commencement of which swing closes switch 28 to operate the relay and close the shutter. At the same time, switches 17, 18 open to disconnect storage condenser 16 from amplifier 15. (Switches 28, 18, 17 are ganged for simultaneous operation by cam 31, and switches 29, 23, 22, 25 similarly by cam 32, the cams both being operated as screen 10 is swung upwards.) At the completion of the swing of screen 10, switch 29 is opened, which interrupts the relay current, thus opening the shutter (i.e. beginning the photographic exposure period). At the same time, switch 25 opens to disconnect the comparison condenser 19 from the negative source 24. Simultaneously, switches 22, 23 close to connect the two condensers 16, 19 together via a variable resistor 20, through which an equalization of charge now takes place. Thus the control grid of the thyratron becomes more and more negative until the tube fires, operating relay 26 to close the shutter 27, which completes the taking of the photograph. The screen 10 can now be swung back into the position of Fig. 1. This closes switch 29 again, so that the thyratron is extinquished. Switch 25 closes again, and 22, 23 re-open, so that the two condensers are once more separated, and the negative bias from contact 24 again applied to the comparison condenser 19. At the end of the swing, switch 28 re-opens and so opens shutter 27 again, and condenser 16 is reconnected to amplifier 15, thus once more reaching the position prior to the photographtaking sequence of operations. Suitable values of the condensers, voltage 24, and resistance 20 are chosen to give an appropriate exposure time, which is inversely proportional to the current density of beam 4. The variable resistance 20 can be adjusted according to the speed of the photographic plate.