741,471. Counting particles by radiation beams. MULLARD RADIO VALVE CO., Ltd. June 13, 1952 [June 27, 1951], No. 15311/51. Class 40 (3). Particles in a specimen are counted by scanning the specimen or a photographic reproduction of it by means such as a cathode-ray tube which produces in a pick-up device such as a photo-electric cell electrical signals which are counted, the arrangement being such that multiple counting of a large particle which extends across more than one scanning line is avoided. Scanning arrangements.- In all the arrangement described a particle which is so wide as to extend across more than one scanning line is detected by a double beam arrangement. This double beam may be produced by a double beam oscilloscope or by a single beam which simulates the effect of a double beam by "wobbling" between the two scanning lines at highfrequency. The two lines are known as the scanning beam and the guard beam. Alternatively the double beam may be produced by resolving a single beam on the end of a cathoderay tube into two separate beams by means of a lens split along a diameter. The Specification also described a system (Fig. 11, not shown) in which two beams of different colours are produced by the use of colour filters or polarizing elements, these beams then scanning the specimen by means of a known mechanical system a separate photo-electric cell with an associated filter or polarizer being provided to recognize each beam. Finally the sample may be scanned directly by the beam of an electron microscope, the effect of the double beam being obtained by the use of "spot-wobble." Circuits.-In the arrangement of Fig. 3, the signal derived from one beam on encountering a particle is used to cancel the signal produced by the other when both beams encounter a particle substantially simultaneously. As shown, the output of the photo-cell is applied simultaneously to the control grids of pentodes 10, 11. The spot wobbling potential which is of square wave-form is applied to the suppressor grid of valve 10 and is inverted and applied to the suppressor grid of valve 11. The effect of this wobble potential is to open valve 10 when the spot is in the scanning beam and valve 11 when the spot is in the guard beam. When the first beam encounters particle 1 (Fig. 1) valve 10 will pass a series of pulses at the wobble frequency to an integrator where they are combined to a single pulse and passed to an amplifier and counter (not shown), but valve 11 will give no output because the particle does not extend across the second beam or guard beam. As both beams approach large particle 2, however, the guard beam, which is arranged to slightly precede the scanning beam, encounters it first so that an output is produced by valve 11 which quenches the amplifier so that the pulse produced at the anode of valve 10 when the scanning beam reaches the pulse is not registered. Thus this particle is not registered until the guard beam scans line d, indicating that the particle has been completely registered. The Specification mentions a modification in which the guaid pulse triggers a flip-flop circuit to quench the amplifier the flip-flop returning to its original state after an interval during which the scanning pulse should occur. In the arrangement of Fig. 4 (not shown), the scanning signal is fed to one switch which feeds it to a counter where it is added at the same time a second switch is set so that when the guard signal arrives it is routed to the counter in such a way as to be subtracted. Thus only the first and last traverses are counted and the total particle count must be divided by two. In the arrangement of Fig. 6 use is made of the fact that when a small particle 1 (Fig. 1) is scanned and the output of the pick-up differentiated a single positive spike followed by a single negative spike is produced whereas wide particle 2 produces two positive spikes followed by two negative ones during each sweep until the last when one positive and one negative pulse are produced. As shown, the pick-up output is differentiated and applied simultaneously to the grids of pentodes V1, V2. Both valves are cut off on their control grids, V1 being also cut off in its suppressor grid. The anode of V1 is connected to the grid of a pentode V4 and a delay network R1, C1, and the anode of V2 is connected to the grid of a triode V3 and a delay network R2, C2. Pentode V4 and triode V3 together form a bi-stable multivibrator and their anodes are connected respectively to the suppressor grids of valves V2, VI. Initially V3 conducts and V4 is cut off. When a small particle is scanned the initial positive pulse causes V2 to conduct the negative pulse produced at its anode blocking V3 which in turn causes V4 to conduct. The valve V1 is now opened on its suppressor grid while valve V2 is cut off at its suppressor. The subsequent negative pulse produces no effect on valves V2, V1 but is passed through a diode 20 to the suppressor grid of V4 which is then cut off. The screen grid current of this valve then rises and this rise is used to register the particle. Multivibrator V3, V4 returns to its original state and valves V1, V2 are reset. When a long particle is encountered the second pulse is a positive one and V which has been opened at its suppressor by the first pulse conducts. The anode potential drop of valve V 1 resets valve V4 so that the multivibrator is restored and V2 is opened. Both of the negative pulses pass to the grid of valve V4 but the valve is cut off at the control grid, hence there is no screen potential drop and the particle is not registered. Still another arrangement is described with reference to Fig. 9, in which pulses of one polarity only are applied to a coincidence switch which tests as to whether a pulse is accompanied by a guard pulse indicating that the particle is a large one so that no registration takes place until the last scan. As shown, the positive pulses produced by the scanning beam are applied through a diode V6 and a pulse inverter V7 to the grid of a triode V8 which forms with a pentode V9 a bi-stable multi-vibrator. The differentiated scanning signal is also applied to the suppressor grid of valve V9. The control grid of valve V9 is connected to the negative H.T. line through a resistor shunted by a triode V10 to the grid of which the guard beam signal is applied. When detecting a small particle the first picture pulse produced by the scanning beam is inverted and causes the multivibrator to flip. The negative pulse produced by the trailing edge of the particle is applied to the suppressor of V9 causing the multivibrator to flop. The drop in screen grid potential of V9 is used to operate a counter holding the grid of valve V9 negative. When a large particle is encountered the guard beam triggers triode V10 before the positive pulse of the scanning 'beam is applied to V6. Thus the grid of V9 is held negative and the multivibrator cannot flip. Thus the subsequent negative pulse produced by the scanning beam does not operate the counter. Specification 727,134 is referred to.