Sept. 6, 1955 R

Sept. 6, 1955 R. MADEY 2,717,316 PULSE LIMITER AND SHAPER Filed Deo. 24, 1952

0"`|8 O V. + l O O V.

|5oov. I6 POWER SUPPLY

INVENTOR. RICHARD MADEY BY

ATTORNEY 2,717,316 Ünited States Patent O1 1C@ Patented Sept. 6, 1955

2 pulse limiter and Shaper circuit for use with a photo multiplier tube which is simple and has a low manu 2,717,316 facturing cost as Well as an improved resolving time. Other objects and advantages of the invention will be PULSE LÍMITER AND SHAPER apparent from the following description and claims con Richard Marley, Berkeley, Calif., assignor to the United sidered together with the accompanying drawing which States of America as represented by the United States is a schematic wiring diagram of the invention. Atomic Energy Commission Referring to the drawing in detail there is provided a scintillation crystal 11 which may be disposed in the Application December 24, 1952, Serial No. 327,856 path of a beam 12 of nuclear particles. As such par 6 Claims. (Cl. Z50-207) ticles pass through the crystal 11 a certain amount of the energy of the particles is lost and appears in the form of light scintillations. To transform the light scintillations into electrical energy, a photomultiplíer The present invention relates to a pulse limiter and tube 13 is positioned with the light-sensitive cathode shaper and more particularly to a circuit for limiting thereof adjacent to the crystal 11. and shaping the output pulses of a photomultiplíer tube. A conventional power supply 14 having a 1500 volt The techniques and equipment for counting the in negative terminal 16, a grounded terminal 17, and a cidence of nuclear particles have been improved con 100 volt positive terminal 1S is utilized to supply operat siderably by the use of scintillation indicators, such as ing voltages for the circuit elements of the invention. stilbene phosphors in combination with photomultiplíer Connected between the negative terminal 16 and the tubes, to detect and translate particle energy into elec grounded terminal 17, there is a resistance voltage di trical energy and by the use of distributed amplifiers and vider 19 having a plurality of taps conventionally con coincidence circuits. ln many applications it is necessary nected to the various elements of the photomultiplíer to limit the amplitude of electric pulses developed at ' tube 13 to render the tube operative. the output of photomultiplíer tubes and, in so doing, , The anode of the photomultiplíer tube 13 is connected the duration of the pulses has been increased, thereby to the grounded terminal 17 through a resistor 21 and decreasing the efliciency of the counting arrangement. directly to the control grid of a pentode type ampliíier An electronic pulse limiter and Shaper circuit to over tube 22. Also connected to the control grid of the am come the above-referenced pulse stretching has been de- ’ pliíier tube 22 is one end of the central conductor or” scribed by L. F. Wouters in U. S. Patent 2,576,661 a short section of coaxial cable 23. Terminating such granted November 27, 1951, The teaching of such patent coaxial cable 23 there is provided a resistor 24 connected involves the combination of a two stage amplifier fol between the other end of the central conductor of the lowed by a diseriminator stage with a positive feedback cable and the outer conductor thereof, which latter con loop connected from the output of the latter stage to -_ ductor is also connected to the grounded terminal 17. the input of the amplifier. The values of the two resistors 21 and 24 and the time The present invention is an improvement in known constant (and therefore length) of the coaxial cable 23 pulse limiter and shaper circuits, such as the above will be discussed in detail hereinafter, as well as a re referenced patent, in that the resolving time or “dead quired characteristic of the amplifier tube 22. time” or” the circuit has been substantially improved and Further connections of the amplifier tube 22 are made a simpler and less costly device has been provided. By by connecting the cathode to the suppressor grid and to the “dead time” is meant that time during which one pulse grounded terminal 17; by connecting the anode to the renders the circuit operative and incapable of receiving positive terminal 18 through two series-connected resistors a subsequent pulse or differentiating between pulses cor 26 and 27; and by connecting the screen grid to the junc responding to ditïerent amounts of energy given up in tion between the two resistors 26 and Z7, which junction is the scintillation indicator. Such advantages have been connected to the grounded terminal 17 through a by-pass achieved by reducing the number of tubes required to capacitor 28. A coupling capacitor 29 is connected be one and achieving the discharge of circuit distributed and tween the anode of the ampliiier tube 22 and an output interelectrode capacitances by utilizing a section of co terminal 31 so that the output voltage of the circuit may be axial cable suitably connected to the anode of the cir 50 taken from between the output terminal and the grounded cuit photomultiplier tube. Such latter connection also terminal 17. serves to render the circuit sensitive to consecutive pulses Now with the elements of the invention interconnected corresponding to light scintillations of differing magni in accordance with the foregoing description and with the tude, as Where a small scintillation occurs after a large power supply 14 suitably energized, the operation of the one but within the decay time of the latter. Ul Ul invention will be considered. The incidence of a nuclear it is therefore an object of the present invention t0 particle upon the crystal 11 results in a light scintillation provide a new and improved pulse Shaper and limiter which reaches the light-sensitive cathode of the photo circuit. multiplier tube 13 to release electrons therefrom. Such Another object of the invention is to provide a cir electrons are attracted to the ñrst dynode where secondary cuit for limiting the output pulses of a photomultiplíer electrons are formed to increase the current flow and this tube in duration and amplitude. action occurs at each successive dynode until a greatly Still another object of the invention is to provide a increased pulse occurs at the anode of the photomulti circuit wherein pulse duration is limited by a section plier tube 13. Because of the pulse at the anode of the of coaxial cable terminated in less than the character photomultiplíer tube 13 a negative pulse is developed istic impedance thereof and pulse amplitude is limited 65 across the resistor 21 which impresses a negative pulse on by a tube having a sharp cut-oit characteristic. the control grid of the amplifier tube 22. A further object of the invention is to provide a cir Because of the inherent characteristics of scintillation cuit for use with scintillation indicators which discrim indicators and interelectrode and distributed capacitances inates between a scintillation of small intensity occurring of the circuit elements, the voltage impressed at the con during the decay time of a scintillation of larger inten 70 trol grid of the amplifier tube 22 has a steeply rising wave sity. front, but a substantially long trailing end. it-will be A still further object of the invention is to provide a readily apparent that the trailing end of a pulse inter 2,717,316 3 4 feres with the operation of the circuit in that a subsequent of a pulse at the anode of the photomultiplier tube 13 pulse, corresponding to a scintillation of lesser intensity, results in a fiow of current through the resistor 21 so may be missed during the time of decay of the former that a negative bias is applied to the control grid of the pulse. To correct for such effect a short length of coaxial tube 22 and at the same time a voltage wave is trans cable 23 is connected to the control grid of the amplifier L'i mitted along the coaxial cable 23. Simultaneously with tube 22 to reflect a voltage of opposite phase to cancel the above, the interelectrode and distributed capacitances out the trailing end of the pulse by rapidly discharging of the circuit elements become charged. As the voltage the capacitances. wave of the coaxial cable 23 reaches the receiving end For proper charging of the section of coaxial cable 23 thereof, a portion of such voltage wave, as determined the sending end thereof (i. e., the end connected to the if) by the value of the resistor 24, is reflected so that such control grid of the amplifier tube 22) should have a re reflected voltage wave upon reaching the sending end of sistance substantially equal in value to the characteristic the cable discharges the interelectrode and distributed impedance of the cable connected thereacross. Thus the capacitanccs. Thus the voltage pulse at the control grid value of the resistor 21 is chosen to be substantially that of the tube 22 has a duration equal to the time for the of the characteristic impedance of the coaxial cable 23. pulse to travel the length of the coaxial cable and be re Another consideration is that of the most effective load fiected back. Because of the sharp cut-off characteristic impedance for the photomultiplier tube i3 and it has of the tube 22 the normal current fiow is sharply cut-off been found that for a fixed operating voltage on the tube by the negative grid bias so that a positive square wave is and for a given energy loss in the crystal ii, the output formed at the anode and coupled to the output terminal voltage from the tube usually can be increased by in 31. creasing the load impedance. Thus the coaxial cable 23 It has been found that by using a stilbene phosphor is chosen to have a high value of characteristic impedance, crystal i1, a type 1F21 photomultiplier tube 13, a load for example, about 100() ohms. resistor 21 having a value of 1G00 ohms, 11/2 inch length The next consideration is that of selecting the proper of type RG65/U coaxial cable 23, a terminating resistor length of coaxial cable which will reflect a wave in sufñ 24 having a value of 330 ohms, and a type 6AH6 vacuum cient time to accomplish the desired effect of reducing the tube 22, the resolving time of the circuit is about 10“8 duration of the input pulse to maintain a maximum de second. Such short resolving time is better than that tection rate. The time limit selected for the present em attainable with circuits known prior to the present inven bodiment of the invention is about iO-8 second and to ac tion and means that the circuit will accept pulses l0*8 complish such limit 11/2 inches of type RG65/U delay second after the occurrence of a preceding pulse. cable is utilized. It will be apparent that other time limits The present invention is useful in counting circuits as may be selected and the length of the type of cable estab a detector and limiter to be used ahead of conventional lished accordingly. amplifier circuits, such as distributed amplifiers and co It has been found that the decay time of the pulses ap incidence circuits, which have suitable resolving times. pearing at the output of the photomultiplier tube 13 is While the salient features of the present invention have shorter than the reflection time of the usually selected been described in detail with respect to one embodiment length of coaxial cable 23. Such finding means that, if it will, of course, be apparent that numerous modifica the receiving end of the cable 23 were short-circuited, the tions may be made within the spirit and scope of the reñected pulse would be 180 degrees out of phase and invention and it is therefore not desired to limit the in have the full amplitude of the original pulse. The result 40 vention to the exact details shown except insofar as they would then be an “overshoot” so that the line would be may be defined in the following claims. charged in the opposite sense from the original pulse. To What is claimed is: correct the foregoing “overshoot” the coaxial cable 23 l. In a pulse limiter and shaper circuit for a photo may be terminated at the receiving end in an impedance multiplier tube circuit, the combination comprising a having a value substantially less than the characteristic r photomultipiier tube, means connected to the elements of impedance of the cable so that the amplitude of the re said tube for impressing operating voltages, a length of flected wave has a value such as to cancel the voltage at coaxial cable with one end of the central conductor con the sending end (in the present embodiment the ter nected to the anode of said tube, the other end of the minating impedance is selected as a resistor 24). From central conductor of said cable connected to a resistive the foregoing considerations, it is seen that the duration impedance having a value such that voltage reliection 0f the input pulses to the amplifier tube 22 is limited to occurs therefrom, and limiting means connected between the selected time for the pulse to charge the coaxial cable the anode of said tube and an output terminal for limiting 23 and be refiected back to the sending end thereof. the amplitude of output pulses. In addition to limiting the duration of the pulses de 2. ln a pulse limiter and shaper circuit for a photo veloped in response to the incidence of nuclear particles multiplier tube circuit, the combination comprising a upon the crystal 11, it is also desirable to limit the pulses photomultiplier tube, means connected to the elements of to a uniform amplitude. The importance of the latter said photomultiplier tube for impressing operating volt feature becomes apparent when it is considered that fur ages, a load resistor connected to the anode of said photo ther amplification of the pulses is normally necessary and, multiplier tube, a length of coaxial cable with the central unless tube saturation is prevented in the following ' conductor connected at one end to said load resistor, a amplifier circuit, the fast resolving time of the circuit will terminating resistor connected to the other end of the be lost. Amplitude limiting of the pulses is accomplished central conductor of said cable to cause voltage reflection, by selecting a tube having a sharp cut-off characteristic and an amplitude limiting circuit connected to said load as the amplifier tube 22. To best utilize the sharp cut-off resistor and to an output terminal whereby pulses devel characteristic of the tube 22 it Will be seen that the volt ii oped by said photomultiplier tube are limited in duration ages impressed on the control grid should be as large as and amplitude at the output of said amplitude limiting possible. In such respect, in the present circuit, the value circuit. of the grid resistor 21 is limited by the value of the char 3. In a pulse limiter and Shaper circuit for a photo acteristic impedance of the coaxial cable 23. rÍhus the multiplier tube circuit, the combination comprising a choice of a coaxial cable 23 having a high value of char 74 photomultiplier tube, means connected to the elements acteristic imperance is advantageous for the limiting of of said photomultiplier tube for impressing operating the amplitude of the input pulses by the amplifier tube 22 voltages, a load resistor connected to the anode of said to provide output pulses at the terminal 31 having a uni photomultiplier tube, a length of coaxial cable with the form amplitude. central conductor connected at one end to said load From the foregoing it will be seen that the occurren@ ’ resistor, said load resistor having a value substantially 2,717,316 5 6 equal to the characteristic impedance of said cable, a resistor, the value of said load resistor being substantially terminating resistor connected to the other end of the equal to the characteristic impedance of said cable, a central conductor of said cable and having a value less terminating resistor connected to the other end of the than the characteristic impedance thereof, and an ampli central conductor of said cable and having a value less tude limiting circuit connected to said load resistor. than the characteristic impedance thereof and to reiiect 4. In a pulse limiter and shaper circuit for a photo a portion of the voltage so as to cancel the voltage multiplier tube circuit, the combination comprising a across said load resistor, an ampliiier tube having the photomultiplier tube, means connected to the elements control grid and cathode respectively connected to either of said photomultiplier tube for impressing operating side of said load resistor, said ampliñer tube having a voltages, a load resistor connected to the anode of said sharp cut-off characteristic, and means connected to said photomultiplier tube, a length of coaxial cable with the amplifier tube for impressing operating voltages to render central conductor connected at one end to said load the tube normally conductive, whereby pulses of limited resistor, a terminating resistor connected to the other duration and amplitude are uniformly developed at the end of the central conductor of said cable to cause voltage anode of said ampliñer tube in response to pulses de reflection, an amplifier tube having the control grid and 15 veloped across said load resistor. cathode respectively connected to the sides of said load 6. The combination of claim 4 wherein the length of resistor, means connected to said ampliiier tube impress said coaxial cable is selected so that twice the time con ing operating voltages to render such tube normally con stant thereof determines the duration of the output pulses. ductive, and means for coupling the anode to cathode voltage of said amplifier tube to an output, 20 References Cited in the ñle of this patent 5. In a pulse limiter and Shaper circuit for a photo UNITED STATES PATENTS multiplier tube circuit, the combination comprising a photomultiplier tube, means connected to the elements 1,939,435 Cockrell ______Dec. 12, 1933 of said photornultiplier tube for impressing operating 2,576,661 Wouters ______Nov. 27, 1951 voltages, a load resistor connected to the anode of said 25 2,610,303 Bell ______Sept. 9, 1952 photomultiplier tube, a length of coaxial cable with the 2,647,436 Shapiro ______Aug. 4, 1953 central conductor connected at one end to said load