US2594703A - Photomultiplier tube circuit - Google Patents

Description

April 5 L. F. WOUTERS 2,594,703

PHOTOMULTIPLIER TUBE V CIRCU-IT Filed March 17, 1951 SWE-EP GENERATOR POWER SUPPLY SUPPLY SQUARE WAVE VOLTAGE GENERATOR INVENTOR. LOUIS E WOUTERS ATTORNEY.

Patented Apr. 29, 1952 UNITED STATES PATENT- OFFICE;

PHOTOMULTIPLIER TUBE omcurr Louis F. Wouters, Oakland, Calif., assignor to the United States of America as represented'by the a United States Atomic Energy Commission Application April 17, 1951, Serial so; 221.396

' 4 Claims. I V 1 This invention relates to a photomultiplier tube circuit and more particularly to a method and apparatus for increasing the output of a photomultiplier tube.

In many circuits utilizing photomultiplier tubes it is necessary to amplify the output of such tubes so that they may be applied to auxiliary circuits. Thus, if the source of light is faint. the electron multiplication of the photomultiplier tube may be insufiicient to raise the output to a readily useful level requiring expensive, as well as, extensive amplifier circuits. In some instances the desire to maintain the pulse shape and resolution time of the output requires additionally complex pulse forming and shaping amplifiers.

The present invention provides a photomultiplier tube circuit which overcomes the aboveoutlined difiiculties by operating the photomultiplier tube at voltages many times the normal operating voltages for such tube. The foregoing is made practical by pulsed operation which prevents deleterious overloading of the photomulti- 'plier tube.

It is therefore an object of the present invention to provide a new and improved photomultiplier tube circuit.

Another object of the invention is to provide a photomultiplier tube circuit which may directly drive an auxiliary circuit.

Still another object of the invention is to provide a photomultiplier tube circuit having a pulsed power supply therefor to provide greater amplification through such tube.

A further object of the invention is to'provide 'a'photomultiplier tube circuit which is simple in construction and which eliminates the necessity oi'am'plifiers in the output-circuit. l

Still further objects and advantages of the invention will be apparent from the following description and claims considered together with the accompanying drawing which is a schematic wiring diagram.

Referring to the drawing in detail there is provided a source of light, such as a scintillation crystal ll, placed in the path of a beam of charged particles I 2. Disposed adjacent the crystal I l is a photomultiplier tube I 3 in such position that light from the crystal falls upon the light-sensitive cathode of the tube; The photomultiplier tube I3, as illustrated in the drawing, has nine dynodes disposed between the cathode and anode and it will be readily apparent that other types of photomultiplier tubes maybe substituted. A'voltag'e divider l6,'com- 2 prising a plurality of similar resistors I1 and similar capacitors I8, is connected between the cathode and penultimate dynode with intermediate connections thereof connected to the first seven dynodes, so that a resistor and a capacitor are parallel-connected between each of such electrodes. In the present embodiment of the 'inven-' tion the penultimate dynode is connected directly to ground by'a lead 2| while the cathode'iscom nected to one side of a'storage capacitor 22 by a lead 23. The other side of the capacitor 22 is connected to the anode of a triode' type tube 26.

the cathode of which is grounded.

As a means for supplying unidirectional voltage to the photomultiplier tube l3 there is provided a conventional power supply -3l-having-a plurality of output voltages; For purposes of illustration the values of such'output voltages will hereinafter be numerically identified by the voltage existing with respect to a grounded terminal 32, but should not be understood as limiting the circuit to the exact voltages set forth. A negative terminal 33 having a value of l00 volts is connected to the control grid of thetube 26 through a biasing resistor 34. A positiveterminal 35 having a value of +2000 volts-is connected through a dropping resistor 36 tothe anode of the tube 25 to supply operating voltage thereto and also a charging voltage to the capacitor 22. A second positive terminal 31 having a value of +250 volts is connected :to the final dynode of the tube l3 to supply asuitable operating potential thereto through a dropping resistor 38. A-third positive terminal 39 having a valueof +500 volts is connected :to the anodeof -the t'ube I3 through adropping resistor 40 to supply a suitableoperating potential thereto; r.

It will be seen that with the powersup'ply' 3l suitably energized that the capacitor 22 will become charged tothe-value of the power supply terminal 35, that is +2000 volts, but that the side of the capacitor connected to the cathode of the tube I3 is at ground potential. Now, if the positively charged side of the capacitor 22 is suddenly connected to ground, the voltage of the cathode of the tube l3 will be'effectively lowered to a value of 2000 volts which will be evenly distributed across the resistors I! to ground. Thus the photomultiplier tube [3 will be operative topass electrons between the cathode and anode by way of the dynodes upon activation of the cathode by lightstriking thereon. To accomplish-such mode of operation a square wave voltage generator 46 of conventionaldesign and having an output sufiicient to overcome the negative grid bias of the tube 26 is connected between the control grid and cathode thereof. The duration of the square wave output of the generator 46 should be short as compared to the time constant of the capacitor 22 and the resistance of the resistors I? so that the high voltage across the elements of the photomultiplier tube will not be damaged by prolonged overvoltage. The repetition rate of the generator 46 is not critical, but should be sufiiciently rapid to energize the circuit for each scintillation of the crystal II in the present embodiment of the invention and still have an average value which is not in excess of the voltage rating of the tube 13.

The output of the photomultiplier tube l3 may be taken from between the final dynode and anode thereof and will be of sufficient magnitude to directly drive an auxiliary circuit. As illustrated in thedrawing a lead 5| is connected to the final dynode of the tube l3 and coupled to one end of a center-grounded resistor 52 by a capacitor 53. A lead 55 is similarly connected to the anode of the tube 13 and coupled to the other end of the resistor 52 by a capacitor 51. Thus a push-pull voltage is developed across the resistor 52 which may then be impressed between the vertical deflecting plates of an oscilloscope 59. The oscilloscope 59 is equipped with a power supply Bl to energize the electron gun and to energize the sweep generator 62 which is in turn connected to the horizontal deflecting plates.

Now consider the operation of the present invention, as described above, with the power supplies 31 and 51 and the square wave generator '46 suitably energized. Under such condition a square wave of voltage at the output of the generator 46 renders the triode tube 26 conductive to ground the positively charged side of the capacitor 22. Thus the cathode of the photomultiplier tube I3 is biased to -2000 volts. Such voltage diil'erence with respect to ground is evenly divided across the voltage divider it so that each successive electrode of the tube I3 is substantially 250 volts more positive than the preceding electrode. The particular type of photomutliplier tube [3 selected for purposes of the description has a maximum rated voltage difference between successive electrodes of approx-imately 150 volts. It is thus seen that the voltage of the electrodes of the tube 13 is in'excess of the rated value so it is necessary to maintain a repetition-rate of. the generator 46 at such a value that the average voltage of the electrodes does not exceed the maximum rated voltage.

While the voltages of the electrodes of the tube 13 render the tube operative a charged particle striking the crystal results in light reaching the cathode of the tube. Such light at the cathode frees electrons which are accelerated at greater than the normal rate to the first dynode because -01 the .highervoltage. Since secondary emission is directly proportional to the acceleration of the electron striking. it will be seen that a greater than normal num r of seconda y e ct o s i result at the first dynode and be attracted to the second dynode. Each of .the successive d n des an e an de e i a y ased so that the electron flow between the final dynode and the anode is much larger than would exist under the condition of rated voltages applied to the tube I3. By coupling the final dynode of the tube $3 to one side of the center-grounded resistor 52 by the capacitor 53 and the anode to the other side of the resistor 52 by the capacitor 51 the unidirectional voltages at such electrodes are blocked and only the varying voltage caused by the electron flow is passed. Such varying voltage establishes a push-pull voltage across the resistor 52 and thereby causes deflection of the electron beam of the oscilloscope 59. Thus a pattern of the magnitude and duration of the light scintillation at the crystal "ll formed on the screen of the oscilloscope 59.

From the foregoing it will be readily apparent that any desired light source may be utilized in the position of the crystal II. It will also be readilynnderstood that numerous types of loads may .be connected to the output of the photomultiplier tube to be driven thereby, in the place of the oscilloscope 59.

While the salient features of this invention have been described in detail with respect to one embodiment it will, of course, be understood that numerous modifications may be made within the spirit and scope of the invention and it is therefore not desired to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is:

1. In a vphotomultiplier tube circuit, the combination comprising a source of light, a photos multiplier tube having an anode, a plurality of dynodes, and a cathode with substantially low values of operating voltages, said tube being disposed adjacent said source to receive light therefrom, a voltage divider having a plurality of equally spaced taps connected between the P8111117 timate dynode and said cathode with the taps respectively connected to the dynodes, said penultimate dynode being connected to ground, said cathode being connected to a storage capacitor, power supply means having a grounded terminal, a positive terminal connected to said capacitor for charging the capacitor to a high value of voltage, and terminals connected to said anode and penultimate electrode for impressingsuitable biasing voltages, and means connected to the power supply side of said capacitor for intermittently grounding said capacitor to apply high potentials to said dynodes with respect to said substantially low values of operating voltages.

2. In a photomultiplier tube circuit, the combination comprising a source of light, a photo: multiplier tube having an anode, -a pluraiityof dynodes, and a cathode with substantially low values of operating voltages, said tube being dis posed adjacent said source to-receive-lighttherefrom, a voltage divider having a, pluralityof equally spaced 'tapsconnected between thepenultimate dynode and said cathode with the tape respectively connected to the dynodes,-said penultimate dynode being connected to ground, said cathode being connected to a storage capacitor, power supply means connected to 'saidcapacitor and to said anode and penultimate-electrode. a triode tube having an anode, control grid, and cathode with the. anode thereof connected, be tween said capaciter and power supply'and the cathode ther of c nn c e to r u d. m ans for applying a negative voltage to the control gridof said triode tube, and a square Wave generator n cted to he c n rol sr d i aid riode-t be for intermittently renderingsaid triode tube con ductive.

3. In a photomultiplier tube circuit, the com bination comprising a source-of light, a photomul ier t e ving an anode, a pl rality or dynodes, and a cathode with substantially low values of operating voltages, said tube being disposed adjacent said source to receive light therefrom, a voltage divider having a plurality of equally spaced taps connected between th penultimate dynode and said cathode with the taps respectively connected to the dynodes, said penultimate dynode being connected to ground, said cathode being connected to one side of a Storage capacitor, power supply means connected to the other side of said capacitor, to said anode, and to said penultimate electrode, and electronic switching means connected to the junction between said capacitor and said power supply for recurrently grounding said capacitor. 7

4. In a photomultiplier tube circuit, the combination comprising a source of light, a photomultiplier tube having an anode, a plurality of dynodes, and a cathode, said tube having substantially low values of operating voltages and being disposed adjacent said source to receive light therefrom, a voltage divider connected at one end to ground and to the penultimate dynode and at the other end to said cathode, said voltage divider having a plurality of taps with equal REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,430,265 Weisglass Nov. 4, 1947 2,576,661 Wouters Nov. 27, 1951

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