1. Field of the Invention
The present invention relates to a voltage division circuit for supplying high voltages to a photomultiplier tube.
2. Description of the Related Art
A photomultiplier tube is a photodetector with a large electron multiplication factor, wherein photoelectrons generated when light is incident on a photocathode are multiplied by multistage dynodes and the resultant electrons are extracted from an anode as a photo detection signal. Due to the large electron multiplication factor, the photomultiplier tubes have been extensively used in various fields such as a light measurement field to measure minute light.
In the light measurement,
(i) a single photomultiplier tube is used to detect whether or not light is received or to detect an amount of light received, and
(ii) a number of photomultipliers are used in conjunction with position CTs (Computer Tomography), gamma cameras, TOFs (Time Of Flight) and the like.
A voltage is applied between the photocathode and the first-stage dynode and also between the succeeding dynodes to activate the photomultiplier tubes. To this effect, a voltage division circuit is employed in which a high voltage applied thereto is subjected to a voltage division to generate voltages to be applied to the photocathode and the respective dynodes.
FIG. 4 is a circuit diagram showing a conventional, very basic voltage division circuit. As shown therein, the voltage division circuit includes a serially-connected resistor array 910 in which a plurality of resistor elements 910.sub.1 through 910.sub.N+2 are connected in series. A high voltage applied between both terminals of the resistor array 910 is divided by the respective resistor values. Divided voltages are applied to the photocathode 991, focusing electrode 992, and dynodes 993.sub.1 through 993.sub.N of the photomultiplier tube 990.
The electron multiplication factor in the photomultiplier tube changes depending on the voltages applied thereto. Further, the electron multiplication factors change for different photomultiplier tubes even if the same voltages were applied between the photocathode and the first-stage dynode and between the succeeding dynodes.
When a single photomultiplier tube is used as described in (i) mentioned above, it is required that the electron multiplication factor be controlled depending on a predicted amount of incident light from the subject to be measured so that the output from the photomultiplier tube may not be saturated. When a number of photomultiplier tubes are used at a time as described in (ii) mentioned above, it is desirable that the electron multiplication factors of all the photomultiplier tubes be approximately equal to one another. Equal electron multiplication factors facilitate configuration of amplifiers to be provided following the photomultiplier tubes in terms of processing photo detection signals output from the photomultiplier tubes.
It has conventionally been proposed to change the level of the high voltage applied to the voltage division circuit or to change the voltage division ratio in the voltage division circuit to change the electron multiplication factor in an individual photomultiplier tube. When changing the electron multiplication factor by such a method, attention should be drawn to the fact that a dynode's collection efficiency, which is a probability in which the photoelectrons emitted from the photocathode enter the first stage dynode, relies upon the voltage applied between the photocathode and the first dynode. FIG. 5 is a graphical representation showing that the dynode's collection efficiency relies upon the voltage applied between the photocathode and the first-stage dynode. As is apparent from FIG. 5, it is required that the voltage applied between the photocathode and the first-stage dynode be maintained at a level higher than a predetermined value in order to maintain the dynode's collection efficiency at a high level.
In view of the foregoing, there have been proposed the following voltage division circuits. One type of the voltage division circuit maintains the voltage applied between the photocathode and the first-stage dynode at a constant level and changes the high voltage applied to the voltage division circuit. Another type of the voltage division circuit changes the voltage division ratio to thus change the electron multiplication factor of the individual photomultiplier tubes.
FIG. 6 shows a conventional voltage division circuit disclosed in Japanese Laid-Open Patent Publication (Kokai) No. HEI-7-142024. The circuit shown therein includes Zener diodes which are used to maintain the voltage applied between the photocathode and the first-stage dynode at constant. In this circuit, a variable resistor 921, Zener diodes 922, 923 and resistor elements 924.sub.1 through 924.sub.N are connected in series. The Zener diodes 922 and 923 maintain the voltages between the photocathode 991 and a focusing electrode 992 and between the focusing electrode 992 and the first-stage dynode 993.sub.1, respectively. As such, the voltage between the photocathode 991 and the first-stage dynode 993.sub.1 is maintained at constant by virtue of the Zener diodes 922 and 923, thereby remaining the dynode's collection efficiency unchanged. To change the electron multiplication factor of the photomultiplier tube, the resistance value of the variable resistor 921 is changed to change the voltage applied between the adjacent two electrodes of the photocathode 991 and the dynodes 993.sub.1 through 993.sub.N.
FIG. 7 shows another conventional voltage division circuit disclosed in Japanese Laid-Open Patent Publication (Kokai) No. HEI-7-142024. This circuit uses a constant voltage source to maintain the voltage between the photocathode and the first-stage dynode at constant. This circuit includes a first serially-connected circuit consisting of a first constant voltage source 931, a second constant voltage source 932, and resistor elements 993.sub.1 through 993.sub.N, and a second serially-connected circuit consisting of resistor elements 934.sub.1 through 934.sub.N, wherein the first and second serially-connected circuits are connected in parallel to each other. The resistor elements 993.sub.1 through 993.sub.N are provided in one-to-one correspondence to the resistor elements 934.sub.1 through 934.sub.N so that the resistor element 993.sub.1 correspond to the resistor element 994.sub.1. Switches 935.sub.1 through 935.sub.N+1 are connected between terminals of the corresponding resistors. A high voltage is applied between the two terminals of the resistor array 993.sub.1 through 993.sub.N.
In the voltage division circuit shown in FIG. 7, the constant voltage sources 931 and 932 maintain the voltages between the photocathode 991 and the focusing electrode 992 and between the focusing electrode 992 and the first-stage dynode 993.sub.1, respectively. That is, the voltage between the photocathode 991 and the first-stage dynode 993.sub.1 is maintained at constant to thereby maintain the dynode's collection efficiency. By individually performing on-off actions of the switches 935.sub.1 through 935.sub.N+1, the electron multiplication factor of the photomultiplier tube 990 is changed.
The conventional voltage division circuits as described above contain the following disadvantages.
In the voltage division circuit disclosed in Japanese Laid-Open Publication No. HEI-7-142024, the Zener diode generates non-negligible noises when the current flowing therein is less than several hundreds microamperes. In order to operate the circuit without introducing unwanted Zener noises, it is required that a current more than several hundreds microamperes be flowed in the Zener diode at all times. This results in a large consumption power in the photomultiplier tube.
In the prior art described above, the voltage between the photocathode and the N-th dynode, that is, the voltage between the photocathode and the anode is changed to change the electron multiplication factor. However, a range of linearity of the anode current relative to the amount of incident light becomes narrow when the voltage between the photocathode and the anode is lowered. Therefore, the output linearity is degraded when the electron multiplication factor is lowered.
Japanese Laid-Open Patent Publication No. HEI-7-142024 is not involved with the above-described problems. Instead, a complicated operation is required to change the electron multiplication factor due to the provision of a large number of switches.