# Patent

## Title
Photodetector using photomultiplier and gain control method

## Abstract

The present invention relates to a photodetector that has a structure capable of realizing a wide range gain adjustment for each of electron multiplier channels respectively assigned to a plurality of light incidence regions of a multi-anode multiplier. The photodetector comprises a multi-anode photomultiplier, and a bleeder circuit unit. The multi-anode multiplier has a dynode unit constituted by N (an integer or no less than 3) dynode plates, and n-th (an integer of no less then 2) dynode plate is constituted by a plurality of control plates respectively corresponding to the multiplier channels. The bleeder circuit unit has a primary section setting each potential of a first to (n−1)-th and (n+1)-th to N-th dynode plates, and a secondary section for individually setting a potential of each control plate at any potential within the range wider than a potential difference between the (n−1)-th and (n+1) dynode plates. By expanding the potential setting range for the control plates rather than the potential difference between the dynode plates adjacent to the n-th dynode plate, the gain of each electron multiplier channel can be controlled by two digits or more.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This is a Continuation-In-Part application of the patent application Ser. No. 11/188,215 filed on Jul. 25, 2005 by the same Applicant, now U.S. Pat. No. 7,115,854.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photodetector using a photomultiplier which enables electron multiplications in electron multiplier channels respectively assigned to a plurality of light incidence regions partitioned on an entrance face plate, and a gain control method for the electron multiplier channels in the photomultiplier applied to the photodetector.
2. Related Background of the Invention
U.S. Pat. No. 5,077,504 discloses a photomultiplier having a single entrance face plate partitioned into a plurality of light incidence regions, and having a structure in which a plurality of electron multiplier sections (each constituted by an anode and a dynode unit comprising a plurality of dynode stages) prepared as processing channels (electron multiplier channels) assigned to the plurality of light incidence regions (having a photocathode formed on the inner surface), are sealed inside a single glass tube. A photomultiplier, having such a structure that a plurality of processing channels are contained within a single glass tube, is generally referred to as a “multi-anode photomultiplier,” and as outputs of the respective channels, electrical signals are taken out from the anodes corresponding to the respective channels.
SUMMARY OF THE INVENTION
The inventors have studied a conventional multi-anode photomultiplier as a photo-sensing device in detail, and as a result, have found problems as follows.
Namely, the multi-anode photomultiplier disclosed in U.S. Pat. No. 5,077,504 has a structure such that a plurality of electron multiplier sections, physically and electrically separated from each other, are housed in a single glass tube. That is, a limit to making the photomultiplier itself compact and the resolution of a photodetector itself including the same could not be improved.
On the other hand, the making of the photomultiplier itself compact by arranging the dynodes of each stage of the plurality of electron multiplier sections as a common dynode (on which electron multiplier holes are respectively provide for electron multiplier channels) may be considered. However, since the electron multiplier holes for channels in each dynode are provided with a common potential, the gain cannot be adjusted in each channel. Applications are thereby restricted significantly.
Additionally, in consideration of applications of the multi-anode photomultiplier to high energy physics and other scientific technological fields (digital signal processing applications) as well as fluorescence analysis, blood analysis, drug development, and other analysis technologies in the field of biotechnology (analog signal processing applications), it is inadequate to simply make the gain uniformity substantially uniform among the respective channels to adjust the detection efficiency of the respective channels. That is, in order to realize a photodetector capable of individually adjusting channel gains over a wider range, a multi-anode photomultiplier, in which the gain of each channel can be controlled by two digits or more while each channel is set at an arbitrary gain, is necessary.
The present invention has been made to resolve the above problems and an object thereof is to provide a photodetector using a photomultiplier that can realize a gain adjustment for each of electron multiplier channels respectively assigned to a plurality of light incidence regions partitioned on an entrance face plate in a more compact structure, and a gain control method for the electron multiplier channels in the photomultiplier applied to the photodetector.
A photodetector according to the present invention comprises a photomultiplier, and a bleeder circuit unit. The photomultiplier, which is applicable to the photodetector, relates to a multi-anode photomultiplier having a plurality of electron multiplier sections respectively prepared for electron multiplier channels assigned to a plurality of light incidence regions partitioned on an entrance face plate. The multi-anode photomultiplier has a sealed container having the entrance face plate partitioned into the plurality of light incidence regions, and a stem opposing the entrance face plate. On the other hand, a gain control method according to the present invention controls the gains for electron multiplier channels in the multi-anode photomultiplier applied to the photodetector.
In the photodetector according to the present invention, the photomultiplier further comprises a photocathode, a plurality of anodes, and a dynode unit together with the above-mentioned sealed container. The photocathode, the plurality of anodes and the dynode unit are disposed inside the sealed container of the photomultiplier. The photocathode emits photoelectrons into the sealed container in response to light having passed through the entrance face plate. The plurality of anodes respectively prepared for the electron multiplier channels assigned to the plurality of light incidence regions partitioned on the entrance face plate and arranged at positions corresponding to the plurality of light incidence regions. The dynode unit is provided between the plurality of anodes and the photocathode are housed.
Here, the dynode unit is constituted by N (an integer of no less than 3) dynode plates laminated from the anodes to the photocathode, and in each dynode plate, one or more electron multiplier holes for the associated channel are positioned, concerning all channels, at positions corresponding to the associated one of the light incidence regions. One electron multiplier section that makes up a single electron multiplier channel is constituted by the electron multiplier holes for the associated channel that are provided in each dynode plate and the anode for the associated channel. In addition, the n-th (an integer of no less than 2) dynode plate in the dynode unit is constituted by a plurality of control plates each having one or more electron multiplier holes for the associated channel and being electrically and physically separated from the others.
In particular, the photodetector according to the present invention is characterized by the bleeder circuit unit for setting at least each of said dynode plates constituting the dynode unit in the photomultiplier to predetermined potentials, and the bleeder circuit unit comprises a primary section prepared for first to (n−1)-th and (n+1)-th dynode plates, and a secondary section prepared for the plurality of control plates constituting the n-th dynode plate. The primary section sets potentials of the first to (n−1)-th and (n+1)-th to N-th dynode plates respectively. The secondary section set potentials of the control plates of the n-th dynode plate respectively, and has a structure, for adjusting gains of the electron multiplier channels respectively, capable of individually setting a potential of the selected one of the control plates at any potential within an operable range, the range being wider than a potential difference between the (n−1)-th dynode plate and the (n+1)-th dynode plate. As an example for individually changing potentials of the control plates, the secondary section may includes one or more external power supplies each capable of controlling a voltage to be applied to the associated one of the control plates.
On the other hand, the gain control method according to the present invention uses the photomultiplier having the above-mentioned structure, and comprises a potential setting by using the primary section, a potential setting by using the secondary section, and a gain adjustment by using the secondary section. That is, the primary section sets potentials of the first to (n−1)-th and (n+1)-th to N-th dynode plates in phtomultiplier such that potentials to be set decrease stepwise from the first dynode plate to the N-th dynode plate. The secondary section sets potentials of the control plates constituting the n-th dynode plate within a potential range wider than a potential difference between the (n−1)-th dynode plate and the (n+1)-th dynode plate. In particular, the secondary section adjusts the gains of the electron multiplier channels respectively by individually changing a potential of the selected one of the control plates over the potential range.
In the photodetector according to the present invention, the control plates, constituting the n-th dynode plate, are supported in a state of being sandwiched, via insulators, by the (n−1)-th dynode plate and the (n+1)-th dynode plate. By this arrangement, each of the control plates making up the n-th dynode plate can be set to an arbitrary potential, thereby enabling realization of a gain adjustment for each channel in a more compact structure.
In the photodetector according to the present invention, the photomultiplier may further comprise a protection electrode provided between the stem and the dynode unit. This protection electrode supports the entirety of the dynode unit via an insulator and is provided with a plurality of through holes each housing the associated one of the anodes individually. In particular, a diameter of each dynode side opening of the protection electrode is preferably narrower than a diameter of each stem side opening of the protection electrode. In this case, since the trajectories of secondary electrons emitted from the final stage of dynode plate in the dynode unit to the anodes are respectively converged every anode, crosstalk among the anodes corresponding to the respective channels is reduced effectively.
In the photodetector according to the present invention, the photomultiplier may further comprise a focusing electrode disposed between the photocathode and the dynode unit. In this case, the focusing electrode is preferably provided with a plurality of through holes each arranged at a position corresponding to the associated one of the channels assigned to the plurality of light incidence regions partitioned on the entrance face plate. Since photoelectrons emitted from a certain region of the entrance face plate will then arrive at a high probability at an electron multiplier hole, which, among the electron multiplier holes of the first dynode plate, corresponds to the channel assigned to the region from which the photoelectrons are emitted, crosstalk among the electron multiplier channels is reduced effectively.
Accordingly, the photodetector according to the present invention, due to such an arrangement, enables each of the control plates constituting the n-th dynode to be set to an arbitrary potential and thus the realization of gain adjustment for each electron multiplier channel in a more compact structure.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an assembly process diagram of a configuration of a photodetector according to the present invention;
FIG. 2 shows a partially broken-away view of the configuration of the photodetector according to the present invention;
FIG. 3 shows an assembly process diagram of a configuration of a photomultiplier applied to the photodetector according to the present invention;
FIG. 4 shows a sectional view of the configuration of the photomultiplier taken along the line I-I in FIG. 1;
FIG. 5 shows a plan view of a rear pin arrangement of the bleeder circuit unit shown in FIG. 1;
FIG. 6 shows a diagram for explaining a configuration of the bleeder circuit unit shown in FIG. 1;
FIGS. 7A and 7B show circuit diagrams of a secondary section in the bleeder circuit unit shown in FIG. 6;
FIGS. 8A and 8B are tables for explaining a gain control method according to the present invention in the case that the secondary section shown in FIG. 7A or 7B is applied to the bleeder circuit unit shown in FIG. 6;
FIG. 9 shows a perspective view for explaining the connection structure between the photomultiplier and the bleeder circuit unit;
FIG. 10 shows a sectional view of a configuration of a protection electrode taken along the line II-II in FIG. 3;
FIGS. 11A to 11C show diagrams for explaining a structure of a dynode plate of each stage;
FIGS. 12A and 12B show diagrams for explaining a structure of each of control plates which are provided in each channel and are sandwiched by the dynode plates shown in FIG. 8;
FIGS. 13A and 13B show other configurations of the secondary section in the bleeder circuit unit shown in FIG. 6;
FIGS. 14A and 14B show views for explaining an operation of the secondary section shown in FIG. 13B;
FIGS. 15A and 15B show equivalent circuits in ON and OFF states of a transistor shown in FIG. 14A;
FIG. 16 shows a table for explaining a gain control method according to the present invention in the case that the secondary section shown in FIG. 13B is applied to the bleeder circuit unit shown in FIG. 6.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, embodiments of a photodetector and a gain control method according to the present invention will be explained in detail with reference to FIGS. 1-6, 7A-8B, 9-10, 11A-15B and 16. In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions.
FIG. 1 shows an assembly process diagram of a configuration of a photodetector according to the present invention, and FIG. 2 shows a partially broken-away view of a configuration the photodetector according to the present invention. In the following embodiment, as a photomultiplier applied to a photodetector, an 8-channel multi-anode type of photomultiplier, in which an entrance face plate is partitioned into eight effective regions, will be explained.
As shown in FIG. 1, the photodetector, which is applied to the photodetector according to the present invention, comprises a case 100, a photomultiplier 200, and a bleeder circuit unit 300. The photomultiplier 200 comprises an entrance face plate 210 which is partitioned into a plurality of light incidence regions each corresponding to a signal channel, a stem 230 which holds a plurality of lead pins 240, and a metal container 220 provided with a flange 220a and housing an electron multiplier section in the interior. The bleeder circuit unit 300 has pipe-like metal sockets 320 (see FIG. 9) respectively housing lead pins 240 that extend to the exterior from the stem 230 of the photomultiplier 200, output terminals 340 for taking out electrical signals from each of the plurality of anodes housed inside metal container 220, fixing members 310a and 310b which engage with the flange 220 of the metal containers 220 in the state that the lead pins 240 of the photomultiplier 200 are housed in metal sockets 320, and adjusters 330 for adjusting variable resistors that constitute a part of the bleeder circuit. The case 100 houses both the photomultiplier 200 and the bleeder circuit unit 300, and has an upper opening 110 which exposes at least part of the entrance face plate 210 in the state of housing the photomultiplier 200, a lower opening 130 which leads output terminals 340 of the bleeder circuit unit 300 out to the exterior of the case 100, and openings 120 which enable the adjustment of each adjuster 330 from the exterior of the case 100.
The state in which the photomultiplier 200 and the bleeder circuit unit 300 are housed in the case 100 from the upper opening 110 is shown in FIG. 2.
The structure of the photomultiplier 200 (applied to the photodetector according to the present invention), shown in FIGS. 1 and 2, will now be explained by using FIGS. 3 and 4. Here, FIG. 3 shows an assembly process diagram of the arrangement of the photomultiplier 200.
As shown in FIG. 3, in the photomultiplier 200, a sealed container comprises the entrance face plate 210 which is partitioned into a plurality of light incidence regions 210a respectively assigned to signal processing channels, the metal container 220 having a flange 220a, and the stem 230 holding the plurality of lead pins 240.
Inside the sealed container, a photocathode 210b, a focusing electrode 250, a dynode unit, and anodes 270 are arranged in the order towards the stem 230 from the side of the entrance face plate 210.
The photocathode 210b is formed on a surface of the entrance face plate 210 at the side positioned inside the sealed container.
The focusing electrode 250 has openings 250a which are provided at positions corresponding to light incidence regions 210a so as to correspond to the respective channels, and spring electrodes 250b which contact the inner wall of the metal container 220 such that the potential of the focusing electrode 250 is set equal to the potential of the metal container 220. The focusing electrode 250 is fixed to the dynode unit via the ceramic spacers SP made of an insulator.
In the embodiment shown in FIG. 3, the dynode unit is constituted by twelve dynode plates DY1 to DY12. The respective dynode plates DY1 to DY12 are laminated via the ceramic spacers SP. In particular, the seventh dynode plate is constituted by control plates DY7-1CH to DY7-8CH, which are electrically and physically separated from each other. In each of the control plates DY7-1CH to DY7-8CH, electron multiplier holes for the associated channel are formed.
The anodes 270, which correspond to the respective channels, are respectively fixed to the stem 230 and each of these anodes 270 is housed in the associated one of the through holes 260a of the protection electrodes 260.
In the state that the dynode unit and the focusing electrode 250 have been installed successively on the protection electrode 260 as a base and with the ceramic spacers SP disposed in between, the photomultiplier 200, with the structure shown in FIG. 4, can be obtained by vacuuming inside of the sealed container constituted by the entrance face plate 210, the metal container 220, and the stem 230. FIG. 4 shows a sectional view of the structure of the photomultiplier 200 taken along line I-I of FIG. 1.
As shown in FIG. 4, the protection electrode 260 supports the entirety of the dynode unit via the ceramic spacers SP and has through holes 260a, which house the plurality of anodes 270 respectively and individually and reduce the electrical influences of the anodes 270 on each other. The focusing electrode 250, arranged between the photocathode 210b and the dynode unit, has the plurality of through holes 250a each corresponding to the associated one of the channels respectively assigned to the light incidence regions 210a partitioned on the entrance face plate 210. The photoelectrons, which are emitted from a region of the incidence surface place 210, by the actions of the focusing electrode 250, reach at a high probability to the electron multiplier hole, among the electron multiplier holes in the first dynode plate, which corresponds to the channel assigned to the region from which the photoelectrons are emitted. Crosstalk among the channels is thus reduced effectively.
FIG. 5 shows a plan view of the rear pin arrangement of the bleeder circuit unit 300. As shown in FIG. 5, at the rear surface of the bleeder circuit unit 300, anode output terminals 340a for respectively taking out electrical signals from the plurality of anodes 270 positioned inside the sealed container of the photomultiplier 200, a ground terminal 340b for grounding the terminals at one side of the bleeder circuit in order to set dynode plates DY1 to DY12 of the respective stages constituting the dynode unit to predetermined potentials, and a terminal 340c for applying −HV to the terminals at the other side of the bleeder circuit are arranged.
FIG. 6 shows a diagram for explaining a configuration of the bleeder circuit unit 300 shown in FIG. 1. The bleeder circuit unit 300 comprises a primary section 310, and a secondary section 320. The primary section 310 has one terminal that is set at −HV and another terminal that is grounded, and a bleeder circuit is arranged between these terminals. The bleeder circuit has terminals respectively for applying to first dynode plate DY1 to sixth dynode plate DY6 and the eighth dynode plate DY8 to twelfth dynode plate DY12 excluding the seventh dynode plate. Between the terminal for the sixth dynode plate DY6 and the terminal for the eighth dynode plate DY8, an adjustment resistance AR1 is arranged so as to maintain the potential difference between these dynode plates DY6 and DY8. In contrast, the secondary section 320 sets potentials of the control plates DY7-1CH to DY7-8CH constituting the seventh dynode plate, respectively. Namely, the secondary section 320 can individually set a potential of the selected one of the control plates DY-1CH to DY7-8CH at any potential within an operable range, the range being wider than a potential difference between the dynode plates DY6 and DY8.
FIG. 7A shows a first configuration of the secondary section 320 applicable to the bleeder circuit unit 300 shown in FIG. 6. In similar to the primary section 310, the secondary section 320 shown in FIG. 7A has one terminal that is set at −HV and the another terminal that is grounded, and adjustment resistances AR2 and AR3 are connected to these terminals in order to restricting a potential range that the secondary section 320 can adjust. Between the adjustment resistances AR2 and AR3, variable resistances VR1 to VR8 are arranged in parallel, and whereby voltages, divided by the variable resistances VR1 to VR8, are respectively applied to the control plates DY7-1CH to DY7-8CH that are electrically and physically separated from each other and constitute the seventh dynode plate. Since the values of variable resistances VR1 to VR8 can be changed by the adjusters 330, the gain adjustment for each channel is enabled by this adjusting mechanism. In this manner, by controlling the values of the adjustment resistances AR2 and AR3, the potential range, that the secondary section 320 can adjust, can be set so as to be wider than the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8.
On the other hand, FIG. 7B show a second configuration of the secondary section 320 applicable to the bleeder circuit unit 300 shown in FIG. 6. The second configuration shown in FIG. 7B differs the first configuration shown in FIG. 7A in the terminal potential. That is, the resistance R6 of the primary section 310, which is positioned between the terminal for the sixth dynode plate DY6 and the terminal for the eighth dynode plate DY8, is divided into the resistances R6a and R6b, and the resistance R9 of the primary section 310, which is positioned between the terminal for the eighth dynode plate DY8 and the terminal for the ninth dynode plate DY9, is divided into the resistances R9a and R9b. And then, one terminal of the secondary section 320 is connected to the intermediate of the resistances R6a and R6b, and another terminal is connected to the intermediate of the resistances R9a and R9b. This second configuration can also makes the adjustable potential range of the secondary section 320 expand rather than the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8. In the second configuration, since the values of variable resistances VR1 to VR8 can be changed by the adjusters 330, the gain adjustment for each channel is also enabled by this adjusting mechanism.
The operation of the gain control method according to the present invention will be explained with reference to FIGS. 8A and 8B. In the following explanation, one of the configurations shown in FIGS. 7A and 7B is applied as the secondary section 320.
First, as shown in FIG. 8A, the primary section 310 applies a voltage of −1000V to the cathode 210b and makes the anodes 270 be grounded. At this time, the primary section 310 applies desirable voltages to the dynode plates DY1-DY6 and DY8-DY12 excluding the seventh dynode plate such that potentials thereof decrease stepwise from the first dynode plate DY1 to the twelfth dynode plate DY12. On the other hand, the secondary section 320 individually applies desirable voltages to the control plates DY7-1CH to DY7-8CH. The secondary section 320 can expand the adjustable potential range for the control plates DY7-1CH to DY7-8CH, as compared with the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8, by individually adjusting the variable resistances VR1 to VR8. In the following, the voltage control of the secondary section 320 will be explained with reference to FIG. 8A.
Here, it is assumed that the maximum gain of each channels in the entire photomultiplier 200 is 1 when each of the control plates DY7-1CH to DY7-8CH is applied with −462V as a reference voltage value. In the case of adjusting the gain of the selected channel to 1/10 with respect to the normalized gain of 1, the gain adjustment can be achieved by applying a minimum voltage of −538V or a maximum voltage of −385V to the associated one (corresponding to the selected channel) of the control plates DY7-1CH to DY7-8CH. At this time, the voltage control width of the secondary section 320 is 154V (±77V with respect to the reference voltage value of −462V). Furthermore, in the case of adjusting the gain of the selected channel to 1/100 with respect to the normalized gain of 1, the gain adjustment can be achieved by applying a minimum voltage of −561V or a maximum voltage of −361V to the associated one (corresponding to the selected channel) of the control plates DY7-1CH to DY7-8CH. At this time, the voltage control width of the secondary section 320 is 200V (±100V with respect to the reference voltage value of −462V), and is wider than the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8. Of course, the secondary section 320 can change its voltage control width so as to become narrower than the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8, such as ±60V or less, ±30V or less, and the like with respect to the reference voltage value of −462V.
As described above, the gain control method according to the present invention, using the secondary section 320 having the first or second configuration, can individually control the gain of each channel by two digits or more.
Next, the fixing of the photomultiplier 200 to the bleeder circuit unit 300 is carried out as shown in FIG. 9. That is, the photomultiplier 200 is moved in the direction indicated by the arrow S1 in this figure. By lead pins 240 becoming inserted into pipe-like metal sockets 320 of the bleeder circuit unit 300, and flange 220a of the metal container 220 of the photomultiplier 200 engaging with fixing members 310a and 310b in this process, the photomultiplier 200 becomes fixed to the bleeder circuit unit 300. FIG. 9 shows a perspective view for describing the structure for connecting the photomultiplier to the bleeder circuit unit.
In this embodiment, the positional relationship between the protection electrode 260 which supports the entirety of the dynode unit, and the anodes 270 which are prepared in correspondence with the respective channels, is set, for example, as shown in FIG. 10. FIG. 10 shows a sectional view of the structure of the protection electrode taken along line II-II of FIG. 3.
As shown in FIG. 10, the anodes 270, which are prepared in correspondence with the respective channels, are respectively housed in the through holes 260a of the protection electrode 260, and in the protection electrode 260, the opening diameters D1 at the dynode unit side are made narrower than the opening diameters D2 at the stem side. Since by this arrangement, the trajectories of the secondary electrons emitted from twelfth dynode plate DY12 (the final dynode plate) of the dynode unit towards the anodes 270 are converged every associated one of the anodes 270, crosstalk among anodes 270 corresponding to the respective channels is reduced effectively.
The arrangement of the respective dynode plates constituting the dynode unit shall now be described.
First, FIG. 11A shows a plan view of the structure of first dynode plate DY1. As shown in FIG. 11A, the first dynode plate DY1 has electron multiplier holes CH1 to CH8, respectively corresponding to the channels assigned to the plurality of the light incidence regions 210 partitioned on the entrance face plate 210. Here, CH1 indicates the electron multiplier holes for the first channel, CH2 indicates the electron multiplier holes for the second channel, CH3 indicates the electron multiplier holes for the third channel, CH4 indicates the electron multiplier holes for the fourth channel, CH5 indicates the electron multiplier holes for the fifth channel, CH6 indicates the electron multiplier holes for the sixth channel, CH7 indicates the electron multiplier holes for the seventh channel, and CH8 indicates the electron multiplier holes for the eighth channel. A secondary electron emission surface is formed at each electron multiplier hole.
The first dynode plate DY1 furthermore has through holes 280a which install the ceramic spacers Sp and are disposed at the peripheries of electron multiplier holes CH1 to CH8 for channels, and has a fixing member 280b which is fixed by welding to the lead pin 240 extending from the stem 230 for setting the first dynode plate DY1 to a predetermined potential. Though the first dynode plate DY1 is shown in FIG. 11A, the second and third dynode plates DY2 and DY3 have the same structure.
Meanwhile, as shown in FIG. 11B, the fourth dynode plate DY4 has, at its outer peripheral portions, a plurality of notch portions 280c with curved surfaces. FIG. 11B shows a plan view of the structure of fourth dynode plate DY4. In similar to the first dynode plate DY1, this fourth dynode plate DY4 has electron multiplier holes CH1 to CH8, respectively corresponding to the channels assigned to the plurality of the light incidence regions 210a partitioned on the entrance face plate 210. Here, CH1 indicates the electron multiplier holes for the first channel, CH2 indicates the electron multiplier holes for the second channel, CH3 indicates the electron multiplier holes for the third channel, CH4 indicates the electron multiplier holes for the fourth channel, CH5 indicates the electron multiplier holes for the fifth channel, CH6 indicates the electron multiplier holes for the sixth channel, CH7 indicates the electron multiplier holes for the seventh channel, and CH8 indicates the electron multiplier holes for the eighth channel. A secondary electron emission surface is formed at each electron multiplier hole.
The fourth dynode plate DY4 furthermore has through holes 280a which install the ceramic spacers SP and arranged at the peripheries of electron multiplier holes CH1 to CH8 for the respective channels, and has a fixing member 280b which is fixed by welding to the lead pin 240 extending from the stem 230 for setting the fourth dynode plate DY4 to a predetermined potential. Though the fourth dynode plate DY4 is shown in FIG. 11B, the fifth, sixth, and eighth to twelfth dynode plates DY5, DY6, and DY8 to DY12 have the same structure.
In particular, at the outer peripheral portions of each of these fourth to sixth and eighth to twelfth dynode plates DY4 to DY6 and DY8 to DY12, the plurality of notched portions 280c are provided in correspondence to the lead pins 240 respectively connected to the plurality of control plates DY7-1CH to DY7-8CH that constitute seventh dynode plate DY7, and are provided with curved surfaces so as to be separated from the lead pins 240 by a predetermined distance. This is because as the photomultiplier 200 is made compact, the distances between metal members, for example, the distances between a dynode plate DY and the lead pins 240 become close inevitably and the possibility of discharge being generated at the edge portions of these metal members becomes high. In the present embodiment, the lead pins 240, connected to control plates DY7-1CH to DY7-8CH must furthermore be introduced in a space in which a plurality of dynode plates are integrated. Thus with this embodiment, by intentionally processing the outer peripheral portions of the fourth dynode plate DY4 to shapes having curved surfaces, the generation of discharge is restrained.
The dynode plates with the above structure are respectively laminated with the ceramic spacers SP arranged in between as shown in FIG. 11C. That is, by installing the ceramic spacers SP at positions of the through holes 280a of a lower dynode plate and then installing an upper dynode plate so that its through holes 280a overlap with the ceramic spacers SP, a dynode unit, in which a plurality of dynode plates are laminated, is obtained.
However, in the photomultiplier applied to the photodetector according to the present invention, the structure of the seventh dynode differs. That is, as shown in FIG. 12A, this seventh dynode plate is constituted by the control plate DY7-1CH having the electron multiplier holes CH1 for the first channel, the control plate DY7-2CH having the electron multiplier holes CH2 for the second channel, the control plate DY7-3CH having the electron multiplier holes CH3 for the third channel, the control plate DY7-4CH having the electron multiplier holes CH4 for the fourth channel, the control plate DY7-5CH having electron multiplier holes CH5 for the fifth channel, the control plate DY7-6CH having the electron multiplier holes CH6 for the sixth channel, the control plate DY7-7CH having the electron multiplier holes CH7 for the seventh channel, and the control plate DY7-8CH having the electron multiplier holes CH8 for the eighth channel. For example, the control plate DY7-2CH is provided with through holes 290a for setting the ceramic spacers SP for maintaining a state of insulation between the dynode plates DY6 and DY8 that are positioned above and below, and with a fixing member 290b that is fixed by welding by solder to the lead pin 240. The other control plates DY7-1CH and DY7-3CH to DY7-8CH have the same structure.
As shown in FIG. 12B, the control plate DY7-2CH with the above-described structure is supported by three ceramic spacers SP disposed between the control plate DY7-2CH and the eighth dynode plate DY8, and by the lead pin 240 extending from the stem 230. Also by the sixth dynode DY6 being installed above the control plate DY7-2CH via the ceramic spacers SP, the control plates DY7-1CH to DY7-8CH that make up the seventh dynode plate are sandwiched, in the state of being separated from each other electrically and physically, between the sixth and eighth dynode plates DY6 and DY8. By this arrangement, each of the control plates DY7-1CH to DY7-8CH that make up the seventh dynode plate can be set to an arbitrary potential, and the gain adjustment for each channel can be realized by a more compact structure. In addition, since extraneous structures can be eliminated, the entirety of the photomultiplier 200 can be made even more compact.
As described above, FIGS. 7A and 7B show the first and second configurations of the secondary section 320 applicable to the bleeder circuit unit 300, whereas the secondary section 320 can be realized by other configurations. For example, FIGS. 13A and 13B show the third and fourth configurations of the secondary section 320 applicable to the bleeder circuit unit 300.
The secondary section 320 of the third configuration, as shown in FIG. 13A, has eight external power supplies (external power supply 1 to external power supply 8), and each of the external power supplies is prepared so as to correspond to the associated one of the control plates DY7-1CH to DY7-8CH. In the third configuration of the secondary section 320, the external power supply 1 to the external-power supply 8 directly apply the control plates DY7-1CH to DY7-8CH, respectively. In the third configuration, these eight external power supplies are included in the secondary section 320.
On the other hand, the fourth configuration of the secondary section 320 is constituted by a plurality of transistor circuits that are arranged in parallel, and each transistor circuit is prepared so as to correspond to the associated one of the control plates DY7-1CH to DY7-8CH. In particular, FIG. 14A shows a transistor circuit for the control plate DY7-1CH. In the transistor circuit for the control plate DY7-1CH, the output voltage applied to the associated control plate DY7-1CH can be adjusted by the input voltage V1CH applied from the external power supply. In the fourth configuration, such an external power supply has eight output ports respectively outputting the input voltages V1CH to V8CH, and it is also included in the secondary section 320.
Specifically, as shown in FIG. 14B, when the external power supply outputs the voltage V1CH below the threshold voltage (+0.7V) of the transistor Tr1, the voltage control for the control plate DY7-1CH becomes a OFF state. In this OFF state of voltage control, the transistor circuit for the control plate DY7-1CH becomes equal to the resistance circuit shown in FIG. 15A, and the control plate DY7-1CH is applied with a minimum voltage of −561V as an output voltage VOUT. This voltage value is lower than the voltage applied to the sixth dynode plate DY6. On the other hand, when the external power supply outputs the voltage V1CH exceed the threshold voltage (+0.7V) of the transistor Tr1, the voltage control for the control plate DY7-1CH transfers into a ON state. In this ON state of voltage control, the transistor circuit for the control plate DY7-1CH becomes equal to the resistance circuit shown in FIG. 15B. That is, the voltage (VOUT) to be applied to the control plate DY7-1CH can be changed from the minimum voltage 0 f-561V to the maximum voltage of −361V, by the external power supply controlling input voltage is applied with a minimum voltage V1CH. Other transistor circuits for the control plates DY7-2CH to DY7-8CH operate in a similar manner as the above-explained transistor circuit for the control plate DY7-1CH.
Next, the operation of the gain control method according to the present invention will be explained with reference to FIGS. 16. In the following explanation, the fourth configurations shown in FIG. 13B is applied as the secondary section 320.
In the fourth configuration, the primary section 310 applies a voltage of −1000V to the cathode 210b and makes the anodes 270 be grounded, in similar to the first and second configurations shown in FIGS. 7A and 7B. Furthermore, the primary section 310 applies, as shown in FIG. 8A, desirable voltages to the dynode plates DY1-DY6 and the dynode plates DY8-DY12 excluding the seventh dynode plate such that potentials thereof decrease stepwise from the first dynode plate DY1 to the twelfth dynode plate DY12. On the other hand, the secondary section 320 individually applies desirable voltages to the control plates DY7-1CH to DY7-8CH. The secondary section 320 can expand the adjustable potential range for the control plates DY7-1CH to DY7-8CH, as compared with the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8, by the external power supply individually adjusting the input voltages V1CH to V8CH. In the following, the voltage control of the secondary section 320 with the fourth configuration will be explained with reference to FIG. 16.
Here, it is assumed that the maximum gain for each channels in the entire photomultiplier 200 is 1 when each of the control plates DY7-1CH to DY7-8CH is applied with −462V as a reference voltage value. In the case of adjusting the gain of the selected channel to 1/10 with respect to the normalized gain of 1, the gain adjustment can be achieved by applying a minimum voltage of −538V or a maximum voltage of −385V to the associated one (corresponding to the selected one) of the control plates DY7-1CH to DY7-8CH, in the ON state of the voltage control. At this time, the output voltage increase is from +23V to +176V (±77V with respect to the reference voltage value of −462V). Furthermore, in the case of adjusting the gain of the selected channel to 1/100 with respect to the normalized gain of 1, the gain adjustment can be achieved by applying a minimum voltage of −561V or a maximum voltage of −361V to the associated one (corresponding to the selected channel) of the control plates DY7-1CH to DY7-8CH, in the ON state of the voltage control. At this time, the output voltage increase is from 0V to +200V (±100V with respect to the reference voltage value of −462V), and is wider than the potential difference between the sixth dynode plate DY6 and the eighth dynode plate DY8.
As described above, the gain control method according to the present invention, using the secondary section 320 having the fourth configuration, can also individually control the gain of each channel by two digits or more.
From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

## Claims

1. A photodetector comprising:
a photomultiplier functioning as a photo-sensing device, said photomultiplier comprising:
a sealed container having an entrance face plate partitioned into a plurality of light incidence regions, and a stem opposing said entrance face plate;
a photocathode emitting photoelectrons into said sealed container in response to light having passed through said entrance face plate;
a plurality of anodes respectively prepared for electron multiplier channels which are assigned to said light incidence regions partitioned on said entrance face plate, said plurality of anodes being respectively positioned inside said sealed container so as to oppose said associated light incidence regions; and
a dynode unit housed in said sealed container, disposed between said anodes and said photocathode and constituted by N (an integer of no less than 3) dynode plates, each of said dynode plates being provided with an electron multiplier hole for the associated channel, concerning all channels, arranged at a position corresponding to the associated one of said plurality of light incidence regions, wherein a n-th (an integer of no less than 2) dynode plate in said dynode unit is constituted by a plurality of control plates each having an electron multiplier hole for the associated channel and being electrically and physically separated from the others; and

a bleeder circuit unit for setting at least each of said dynode plates constituting said dynode unit in said photomultiplier to predetermined potentials, said bleeder circuit unit comprising:
a primary section setting potentials of a first to (n−1)-th and (n+1)-th to N-th dynode plates respectively; and
a secondary section setting potentials of said control plates constituting said n-th dynode plate respectively, said secondary section having a structure, for adjusting gains of the electron multiplier channels respectively, capable of individually changing a potential of the selected one of said control plates over the potential range that is wider than a potential difference between said (n−1)-th dynode plate and said (n+1)-th dynode plate.

2. A photodetector according to claim 1, wherein, in said photomultiplier, said control plates are supported in a state of being sandwiched, via insulators, by said (n−1)-th dynode plate and said (n+1)-th dynode plate.

3. A photodetector according to claim 1, wherein said photomultiplier further includes a protection electrode provided between said stem and said dynode unit, and having a plurality of through holes each individually housing the associated one of said anodes.

4. A photodetector according to claim 1, wherein said photomultiplier further includes a focusing electrode provided between said photocathode and said dynode unit, and having a plurality of through holes each arranged at a position corresponding to the associated one of said light incidence regions.

5. A photodetector according to claim 1, wherein said secondary section includes one or more external power supplies each capable of controlling a voltage to be applied to the associated one of said control plates.

6. A method of controlling channel gains in a photomultiplier applied to a photodetector according to claim 1, comprising the steps of:
setting, by using said primary section included in said bleeder circuit unit, potentials of said first to (n−1)-th and (n+1)-th to N-th dynode plates in said photomultiplier such that potentials to be set decrease stepwise from said first dynode plate to said N-th dynode plate;
setting, by using said secondary section included in said bleeder circuit unit, potentials of said control plates constituting said n-th dynode plate within a potential range wider than a potential difference between said (n−1)-th dynode plate and said (n+1)-th dynode plate; and
adjusting the gains of the electron multiplier channels in said photomultiplier respectively by said secondary section individually changing a potential of the selected one of said control plates over the potential range.