Side-on type photomultiplier

Abstract: The present invention relates to a versatile side-on type photomultiplier comprising a structure for improving the uniformity in light receiving sensitivity. Disposed on the outer peripheral surface of a sealed envelope of this photomultiplier is a restricting member which guides light to be detected into, of the light receiving surface of a photocathode, an effective region where the light receiving sensitivity is high, thereby restraining the light to be detected from reaching the outside of the effective region.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a light detecting apparatus for detecting light incident thereon and, in particular, to a so-called side-on type photomultiplier having a photocathode which is inclined with respect to the direction of incidence of the light. Related Background Art For instance, Japanese Patent Publication No. 2-22334 discloses an example of side-on type photomultipliers. In the photomultiplier disclosed in this publication, light passing through a slit is guided through an imaging lens onto a reflection type photocathode. Such a configuration shortens the electron transit time between the photocathode and the dynode of the first stage, thereby reducing the fluctuation in electron transit time. SUMMARY OF THE INVENTION Having studied the conventional side-on type photomultiplier having the above-mentioned configuration, the inventors have found the following problems. Namely, in the conventional photomultiplier, since the imaging lens is disposed as a member separated from the photomultiplier, it is necessary for the imaging lens and the photocathode to be precisely aligned, in terms of optical axis, with respect to the light to be detected. In particular, this optical axis alignment is necessary after an operation for replacing the photomultiplier. Also, an additional apparatus is necessary so as to be utilized for the optical axis alignment of the imaging lens with the photocathode, thus affecting usability. Further, since a part of light which should reach the photocathode is eliminated by a slit plate, the conventional photomultiplier may not be suitable for such a use as measurement of weak light, thus lacking in versatility (restricting its use). In order to overcome the above-mentioned problems, it is an object of the present invention to provide a side-on type photomultiplier comprising a structure excellent in versatility and a structure for improving the uniformity in its light receiving sensitivity. The side-on type photomultiplier according to the present invention guides light to be detected into a reflection type photocathode provided within a sealed envelope; cascade-multiplies by an electron multiplier comprising a plurality of stages of dynodes, photoelectrons emitted from the reflection type photocathode; and collects thus multiplied secondary electrons at an anode as output electric signals. Specifically, the side-on type photomultiplier according to the present invention comprises, at least, an envelope made of a material transparent to light to be detected; a photocathode for emitting photoelectrons in response to said light to be detected reaching said photocathode after being transmitted through said envelope, said photocathode having a light receiving surface with a predetermined area, said photocathode being accommodated in said envelope and supported by an electrode member so as to be inclined with respect to a direction of incidence of said light to be detected; a lens for limiting an area of a light incident region on said photocathode, where said light to be detected is to reach, so as to make the area of said light incident region smaller than the area of said light receiving surface of said photocathode, said lens being positioned outside said envelope; and an adhesive, provided between said lens and an outer peripheral surface of said envelope, for securing said lens onto said outer peripheral surface of said envelope, said adhesive being transparent to said light to be detected, the photomultiplier characterised by; the lens including a lens element having a positive refracting power, said lens element having a bonding surface substantially corresponding to a shape of said outer peripheral surface of said envelope. Here, the photomultiplier is of side-on type in which light to be detected is made incident thereon from a side face of the envelope. Accordingly, the above-mentioned photocathode is a reflection type photocathode. This reflection type photocathode emits photoelectrons into a direction opposite to the advancing direction of the light to be detected. Preferably, the above-mentioned lens is made of a material transparent to the light to be detected. The above is provided in order to improve the contact between the condenser lens and the envelope and reduce the optical coupling loss therebetween. The condenser lens has a bonding surface, which may be a concave surface having a shape matching that of the outer peripheral surface, which may be a convex surface of the envelope. Further, since this condenser lens is attached beforehand to the envelope of the photomultiplier by the adhesive provided therebetween, neither operation nor apparatus for effecting the optical axis alignment of the condenser lens is necessary. Preferably, the condenser lens includes a cylindrical lens having a bonding surface with a shape substantially identical to that of the light incident region (on the outer peripheral surface) of the envelope, and a cylindrically-curved light entrance surface facing the bonding surface. When such a cylindrical lens is employed, the light to be detected can be collected, in a slit form, within the effective region on the photocathode, thus elongating the form of the collected light on the photocathode in its longitudinal direction so as to match the long form of the photocathode. Accordingly, the form of the area of the photocathode 9 on which the collected light reaches can match the long form of the dynode in each stage, thus allowing the electron multiplying region of each dynode to be utilized efficiently. Also, it becomes unnecessary to perform an operation for inserting a slit plate between an object to be measured and the photomultiplier, and the axial alignment of the slit in the slit plate with the photocathode. Here, "cylindrical lens" refers to a lens having at least one surface formed like a part of a cylinder and yielding astigmatism such that a point of light extends into a line. Also, the above-mentioned condenser lens may include a hemispherical lens having a bonding surface with a shape substantially identical to the outer peripheral surface of the envelope and a spherically-curved light entrance surface. Since the light to be detected can be collected onto the photocathode in a spot form, such an arrangement is effective for detecting weak light in particular. 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 scope of the invention will be apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view for explaining an assembling step of a first embodiment of the side-on type photomultiplier according to the present invention; Fig. 2 is a perspective view showing a structure of a lens element (restricting member) made of a plastic material, which is applicable to the side-on type photomultiplier shown in Fig. 1; Fig. 3 is a sectional view of the first embodiment taken along line I-I in Fig. 1; Fig. 4 is a view for explaining a function of the lens element (restricting member) employed in the photomultiplier and light detecting apparatus according to the present invention, which corresponds to the sectional view of the first embodiment taken along line I-I in Fig. 1; Fig. 5 is a perspective view showing the first embodiment of the side-on type photomultiplier according to the present invention and a light detecting apparatus to which the first embodiment is applied; Fig. 6 is a measurement system for measuring a sensitivity characteristic of the side-on type photomultiplier according to the present invention; Fig. 7 is a view showing a configuration of a bleeder circuit and power supply in the measurement system of Fig. 6; Fig. 8 is a graph showing respective anode outputs of side-on type photomultipliers with and without a lens element (restricting member) measured by the measurement system shown in Fig. 6; and Fig. 9 is a perspective view for explaining an arrangement of a second embodiment of the side-on type photomultiplier in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, preferred embodiments of the side-on type photomultiplier according to the present invention will be explained in detail with reference to Figs. 1 to 9. Fig. 1 is a perspective view for explaining an assembling step of a first embodiment of the side-on type photomultiplier according to the present invention. In this drawing, the side-on type photomultiplier includes a sealed envelope 1 transparent to light to be detected. This sealed envelope 1 is formed as a transparent cylinder whose upper and lower ends are closed, while comprising borosilicate glass, UV glass, silica glass, or the like. In the sealed envelope 1, insulator substrates 2a and 2b made of ceramics or the like are respectively disposed at upper and lower portions thereof, such that various kinds of electrodes are supported as being held between a pair of the insulator substrates 2a and 2b. Secured to the bottom portion of the sealed envelope 1 is a pin base 3 made of a resin. This pin base 3 is provided with a plurality of pin terminals 3a, by which the various kinds of electrodes are lead to the outside. As shown in Figs. 1 and 3, supported by a pair of the insulator substrates 2a and 2b therebetween are a reflection type photocathode 9 supported by an electrode plate 5 so as to be inclined with respect to the direction of incidence of the light to be detected (collimated light) that is indicated by depicted arrow A10; an electron multiplier 6 comprising a plurality of stages of dynodes 6a to 6i for cascade-multiplying a photoelectron emitted from the photocathode 9; and an anode 7 for collecting thus multiplied electrons (secondary electrons) as output signals. Further disposed between a light incident region 4 on the outer peripheral surface of the envelope 1 and the photocathode 9 is a grid electrode 8 for securely guiding the photoelectrons emitted from the photocathode 9 into the dynode 6a of the first stage. This grid electrode 8 is set to the same potential as the photocathode 9. Also, the photocathode 9 is formed on the electrode plate 5 and faces the light incident region 4 of the envelope 1. Also, in this embodiment, a cylindrical lens 10, as a condenser lens, is secured to the light incident region 4 on the outer peripheral surface (convex surface) of the envelope 1 with an adhesive 30 disposed therebetween. This cylindrical lens 10 is made of a material (transparent to the light to be detected) identical to that of the envelope 1, while having a bonding surface 10a with a radius of curvature substantially identical to that of the outer peripheral surface of the envelope 1 and a cylindrically-curved light entrance surface 10b with a radius of curvature smaller than that of the bonding surface 10a. Accordingly, when the bonding surface 10a of the cylindrical lens 10 coated with the adhesive 30 of epoxy resin type (e.g., product No. 1565 manufactured by Cemedine Co., Ltd.) is pressed against the light incident region 4 of the envelope 1, the cylindrical lens 10 can securely be bonded to the outer peripheral surface of the envelope 1 (see Fig. 5). Here, either the whole surface or only marginal portions of the bonding surface 10a of the cylindrical lens 10 may be coated with the adhesive 30. The condenser lens may also be a plastic cylindrical lens 50 shown in Fig. 2 obtained by injection molding. This cylindrical lens 50 has side surfaces 51 and 52 formed by cutting both sides of the cylindrical lens 50. Further, the radius of curvature of the light entrance surface 10b of the cylindrical lens 10 is selected such that, as shown in Fig. 4, the light incident on the cylindrical lens 10 substantially forms a focal point in an effective region A of the photocathode 9. When the cylindrical lens 10 like this is utilized, the light to be detected can be collected into a slit form on the effective region A of the photocathode 9. Thus, the form of collected light on the photocathode 9 is elongated in its longitudinal direction so as to match the long form of the photocathode 9. Accordingly, when the part generating photoelectrons is formed like a long slit, the long electron multiplying region produced by each of the dynodes 6a to 6i can effectively be utilized. Also, the condenser lens can guide a part of collimated light, which cannot reach the photocathode 9 by itself, to the effective region A. It indicates that an incident region D of the light to be detected is enlarged, thereby the photomultiplier can sufficiently be applied to such a use as measurement of weak light. As shown in Fig. 5, a light detecting apparatus to which the side-on type photomultiplier according to the present invention is applied comprises a collimator 40 for collimating the light to be detected in order to sufficiently obtain the above-mentioned function of the condenser lens. Fig. 6 is view showing a measurement system for measuring the uniformity in light receiving sensitivity of a side-on type photomultiplier which is an object to be measured. The measurement system shown in Fig. 6 comprises, at least, a light source 600; a spectroscope 500 for selecting a light component with a predetermined wavelength from the light emitted from the light source 600; a collimator 400 for collimating the light component selected by the spectroscope 500; a black box 300 accommodating a photomultiplier 100 (including photomultipliers with and without the lens element 10) which is the object to be measured; a stage 200 for relatively moving the object to be measured with respect to a beam B10 emitted from the collimator 400; a power supply 700 for supplying a desired voltage to the object to be measured; a bleeder circuit 900 for dividing the voltage supplied from the power supply 700; and an ammeter 800 for detecting the output signals obtained from the anode of the object to be measured 100. Here, the stage 200 on which the object to be measured 100 is mounted and the bleeder circuit 900 are accommodated in the black box 300. The stage 200 moves the object to be measured 100 in the directions indicated by depicted arrows C10 (directions perpendicular to the paper surface) and in the directions indicated by depicted arrows C11 (directions orthogonal to the directions indicated by C10). As shown in Fig. 7, the bleeder circuit 900 comprises a plurality of resistors connected in series, thereby dividing the voltage supplied from the power supply 700. Here, the above-mentioned effective region A is, in the whole surface of the photocathode 9, not only an area which has a high sensitivity but also an area where stray electrons are hard to occur. This effective region A is an area which is near the dynode 6a of the first stage, is positioned near the center axis side of the envelope 1, and is far from the grid electrode 8 having the same potential. Namely, as can also be seen from Figs. 2 and 3, the effective region A refers to, in the photocathode 9, an area which extends from near the center portion toward the dynode 6a of the first stage and where the light receiving sensitivity (anode output) in the width directions (directions indicated by arrows C10 in Fig. 6) is not lower than 80%. Here, there are also cases where the effective area A is determined as an area in which the light receiving sensitivity in the width directions (C10) is not lower than 90%. Since the place for generating each photoelectron is restricted to a narrow area referred to as the effective region A, there is little deviation among times at which individual photoelectrons are generated. Also, since the photoelectrons are generated at places close to each other, fluctuation in electron transit time can be made very small. Further, even when the position of the light source fluctuates to a certain extent, thereby somewhat changing the position of light incident on the condenser lens, since the light is collected at the effective region A for photoelectrons, in combination with little fluctuation in electron transit time, the output from the anode fluctuates very little. Moreover, even when the positioning of the photocathode with respect to the object to be measured is not precisely effected, light can be collected at an appropriate position of the photocathode 9 due to the condensing action of the condenser lens, thereby facilitating the optical axis alignment of the object to be measured with the photocathode 9. Thus, a small shift in the optical axis hardly affects the uniformity in light receiving sensitivity. Such a condensing action is effective, in particular, for weak light such as chemiluminescence, bioluminescence, or fluorescence, thereby contributing to improvement in S/N. Besides, since the condenser lens is firmly bonded to the outer peripheral surface of the envelope in the side-on type photomultiplier of the present invention, operations for making a product are easy to effect, and the yield of the product is quite favorable. Also, when the form or bonding position of the condenser lens is appropriately changed in response to the output sensitivity characteristic of the photomultiplier before the mounting of the lens, a product can easily be made in conformity with a need. Next, the inventors measured changes in light receiving sensitivity between photomultipliers with and without a condenser lens by using the measurement system shown in Figs. 6 and 7. Specifically, the wavelength of the light to be detected was 400 nm, whereas its spot diameter was 1 mm. The condenser lens used was a cylindrical lens having a width of 15 mm (in the directions indicated by C10 in Fig. 6) and a length of 26 mm (in the directions indicated by C11 in Fig. 6). Here, the radius of curvature of the light entrance surface 10b was designed such that the collimated light to be detected could reach into the effective region A. The scanning pitch of the spot light (having a wavelength of 400 nm and a spot diameter of 1 mm) in the width directions C10 was 1 mm. On the other hand, the scanning pitch of the spot light (having a wavelength of 400 nm and a spot diameter of 1 mm) in the length directions C11 was also 1 mm. By connecting a plurality of 100-kΩ resistors in series, the bleeder circuit 900 equally divided the applied voltage. An output terminal of the anode 7 is electrically connected to the ammeter 800, whereas a voltage of -750 V was applied to the photocathode 9 and the grid electrode 8. Fig. 8 includes graphs each showing a relationship between the incident position of the spot light (with a spot diameter of 1 mm) and the anode output measured under the condition mentioned above. In these graphs, solid and dashed lines respectively indicate measured results of the photomultipliers with and without the condenser lens. As can be seen from the upper-side graph in Fig. 8, the photomultiplier without the condenser lens can hardly measure the light to be detected incident on the outside of the effective region A. In the photomultiplier with the condenser lens, by contrast, a wide range of the light to be measured is guided by the condenser lens along the width directions C10 into the effective region A, thereby improving the uniformity in light receiving sensitivity. Here, as can be seen from the right-side graph in Fig. 8, due to the forms of the photocathode 9 and dynodes 6a to 6i, no remarkable difference could be found in the light receiving sensitivity along the length directions C11 between the cases with and without the condenser lens. The present invention should not be restricted to the first embodiment mentioned above. For example, the condenser lens may be a hemispherical lens. Fig. 9 is a perspective view showing an arrangement of a second embodiment of the side-on type photomultiplier (to which a hemispherical lens 12 is applied) according to the present invention. In the second embodiment, the hemispherical lens 12 is made of a material identical to that of the envelope 1 and comprises a bonding surface 12a matching the shape of the outer peripheral surface of the envelope 1 (i.e., having a radius of curvature substantially identical to that of the outer peripheral surface) and a spherically-curved light entrance surface 12b with a radius of curvature smaller than that of the bonding surface 12a. Accordingly, when the bonding surface 12a of the spherical lens 12 coated with an epoxy resin type adhesive is pressed against the light incident region 4 on the outer peripheral surface of the envelope 1, the hemispherical lens 12 can securely be bonded to the outer peripheral surface of the envelope 1. Here, either the whole surface or only marginal portions of the bonding surface 12a of the hemispherical lens 12 may be coated with the adhesive. Further, the radius of curvature of the light entrance surface 12b of the hemispherical lens 12 is selected such that the light incident on the hemispherical lens 12 substantially forms a focal point in the effective region A of the photocathode 9. Also, when the hemispherical lens 12 is utilized, the collimated light to be detected can be collected into a spot-like form on the effective region A of the photocathode 9. Selected as the location of this spot-like collected light portion is the center part on the effective region A where the light receiving sensitivity (anode output) in the length directions C11 is particularly high (see Fig. 8). When the light is thus substantially collected like a point, very weak light to be measured can securely be detected. Of course, the envelope 1 and the condenser lenses 10 and 12 may be made of materials such as plastics (see Fig. 2) which are different from each other. Also, the bonding surfaces 10a and 12a of the condenser lenses 10 and 12 may be flat surfaces in conformity with the shape of the outer peripheral surface of the envelope 1. Of course, the adhesive is not restricted to the epoxy resin type. As explained in the foregoing, in accordance with the present invention, a condenser lens is firmly bonded to the outer peripheral surface of the envelope. This condenser lens is disposed at a position where the incident light can be condensed onto, of the photocathode, the effective region having a high sensitivity, thereby a side-on type photomultiplier with a high versatility and a high uniformity in light receiving sensitivity can be realized. Also, after the electron multiplier is installed within the envelope, the condenser lens can firmly be bonded to the sealed envelope as a separate member. Further, worthy of special mention in the photomultiplier integrated with a lens according to the present invention are effects that uniformity in light receiving sensitivity is remarkably improved by a very simple amelioration in which a condenser lens is firmly bonded to a conventional side-on type photomultiplier. From the invention thus described, it will be obvious that the invention may be varied in many ways. Further details of aspects of the present invention can be found in Japanese Application No. 237022/1996.

Claims: A side-on type photomultiplier comprising: an envelope (1) made of a material transparent to light to be detected; a photocathode (9) for emitting photoelectrons in response to said light to be detected reaching said photocathode (9) after being transmitted through said envelope (1), said photocathode (9) having a light receiving surface with a predetermined area, said photocathode (9) being accommodated in said envelope (1) and supported by an electrode member (5) so as to be inclined with respect to a direction of incidence of said light to be detected; a lens (10) for limiting an area of a light incident region (4) on said photocathode (9), where said light to be detected is to reach, so as to make the area of said light incident region (4) smaller than the area of said light receiving surface of said photocathode, said lens (10) being positioned outside said envelope (1); and an adhesive (30), provided between said lens (10) and an outer peripheral surface of said envelope (1), for securing said lens (10) onto said outer peripheral surface of said envelope (1), said adhesive (30) being transparent to said light to be detected, the photomultiplier characterised by ; the lens (10) having a positive refracting power, said lens having a bonding surface substantially corresponding to a shape of said outer peripheral surface of said envelope (1). A side-on type photomultiplier according to claim 1, wherein said lens is a cylindrical lens having said bonding surface on a side facing said outer peripheral surface of said envelope. A side-on type photomultiplier according to claim 1, wherein said lens is a hemispherical lens having said bonding surface on a side facing said outer peripheral surface of said envelope. A light detecting apparatus comprising: a side-on type photomultiplier according to claim 1; and a collimator for collimating light to be detected which is to be made incident on said side-on type photomultiplier.

Inventors: GOTO TAKASHI, KIMURA SUENORI, KUME HIDEHIRO, TACHINO MASUMI, GOTO, TAKASHI, KIMURA, SUENORI, KUME, HIDEHIRO, TACHINO, MASUMI
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1997
Main classification: H01J43
IPC codes: H01J43