Photomultiplier

Abstract: A photomultiplier of head-on type has a transparent tubular glass bulb (1) and a principal photocathode (3) formed on the internal surface of a closed end of the glass bulb, that is, on the internal surface of light entrance window. In addition, this photomultiplier has a side photocathode (10) formed on the entire internal surface of side wall of glass bulb in the region adjacent to the principal photocathode. A reflection film (14) is formed on the outer surface of side wall of glass bulb as being opposed to the side photocathode. Some light entering the peripheral portion of light entrance window of glass bulb and traveling toward the outer surface of side wall is reflected inwardly by the reflection film and reaches the side photocathode to be converted into photoelectrons. The photoelectrons are guided to the electron multiplying unit (5) in glass bulb, where the photoelectrons are multiplied and detected by an anode (13).

Description: The invention relates to a photomultiplier. More specifically the invention relates to a photomultiplier in which light to be detected is incident through a closed end of a tubular closed container, a so-called head-on or end-on type photomultiplier. Fig. 1 shows a head-on type photomultiplier which is often conventionally used. This photomultiplier has a light entrance window 2 which is a closed end of a glass bulb 1 of a transparent tubular closed container and through which light L to be detected enters the container. The incident light L passing through the light entrance window 2 reaches a photocathode 3 formed inside the light entrance window 2, whereby photoelectrons are emitted. A focusing electrode 4 guides the emitted photoelectrons to an electron multiplying unit 5. The electron multiplying unit 5 as shown is comprised of a plurality of box-and-grid type dynodes 6 combined in multiple stages. The electron multiplying unit 5 successively multiplies the photoelectrons by the secondary emission effect, and the thus multiplied electrons are collected as output signal by an anode 7. In the conventional photomultiplier as described, aluminum is vapor-deposited over an internal surface of a side wall 8 of the glass bulb 1 in order to maintain the potential of photocathode 3. The aluminum vapor-deposited film is connected with the photocathode 3 at a part thereof. The aluminum vapor-deposited film 9 is formed over the entire internal surface of side wall 8 within the region between the light entrance window 2 and the focusing electrode 4 to prevent light from the side from entering the glass bulb 1. The existence of aluminum vapor-deposited film 9, however, causes a decrease in sensitivity of photomultiplier, because among light incident into an effective area in the light entrance window 2 light L' incident obliquely and outwardly at the periphery of effective area cannot be detected. As clearly shown in Fig. 2, a part of light L' incident obliquely at the periphery of light entrance window 2 goes out from the side wall 8 of glass bulb 1. The remainder is reflected by the external surface of side wall 8 toward inside the glass bulb 1, but is stopped from entering the inside by the aluminum vapor-deposited film 9. A countermeasure against this problem may be use of a photomultiplier having a side photocathode 10 which is formed by extending the photocathode 3 to above the internal surface of side wall 8 of glass bulb 1, as shown in Fig. 3. Using the photomultiplier of such type, a part of light L' could go into the side photocathode 10 and be detected as electrons. It is, however, impossible that all light L' impinge on the side photocathode 10, and therefore a part thereof is inevitably lost out of the side wall 8. In addition, the photomultiplier of Fig. 3 also detects light L" going toward the side of glass bulb 1, which hinders detecticn of only light coming into the front of light entrance window 2. The invention aims to provide a high-sensitive photomultiplier which can efficiently convert light entering a light entrance window into photoelectrons. According to the invention there is provided a photomultiplier comprising: a transparent tubular closed container; a photocathode formed on an internal surface of a closed end of said closed container; a side photocathode formed to cover a substantially entire internal surface of a side wall of said closed container in a predetermined region adjacent to said photocathode; a reflection film; an electron multiplying unit provided in said closed container, for multiplying electrons emitted from said photocathode when said photocathode receives light entering said closed end; and a focusing electrode having an aperture for focusing the electrons emitted from said photocathode and guiding the electrons to said electron multiplying unit, said focusing electrode being disposed between said photocathode and said electron multiplying unit in said closed container, characterised in that the reflection film is formed on an outer surface of said side wall of said closed container such that said reflection film is substantially opposed-to said side photocathode, said reflection film having a reflective surface on said side wall. In an embodiment to be described the side photocathode extends from the peripheral edge of the photocathode to a certain level between the focusing electrode and the photocathode such that electrons emitted from the side photocathode are guided to the electron multiplying unit through the aperture of the fccusing electrode. Further, in the embodiment, the reflection film extends at a predetermined length from the edge of outer surface of the side wall on the closed end side toward the other edge. A preferable reflection film is one formed by winding a reflective tape with a reflective surface on its adhesive face round the outer surface of the side wall of closed container. Alternatively, the reflection film may be formed by vapor-depositing aluminum onto the outer surface of the side wall of closed container, or by coating the outer surface of the side wall of closed container with barium sulfate. The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings. In the drawings: Fig. 1 is an end elevation to,schematically show a photomultiplier which is conventionally often used; Fig. 2 is an enlarged view to show the portion A in Fig. 1 as enlarged; Fig. 3 is an end elevation to schematically show another example of conventional photomultiplier; Fig. 4 is an end elevation to schematically show an embodiment of a photomultiplier according to the present invention; Fig. 5 is a perspective view to show a box-and-grid type dynode used in the photomultiplier of Fig. 4; and Fig. 6 is an enlarged view to show the portion B in Fig. 4 as enlarged. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which same or corresponding portions are denoted by same numerals. The terms "upper" or "lower" in the following description refer to upper or lower in each drawing. Fig. 4 shows a photomultiplier of so-called head-on type embodying the invention. In Fig. 4, reference numeral 1 denotes a transparent tubular closed container. Specifically, the container is a transparent cylindrical glass bulb closed at the both ends. A light entrance window 2 is formed at the upper closed end of glass bulb 1. A photocathode (as will be referred to as "principal photocathode") 3 is formed inside the light entrance window 2. The photocathode is a film of photoemitter, which is made of antimony (Sb) and an alkali metal (for example cesium (Cs), potassium (K), rubidium (Rb), etc.) or made of tellurium (Te) and an alkali metal. The photoemitter film is deposited over the entire internal surface of glass bulb 1. However, the portion of a photoemitter film lower than aluminum vapor-deposited film 9 is omitted in Fig. 4, because only the upper portion of photoemitter film than the upper edge of aluminum vapor-deposited film 9 serves as photocathode. A focusing electrode 4 is disposed inside the glass bulb 1 and at a position where it is opposed to the light entrance window 2. The focusing electrode 4 has an aperture 11, toward which photoelectrons emitted from the principal photocathode 3 are focused and through which the photoelectrons are guided to an electron multiplying unit 5. There are various types of electron multiplying unit 5 which can be employed herein. This embodiment employs box-and-grid type dynodes 6 arranged in multiple stages. Each box-and-grid type dynode 6 is of quarter cylinder as shown in Fig. 5, a partition face 6a of which is an entrance plane of electrons and the other partition face 6b of which is an exit plane thereof. A grid 12 of fine wires are arranged in the entrance plane 6a. The electrons are surely guided into inside the dynode by this grid 12. Electrons entering the entrance plane 6a impinge on the internal surface of quarter-cylinder side wall 6c, whereby secondary electrons are emitted. The secondary electrons are released from the exit plane 6b. The thus released secondary electrons then enter next dynode 6. The secondary electrons successively multiplied are collected by a mesh anode 7 located at the front surface of plate dynode 13 in the final stage. The peripheral portion of the principal photocathode 3 is extended by a predetermined length toward the focusing electrode 4 along the internal surface of side wall 8 of glass bulb 1. This extended portion constitutes a side photocathode 10 covering the entire upper internal surface of side wall 8. In this embodiment the lower edge of side photocathode 10 lies halfway between the principal photocathode 3 and the focusing electrode 4. In this arrangement, even photoelectrons emitted from the lowermost portion of side photocathode 10 can enter the electron multiplying unit 5 through the aperture 11 of focusing electrode 4. Also, the aluminum vapor-deposited film 9 for keeping the potential of photocathodes 3, 10 is formed over the entire internal surface of side wall 8 of glass bulb 1 in the region ranging from the lower edge of side photocathode 10 to a level of focusing electrode 4. Further, a reflection film 14 with internal surface of reflective surface is formed on a part of the external surface of glass bulb side wall 8 on the side of light entrance window 2, that is, on the upper portion of the outer surface such that it surrounds the entire side photocathode 10 while being opposed to the side photocathode 10. Preferably, the upper edge of reflection film 14 is located at the same level as the upper edge of the outer surface of side wall 8, and the lower edge of the film 14 is located slightly below the lower edge of side photocathode 10. The reflection film 14 may be formed by winding a reflective tape such as titanium oxide and aluminum round the side wall 8 of glass bulb 1, or by vapor-depositing aluminum or an equivalent on the side wall 8. Alternatively, the reflection film 14 may be formed by coating the side wall 8 with barium sulfate or an equivalent. In the arrangement as described, when light L enters the central portion of light entrance window 2, the light L passes through the light entrance window 2 and then impinges on the principal photocathode 3, whereby photoelectrons are emitted. The photoelectrons are guided by the focusing electrode 4 to the electron multiplying unit 5 and successively multiplied in the electron multiplying unit 5 by the secondary emission effect. The electrons are finally collected by the anode 7. This operation is the same as that in the conventional structure. If light L' enters the peripheral portion of light entrance window 2 obliquely and outwardly, the light L' advances in the side wall 8 of glass bulb 1 and is reflected by the reflective surface of reflection film 14, as shown in Fig. 6, which is an enlarged view of portion B in Fig. 4. This reflected light travels inwardly in the side wall 8 of glass bulb 1 and impinges on the side photocathode 10 formed on the internal surface of side wall. As a result, photoelectrons are emitted from the side photocathode 10. The photoelectrons are guided by the focusing electrode 4 to the electron multiplying unit 5 in the same manner as the light L and finally collected by the anode 7. In this manner the light L' entering the peripheral portion of light entrance window, which could not be detected in the conventional procedure, can also be detected in the present invention, whereby the sensitivity of photomultiplier can be improved. Following Table 1 is a table of results in experiment in which cathode lumen sensitivities Sk are compared for each of plural photomultipliers of the type shown in Fig. 1, different in size from each other, between samples with no reflection film and samples with reflection film formed on the outer surface of side wall of glass bulb. Also, Table 2 compares cathode lumen sensitivities of photomultipliers of the type shown in Fig. 3 different in size from each other with cathode lumen sensitivities of the photomultipliers with reflection film, that is, with cathode lumen sensitivities of photomultipliers embodying the invention. No. Without reflection film With reflection film Sensitivity up rate (%) Sk (µA/lm) Sk (µA/lm) (1) 172 173 0.58 (2) 158 158 0 (3) 190 191 0.53 (4) 175 175 0 (5) 162 161 -0.62 Average 171.4 171.6 0.10 No. Without reflection film With reflection film Sensitivity up rate (%) Sk (µA/lm) Sk (µA/lm) (6) 162 167 3.09 (7) 158 163 3.16 (8) 158 162 2.53 (9) 153 157 2.61 (10) 145 150 3.44 Average 155.2 159.8 2.97 As seen from the above tables, the simple provision of reflection film 14 rarely improves the sensitivity of photomultipliers, but the combination of reflection film 14 with the side photocathode 10 can greatly improve the sensitivity. According to the above described embodiment, the photocathode is formed also on the internal surface of side wall in head-on type photomultiplier and the reflection film is on the outer surface of side wall as surrounding the side photocathode, whereby incident light into the peripheral portion of light entrance window, which leaked out in the conventional procedure, can be guided to impinge on the photocathode so as to enlarge the effective area of photocathode. Accordingly, even with photomultiplier of same size a quantity of detected light increases as compared with those in the conventional structures, whereby the sensitivity of photomultiplier can be greatly improved. There are of course various modifications that may be made to the above described embodiment without departing from the invention. The scope of the invention is defined by the appended claims.

Claims: A photomultiplier comprising: a transparent tubular closed container (1); a photocathode (3) formed on an internal surface of a closed end (2) of said closed container; a side photocathode (10) formed to cover a substantially entire internal surface of a side wall (8) of said closed container in a predetermined region adjacent to said photocathode; a reflection film (14); an electron multiplying unit (5) provided in said closed container, for multiplying electrons emitted from said photocathode when said photocathode receives light entering said closed end; and a focusing electrode (4) having an aperture for focusing the electrons emitted from said photocathode (3) and guiding the electrons to said electron multiplying unit (5), said focusing electrode being disposed between said photocathode and said electron multiplying unit in said closed container, characterised in that the reflection film (14) is formed on an outer surface of said side wall (8) of said closed container such that said reflection film is substantially opposed to said side photocathode (10), said reflection film having a reflective surface on said side wall. A photomultiplier according to Claim 1, wherein said side photocathode (10) extends from a peripheral edge of said photocathode to a certain level between said focusing electrode (4) and said photocathode (3) such that the electrons emitted from said side photocathode (10) are guided to said electron multiplying unit (5) through said aperture of said focusing electrode. A photomultiplier according to Claim 1 or 2, wherein said reflection film extends at a predetermined length from an edge of outer surface of said side wall on a side of said closed end toward the other side. A photomultiplier according to Claim 1 or 2 or 3, wherein said reflection film (14) is formed by winding a reflective tape having a reflective surface on an adhesive face thereof round the outer surface of said side wall (8) of said closed container. A photomultiplier according to Claim 1 or 2 or 3, wherein said reflection film (14) is formed by vapor-depositing aluminum over the outer surface of said side wall (8) of said closed container. A photomultiplier according to Claim 1 or 2 or 3, wherein said reflection film (14) is formed by coating the outer surface of said side wall (8) of said closed container with barium sulfate.

Inventors: KIMURA SUENORI, OHSUGI NOBUYUKI, TOTSUKA TOSHIHARU, KIMURA, SUENORI, OHSUGI, NOBUYUKI, TOTSUKA, TOSHIHARU, KIMURA, SUENORI, HAMAMATSU PHOTONICS K.K., OHSUGI, NOBUYUKI, HAMAMATSU PHOTONICS K.K., TOTSUKA, TOSHIHARU, HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: H01J43
IPC codes: H01J43, H01J40